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ALDEHYDES and KETONES

Objective : To study some chemical reactions that are used to distinguish between

aldehydes and ketones , especially oxidation of aldehydes.

The carbonyl group :

Aldehydes have at least one hydrogen attached to the

carbonyl group.

;

Ketones have two carbons attached to the carbonyl group.

Physical Properties

The carbonyl group is moderately polar, but it doesn’t have any hydrogen atoms

bonded to the oxygen atom, so it cannot hydrogen bond between molecules.

Because of the polarity of the C=O group, these groups can interact, but the attraction is not as strong as hydrogen bonding. This makes the boiling point of aldehydes and ketones higher than alkanes, but lower than alcohols.

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The C=O group can hydrogen bond with water molecules because the oxygen atom

in the carbonyl group has a partial negative charge that attracts the partial positive

charge of a hydrogen atom in the water molecule.

This makes low molecular weight aldehydes and ketones water soluble (they have

small hydrophobic sections).

Important ALDEHYDES and KETONES

Acetone Formaldehyde

Formaldehyde

Gas at room temperature

Formalin – 37% aqueous solution

used as a disinfectant and to preserve biological specimens.

Acetone

Important organic solvent in the chemical laboratory

nail polish remover

In the Industry

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• Progesterone and testosterone (female and male sex hormones) are ketones.

In flavorings

Some aldehydes and ketones are very fragrant and are used in flavorings.

Vanillin (vanilla)

Cinnamaldehyde (cinnamon)

Citral (citrus flavoring)

Camphor (medicinal odor)

In the Human

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Aldehyde and Ketone Reactions

Distinguish between Aldehydes and Ketones

The difference in reactivity toward oxidation is the chief reason why aldehydes and ketones are classified in separated families The oxidation of aldehydes can be performed with a mild oxidizing agent, such as Ag in ammonia solution

which used in the Tollen's test or Cu in alkaline solution which used in Fehling's test.

Tollen's test : Freshly prepared

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In the presence of aldehydes, Tollens’ reagent produces a silver coating on glass.

Reagents :

1- (10% ) Silver nitrate ( AgNO3 ).

2- (10% ) Sodium hydroxide ( NaOH ).

3- (30% v/v) Ammonium hydroxide ( NH4OH )

Principle of the test

Most aldehydes reduce Ammoniacal silver nitrate solution to give a precipitate of silver metal

(pronounced coating on test tube wall), and the aldehyde is oxidized to carboxylic acid.

procedure

In two clean test tube put :-

1- Tollen's reagent { a few drops AgNO3 + 1 drop NaOH ( brown ppt. formed ) + drop by

drop NH3until brown ppt. disappear [colorless] } .

2- Added to the 1st one 5 drops of acetaldehyde then added to the second one 5 drops of

acetone , wait 1 min , Silver mirror deposited on the side the test tube in aldehyde (because

aldehyde contain H atom weak acidic ) and in the 2nd tube no reaction obtain because

ketone is not contain H atom weak acidic.

Fehling test:

Fehling solution consist of :-

1- Fehling solution one : CuSO4 cupper sulfate .

2- Fehling solution two : sodium potassium tartarate + NaOH .

2 AgNO3+ 2 NaOH Ag

2O + H

2O + 2 Na⁺ + 2 NO3¯

Ag2O + 4 NH

3 + H

2O 2 Ag(NH

3)2 + 2 OH¯

2 [ Ag(NH3)2 OH ] + RCHO RCOO

-

NH+

4 + 2 Ag + H

2O + 3 NH

3

RCOR + [ Ag(NH3)2 OH ] No Reaction

2 AgNO3+ 2 NaOH Ag

2O + H

2O + 2 Na⁺ + 2 NO3¯

Ag2O + 4 NH

3 + H

2O 2 Ag(NH

3)

2 + 2 OH¯

2 [ Ag(NH3)

2 OH ] + RCHO RCOO

-

NH+

4 + 2 Ag + H

2O + 3 NH

3

RCOR + [ Ag(NH3)

2 OH ] No Reaction

2 AgNO3+ 2 NaOH Ag

2O + H

2O + 2 Na⁺ + 2 NO3¯

Ag2O + 4 NH

3 + H

2O 2 Ag(NH

3)

2 + 2 OH¯

2 [ Ag(NH3)

2 OH ] + RCHO RCOO

-

NH+

4 + 2 Ag + H

2O + 3 NH

3

RCOR + [ Ag(NH3)

2 OH ] No Reaction

ketone

Aldehyde

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Principle of Fehling test :

Aldehyde act as reducing agent it can reduce the deep blue color of Fehling solution into red

ppt. of cuprous oxide.

Procedure :

In two clean test tube put :-

1- ( 5 ) drops of Fehling one.

2- ( 5 ) drops of Fehling two.

Mixed well , then added to the first one 5 drops of acetaldehyde and to the second 5 drops of

acetone , mixed well , then put the tubes in a boiling water bath for 3-5 min. then red ppt.

will be appears in the 1st tube indicate the presence of aldehyde (because it contain H weak

acidic ) , and in the 2nd tube no change in color (remain blue color ) because it contain

ketone (absence of H weak acidic ) .

Results

Conclusion Observation Test

Tollen's Test

Presence of H +ve Silver mirror Acetaldehyde

Absence of H No reaction Acetone

Fehling Test

Presence of H +ve Red ppt. Acetaldehyde

Absence of H Blue color (No reaction ) Acetone