26.0 Compounds containing the carbonyl group

Question 1

How can the following be formed from an
alcohol?
a) Ethanal
b) propanone

Answer 1

a) Warm ethanol with acidified K2Cr2O7, distil product as it forms
b) Heat propan-2-ol with acidified K2Cr2O7, reflux

Question 2

Describe two tests to distinguish aldehydes
from ketones, with expected observations.

Answer 2

Tollen’s reagent: aldehyde forms silver mirror, ketone no reaction.
Fehling’s solution: aldehyde forms brick-red ppt, ketone no reaction

Question 3

What reagent(s) are needed to reduce an
aldehyde/ketone to an alcohol? How would
these be represented in a symbol equation?

Answer 3

NaBH4
[H]

Question 4

Why does nucleophilic addition of KCN
(followed by acid) to ethanal result in a mixture
of enantiomers?

Answer 4

Nucleophile attacks Cd+ from above or below C=O plane. 50/50 chance.

Question 5

CH3COOH + NaHCO3 → ?

Answer 5

CH3COONa + CO2 + H2O

Question 6

Give reagents and conditions for formation of
propyl ethanoate.

Answer 6

Propanol + ethanoic acid, H2SO4 catalyst, warm

Question 7

Suggest two methods for hydrolysing an ester,
give equations (eg propyl ethanoate)

Answer 7

Acid catalysed:
CH3COOCH2CH2CH3 + H2O ⇌ CH3COOH + CH3CH2CH2OH
Alkaline (reaction completes)
CH3COOCH2CH2CH3 + NaOH → CH3COONa + CH3CH2CH2OH

Question 8

Give examples of the two types of acylating
agent. State the advantages/disadvantages of
each.

Answer 8

Acid chloride RCOCl (Faster reaction rate, forms corrosive HCl by product, reacts vigorously with water)
Acid anhydride (RCO)2O (Slower reaction rate, forms carboxylic acid by product)

Question 9

Give the four nucleophiles that can be acylated.
Identify the position of the lone pair.

Answer 9

Water H2Ö alcohol RÖH
Ammonia N̈H3 amine RN̈H2

Question 10

What are the key steps in recrystallisation
aspirin and the reason for each step?

Answer 10

1.Dissolve the aspirin in minimum volume of hot ethanol.
2. Cool solution so that solubility decreases and crystals form.
3. Filter under reduced pressure to separate crystals from impure solution.
4. Allow to dry.

Question 11

Describe how to test purity by melting point.

Answer 11

Use electric melting point apparatus. Increase melting point, more slowly as you near the expected melting point. Record the start and finish temperature for melting.