Aldehydes and Ketones

Aldehydes and Ketones

The connection between the structures of alkenes and alkanes was previously established, which noted that we can transform an alkene into an alkane by adding an H2 molecule across the C=C double bond.

The driving force behind this reaction is the difference between the strengths of the bonds that must be broken and the bonds that form in the reaction. In the course of this hydrogenation reaction, a relatively strong H--H bond (435 kJ/mol) and a moderately strong carbon-carbon pi bond (approx.270 kJ/mol) are broken, but two strong C--H bonds (439 kJ/mol) are formed. The reduction of an alkene to an alkane is therefore an exothermic reaction.

What about the addition of an H2 molecule across a C=O double bond?

Once again, a significant amount of energy has to be invested in this reaction to break the H--H bond (435 kJ/mol) and the carbon-oxygen pi bond (approx.375 kJ/mol). The overall reaction is still exothermic, however, because of the strength of the C--H bond (439 kJ/mol) and the O--H bond (498 kJ/mol) that are formed.

The addition of hydrogen across a C=O double bond raises several important points. First, and perhaps foremost, it shows the connection between the chemistry of primary alcohols and aldehydes. But it also helps us understand the origin of the term aldehyde. If a reduction reaction in which H2 is added across a double bond is an example of a hydrogenation reaction, then an oxidation reaction in which an H2 molecule is removed to form a double bond might be called dehydrogenation. Thus, using the symbol [O] to represent an oxidizing agent, we see that the product of the oxidation of a primary alcohol is literally an "al-dehyd" or aldehyde. It is an alcohol that has been dehydrogenated.

This reaction also illustrates the importance of differentiating between primary, secondary, and tertiary alcohols. Consider the oxidation of isopropyl alcohol, or 2-propanol, for example.

The product of this reaction was originally called aketone, although the name was eventually softened to azetone and finally acetone. Thus, it is not surprising that any substance that exhibited chemistry that resembled "aketone" became known as a ketone.

Aldehydes can be formed by oxidizing a primary alcohol; oxidation of a secondary alcohol gives a ketone. What happens when we try to oxidize a tertiary alcohol? The answer is simple: Nothing happens.

There aren't any hydrogen atoms that can be removed from the carbon atom carrying the --OH group in a 3º alcohol. And any oxidizing agent strong enough to insert an oxygen atom into a C--C bond would oxidize the alcohol all the way to CO2 and H2O.

A variety of oxidizing agents can be used to transform a secondary alcohol to a ketone. A common reagent for this reaction is some form of chromium(VI)--chromium in the +6 oxidation state -- in acidic solution. This reagent can be prepared by adding a salt of the chromate (CrO42-) or dichromate (Cr2O72-) ions to sulfuric acid. Or it can be made by adding chromium trioxide (CrO3) to sulfuric acid. Regardless of how it is prepared, the oxidizing agent in these reactions is chromic acid, H2CrO4.

The choice of oxidizing agents to convert a primary alcohol to an aldehyde is much more limited. Most reagents that can oxidize the alcohol to an aldehyde carry the reaction one step further -- they oxidize the aldehyde to the corresponding carboxylic acid.

A weaker oxidizing agent, which is just strong enough to prepare the aldehyde from the primary alcohol, can be obtained by dissolving the complex that forms between CrO3 and pyridine, C6H5N, in a solvent such as dichloromethane that doesn't contain any water.

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The Nomenclature of Aldehydes and Ketones

The common names of aldehydes are derived from the names of the corresponding carboxylic acids.

The systematic names for aldehydes are obtained by adding -al to the name of the parent alkane.

The presence of substituents is indicated by numbering the parent alkane chain from the end of the molecule that carries the --CHO functional group. For example,

The common name for a ketone is constructed by adding ketone to the names of the two alkyl groups on the C=O double bond, listed in alphabetical order.

The systematic name is obtained by adding -one to the name of the parent alkane and using numbers to indicate the location of the C=O group.

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Common Aldehydes and Ketones

Formaldehyde has a sharp, somewhat unpleasant odor. The aromatic aldehydes in the figure below, on the other hand, have a very pleasant "flavor." Benzaldehyde has the characteristic odor of almonds, vanillin is responsible for the flavor of vanilla, and cinnamaldehyde makes an important contribution to the flavor of cinnamon.

   

Aldehydes and ketones play an important role in the chemistry of carbohydrates. The term carbohydrate literally means a "hydrate" of carbon, and was introduced to describe a family of compounds with the empirical formula CH2O. Glucose and fructose, for example, are carbohydrates with the formula C6H12O6. These sugars differ in the location of the C=O double bond on the six-carbon chain, as shown in the figure below. Glucose is an aldehyde; fructose is a ketone.

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Thin Layer Chromatography - TLC

TLC is a simple, quick, and inexpensive procedure that gives the chemist a quick answer as to how many components are in a mixture. TLC is also used to support the identity of a compound in a mixture when the Rf of a compound is compared with the Rf of a known compound (preferrably both run on the same TLC plate).

A TLC plate is a sheet of glass, metal, or plastic which is coated with a thin layer of a solid adsorbent (usually silica or alumina). A small amount of the mixture to be analyzed is spotted near the bottom of this plate. The TLC plate is then placed in a shallow pool of a solvent in a developing chamber so that only the very bottom of the plate is in the liquid. This liquid, or the eluent, is the mobile phase, and it slowly rises up the TLC plate by capillary action.

As the solvent moves past the spot that was applied, an equilibrium is established for each component of the mixture between the molecules of that component which are adsorbed on the solid and the molecules which are in solution. In principle, the components will differ in solubility and in the strength of their adsorption to the adsorbent and some components will be carried farther up the plate than others. When the solvent has reached the top of the plate, the plate is removed from the developing chamber, dried, and the separated components of the mixture are visualized. If the compounds are colored, visualization is straightforward. Usually the compounds are not colored, so a UV lamp is used to visualize the plates. (The plate itself contains a fluor which fluoresces everywhere except where an organic compound is on the plate.)

The procedure for TLC, explained in words in the above paragraphs, is illustrated with photographs on the TLC Procedure page.

TLC Adsorbent

In the teaching labs at CU Boulder, we use silica gel plates (SiO2) almost exclusively. (Alumina (Al2O3) can also be used as a TLC adsorbent.) The plates are aluminum-backed and you can cut them to size with scissors. Our plates are purchased ready-made from EM Sciences or from Scientific Adsorbents. The adsorbent is impregnated with a fluor, zinc sulfide. The fluor enables most organic compounds to be visualized when the plate is held under a UV lamp. In some circumstances, other visualization methods are used, such as charring or staining.

TLC Solvents or Solvent Systems

Choosing a solvent is covered on the Chromatography Overview page. The charts at the bottom of that page are particularly useful.

Interactions of the Compound and the Adsorbent

The strength with which an organic compound binds to an adsorbent depends on the strength of the following types of interactions: ion-dipole, dipole-dipole, hydrogen bonding, dipole induced dipole, and van der Waals forces. With silica gel, the dominant interactive forces between the adsorbent and the materials to be separated are of the dipole-dipole type. Highly polar molecules interact fairly strongly with the polar Si—O bonds of these adsorbents and will tend to stick or adsorb onto the fine particles of the adsorbent while weakly polar molecules are held less tightly. Weakly polar molecules thus generally tend to move through the adsorbent more rapidly than the polar species. Roughly, the compounds follow the elution order given on the Chromatography Overview page.

The Rf value

Rf is the retention factor, or how far up a plate the compound travels. See the Rf page for more details:

Visualizing the Spots

If the compounds are colored, they are easy to see with the naked eye. If not, a UV lamp is used (see the Procedure page).

Troubleshooting TLC

All of the above (including the procedure page) might sound like TLC is quite an easy procedure. But what about the first time you run a TLC, and see spots everywhere and blurred, streaked spots? As with any technique, with practice you get better. One thing you have to be careful Examples of common problems encountered in TLC:

  • The compound runs as a streak rather than a spot

    The sample was overloaded. Run the TLC again after diluting your sample. Or, your sample might just contain many components, creating many spots which run together and appear as a streak. Perhaps, the experiment did not go as well as expected.

  • The sample runs as a smear or a upward crescent.

    Compounds which possess strongly acidic or basic groups (amines or carboxylic acids) sometimes show up on a TLC plate with this behavior. Add a few drops of ammonium hydroxide (amines) or acetic acid (carboxylic acids) to the eluting solvent to obtain clearer plates.

  • The sample runs as a downward crescent.

    Likely, the adsorbent was disturbed during the spotting, causing the crescent shape.

  • The plate solvent front runs crookedly.

    Either the adsorbent has flaked off the sides of the plate or the sides of the plate are touching the sides of the container (or the paper used to saturate the container) as the plate develops. Crookedly run plates make it harder to measure Rf values accurately.

  • Many, random spots are seen on the plate.

    Make sure that you do not accidentally drop any organic compound on the plate. If get a TLC plate and leave it laying on your workbench as you do the experiment, you might drop or splash an organic compound on the plate.

  • No spots are seen on the plate.

    You might not have spotted enough compound, perhaps because the solution of the compound is too dilute. Try concentrating the solution, or, spot it several times in one place, allowing the solvent to dry between applications. Some compounds do not show up under UV light; try another method of visualizing the plate. Or, perhaps you do not have any compound because your experiment did not go as well as planned.

    If the solvent level in the developing jar is deeper than the origin (spotting line) of the TLC plate, the solvent will dissolve the compounds into the solvent reservoir instead of allowing them to move up the plate by capillary action. Thus, you will not see spots after the plate is developed.

  • You see a blur of blue spots on the plate as it develops.

    Perhaps, you used an ink pen instead of a pencil to mark the origin?

Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2010. The information on these pages is available for academic use without restriction.

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Thin Layer Chromatography

A Description

Preparing the Chamber

Preparing Plates for Development

Developing the Plates

Identifying the Spots

Interpreting Data

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A Description

Thin layer chromatography (TLC) is a method for identifying substances and testing the purity of compounds. TLC is a useful technique because it is relatively quick and requires small quantities of material.

Equipment used in a thin layer chromatography experiment.

Separations in TLC involve distributing a mixture of two or more substances between a stationary phase and a mobile phase. The stationary phase is a thin layer of adsorbent (usually silica gel or alumina) coated on a plate. The mobile phase is a developing liquid which travels up the stationary phase, carrying the samples with it. Components of the samples will separate on the stationary phase according to how much they adsorb on the stationary phase versus how much they dissolve in the mobile phase.

Video: TLC process ( 5.83 M )




Preparing the Chamber

To a jar with a tight-fitting lid add enough of the appropriate developing liquid so that it is 0.5 to 1 cm deep in the bottom of the jar. Next, place a piece of filter paper into the jar so that it lines the walls and is immersed in the liquid. Why?

Close the jar tightly, and let it stand for about 30 minutes so that the atmosphere in the jar becomes saturated with solvent.

Video: Preparing the chamber ( 6.69 M )


This jar has been standing for 30 minutes. Why is it not ready to be used in a TLC experiment?

Answer




Preparing the Plates for Development

With a pencil, etch two small notches into the adsorbent about 2 cm from the bottom of the plate. The notches should be on the edges of the plate, and each notch should be the same distance up from the bottom of the plate. The notches must be farther from the bottom of the plate than the depth of the solvent in the jar. Using a drawn-out capillary tube, spot the samples on the plate so that they line up with the notches you etched.

Video: Spotting a sample ( 3.66 M ) Text description

If more sample is needed on the plate for the experiment, the sample may be re-spotted.

Video: Re-spotting a sample ( 1.38 M ) Text description


Question: What is wrong with the plate shown below?

Answer




Developing the Plates

After preparing the development chamber and spotting the samples, the plates are ready for development. Be careful to handle the plates only by their edges, and try to leave the development chamber uncovered for as little time as possible.

Video: Developing plates ( 6.61 M ) Text description

When the plates are removed from the chamber, quickly trace the solvent front (the highest solvent level on the plate) with a pencil.

Predict what will happen when this plate is developed in the chamber.

Answer




Identifying the Spots

If the spots can be seen, outline them with a pencil.

If no spots are obvious, the most common visualization technique is to hold the plate under a UV lamp (CAUTION: Do not look directly into the lamp.) Many organic compounds can be seen using this technique, and many commercially made plates often contain a substance which aids in the visualization of compounds.

Commercial TLC plate after development
in normal lighting.
Same TLC plate held under a UV lamp -
Note the appearance of additional spots.

A student removes his TLC plate from the chamber after the solvent reaches the etched pencil mark, but he cannot see any spots on the plate. What technique would you suggest to help him identify the spots?

Answer




Interpreting the Data

The Rf value for each spot should be calculated. Rf stands for "ratio of fronts" and is characteristic for any given compound on the same stationary phase using the same mobile phase for development of the plates. Hence, known Rf values can be compared to those of unknown substances to aid in their identifications.

(Note: Rf values often depend on the temperature and the solvent used in the TLC experiment; the most effective way to identify a compound is to spot known substances next to unknown substances on the same plate.)

In addition, the purity of a sample may be estimated from the chromatogram. An impure sample will often develop as two or more spots, while a pure sample will show only one spot.


Calculate the Rf values for the spots on the TLC slide below.

Answer

Which of the samples spotted on the TLC plate below were definitely composed of more than one substance?

Answer



Related Modules: Paper Chromatography, Gas Chromatography

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Element Analysis

These methods are available to detect isotope types and concentrations, of a majority of elements in the periodic table - some, even at trace levels.

Inductively Coupled Plasma-Emission Spectrometry (ICP)

  • Analysis of solutions or dissolved solids
  • Quantitative determination of 20 to 28 elements simultaneously
  • Detection limits in the parts-per-million (ppm) range

Typical applications: Sample types previously processed include wooden boards, plant tissue, soils, proteins, bones, human tissue, fish, snails, clams, wastewater, and ocean water

VG Plasma Quad 3 Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)

  • Analysis of solutions or dissolved solids
  • Rapid, multi-elemental analysis capability covering most elements in the periodic table
  • Detection limits in the parts-per-billion to parts-per-trillion range
  • Requires a minimum sample of 2 ml

Typical applications: To date, our lab has used this method to analyze protein, soil, water and plant samples. Other applications include detection of trace elements in a wide variety of aqueous matrices (drinking water, river, lake and ground water, waste water and effluent, and seawater) in solids after digestion (sediment, soil, sludge, road dust, air particulate matter, plant tissue and grain, rocks and minerals, etc.) and in samples of body fluids (blood, plasma, and urine)

Carbon, Hydrogen, and Nitrogen Analysis

Rapid, simultaneous determination of total carbon, hydrogen and nitrogen content of non-aqueous samples

  • Requires 1-3 mg of dry, ground plant or animal tissue and 200 mg of dry 18-40 mesh soils
  • Instrumentation: Perkin-Elmer 2400 Carbon, Hydrogen, Nitrogen Analyzer (CHN)

Typical applications: plants, soils, forestry, water, crystalline compounds, seston, complex carbohydrates, and plastics

Nutrient Analysis

Available chemistries are ammonia, chloride, nitrite, sulfate, ortho phosphate, alkalinity, total nitrogen and total phosphorus.

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Typical applications: water, wastewater

Other Services

Other equipment includes a microwave digestion system, a freeze-dryer for lyophilizing tissue, and a jar mill for grinding samples for low-level metal analysis. The laboratory uses several EPA-approved and AOAC methods for preparation and analysis.

For further information contact Becky Auxier (706) 542-6031 auxier@uga.edu

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Chemistry 2283g Experiment 1 – Alkyl Halides

1-1

EXPERIMENT 1: Preparation and Reactivity of Alkyl Halides

Relevant Sections in the text (Wade, 7th ed.)

6.7 (p. 228) Reactions of alkyl halides

6.8 – 6.12 (p. 229 – 242) The SN2 reaction: generality, factors affecting SN2 reactions,

substrate reactivity, mechanism

6.13 – 6.14 (p. 243 – 249) The SN1 reaction: mechanism, stereochemistry

General Concepts

The most common of the variety of methods available for preparing alkyl halides is the replacement

of the hydroxyl group of an alcohol. This type of reaction is representative of an important class of reactions

in organic chemistry called nucleophilic aliphatic substitution reactions. There are numerous ways of doing

this transformation, and we will discuss these in lecture. In this experiment you will convert an alcohol to an

alkyl halide by reaction with a hydrohalic acid. The overall reaction is shown in equation 1; however the

mechanism of the reaction depends on the structure of the alkyl group bearing the functional group being

replaced.

ROH + H-X RX + H2O ( 1 )

The reaction may occur by one of two mechanisms designated SN1 or SN2. Which mechanism operates

depends on the structure of the R group and the reaction conditions. The first step in both mechanisms is the

protonation of the alcohol to form an oxonium ion, converting the OH group into a good leaving group. What

happens next depends on the nature of the alkyl group, R. If R is a group that readily forms a carbocation,

then the slow, rate-determining step is the loss of a water molecule from the oxonium ion. Once formed, the

carbocation then reacts rapidly with a halide ion to form the alkyl halide.

SN1 Mechanism:

The first step is protonation of the alcohol, followed by the second step which is the formation of the

carbocation via the oxonium ion. This second step is the slow step (rate-determining).

HO

R4 H

R1

R3

R2

X H H2O

R4 H

R1

R3

R2

+ X

oxonium ion

RDS

H

R4

R1

R3

R2

planar carbocation

+ H2O

The third step is the attack of the planar carbocation by X- (in our case the halide ion).

H

R4

R1

R3

R2

X

X

R4 H

R1

R3

R2

Chemistry 2283g Experiment 1 – Alkyl Halides

1-2

This mechanism is followed when R is a tertiary alkyl group and is called SN1 (substitution, nucleophilic,

unimolecular in the rate-determining step). An SN1 mechanism may also be involved when R is a secondary

group or when R can form a resonance-stabilized carbocation, such as an allyl or benzyl cation (the more

stable the carbocation, the more likely the SN1 mechanism). If R is a primary alkyl group, then substitution

occurs generally without formation of the carbocation by an SN2 mechanism (why no carbocation? Because

1o

carbocations are generally unstable). In this case a halide ion attacks the primary carbon atom of the

oxonium ion in the rate-determining step, and the alkyl halide forms directly (why is this mechanism not

favorable when R = tertiary?). Because two species, the nucleophile (X-) and the substrate (the protonated

alcohol), are involved in the rate-determining step, the mechanism is designated SN2. When R is a

secondary group, an SN1 or an SN2 mechanism may be involved, depending on the reaction conditions.

SN2 Mechanism:

HO

H H

R1

H

R2

X H H2O

H H

R1

H

R2

X

X H

H R1

H R2

+ H2O

Experiment: Preparation and Reactivity of Alkyl Halides

In this experiment you will prepare 1-bromobutane (1-butyl bromide) from 1-butanol shown in

equation 2 (by what mechanism do you expect this substitution reaction to occur?). Compound 1-butanol is

a primary alcohol and thus one might expect that the reaction would proceed by an SN2 reaction. However,

when a strong, non-nucleophilic, acid is used as a solvent the conditions are very polar and the reaction can

proceed via an SN1 reaction. You will use the NMR spectrum of your product and gas chromatography

results to make decisions about which mechanism is operating.

NaBr / H2SO4

CH3CH2CH2CH2OH CH3CH2CH2CH2OH ( 2 )

The purity of the product (and any possible side products) will be assessed by gas chromatography and the

identity of the product will be determined by IR, 1H, and 13C NMR spectroscopy. You will also evaluate the

reactivity of your product and other substrates using a number of qualitative tests in Part B.

Chemistry 2283g Experiment 1 – Alkyl Halides

1-3

PART A: Preparation of 1-Bromobutane

Place 20.0 g of sodium bromide (NaBr) in a 250 mL

round-bottom flask. Add 15 mL of water and 15 mL of 1-

butanol. Mix thoroughly and cool the flask in an ice bath.

Slowly add 15 mL of concentrated sulfuric acid (conc.

H2SO4) to the solution. Remove the flask from the ice

bath, add a couple of boiling chips, and attach a reflux

condenser (Figure 1). Your TA will discuss assembly of

the apparatus.

Heat the flask with a heating mantle until most of the

salts have dissolved and the solution is at a gentle reflux

(what does “reflux” mean?). Note the appearance of two

layers (lower layer is the alkyl bromide). Continue the

reflux for 45 min.

Equip the flask for simple distillation with a heating mantle

(Figure 2). Your TA will discuss assembly of the apparatus.

Distil the mixture rapidly into an ice-cooled flask until the

head temperature reaches 120 °C. (Codistillation of 1-

bromobutane and water occurs, and the increased boiling

point is due to the codistillation of sulfuric acid and

hydrobromic acid with water).

Transfer the distillate to a separatory funnel with a closed stockcock! and perform an extraction. Add

25 mL of water to the organic layer in the funnel. Invert the funnel and point into the fume hood and

away from any other people. Remember to shake the funnel gently with venting! Separate and label

each layer carefully.

Wash the organic layer with 15 mL of saturated sodium bicarbonate (NaHCO3) solution and then again

with 15 mL of saturated sodium chloride solution. Collect the organic layers and discard the aqueous

layers.

Figure 1. Reflux setup

EXP

ex

Figure 2. Distillation setup

EXP

ex

Chemistry 2283g Experiment 1 – Alkyl Halides

1-4

Transfer the cloudy 1-bromobutane layer to a small Erlenmeyer flask, and dry it with anhydrous

magnesium sulfate (MgSO4). After allowing drying for about 10 minutes, gravity filter the mixture into a

clean, dry and tared (i.e. preweighed) 50 mL round-bottomed flask.

Determine the yield of crude product.

After adding a couple of new boiling chips, equip the flask for simple distillation again with a heating

mantle. Collect the product that comes off when boiling at 90-103 oC into a clean and dry tared flask.

Calculate the yield of the purified product.

Analyze your product by gas chromatography on a nonpolar column (SF-96 or SE-30 is satisfactory).

Obtain the retention times of standard samples of 1- and of 2-bromobutane.

Obtain an IR spectrum of your product and of 1-butanol and discuss the differences observed (you will likely

have to add some drying reagent to remove traces of water). Prepare a sample of your product for NMR

analysis (as you did in Chem 2273a experiment 3). You will be provided with the relevant spectra of 1-

butanol and the authentic NMR spectra of the two possible products. Assign the signals as completely as

possible and discuss the differences between the two spectra. In your report discuss the relative

percentages of 1- and 2-bromobutanes in your product in terms of the mechanism(s) of the reaction. You will

get this information by analyzing your GC trace and your NMR spectrum of your product.

Gas Chromatography

Gas chromatography is a versatile and widely used method for the examination of volatile mixtures by

distributing each component in the mixture between a gas phase and either a liquid or solid phase. The

method can be used to follow the course of a reaction both qualitatively and quantitatively, to separate and

isolate components of a mixture, to examine purity, and to assist in the identification of compounds.

The compound mixture is passed through a heated column (glass capillary tubing) as a vapour, surrounded

by an inert carrier gas. The column is coated with a stationary phase that varies in type and structure

depending on the type of separation required. The separation process is readily understood in qualitative

terms. Consider a two-component mixture entering the column. Each component will be partitioned between

the gas phase and the stationary liquid phase depending on the volatility of the component and its affinity for

the stationary phase. Since each component will have a different distribution between the two phases, the

most volatile/least soluble in the stationary phase will be sent through the column more rapidly than the

other, where separation is achieved.

Chemistry 2283g Experiment 1 – Alkyl Halides

1-5

PART B: SN1 and SN2 Reactivity

(i) Reactivity of Alkyl Halides Towards Sodium Iodide: Test for SN2

In this part of the experiment, you will test the reactivity of several alkyl halides in an SN2 reaction.

Iodide ion (I-) is an effective nucleophile in SN2 displacements. In acetone solution, other alkyl halides (alkyl

chlorides or bromides) can be converted to alkyl iodides easily by this method.

I + R Cl R I + Cl

Although one might expect such a reaction to be reversible, it can be driven to formation of R-I by using

anhydrous acetone as the solvent. Sodium iodide (NaI) is soluble in this solvent, but sodium chloride and

sodium bromide are not. If a reaction occurs, a precipitate of sodium chloride or sodium bromide forms and

thus the ion is not available in solution for the reverse reaction. The displacement mechanism involves a

one-step, concerted, SN2 reaction. Therefore, the reaction occurs most quickly when attack at the carbon that

bears the halogen (X) is least hindered sterically. For alkyl halides, the order of reactivity is primary >

secondary > tertiary.

(ii) Lucas Test: Test for SN1

The reactivity of the product will be evaluated using the Silver Nitrate Test. The Lucas Test will also be

used to distinguish the reactivity between various alcohols. The Lucas Test utilizes a reagent containing

hydrochloric acid and zinc chloride, which converts alcohols to alkyl chlorides. Primary alcohols do not react,

secondary alcohols react fairly quickly, and tertiary alcohols react very rapidly. A positive test depends on the

fact that the alcohol is soluble in the reagent, whereas the alkyl chloride is not; thus the formation of a second

layer or an emulsion constitutes a positive test. This is a good test for the feasibility of a particular alcohol to

undergo an SN1 reaction. The overall reactions are as follows:

RCH2OH

ZnCl2

+ HCl No reaction

R2CHOH

ZnCl2

+ HCl

R3COH

ZnCl2

+ HCl R3CCl + H2O

R2CHCl + H2O

Primary:

Secondary:

Tertiary:

(iii) Silver Nitrate Test: Test for SN1

The Silver Nitrate Test allows for the identification of alkyl halides by observing them in an alcoholic silver

nitrate environment. The rate at which the silver halide salt precipitate forms is characteristic of different

types of alkyl halides. You will test the reactivity of several alkyl halides in a SN1 reaction. Organic halides

may react with ethanol to form ethyl ethers. When the ethanol contains silver ion, the rate of reaction

Chemistry 2283g Experiment 1 – Alkyl Halides

1-6

increases because the silver ion acts as an electrophile toward the halogen and helps to break the carbonhalogen

bond. Alkyl chlorides yield an observable silver chloride precipitate, which is insoluble in ethanol and

thus provides a indicator that a reaction has occurred.

R Cl + CH3CH2OH + Ag+ CH3CH2OR + AgCl (ppt) + H+

In this case, the reaction mechanism is SN1, the slow step being the breaking of the carbon-halogen bond.

The carbocation then reacts rapidly with alcohol to form the ether.

Organic halide reactivity parallels the stability of the corresponding carbocations. For saturated alkyl groups,

this order is tertiary > secondary> primary. Tertiary, allylic, and benzylic alcohols react very quickly, while

secondary alkyl halides react fairly quickly. Primary as well as aryl, alkenyl (vinyl), and alkynyl halides do not

react at all. In addition, alkyl iodides and bromides react faster than chlorides, which sometimes require

heating.

Experimental Procedures Part B:

(i) Sodium Iodide Test: SN2 Reactivity

Test each of the following organic bromides:

- 1-bromobutane - allyl bromide (3-bromopropene)

- 2-bromobutane - 2-bromo-2-methylpropene (tert-butylbromide)

- bromobenzene - your product from Part A

Dry test tubes must be used for this experiment. Use the disposable small test tubes.

Add 2 drops of the liquid to be tested to 1 mL of a 15% solution of sodium iodide in dry acetone. Mix the

contents and note the order of reactivity by looking at how long it takes for a precipitate to form. The

solution may turn slightly yellow – this is NOT due to the SN2 reaction. (Hint: SN2 reactions are slower

than SN1 reactions. Save your test-tubes for 45-60 minutes and check occasionally for any changes

before discarding).

Tabulate the results of your qualitative tests including detailed observations. Discuss the results in terms

of the reaction mechanism.

IMPORTANT! Clean dry test tubes from oven must be used for these tests.

Tests are to be performed in the fume hood; whenever transporting your

test tube ensure they are safely stoppered with a cork.

Chemistry 2283g Experiment 1 – Alkyl Halides

1-7

(ii) Lucas Test: Reactivity of Alcohols

Conduct the test on each of the following alcohols:

- methanol - 1-butanol - benzyl alcohol

- ethanol - 2-butanol - 2-methylpropan-2-ol (tert-butanol)

Add ~1 mL of the hydrochloric acid-zinc chloride reagent (Lucas reagent) to 4-5 drops of the alcohol

compound in a small clean and dry test tube.

Stopper the tube, shake, and note any change observed. Also record the time required for the formation

(if any) of an alkyl chloride (second layer or an emulsion). If no reaction is observed after 5-10 min at

room temperature, heat gently using a steam bath for 2-3 min.

Tabulate the results of your qualitative tests including detailed observations. Discuss the results in terms

of the reaction mechanism.

(ii) Silver Nitrate Test: Reactivity of Alkyl Halides

Conduct the test on each of the following alkyl halides:

- 1-bromobutane -1-bromo-2-methylpropane (iso-propylbromide)

- your product - 2-bromo-2-methylpropane (tert-butylbromide)

- bromobenzene - benzyl bromide

Add 1 drop of the alkyl halide to 2 mL of a 0.1 M solution of silver nitrate in 95% ethanol in a small test

tube. Stopper, shake, and note any change (and time required). If no reaction is observed within 5 min

at room temperature, warm the mixture in a steam bath and observe any change.

When noting the color of any precipitate, keep in mind that silver chloride is white, silver bromide is pale

yellow, and silver iodide is yellow.

Add several drops of 1 M nitric acid solution to any precipitate, and note any changes.

If the precipitate remains, it is a silver halide. Silver halides are insoluble in acid, while the silver salts of

organic acids are soluble. Also, silver hydroxide (oxide) is insoluble but dissolves in dilute acid. Tabulate

the results of your qualitative tests including detailed observations. Discuss the results in terms of the

reaction mechanism.

CAUTION: Avoid skin contact with the silver nitrate solution. It will form a

dark, hard-to- remove stain where it comes into contact with skin.

At the end of the laboratory please be sure your clean test tubes are placed

in the oven for your colleagues!

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Carbohydrate Identification Lab

Lab Partner_______________________________ Pledge______________________

1. Present the all of the results of the carbohydrate identification lab in a single, comprehensive table. The clarity of this presentation is important!

2. Unknown #_________ Unknown ID____________________________________

3. Draw the cyclic form(s) of D fructose.

4. Suppose you have a sample of ribose. What test results would you expect for the Bial, Barfoed's, Seliwanoff and Benedict's tests?

5. Suppose you are given an unknown and find the following test results. What type of sugar do you have? Give the general type, not a specific compound.

Bial test - red brown ppt, forms immediately

Barfoed's test - red ppt the forms in 2.5 minutes

Seliwanoff test - red color appears in 45 seconds

Benedict's test - brown ppt forms in 2 minutes

نوشته شده توسط رقیه خان محمدی و طاهره فاضلی در ساعت 13:49 | لینک  | 

Ninhydrin Test



 

Amino Acid

Ninhydrin test for amino acids.


 

Procedure

Add about 2 mg of the sample to 1 mL of a solution of 0.2 g of ninhydrin (1,2,3indanetrione monohydrate) in 50 mL of water. The test mixture is heated to boiling for 15-20 sec; This reaction is important not only because it is a qualitative test, but also because it is the source of the absorbing material that can be measured quantitatively by an automatic amino acid analyzer. This color reaction is also used to detect the presence and position of amino acids after paper chromatographic separation.

Positive Test

A blue to blue-violet color is given by a-amino acids and constitutes a positive test. Other colors (yellow, orange, red) are negative.

Complications

Proline, hydroxyproline, and 2-, 3-, and 4-aminobenzoic acids fail to give a blue color but produce a yellow color instead.

Ammonium salts give a positive test. Some amines, such as aniline, yield orange to red colors, which is a negative test


نوشته شده توسط رقیه خان محمدی و طاهره فاضلی در ساعت 13:48 | لینک  | 

Lab Activity

Carbohydrates Identification

Learning Objectives:

U Students will perform standard chemical identification tests for organic compounds.

U Students will relate indicator reactions to the presence of organic nutrients.

U Students will recognize and apply a standard for comparison.

Focus Question:

Can carbohydrates be identified using one or more indicators?

Background:

e All living organisms are made of carbohydrates, lipids, proteins, and nucleic acids. These substances are

called organic compound because they all contain carbon, hydrogen and oxygen. Substances or

compound that supply your body with energy and the building blocks of macro-molecules are called

nutrients. The food you eat contains nutrients important to your body. Sugars and starches make up

a group of organic compound called carbohydrates, which are important in supplying your body with

energy. Some starches provide your body with indigestible fiber, or roughage, which aids digestion.

Organic compounds called proteins are important for growth and repair. Lipids are organic

compounds that can supply as much as four times the amount of energy as carbohydrates or proteins.

eYou can perform qualitative tests to identify the presence of organic compounds in food using indicators,

chemical substances that react in a certain way when a particular substance is present. Be careful

when using indicators. Many can stain your skin. Pay attention to any lab caution. Lab apron and

safety goggles are mandatory for this lab.

eBenedict’s solution is used to identify the presence of reducing sugars, such as glucose. Benedict’s solution

will turn a food substance containing a monosaccharide orange. Lugol’s iodine solution is used to

identify the presence of starch. Iodine solution will identify a polysaccharide by turning dark purple

to black.

e In this activity you are going to test 6 food items for the presence of monosaccharide,

disaccharide or polysaccharide. Before beginning the activity form a hypothesis of which

type of carbohydrate each food item contains and which indicator will identify it..

CAUTION/SAFETY INSTRUCTIONS:

1. For this lab you must wear safety goggles and lab aprons. Iodine will stain your clothes.

2. DO NOT HEAT IODINE. Heating iodine creates a poison that can make you sick.

3. Do not taste nor eat any of the lab chemical nor food substances.

Materials:

Solutions Indicators Equipment Food Solutions

monosaccharide Benedict’s 6 - test tubes honey

disaccharide Iodine 1 - 600 mL beaker liquid oats

polysaccharide hot plate table sugar

Test tube rack apple juice

Test tube holder milk

test tube brush rice water

9 - small pipette

10 ml graduated cylinder

small funnel

PROCEDURE:

PART 1: Chemical Tests on Known Carbohydrates

1.0 Benedict's Test on Knowns

1.1 Fill a 600 mL beaker half full of water.

1.2 Place the beaker on a hot plate to boil.

1.3 Number three clean test tubes 1 to 3. Also include your group number.

1.4 Add the following:

tube 1 add 2mL of monosaccharide solution

tube 2 add 2mL of disaccharide solution

tube 3 add 2mL of polysaccharide solution

1.5 Add 1 mL of Benedict's solution to each test tube.

1.6 Place the three test tube into the hot water bath for three minutes or until color change

has occurred. (Caution: Overheating can give wrong result.)

1.7 Use a test tube holder to remove the tubes from the hot water.

1.8 Observe and record any color changes in data table 1.

2.0 Iodine Test on Knowns

2.1 Number three clean test tubes 1 to 3. Also include your group number.

2.2 Add the following:

tube 1 add 2mL of monosaccharide solution

tube 2 add 2mL of disaccharide solution

tube 3 add 2mL of polysaccharide solution

2.3 Add 4 drops of iodine solution to each tube.

2.4 Mix the contents of each tube by gently swirling.

2.5 Observe and record any color changes in data table 1.

PART 2: Chemical Tests on Unknown Carbohydrates

3.0 Benedict's Test on Unknowns

3.1 Number six clean test tubes 1 to 6. Also include your group number.

3.2 Add the following:

tube 1 add 2mL of honey

tube 2 add 2mL of liquid oats

tube 3 add 2mL of table sugar solution

tube 4 add 2mL of apple juice

tube 5 add 2mL of milk

tube 6 add 2mL of rice water

3.3 Add 1mL of Benedict's solution to each test tube.

3.4 Place the six test tube into the hot water bath for at least three minutes.

3.5 Use a test tube holder to remove the tubes from the hot water.

3.6 Observe and record any color changes in data table 2.

4.0 Iodine Test on Unknowns

4.1 Number six clean test tubes 1 to 6. Also include your group number.

4.2 Add the following:

tube 1 add 20 drops of honey

tube 2 add 20 drops of liquid oats

tube 3 add 20 drops of table sugar solution

tube 4 add 20 drops of apple juice

tube 5 add 20 drops of milk

tube 6 add 20 drops of rice water

4.3 Add 4 drops of iodine solution to each tube.

4.4 Mix the contents of each tube by gently swirling.

4.5 Observe and record any color changes in data table 2.

Data Section

e Make a data table for procedure part 1.

e Make a second data table for procedure part 2.

Analysis Section

Write the analysis questions in blue ink. Leave room to write your answers. Answers must be written

in pencil.

1. List the 3 types of carbohydrates.

2. List the 3 elements found in all carbohydrates.

3. "Mono-" means one, "di-" means two, and "poly-" means many. Why are these terms used in

describing the three types of sugars?

4. Explain why the known sample were tested first.

5. How can you tell by using Benedict's solution if a sugar is a monosaccharide, disaccharide, or

polysaccharide?

6. List the foods substances from your data that Benedict’s solution identified as monosaccharide.

7. How can you tell by using Iodine solution if a sugar is a monosaccharide, disaccharide, or

polysaccharide?

8. List the food substances from your data that Iodine identified as a polysaccharide.

9. A certain sugar has no change in color when tested with Benedict's solution. How can you tell

what type of saccharide it is?

10. A certain sugar has a color change in Benedict's solution. How can you tell what type of

saccharide it is?

11. List the food substances, if any, that is contains disaccharide.

12. List 3 examples of foods that are monosaccharides, disaccharides, and polysaccharide that is

not mentioned in this activity.

Conclusion

Write a conclusion in essay form with at least 4 paragraphs. The following should be in your conclusion:

The first paragraphs should contain the following:

- state the purpose/objective

- state the hypothesis

- very briefly summarize the experiment

The second paragraph should contain:

- state the results by quoting significant data. Do not re-write all the data. You can refer the reader to the

data table to view all the data. If you found something interesting about the data you can

mentioned it in this paragraph, give specific examples.

The third paragraph should verify or refute the hypothesis:

- verification of the hypothesis means does the data support the hypothesis, explain how by giving

examples

- refutation of the hypothesis means the data did not support the hypothesis, explain why not, give

example and re-state a new hypothesis that matches the data.

The final paragraph is a summary paragraph

-again state the objective/purpose.

- was the objective/purpose achieved if so explain how and if not explain why not.

-state what you learned in this activity

Concepts and Standards Section

Write the concept and standard/s that applies to this lab activity.

Explanation of Standard/s Section

In your own words, using your newly learned knowledge, explain the standard. If you have written more

than one standard, only choose one to explain.

نوشته شده توسط رقیه خان محمدی و طاهره فاضلی در ساعت 13:44 | لینک  | 

Iodine Test



Alkene

Iodine test for alkenes.

Ether

Iodine test for ethers.


Procedure

Add 0.25 mL or 0.25 g of the unknown sample to 0.5 mL of the iodine in methylene chloride solution. If an ether is present, the purple solution becomes tan in color. Aromatic hydrocarbons, saturated hydrocarbons, fluorinated hydrocarbons, and chlorinated hydrocarbons do not react. Unsaturated hydrocarbons produce a light tan solid, while retaining the purple color of the iodine solution.

Iodine in Methylene Chloride Solution: Add a couple of crystals of iodine to 100 mL of methylene chloride. Stopper the flask tightly.

Positive Test

alkene - the purple solution becoming tan in color is a positive test.

ether - production of a light tan solid in the purple solution is a positive test.

Complications

Some alcohols and ketones give a positive test.

Some compounds with nonbonded electron pairs or p-bonded electrons form charge transfer complexes with iodine yielding brown solutions.

نوشته شده توسط رقیه خان محمدی و طاهره فاضلی در ساعت 13:38 | لینک  | 

Bromine Test



Alkene

Bromine test for alkenes.

Alkyne

Bromine test for alkynes.


Procedure

In a hood, 0.1 g or 0.2 mL of the unknown is added to 2 mL of carbon tetrachloride, and a 5% solution of bromine in carbon tetrachloride is added drop by drop, with shaking, until the bromine color persists.

Positive Test

Discharging of the bromine color without the evolution of hydrogen bromide gas is a positive test.

Complications

Should be employed in conjunction with Baeyer test (dilute KMNO4).

Electron-withdrawing groups in the vinylic position can slow down bromine addition to the point that a negative test is erroneously produced.

Tertiary amines (like pyridine) form perbromides upon treatment with bromine and lead to false positive tests.

Aliphatic and aromatic amines discharge the bromine color without the evolution of HBr gas.

نوشته شده توسط رقیه خان محمدی و طاهره فاضلی در ساعت 13:36 | لینک  |