Aldehydes and Ketones

Question 1

What is a primary alcohol oxidised to?

Answer 1

An aldehyde

Question 2

What is a secondary alcohol oxidised to?

Answer 2

A ketone

Question 3

Can an aldehyde be oxidised? If so, what to?

Answer 3

Yes. It can be oxidised to a carboxylic acid

Question 4

Can a ketone be oxidised? If so, what to?

Answer 4

No, it cannot be oxidised further.

Question 5

Acidified potassium dichromate is added to a substance. The colour changes from orange to green. What type of substance is it?

Answer 5

An aldehyde

Question 6

There are three tests to distinguish between an aldehyde and a ketone. What are they, and what are the changes you’d see?

Answer 6

Acidified potassium dichromate + heat under reflux - Ald orange to green, Ket nvc
Tollen’s reagent + gently heat - Ald colourless to silver ppt, Ket nvc
Fehling’s soln. + gently heat - Ald blue to red ppt, Ket nvc

Question 7

Give a balanced equation for the reduction of ethanal

Answer 7

CH3COH + 2[H] –> CH3CH2OH

Question 8

Carbonyls can be reduced using NaBH4 in aqueous ethanol. They can also be reduced using another method. State the name of this other method, the reagents used, and the conditions.

Answer 8

Catalytic Hydrogenation.
Reagents: hydrogen and nickel catalyst
Conditions: high pressure

Question 9

Hydroxynitriles can be formed from aldehydes and ketones. Give the mechanism, the reagents and the conditions.

Answer 9

Mechanism: Nucleophilic addition
Reagent: sodium cyanide (NaCN) and dilute H2SO4
Conditions: room temp. and pressure

Question 10

CH3COCH3 is reacted with NaCN and another reagent under room temp. and pressure. Name the other reagent and the product formed.

Answer 10

Dilute H2SO4

2-hydroxy-2-methylpropanenitrile

Question 11

Nucleophilic addition of HCN to aldehydes and ketones results in the formation of a racemate. Why?

Answer 11

The middle step of the reaction is a trigonal planar carbonyl (2D). The H+ can attack from either above or below, resulting in two different enantiomers. Because the carbonyl has equal chance of being

Question 12

Give two methods of reduction

Answer 12

NaBH4 in aq ethanol

Catalytic hydrogenation

Question 13

What is a suitable oxidising agent?

Answer 13

Acidified potassium dichromate

Question 14

The reduction of an aldehyde or ketone involves what type of reaction mechanism?

Answer 14

Nucleophilic addition

Question 15

Describe and explain the nucleophilic addition of a nucleophile to propanal

Answer 15

The C=O is polar, with O being more electronegative and so being more -ve
The -ve nucleophile is attracted to the slightly +ve C
The C can only have 4 bonds, so breaks the double bond, allowing Nu to join
O takes the electrons, making it negative, which allows it to attract an electrophile

Question 16

Why will the nucleophilic addition reaction of carbonyl compounds (eg propanal) produce a racemic mixture?

Answer 16

It is a trigonal planar, meaning the attacking species is equally likely to attack from the top or the bottom

Question 17

Name a mechanism for the reaction of an aldehyde with HCN

Answer 17

Nucleophilic addition

Question 18

Some were asked to distinguish between a ketone and an aldehyde. One suggested looking at the optical activity. Why would this not work?

Answer 18

The ketone is not optically active

An aldehyde can be optically active, but is formed in a racemic mixture, and so any effects will be cancelled out

Question 19

Someone wants to use acidified potassium dichromate to distinguish between an aldehyde and a ketone. What are the conditions?

Answer 19

Heat under reflux

Question 20

What test produces a silver mirror effect when an aldehyde is present?

Answer 20

Tollen’s reagent

Question 21

Someone wants to use Tollen’s reagent to distinguish between an aldehyde and a ketone. What else would they have to do to the substance? (conditions)

Answer 21

Heat gently

Question 22

What colour change is observed when Tollen’s reagent is added to an aldehyde?

Answer 22

Colourless to silver precipitate

Question 23

What colour change is observed when Tollen’s reagent is added to a ketone?

Answer 23

No visible change

Question 24

Someone wants to use Fehling’s solution to distinguish between an aldehyde and a ketone. What else would they have to do to the substance? (conditions)

Answer 24

Gently heat

Question 25

What colour change is observed when Fehling’s solution is added to an aldehyde?

Answer 25

Blue to red

Question 26

What colour change is observed when Fehling’s solution is added to a ketone?

Answer 26

No visible change

Question 27

What colour change is observed when acidified potassium dichromate is added to an aldehyde?

Answer 27

Orange to green

Question 28

What colour change is observed when acidified potassium dichromate is added to a ketone?

Answer 28

No visible change

Question 29

What are the reagents for the formation of hydroxynitriles from aldehydes/ketones?

Answer 29

Sodium cyanide (NaCN) and dilute H2SO4

Question 30

What is the reaction mechanism for the formation of hydroxynitriles from aldehydes/ketones?

Answer 30

Nucleophilic addition

Question 31

What are the conditions for the formation of hydroxynitriles from aldehydes/ketones?

Answer 31

Room temp. and pressure

Question 32

Give the reagents for the catalytic hydrogenation of an aldehyde

Answer 32

Hydrogen

Question 33

Give the conditions for the catalytic hydrogenation of an aldehyde

Answer 33

Nickel catalyst
150*C
High pressure