Chemistry 3.3.7 Optical isomerism and 3.3.8 Aldehydes and ketones

Strongest intermolecular forces that aldehydes can form

Dipole-dipole forces

Conditions required to oxidise a 1° alcohol to an aldehyde

Distil - don’t want the temperature too high or the aldehyde will evaporate

Conditions for oxidation of a 2° alcohol to a ketone

Reflux

Conditions for oxidation of an aldehyde to a carboxylic acid

Reflux

Conditions for oxidation of a 1° alcohol to a carboxylic acid

Reflux

Why can’t tertiary alcohols be oxidised?

There are no available H atoms for removal on the C with the OH group attached

Why can’t ketones be oxidised further?

There are no available H atoms on the carbonyl

Fehling’s solution positive result

Blue → brick red precipitate if aldehyde is present

Tollens’ reagent positive result

Formation of a silver mirror if aldehyde is present

Stereoisomerism

Molecules with the same structural formula, but different 3D arrangement of atoms in the space

Chiral centre

A carbon atom with 4 different atoms/groups attached. Indicated using *

When does optical isomerism occur?

When a molecule has a chiral centre/chiral carbon

What are enantiomers?

A pair of non-superimposable mirror image molecules (a pair of optical isomers)

Enantiomer properties

• Same physical properties

• Same chemical properties when reacting with molecules that do not display optical isomerism

• Rotate plane-polarised light in opposite directions

Racemic mixture/racemate

A mixture composed of equal amounts of both enantiomers. Optically inactive because opposite rotational effects will cancel out.

Optically inactive

Has no effect on plane polarised light

Why do racemic mixtures occur as a product of nucleophilic addition with aldehydes and ketones?

Carbonyl group is planar so attack by nucleophile is equally likely from either side