Aldehydes and ketones

Question 1

Describe how separate samples of two enantiomers can be distinguished

Answer 1

Plane-polarised light.

Rotated in opposite directions.

Question 2

What are optical isomers

Answer 2

Non - superimposable mirror images

Question 3

When do optical isomers occur

Answer 3

when a carbon atom bonds to 4 different groups

Question 4

what is a racemic mixture

Answer 4

a mixture which contains equal quantities of enantiomers

Question 5

Why can nucleophilic addition to aldehydes and asymmetrical ketones result in a racemic mixture of optical isomers?

Answer 5

*The bonding is about a group in an aldehyde and ketone is planar.

*This means there is a 50:50 chance of the nucleophile attacking from one side of the molecule or the other.

*This results in equal proportions of each optical isomer forming, and so a racemic mixture is formed.

Question 6

Draw the structures of the three branched-chain alkenes with molecular formula C5H10

Answer 6

1) H3C=C(CH3)CH2CH3
2) CH3C(CH3)=CHCH3
3) CH3CH(CH3)CH=CH2

Question 7

Draw the structures of the three dibromoalkanes, C5H10Br2, formed when these three alkenes react with bromine.

Answer 7

1) H2C(BR)CBr(CH3)CH2CH3
2) H3CCBr(CH3)CH(Br)CH3
3) H3CC(CH3)CH(Br)CH2(Br)

Question 8

State the relationship between two chiral molecules with the same structural formula.

Answer 8

Non-superimposable mirror images

Question 9

Butanone is reduced in a two-step reaction using NaBH4 followed by dilute hydrochloric acid.

(a) Write an overall equation for the reduction of butanone using [H] to represent the reductant.

Answer 9

CH3CH2COCH3 + 2[H] -> CH3CH2CH(OH)CH3

Question 10

A student attempted to reduce a sample of 2-methylbutanal but added insufficient NaBH4
The student confirmed that the reduction was incomplete by using a chemical test.
Give the reagent and observation for the chemical test.

Answer 10

Reagent : conc H2SO4
observation : heat