Hemiacetals and acetals

Question 1

What is the rate of reaction for the formation of hemiacetals like, and which catalytic conditions can be used

Answer 1

Formation from aldehydes/ketones is slow. Both acidic and basic catalytic conditions can be used

Question 2

Draw the mechanisms for the formation of a hemiacetal under acidic and basic conditions

Answer 2

Check notes

Question 3

Cyclic acetals?

Answer 3

If a species has a carbonyl functional group and an alcohol functional group with enough carbons between them to form a 5 or 6 membered ring, intramolecular acetal formation can occur under the correct catalytic conditions

Question 4

Under what conditions can an acetal be synthesised and why

Answer 4

Only under acidic conditions. Acetals (more specifically ketals) are stable to base and so cannot be synthesised under these conditions

Question 5

What is the key feature of the acetal formation mechanism

Answer 5

The oxonium ion intermediate

Question 6

Draw the mechanism for the formation of an acetal from a hemiacetal

Answer 6

Check notes

Question 7

Which acid is often used to catalyse the formation of acetals. Also draw it

Answer 7

TsOH (toluene sulfonic acid)

Question 8

How can the efficiency of acetal formation be improved

Answer 8

The reaction is an equilibrium (however the product is not, it wont fall pack into the aldehyde). Dean Stark apparatus will push equilibrium to the products

Question 9

Properties of cyclic acetals relative to acyclic acetals

Answer 9

They are harder to hydrolyse but easier to synthesise

Question 10

Mechanism for the formation of a cyclic acetal using a di-alcohol and aldehyde species

Answer 10

Check notes

Question 11

What makes acetals good protecting groups?

Answer 11

They are stable to nucleophilic attack unlike carbonyls. Can often help prevent intramolecular reactions from occurring