Aldehydyes and Ketones (Organic) (complete)

Question 1

Give the tests for comparing aldehydes and ketones.

Answer 1

• Tollens Reagant - Aldehydes: gives a silver mirror, Ketons: no visible reaction.
• Fehlings solution - Aldehydes: blue solution gives brick-red precipitate, Ketones: no visible reaction (remains blue solution)

Question 2

Give the test for identifying 1˚ and 2˚ alcohols and aldehydes.

Answer 2

• Add Acidified potassium dichromate - goes from orange solution to green solution.

Question 3

Describe the two methods for turning 1˚ alcohol into a carboxylic acid.

Answer 3

1- Reflux: add acidified potassium dichromate and sulphuric acid to the 1˚ alcohol and reflux the product.
2- Heat and distill off product: add acidified potassium dichromate and sulphuric acid and heat and distill of product, then add the same again (or tollens/fehlings and H2SO4) and heat or reflux. (oxidation)

Question 4

Describe the reaction for turning aldehydes into 1˚ alcohols.

Answer 4

• Add NaBH4 or LiAlH4 to the aldehyde. (reduction)

Question 5

Describe the reaction to turning aldehydes 2˚ alcohol into a ketone and back again.

Answer 5

1- Add Acidified potassium dichromate and sulphuric acid to the 2˚ alcohol and heat or reflux. (oxidation)
2- Add NaBH4 to the ketone. (reduction)

Question 6

Why will NaBH4 not react (reduce) with an alkene?

Answer 6

• The C=C is electron rich so attracts electrophiles and repels nucleophiles.

Question 7

Why is a racemic mixture formed during the nucleophilic addition of HCN to and aldehyde/ketone?

Answer 7

• Due to the planar nature of the C=O group the CN- ion has an equal chance of attaching to the delta+ of the C atom from above or below, producing 50/50 of either possible molecule formed.