Aldehydes, Ketones And Carboxylic Acids

Question 1

Explain the high polarity of the carbonyl group 

Answer 1

• the carbon oxygen double bond is polarized due to higher electronegativity of oxygen relative to carbon.
• hence the car carbon is electrophilic that is acid and carbon oxygen is a nucleophilic. That is Lewis base center.
• carbon compounds have substantial dipole moments and are polar than ethers.
• the high polarity of carbon group is explained on the basis of the resonance involving a neutral A and dipolar B structures as shown

Question 2

Aldehyde are generally more reactive than ketones in nucleophilic addition reaction

Answer 2

Due to steric and electronic reasons
- Sterically the presence of two relatively large substituents in ketones, hinders the approach of nucleophiles to the carbonyl carbon then in Aldehydes that have only one such groups.
- electronically, two alkyl groups reduce the electrophilicity of the carbonyl carbon in ketones, more effectively than in aldehydes.

Question 3

Phenols are less acidic than carboxylic acids

Answer 3

• the conjugate base of carboxylic acid, a carboxylate ion is stabilized by two equivalent resonance structures in which the negative charge at the more electronegative oxygen atom. The conjugate base of phenol, phenoxide ion has non-equivalent resonance structures in which, the negative charge is at the less electronegative carbon atom.
• the negative charge is delocalized over the electronegative oxygen atom in carboxylate ion, whereas it is less effectively delocalized over one oxygen atom and less electronegative carbon atom in the phenoxide ion.

Question 4

The boiling points of aldehyde and ketone are higher than hydrocarbons and ether of comparable molecular masses

Answer 4

It is due to weak molecular association in aldehyde and ketones arising out of dipole dipole interactions.

Question 5

The boiling point of ADH and data of alcohols of similar molecular masses due to

Answer 5

Absence of intermolecular hydrogen bonding 

Question 6

The boiling point of ADH and data of alcohols of similar molecular masses due to

Answer 6

Absence of intermolecular hydrogen bonding

Question 7

Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer.

Answer 7

The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in
propanal. The polarity of the carbony! group is reduced in benzaldehyde due to resonance as shown below and hence it is less reactive than propanal.

Question 8

Acidity of alpha hydrogen atoms of carbonyl compound

Answer 8

Is due to the strong electron withdrawing effect of carbon group and the resonance stabilization of the conjugate base

Question 9

Carboxylic acid are higher boiling liquids than aldehydes

Answer 9

• this is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding.
• the hydration bonds are not broken completely even in the vapor phase. In fact, most of the carboxylic acid exist as dimer in vapor face or in aprotic solvent.

Question 10

Simple aliphatic carboxylic acids having up to four carbon atoms are miscible in water

Answer 10

Formation of hydrogen bond in water

Question 11

Higher carboxylic acid are practically insoluble in water

Answer 11

• the solubility decreases with increase in the number of carbon atoms.
• Higher carboxylic acid are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part.

Question 12

Direct attachment of group such as phenyl, or vinyl to the carboxylic acid increases the acidity of the corresponding carboxylic acid

Answer 12

This is because of greater electronegativity of SP2 hybridized carbon to which carboxyl carbon is attached. The presence of electron withdrawing group on phenyl of aromatic carboxylic acid increases their acidity electron donating group decrease the acidity.

Question 13

Why aromatic carboxylic acid do not undergo friedal craft reaction

Answer 13

Aromatic carboxylic acid undergo electrophilic substitution reaction in which the Carboxylic group acts as the deactivating and meta directing group. They however, do not undergo friedal craft reaction because the carboxylic group is deactivating and the catalyst aluminum chloride which is a Lewis acid gets bonded to the carboxyl group.

Question 14

Carboxylic acid are more acidic than phenol

Answer 14

• the conjugate base of carboxylic acid, a carboxylate ion is stabilized by two equivalent resonant structures in which the negative charge is at the more electronegative oxygen atom. The conjugate base of phenol, phenoxide ion has non-equivalent structures in which the negative charge is at the less electronegative carbon atom.
• the negative charge is the delocalized over the electronic oxygen atom in the carboxylate ion, whereas it is less effectively, delocalized over one oxygen atom and less less electron negative carbon atom in phenoxide ion.