Carbonyl Compounds

What is a carbonyl compound?

An organic compound that contains a carbonyl group (C=O)

What is the carbonyl functional group?

The –CO group containing a carbon–oxygen double bond

Which types of compounds are the most common carbonyl compounds?

Aldehydes and ketones

Why is the C=O bond polar?

Oxygen is more electronegative than carbon, creating a polar bond

Why can aldehydes and ketones dissolve in water?

The oxygen lone pair forms hydrogen bonds with water

Why can aldehydes and ketones not hydrogen bond with each other?

They lack a δ+ hydrogen bonded to O, N, or F

What intermolecular forces act between aldehyde and ketone molecules?

Van der Waals forces

What is an aldehyde?

A carbonyl compound with the functional group –CHO

Where is the carbonyl group located in an aldehyde?

On a terminal carbon atom attached to only one other carbon

How are aldehydes produced from alcohols?

By oxidation of primary alcohols

What is a ketone?

A carbonyl compound with a carbonyl group bonded to two carbon atoms

Where is the carbonyl group located in a ketone?

On a carbon atom attached to two other carbons

How are ketones produced from alcohols?

By oxidation of secondary alcohols

Which alcohols can be oxidised easily?

Primary and secondary alcohols

Which alcohols are not easily oxidised?

Tertiary alcohols

What oxidising agent is used to oxidise alcohols in the lab?

Acidified potassium dichromate(VI)

What is the formula of potassium dichromate(VI)?

K₂Cr₂O₇

What acid is used to acidify potassium dichromate(VI)?

Dilute sulfuric acid (H₂SO₄)

What colour change shows dichromate(VI) has been reduced?

Orange to green

Why does potassium dichromate(VI) change colour during oxidation?

The oxidation state of chromium decreases

What product forms when a primary alcohol is oxidised and distilled?

An aldehyde

What product forms when a primary alcohol is oxidised under reflux?

A carboxylic acid

What product forms when a secondary alcohol is oxidised?

A ketone

What symbol represents an oxidising agent in organic equations?

[O]

What symbol represents a reducing agent in organic equations?

[H]

What type of reaction reduces carbonyl compounds back to alcohols?

Reduction by nucleophilic addition

What reducing agent is commonly used for carbonyl compounds?

Sodium borohydride (NaBH₄)

What nucleophile does NaBH₄ provide?

H⁻ (hydride ion)

What forms when an aldehyde is reduced?

A primary alcohol

What forms when a ketone is reduced?

A secondary alcohol

Why is dilute acid added after NaBH₄ reduction?

To release the alcohol from the salt formed

What nucleophile can add to carbonyl compounds to extend the carbon chain?

CN⁻

What is the product of CN⁻ addition to a carbonyl compound?

A hydroxynitrile

Why does CN⁻ addition increase carbon chain length?

The CN group adds one extra carbon atom

Why is NaCN used instead of HCN directly?

HCN is toxic and difficult to store

Why do hydroxynitriles often show optical isomerism?

They contain a chiral carbon centre

Why are two enantiomers formed in CN⁻ addition?

The nucleophile can attack from either side of the planar C=O bond

What does the 2,4-DNPH test detect?

The presence of a carbonyl group

What is a positive result for the 2,4-DNPH test?

A yellow, orange, or red precipitate

Which compounds give a positive 2,4-DNPH test?

Aldehydes and ketones

Why do alcohols not react with 2,4-DNPH?

They do not contain a carbonyl group

What is formed in the reaction with 2,4-DNPH?

A 2,4-dinitrophenylhydrazone

Why are melting points useful for identifying carbonyl compounds?

Their derivatives have sharp melting points

How can 2,4-DNPH help identify a specific aldehyde or ketone?

By comparing melting points with databook values

What does Tollens’ test distinguish between?

Aldehydes and ketones

What is a positive Tollens’ test result?

A silver mirror forming on the test tube

Which compounds give a positive Tollens’ test?

Aldehydes

Which compounds do not react in Tollens’ test?

Ketones

What is Tollens’ reagent made from?

Ammoniacal silver nitrate

What happens to the aldehyde in Tollens’ test?

It is oxidised to a carboxylic acid

What happens to silver ions in Tollens’ test?

They are reduced to silver metal