Aldehydes, ketones and carboxylic acids 2

Question 1

explain the mechanism of nucleophilic addition reactions

Answer 1

• A nucleophile attacks the electrophilic carbon atom of the polar
  carbonyl group from a direction approximately perpendicular
  to the plane of sp2
  hybridised orbitals of carbonyl carbon (Fig.
  8.2).
• The hybridisation of carbon changes from sp 2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced.

-This intermediate captures a proton from the
reaction medium to give the electrically neutral
product. The net result is addition of Nu– and H+
across the carbon oxygen double bond.

Question 2

why are aldehydes more reactive towards NAR than ketones, despite both having carbonyl group

Answer 2

Aldehydes are generally more reactive than ketones in
nucleophilic addition reactions due to steric and electronic
reasons.

• Sterically, the presence of two relatively large
  substituents in ketones hinders the approach of the incoming nucleophile to carbonyl carbon than in aldehydes having only one such
  substituent.
• Electronically, aldehydes are more reactive than
  ketones because two +I alkyl groups reduce the electrophilicity of
  the carbonyl carbon more effectively than in former.

Question 3

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

Answer 3

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 carbonyl
group is reduced in benzaldehyde due
to resonance as shown below and
hence it is less reactive than propanal.

Question 4

uses of aldehydes and ketones (basic)

Answer 4

In chemical industry aldehydes and ketones are used as solvents,
starting materials and reagents for the synthesis of other products.
Acetone and
ethyl methyl ketone are common industrial solvents. Many aldehydes
and ketones, e.g., butyraldehyde, vanillin, acetophenone, camphor, etc.
are well known for their odours and flavours.

Question 5

formaldehyde

Answer 5

Formaldehyde is well known as formalin (40%) solution used to preserve
biological specimens and to prepare bakelite (a phenol-formaldehyde
resin), urea-formaldehyde glues and other polymeric products.

Question 6

acetaldehyde

Answer 6

Acetaldehyde is used primarily as a starting material in the manufacture
of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.

Question 7

benzaldehyde

Answer 7

Benzaldehyde is used in perfumery and in dye industries.