Aldehydes and Keytones

What is the functional group in aldehydes and ketones?

The carbonyl group (C=O)

What is the general formula of a ketone?

R-CO-R'

Why is a number not needed to locate the aldehyde group?

The aldehyde group can only occur at the end of a carbon chain

Why is the carbonyl group polar?

The large difference in electronegativity between carbon and oxygen creates a dipole

Why do carbonyl compounds have higher boiling points than alkanes?

They have permanent dipole-dipole forces between molecules whereas alkanes only have London forces/ van der waals

Why do carbonyl compounds have lower boiling points than alcohols?

Alcohols can form hydrogen bonds between molecules which are stronger than dipole-dipole forces

What happens to solubility in water as the carbon chain length of the aldehydes and ketones increases?

Solubility decreases because the non-polar hydrocarbon chain disrupts hydrogen bonding between the water molecules and the carbonyl group

What type of reactions do carbonyl compounds undergo?

Nucleophilic addition reactions

Why can carbonyl compounds undergo addition reactions?

They are unsaturated due to the C=O double bond.

Why are carbonyl compounds attacked by nucleophiles?

The carbon atom in the C=O bond is electron deficient due to the electronegativity difference

What reagent is used to distinguish between aldehydes and ketones?

A weak oxidizing agent such as Fehling's solution or Tollens' reagent

What is observed when an aldehyde is warmed with Fehling's solution? and what colour does it start as?

A brick red precipitate of copper(I) oxide forms from a deep blue solution of copper(II) oxide at start.

What is observed when a ketone is warmed with Fehling's solution?

No reaction the solution remains blue

What is observed when an aldehyde is warmed with Tollens' reagent?

A silver mirror forms on the inside of the test tube

What is observed when a ketone is warmed with Tollens' reagent?

No reaction

What is the active ion in Tollens' reagent?

The diamminesilver(I) complex ion [Ag(NH₃)₂]⁺

What happens to the aldehyde in the silver mirror test?

It is oxidized to a carboxylic acid

What happens to the silver ions in the silver mirror test?

They are reduced to metallic silver

What is the product when an aldehyde is oxidized?

A carboxylic acid

Why can't ketones be oxidized easily by weak oxidizing agents into carboxylic acids?

Oxidation would require breaking a C-C bond which is too difficult

What reducing agent is commonly used to reduce aldehydes and ketones?

Sodium tetrahydrodiborate(III) also called sodium borohydride NaBH₄

What is the nucleophile generated by NaBH₄?

The hydride ion :H⁻

What is the product when an aldehyde is reduced?

A primary alcohol

What is the product when a ketone is reduced?

A secondary alcohol

Does NaBH₄ reduce C=C bonds?

No the hydride ion is repelled by the high electron density of the C=C bond

What type of reaction is the reduction of a carbonyl with NaBH₄?

Nucleophilic addition (because the hydride ion acts as a nucleophile)

What is the product when an aldehyde or ketone reacts with HCN?

A hydroxynitrile

Why is the reaction with HCN important in organic synthesis?

It increases the length of the carbon chain by one carbon

Why does the reaction of an aldehyde with HCN form a racemic mixture?

The CN⁻ ion can attack from above or below the planar C=O group forming two enantiomers equally, produces a chiral compound

What is the old name for propanone commonly used in nail varnish remover?

Acetone

What common oxidising agent is used to oxidise aldehydes?

Acidified (dilute sulfuric acid) potassium dichromate