Ketones and Aldehydes

Question 1

The Structure of Aldehydes

Answer 1

● when a carbonyl group is bonded to a hydrogen atom and a hydrocarbon group (or a second hydrogen atom), the compound is an aldehyde
● the simplest aldehyde, formaldehyde (methanal) has the carbonyl group bonded to two hydrogen atoms
● in all other aldehydes, the carbonyl group is bonded to one hydrogen atom and one atom carbon

Question 2

The Naming of Aldehydes

Answer 2

● the common name for an aldehyde is derived from the parent name of the corresponding carboxylic acid
● this is done by dropping the word “acid”, and changing the suffix –ic or –oic to –aldehyde
e.g. formaldehyde
formic acid
e.g acetaldehyde
acetic acid

Question 3

The Structure of Ketones

Answer 3

● when the carbon atom of the C=O group is bonded to two hydrocarbon (R-) groups, the compound is a ketone
● the simplest ketone, propanone (acetone) has the carbonyl group bonded to two methyl (CH3-) groups

Question 4

The Naming of Ketones

Answer 4

● common names for ketones are derived by naming the alkyl or aryl group bonded to the carbonyl group as a separate word, followed by the word “ketone”
● groups are generally listed in order of increasing atomic weight

Question 4

The Naming of Ketones

Answer 4

● common names for ketones are derived by naming the alkyl or aryl group bonded to the carbonyl group as a separate word, followed by the word “ketone”
● groups are generally listed in order of increasing atomic weight

Question 5

How to distinguish between aldehydes and ketones

Answer 5

The “silver mirror” test
● Reaction with Tollens reagent
What is Tollens reagent?
- It is a solution of silver oxide in dilute ammonia solution
- It is an oxidising reagent
- It oxidises aldehydes, but not ketones

Question 6

experimental procedure of Tollens reagent

Answer 6

● dissolve the compound in ethanol, and then add Tollens reagent
● a positive result is indicated by the formation of a “silver mirror”, deposited on the side of the reaction vessel
● a positive result indicates the presence of an aldehyde. A negative result suggests the presence of a ketone

Question 7

Aldehydes and ketones typically undergo………

Answer 7

1. Oxidation
2. Reduction

Question 8

Oxidation of Aldehydes

Answer 8

1. Tollens reagent, Ag2O
2. H2CrO4 (chromic acid)

Question 8

Reduction of Aldehydes and Ketones

Answer 8

● the reduction of aldehydes and ketones leads to the formation of alcohols
● the reduction reaction involves the gain of hydrogen

Question 9

name reducing agents of aldehydes and ketones

Answer 9

1. Hydrogen gas (H2)
2. Sodium borohydride (NaBH4)
3. Lithium aluminium hydride (LiAlH4)

Question 10

examples of reduction of Aldehydes and Ketones

Answer 10

• cylcohexanone to cyclohexanol
• butan-2-one to butan-2-ol
• butanal to butan-1-ol