Aldehydes and Ketones

Question 1

Aldehydes and ketones functional group

Answer 1

carbonyl (C=O)
Aldehydes contain the carbonyl group bonded to at least one hydrogen atom
Ketones contain the carbonyl group bonded to two carbon atoms

Question 2

Aldehydes often called and name derived from

Answer 2

formyl or methanoyl group
dehydration of alcohols

Question 3

Hydrogen substituents

Answer 3

carbon atom of this group has two remaining bonds that may be occupied by hydrogen, alkyl or aryl substituents.
If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen, the compound is a ketone

Question 4

If the aldehyde moiety (-CHO) is attached to a ring

Answer 4

carbaldehyde is added to the name of the ring. The carbon attached to this moiety will get the #1 location number in naming the ring.

Question 5

prep of aldehydes

Answer 5

oxidise primary alcohols using pyridinium chlorochromate
alkenes with a vinylic hydrogen can undergo oxidative cleavage when treated with ozone yielding aldehydes

Question 6

prep of ketones

Answer 6

oxidation of secondary alcohols
reagents: Na2Cr2O7, KMnO4, CrO3

Question 7

Boiling Points of Aldehydes and Ketones

Answer 7

higher than that of non-polar compounds (hydrocarbons) but lower than those of corresponding alcohols and carboxylic acids as aldehydes and ketones do not form H-bonds with themselves

Question 8

Solubility of Aldehydes and Ketones

Answer 8

up to 4 carbons of aldehydes and ketones are soluble in water due to H-bonding. The higher members do not dissolve in water because the hydrocarbon part is larger and resists the formation of hydrogen bonds with water molecules

Question 9

Chemical Properties of Aldehydes and Ketones

Answer 9

Both aldehydes and ketones contain carbonyl group, therefore they undergo the same reactions like nucleophilic addition reactions, oxidation, reduction, halogenation etc

Question 10

Nucleophilic Addition Reactions in Aldehydes

Answer 10

Aldehydes are more susceptible to nucleophilic attack than ketones because of both steric and electronic effects. In aldehydes, the relatively small hydrogen atom is attached to one side of the carbonyl group, while a larger R group is affixed to the other side.

Question 11

Nucleophilic Addition Reactions in Ketones

Answer 11

R groups are attached to both sides of the carbonyl group. Thus, steric hindrance is less in aldehydes than in ketones.

Question 12

Nucleophilic Addition Reactions in Aldehydes and Ketones electronically

Answer 12

aldehydes have only one R group to supply electrons toward the partially positive carbonyl carbon, while ketones have two electron‐ supplying groups attached to the carbonyl carbon. The greater amount of electrons being supplied to the carbonyl carbon, the less the partial positive charge on this atom and the weaker it will become as a nucleus.

Question 13

Reactivity of Aliphatic Aldehydes and Ketones

Answer 13

Aliphatic aldehydes are more reactive than ketones:
1. Inductive effect : Alkyl groups are electron donating in nature (i.e., show +I-Effect). Hence, the presence of alkyl groups attached to the carbonyl group increases the electron density on the carbonyl carbon thereby decreases its reactivity towards nucleophilic addition reactions.
2. Steric hindrance : The more the number and the bigger the size of the alkyl group, the more difficult for a nucleophile to attack due to steric hindrance (crowding).

Question 14

Reactivity of Aromatic Aldehydes and Ketones

Answer 14

exhibit electron donating resonance which increases the electron density on the carbonyl carbon
carbonyl carbon becomes less electrophilic, and hence is less susceptible to nucleophilic attack more reactive than aromatic ketones.
The reactivity of aromatic aldehydes and ketones follows the order :
C6H5CHO > C6H5COCH3 > C6H5COC6H5

Question 15

Oxidation Reaction

Answer 15

Aldehydes can be easily oxidised to carboxylic acids due to the presence of a hydrogen atom on carbonyl group which can be easily converted to OH group. Since ketones do not have any hydrogen atom attached to the carbonyl group, they cannot be oxidised easily, and therefore, strong oxidising agents are required for this purpose.

Question 16

Reduction of Aldehydes and Ketones

Answer 16

Aldehydes and ketones can be reduced to a variety of compounds such as alcohols, hydrocarbons etc.

Question 17

Tollens Test

Answer 17

mixture of silver nitrate and ammonia
silver ion Ag+ is reduced to solid silver, Ag(s)
solid silver forms a film on the inner wall of the test tube, resembling a silver mirror
aldehydes are able to be oxidized into a carboxylic acid while ketones cannot

Question 18

Fehling’s Solution

Answer 18

compound to be tested is added to the Fehling’s solution and the mixture is heated
Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are α-hydroxy ketones

Question 19

Uses of aldehydes

Answer 19

synthesize other compounds
less important in industrial synthesis

Question 20

Formaldehyde

Answer 20

made predominantly by the oxidation of methanol
gas but is generally handled as a 37 percent solution in water, called formalin
used in tanning, preserving, and embalming and as a germicide, fungicide, and insecticide for plants and vegetables, but its largest application is in the production of certain polymeric materials

Question 21

Uses of Ketones

Answer 21

acetone which is an excellent solvent for a number of plastics and synthetic fibres
household = nail paint remover and paint thinner
medicine = chemical peeling and for acne treatments