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

Answer 8

• Aldehydes are reduced to form 1° alcohols, and ketones are reduced to form 2° alcohols
• They are warmed with aqueous NaBH4, which acts as a reducing agent

Question 9

Give the equation for the reduction of an aldehyde.

Answer 9

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

Question 10

Give the equation for the reduction of a ketone.

Answer 10

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

Question 11

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

Answer 11

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

Question 12

Describe the mechanism for the reduction of aldehydes and ketones.

Answer 12

• H- lone pair attracted and donated to Cδ+ ​
• Dative covalent bond forms between H- and Cδ+​
• Pi-bond breaks by heterolytic fission, forming negatively charged intermediate​
• O- from intermediate donates lone pair to H-atom in water
• Intermediate has been protonated (H+, a proton, has been added) to form an alcohol

Question 13

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

Answer 13

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

Question 14

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

Answer 14

• It increases the length of the carbon chain

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

How are hydroxynitriles named?

Answer 17

• Any compound with C≡N is called a nitrile (ethanenitrile, propanenitrile), and the carbon with the triple bond to nitrogen is carbon number 1
• The same carbon atom that forms a bond with the cyanide ion is also bonded to a -OH group, which is where the prefix comes from

Question 18

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

Answer 18

• The CN- lone pair attracted and donated to Cδ+
• A dative covalent bond forms between CN- and Cδ+
• Pi-bond breaks by heterolytic fission, forming negatively charged intermediate
• O- from intermediate donates lone pair to H+ ion (is protonated) to form a hydroxynitrile

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