3.8 Aldehydes and Ketones

Question 1

Draw the displayed formula for ethanal.

Answer 1

(see page 1 in the chemrevise revision guide)

Question 2

What is the functional group for an aldehyde?

Answer 2

C=O on the end of a chain with an H attached to the chain too.

(see page 1 in the chemrevise revision guide)

Question 3

Draw the displayed formula for propanone.

Answer 3

(see page 1 in the chemrevise revision guide)

Question 4

What is the functional group for an ketone?

Answer 4

If the C=O is in the middle of the chain.

see page 1 in the chemrevise revision guide

Question 5

What is a carbonyl?

Answer 5

Carbonyls are compounds with a C=O bond.

They can be either aldehydes or ketones.

Question 6

How do carbonyls bond?

Answer 6

Intermolecular forces in carbonyls.

Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole forces.

Question 7

How soluble are carbonyls in water?

Answer 7

The smaller carbonyls are soluble in water because they can form hydrogen bonds with water.

Question 8

Do carbonyls undergo addition reactions easily?

Answer 8

In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily.

Question 9

Are carbonyls polarised?

Answer 9

The C=O bond is polarised because O is more electronegative than carbon. The positive carbon atom attracts nucleophiles.

(see page 1 in the chemrevise revision guide)

Question 10

What is an oxidising agent that causes alcohols and aldehydes to oxidise?

Answer 10

potassium dichromate, K2Cr2O7

Question 11

When a primary alcohol is oxidised what is its functional group change?

Answer 11

primary alcohol -> aldehydes

Question 12

When an aldehyde is oxidised what is its functional group change?

Answer 12

aldehyde -> carboxylic acid

Question 13

When a secondary alcohol is oxidised what is its functional group change?

Answer 13

secondary alcohol -> ketone

Question 14

When a tertiary alcohol is oxidised what is its functional group change?

Answer 14

tertiary alcohols do not oxidise.

Question 15

When a ketone is oxidised what is its functional group change?

Answer 15

ketones do not oxidise.

Question 16

What are the reagents and conditions when oxidising an aldehyde?

Answer 16

reagent - potassium dichromate (VI) solution and dilute sulphuric acid.

conditions - heat under reflux

Question 17

Draw the equation when oxidising propanal.

Answer 17

(see page 1 in the chemrevise revision guide)

Question 18

For oxidising aldehydes with Tollen’s reagent, what are the:

• Reagents
• Conditions
• Reaction
• Observation
• Equation

Answer 18

(see page 1 in the chemrevise revision guide)

Question 19

For oxidising aldehydes with Fehling’s solution, what are the:

• Reagents
• Conditions
• Reaction
• Observation
• Equation

Answer 19

(see page 1 in the chemrevise revision guide)

Question 20

What a 2 reducing agents for carbonyls?

Answer 20

• NaBH4 (sodium tetrhydridoborate)
• LiSlH4 (lithium tetrahydridoaluminate)

Will reduce carbonyls to alcohols.

Question 21

What is the change in functional group when Aldehydes are reduced?

Answer 21

aldehyde -> primary alcohol

Question 22

What is the change in functional group when ketones are reduced?

Answer 22

ketone -> secondary alcohol

Question 23

Draw the equation for the reduction of propanal.

Answer 23

(see page 2 in the chemrevise revision guide)

Question 24

Draw the equation for the reduction of propanone.

Answer 24

(see page 2 in the chemrevise revision guide)

Question 25

Draw the mechanism for the reduction of propanone.

Answer 25

(see page 2 in the chemrevise revision guide)

Question 26

Other than a reducing agent, what is another way carbonyls can be reduced?

Answer 26

Catalytic hydrogenation

Question 27

What are the reagents and conditions for catalytic hydrogenation of carbonyls?

Answer 27

Reagent - hydrogen and nickel catalyst
Conditions - high pressure

(see page 2 in the chemrevise revision guide for some example equations)

Question 28

When adding hydrogen cyanide to a carbonyl what is:

• The change in functional group?
• The reagents?
• The conditions?
• The mechanism?

Answer 28

carbonyl -> hydroxynitrile

reagent - sodium cyanide (NaCN) and dilute sulfuric acid.
The NaCN supplies the nucleophile CN- ions.
The H2SO4 acid supplies H+ ions needed in second step of the mechanism.

conditions - room temperature and pressure

mechanism - nucleophilic addition

Question 29

Draw the equation of the formation of 2-hydroxy-2-methylpropanenitrile from a carbonyl

Answer 29

(see page 2 in the chemrevise revision guide)

Question 30

Draw the equation of the formation of 2-hydroxypropanenitrile from a carbonyl

Answer 30

(see page 2 in the chemrevise revision guide)

Question 31

Why would you use KCN/ NaCN instead of HCN when forming a hydroxynitrile?

Answer 31

We could use HCN but it is a toxic gas that is difficult to contain.
KCN/NaCN are still, however, toxic, because of the cyanide ion.

Question 32

Draw the mechanism for formation of a 2-hydroxypropanenitrile form a carbonyl.

Answer 32

(see page 2 in the chemrevise revision guide)

Question 33

Draw the basic displayed formula of the structure of any hydroxynitrile

Answer 33

(see page 2 in the chemrevise revision guide)

Question 34

How is a racemate formed from a carbonyl?

Draw the mechanism reaction.

Answer 34

(see page 3 in the chemrevise revision guide)