3.3.8 Aldehydes and Ketones

Question 1

Can aldehydes hydrogen bond with other aldehyde molecules?

Answer 1

No

Question 2

Which molecules can aldehydes hydrogen bond to?

Answer 2

Alcohols and water

Question 3

What are the differences between aldehydes when the chain length gets longer?

Answer 3

Less polar

Increased vdw forces

Question 4

How is an aldehyde formed from an alcohol?

Answer 4

Alcohol + hot potassium dichromate + sulfuric acid

Question 5

Why is a numbering system not need for an aldehyde?

Answer 5

It always occurs at the end of the chain

Question 6

How do ketones form hydrogen bonds with water?

Answer 6

The hydrogen bonds form between the electronegative oxygen atom on the ketone and the positive hydrogen atom on the water molecule

Question 7

How does the C=O bond affect the rest of the aldehyde / ketone?

Answer 7

It is a very strongly polar bond with the carbon being positive so there are permanent dipole-dipole forces between the molecules

Question 8

Are aldehydes and ketones soluble?

Answer 8

Shorter chains are completely soluble because they form hydrogen bonds

Question 9

How reactive are carbonyl compounds?

Answer 9

It is strong but almost all the reactions of carbonyl compounds involve the C=O bond
This is because the difference in electronegativity makes the bond very polar
Nucleophilic reagants attach the positive carbon bond
Addition reactions are also possible

Question 10

What are carbonyl compounds?

Answer 10

Compounds with a C = O bond

Question 11

What are the reactants for the addition of hydrogen cyanide?

Answer 11

Sodium or potassium cyanide

Dilute hydrochloric acid

Question 12

What is the reaction mechanism for the addition of hydrogen cyanide to a ketone?

Answer 12

See card

Question 13

What is the reaction mechanism for the addition of hydrogen cyanide to an aldehyde?

Answer 13

See card

Question 14

What are hydroxynitriles?

Answer 14

Aldehyde that have undergone the addition of hydrogen cyanide

Question 15

How are enantomers produced during the addition of hydrogen cyanide to aldehyde and ketones?

Answer 15

When carried out with an aldehyde or an unsymmetrical ketones, the CN ion may attack from above or below the C=O group

Question 16

Can aldehydes be oxidised?

Answer 16

Aldehydes can be oxidised to carboxylic acids
[O] = oxidising agent
The oxidising agent is acidified potassium dichromate

Question 17

Can ketones be oxidised?

Answer 17

No because a C-C bond must be broken

Question 18

What is the Fehling’s test?

Answer 18

Fehling’s A contains Cu2+ ions and is blue
Fehling’s B contains an alkali and a complexing agent
When an aldehyde is warmed with Fehling’s solution, a red precipitate of copper (1) oxide is produced as the copper (2) oxide oxidises the aldehyde to a carboxylic acid and is itself reduced to copper (1)
Ketones don’t react

Question 19

What is the silver mirror test?

Answer 19

Tollens’ reagent contains the complex Ag(NH3)2
When an aldehyde is warmed with Tollens’ reagent
Aldehydes are oxidised to carboxylic acids
Ag+ is reduced to metallic silver

Question 20

How do you name an aldehyde?

Answer 20

Count the carbon as part of the root

e.g. CH2O = methanal

Question 21

What intermolecular forces occur between aldehyde molecules?

Answer 21

The C = O bond is highly polar so dipole - dipole forces are formed which gives a higher melting point than an alkane of the same length