AS Level Chemistry 9701

17. Carbonyl compounds

Written by: Pranav I
Formatted by: Pranav I

Carbonyl group → C=O
In aldehydes, the C in the carbonyl group is bonded to a carbon atom and a hydrogen atom (at the end of the carbon chain)
In ketones, the C in the carbonyl group is bonded to two other carbon atoms
Names of aldehydes → alkanal
Names of ketones → alkanone (position of the carbonyl group must be indicated in larger molecules)

#	Reagent	Acts on	Reaction type	Conditions	Example
1	K₂Cr₂O₇	Primary alcohols	Oxidation	Acidified, heat & distill off	\[ \text{CH}_3\text{CH}_2\text{OH} \overset{\text{K}_2\text{Cr}_2\text{O}_7}{\rightarrow} \text{CH}_3\text{COOH} \]
1	K₂Cr₂O₇	Secondary alcohols	Oxidation	Acidified & heat	\[ \text{CH}_3\text{CH(OH)CH}_3 \overset{\text{K}_2\text{Cr}_2\text{O}_7}{\rightarrow} \text{CH}_3\text{COCH}_3 \]

\[
\text{CH}_3\text{CHO} + 2[\text{H}] \rightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH}_2\text{OH}
\]

\[
\text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH(OH)CH}_3
\]

🚨 LiAlH4 in dry ether can also be used, but it is more common for carboxylic acids

Addition takes place across the C=O bond → attacked by a nucleophile
HCN is generated in situ by the reaction of KCN and dilute H₂SO₄
Nitrile group added → increase in the number of C in the original carbonyl compound

\[
\text{CH}_3\text{CH}_2\text{CHO} + \text{HCN} \rightarrow{\text{CN}^-} \text{CH}_3\text{CH}_2\text{CH(OH)CN}
\]

>C=O is polarised → high electronegativity of the oxygen atom
- C has δ+ and O has δ-
- Open to attack by a nucleophile
Step 1
- CN^– nucleophile attacks the δ+ C atom
- One bond in C=O breaks heterolytically
- O gains a negative charge
Step 2
- Lone pair of electrons from O^– attacks H⁺
- This forms a hydroxy group

🚨 Carboxylic acids and esters do not form precipitates with 2,4-DNPH

Observation: deep-orange precipitate forms
- Precipitate can be purified by recrystallization
- The melting point can be measured experimentally
- Compound can be identified by referring to melting point data
- Specific aldehydes or ketones can be identified

Tests involve mild oxidation
Reagent: Tollens’ reagent
- Aqueous solution of silver nitrate in excess ammonia solution
Observation: silver mirror
- Ag⁺ ions act as a mild oxidizing agent → oxidize aldehyde to form a carboxylate ion
- Ag⁺ ions themselves are reduced to Ag atoms (form a mirror on the inside of the tube)
Reagent: Fehlings’ reagent
- Alkaline solution containing copper(II) ions
Observation: clear blue solution produces a red precipitate
- Cu²⁺ ions act as an oxidizing agent → oxidize aldehyde to form a carboxylate ion
- Cu²⁺ ions are reduced to Cu⁺ ions

🚨 No changes are observed with a ketone since it cannot be oxidized further

Reagent: Iodine → I_2(aq)
Conditions: alkaline, aqueous & warming
Tri-iodomethane (CHI₃) forms as a yellow precipitate with compounds containing the CH₃CO- (methyl ketone) group
- Also known as iodoform

Step 1: the carbonyl compound is halogenated → iodine atoms replace the three H atoms of CH₃
Step 2: the intermediate is hydrolyzed to form the yellow precipitate of triiodomethane, CHI₃

Tri-iodomethane test can also be used to identify the presence of a secondary alcohol, with the -OH on the C next to a methyl group
The CH₃CH(OH)- group is oxidized to form a methyl ketone, CH₃COR
CHI₃ is then formed by the two steps described in the previous section for the CH₃CO- group
Products: CHI₃ + RCOO^–Na⁺