Chapter 26: Carbonyl Compounds

Question 1

What functional group do aldehydes and ketones have?

Answer 1

• C=O, known as the carbonyl functional group

Question 2

Can aldehydes and ketones be oxidised, and why?

Answer 2

• Only aldehydes can be oxidised
• Ketones are unreactive (they don’t have the H group to be removed, and oxidation can also be defined as the loss of hydrogen)

Question 3

Give the equation and the conditions for the oxidation of an aldehyde. How is this reaction different to the oxidation of alcohols, and why?

Answer 3

• Aldehyde + [O] -> carboxylic acid
• Conditions: reflux, acidified potassium dichromate (VI)
• Water is not formed as it was already formed in the step going from an alcohol to an aldehyde

Question 4

What observation can be made during the oxidation of aldehydes?

Answer 4

• The dichromate ions change colour from orange to green

Question 5

What influences the reactivity of aldehydes and ketones, and in what ways? Therefore, what mechanism do aldehydes and ketones undergo?

Answer 5

• The C=O double bond
• It is polar, so the positive dipole on the carbon atom attracts nucleophiles
• It is a double bond, so it undergoes addition reactions
• Nucleophilic addition

Question 6

Explain the difference in reactions between alkenes and aldehydes and ketones.

Answer 6

• C=C in alkenes is non-polar, so they react with electrophiles via electrophilic addition
• C=O in carbonyl compounds is polar

Question 7

What does [H] represent?

Answer 7

• Reducing agent

Question 8

What is another way of describing reduction?

Answer 8

• The gain of hydrogen (formation of more C-H bonds)

Question 9

What are aldehydes and ketones reduced to form, and how is this done?

Answer 9

• Aldehydes are reduced to form 1° alcohols, and ketones are reduced to form 2° alcohols
• Aqueous NaBH4 acts as the reducing agent

Question 10

Give the equation for the reduction of an aldehyde.

Answer 10

• Aldehyde + 2[H]
• Arrow: NaBH4/ H2O
• Primary alcohol

Question 11

Give the equation for the reduction of a ketone.

Answer 11

• Ketone + 2[H]
• Arrow: NaBH4/ H2O
• Secondary alcohol

Question 12

What acts as the nucleophile in the reduction of aldehydes and ketones with NaBH4?

Answer 12

• The hydride ion (H- with a lone pair)

Question 13

Describe the mechanism for the reduction of aldehydes and ketones.

Answer 13

• Check the mechanism

Question 14

Name another compound carbonyl compounds undergo nucleophilic addition reactions with. What part exactly is the nucleophile?

Answer 14

• HCN (hydrogen cyanide)
• The cynanide ion- C≡N, and C has a lone pair and negative charge

Question 15

Why is the reaction of HCN with aldehydes/ ketones important?

Answer 15

• It increases the length of the carbon chain

Question 16

Give the equation of the reaction of aldehyes/ ketones with hydrogen cyanide.

Answer 16

• Aldehyde/ ketone + HCN
• Arrow: NaCN/ H2SO4
• hydroxynitrile

Question 17

Explain the reaction conditions for the reaction of aldehydes and ketones with HCN.

Answer 17

• HCN is too dangerous, so NaCN and H2SO4 are used to produce it in the reaction

Question 18

Describe the mechanism for the reaction of aldehydes and ketones with hydrogen cyanide.

Answer 18

• Check the mechanism

Question 19

How can carbonyl compounds be tested for?

Answer 19

• Add 2,4-DNP
• If an aldehyde or ketone is present, a yellow-orange precipitate will form

Question 20

After proving the presence of a carbonyl compound, how can you identify the compound?

Answer 20

• From the melting point of its DNP derivative; the compound it forms with 2,4-DNP
• To find the melting point, you have to follow the method for purifying an organic solid
• You then compare the melting point value against the values in a data base

Question 21

How can you distinguish between aldehydes and ketones?

Answer 21

• Add Tollens’ reagent and warm gently in a water bath
• Aldehydes will form a silver mirror precipitate as they are oxidised
• Ketones cannot be oxidised, so they don’t react and will stay colourless

Question 22

Give equations and an explanation for the reaction of aldehydes with Tollens’ reagent.

Answer 22

• Ag+(aq) + e- -> Ag(s)
• Aldehyde + [O] -> carboxylic acid
• The silver ion acts as the oxidising agent, and is reduced