6.1.2 Carbonyl Compounds

What is the carbonyl group?

C=O

What is the functional group and general formula of an aldehyde?

RCHO; the carbonyl carbon is bonded to H and R.

What is the functional group of a ketone?

RCOR′; the carbonyl carbon is bonded to two carbon groups.

How do you name aldehydes?

Use the -al suffix; the C=O is at the end of the carbon chain.

How do you name ketones?

Use the -one suffix and specify the position of the C=O.

What intermolecular forces do molecules containing a carbonyl group have?

Permanent dipole-dipole interactions due to the polar C=O bond.

Why is the C=O bond polar?

Oxygen is more electronegative than carbon, so oxygen is δ− and carbon is δ+.

Are carbonyl compounds soluble in water?

Yes; they can form hydrogen bonds between water molecules and the oxygen of the C=O group.

What happens to the solubility of carbonyl compounds as carbon chain length increases?

Solubility decreases.

Which bond in carbonyl compounds is usually involved in reactions?

The C=O bond.

Why is the C=O bond involved in reactions?

It is highly polar due to the large difference in electronegativity between carbon and oxygen.

What is the strongest bond in carbonyl compounds?

C=O

What type of reaction occurs when a nucleophile reacts with a carbonyl compound?

Nucleophilic addition.

What happens during nucleophilic addition to a carbonyl group?

The nucleophile attacks the δ+ carbon of the C=O group and the π bond electrons move onto oxygen.

What is the mechanism for nucleophilic addition of HCN to a carbonyl compound?

CN− attacks the δ+ carbon of the C=O group; the C=O π electrons move onto oxygen; the resulting intermediate is protonated by HCN.

Why is addition of HCN important?

It increases the carbon chain length by one carbon atom, allowing more useful molecules to be made.

Will the product of HCN addition to a carbonyl compound have optical isomers?

Yes. The carbonyl carbon is planar, so CN− can attack from either side, producing two enantiomers.

What is the product when HCN is added to a carbonyl compound called?

A hydroxynitrile.

What two functional groups are present in a hydroxynitrile?

OH and CN.

What is Fehling's solution?

A solution containing copper complex ions.

What colour is Fehling's solution?

Blue.

What happens when an aldehyde is added to Fehling's solution?

Cu2+ is reduced to Cu+ and a brick-red precipitate forms.

What happens when a ketone is added to Fehling's solution?

No visible change; the solution remains blue.

How do you test for a carbonyl compound?

Use Brady's reagent, 2,4-dinitrophenylhydrazine.

What is the positive result with Brady's reagent?

An orange precipitate forms.

What is in Tollens' reagent?

Silver complex ions in a colourless solution.

What happens when an aldehyde is added to Tollens' reagent?

A silver mirror forms because Ag+ is reduced to Ag(s).

What happens when a ketone is added to Tollens' reagent?

No visible change.

What is another oxidising agent for alcohols and aldehydes?

Acidified potassium dichromate(VI), using H2SO4 and K2Cr2O7.

What colour change occurs with acidified potassium dichromate(VI)?

Orange to green.

What reducing agent is used for aldehydes and ketones?

NaBH4, sodium tetrahydridoborate(III).

What ion does NaBH4 release in solution?

H−

What type of mechanism is the reduction of an aldehyde using NaBH4?

Nucleophilic addition.

What happens when an aldehyde is reduced?

It forms a primary alcohol.

What happens when a ketone is reduced?

It forms a secondary alcohol.

What is the equation for reduction of pentan-2-one?

CH3COCH2CH2CH3 + 2[H] → CH3CH(OH)CH2CH2CH3

What is the equation for reduction of 3-methylbutanal?

CH3CH2CH(CH3)CHO + 2[H] → CH3CH2CH(CH3)CH2OH

How do you convert an aldehyde into a carboxylic acid?

Oxidise the aldehyde using Cr2O7^2−/H+, such as K2Cr2O7/H2SO4.