O- Aldehydes & Ketones =)

Question 1

Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene. (3)

Answer 1

• H- ion attracted to δ+ C
• electron rich C=C
• H- ion repelled by C=C

Question 2

What kind of ketone forms a racemate on reduction?

Answer 2

asymmetric

Question 3

How many structural isomers, which are aldehydes, have the molecular formula C5 H10 O? (1)

Answer 3

4
(5C in chain, 4 w/ methyl group x2, 3)

Question 4

Which one of the following is not a suitable method for the preparation of ethanol?
A. oxidation of ethene
B. hydration of ethene
C. reduction of ethanal
D. hydrolysis of bromoethane

Answer 4

A
(for D, lone pair on O attacks δ+ C in C-Br, e- goes onto Br)

Question 5

Ethanal reacts with potassium cyanide, followed by dilute acid, to form 2-hydroxypropanenitrile.
The 2-hydroxypropanenitrile formed is a mixture
of equal amounts of two isomers.
Explain how the structure of ethanal leads to the formation of two isomers. (2)

Answer 5

• planar carbonyl
• equal chance of attack from each side

x flat molecule!

Question 6

Solution X reacts with liquid ketones to form a crystalline solid.
This reaction can be used to identify a ketone if the crystalline solid is separated, purified by recrystallisation, and the melting point determined.
Describe how the crystalline solid is separated and purified. (5)

Answer 6

1. filter
2. dissolve in min vol
3. of hot solvent
4. cool/ leave to crystallise + filter (under reduced pressure)
5. wash w/ cold solvent & dry

Question 7

This hydroxynitrile is usually made by reaction of propanone with KCN followed by dilute acid, instead of with HCN.
Suggest a reason, other than safety, why KCN is used instead of HCN. (1)

Answer 7

HCN weak

Question 8

State the expected observation for a positive result for Fehling’s solution with an aldehyde. (1)

Answer 8

blue –> (brick) red ppt