Chemistry 2412LAldehydes/Ketones pre-lab lecture

Properties of Aldehydes and Ketones• In this experiment you will be investigating the chemical

properties of aldehydes and ketones.• Although aldehydes and ketones are very similar in

structure and reactivity, there are some tests that can distinguish between the two functional groups.

• You will observe results of 4 tests on known compounds, and use your observations to identify an unknown compound. You will also determine the IR spectrum of your known. The unknown will be an aldehyde or a ketone. If it is a ketone, you will need to determine if it is a methyl ketone.

Procedure Notes• The four tests are:

1. 2,4-Dinitrophenylhydrazone formation2. Semicarbazone formation3. Oxidation4. Iodoform test

• Some pictures of these test that may be helpful to you can be found at this link.

Procedure Notes• Aldehydes and ketones react with a number of

nitrogen-containing compounds through nucleophilic addition and subsequent loss of water to give products that have a carbon-nitrogen double bond.

• These reactions are useful in distinguishing aldehydes and ketones from other functional groups.

• If the precipitate formed is isolated and the melting point determined, specific aldehydes and ketones can be identified.

• Two reagents that frequently yield crystalline derivatives are 2,4-dinitrophenylhydrazine and semicarbazide. You will use these reagents in tests 1 and 2.

Procedure Notes1. 2,4-Dinitrophenylhydrazone formation:

– 2,4-Dinitrophenylhydrazine reacts with ketones and aldehydes, and is used to distinguish these functional groups from other carbonyl compounds, such as esters, which do not react.

– The formation of a red or orange precipitate (a 2,4-dinitrophenylhydrazone) constitutes a positive test.

NO2

NO2

NHH2NC O

R

R

+ NO2

NO2

NHNC

R

R

+ H2O

ketone or aldehyde 2,4-dinitrophenylhydrazine 2,4-dinitrophenylhydrazone

(a solid)

Procedure Notes2. Semicarbazone formation:

– Semicarbazide reagent reacts with ketones and aldehydes to form solid semicarbazones.

CNHH2NC O

R

R

+ CNHNC

R

R

+ H2O

ketone or aldehyde semicarbazide semicarbazone

(a solid)

O

NH2

O

NH2

Procedure Notes3. Oxidation using chromic acid:

– This test is based on the reduction of chromium(VI), which is orange, to chromium(III), which is green, when an organic compound is oxidized by the chromium reagent.

– A change in color of the reagent from orange to blue-green represents a positive test.

– Aldehydes are oxidized by the reagent to carboxylic acids; ketones are not oxidized at all by the reagent. As a result, this reaction can be used to distinguish aldehydes from ketones.

C

O

R H

H2CrO4

C

O

R OH

C

O

R R

H2CrO4No reaction

Procedure Notes4. Iodoform test:

– Methyl ketones (RCOCH3) can be distinguished from other ketones.

– Methyl ketones react rapidly with iodine under basic conditions to give a carboxylate (RCO2

-) and iodoform (CHI3).– Iodoform is a pale yellow solid, and its

appearance indicates a positive test.

R C CH3

O

+ 3 I2 + 4 NaOH R C O

O

Na + CHI3 + 3 NaI + 3 H2O

Safety Notes• Although only small amounts of reagents

are used in this experiment, some are strongly acidic or basic and appropriate care should be taken in handling them.

• If any of chemicals are spilled on the skin rinse thoroughly with water, then wash with soap and water.

• Waste from the iodoform test must go in the halogenated waste container. All other waste may go in the non-halogenated waste container.