Carbonyl

Question 1

Preparation of aldehydes

Answer 1

1. From primary alcohols
   H2SO4(aq),K2Cr2O7(aq), heat with imm. Distillation ( From primary alcohols)

Question 2

Preparation of ketones

Answer 2

1. From secondary alcohols
   H2SO4(aq), KMnO4(aq)/K2Cr2O7(aq), heat (from secondary alcohols)
2. Benzene
   Benzene and acyl chloride (RCOCl) -Aromatic ketones can be prepared
3. Oxidation of terminal alkene with two R groups
   H2SO4(aq) KMnO4(aq) heat Oxidation of terminal alkene attached to two R groups (R’RC=C)

Question 3

Reactions aldehyde

Answer 3

1. Reduction
   LiAlH4 in dry ether/ NaBH4 in methanol/ H2/Ni heat to form primary alcohol
2. Oxidation
   K2Cr2O7/KMnO4(aq), H2SO4(aq), heat to form carboxylic acid
3. Nu: add
   Cold HCN in trace amounts of NaCN/ NaOH to form nitrile (CRHOHCN)

Question 4

Reactions ketone

Answer 4

1. Reduction
   LiAlH4 in dry ether, NaBH4 in methanol, H2/Ni heat to form secondary alcohol
2. Oxidation of aromatic ketones
   Aromatic ketones undergo side chain oxidation to form benzoic acid
   H2SO4 (aq), KMnO4(aq), heat
3. Nu: add
   Cold HCN, trace NaOH,NaCN to give (CRR’OHCN)

Question 5

Why carbonyl cmpds are susceptible to nucleophilic attack

Answer 5

Carbonyl C is bonded to highly electronegative O which withdraws electron from C=O bond causing the C to acquire partial positive charge and become electron deficient hence being susceptible to attack by an electron rich nucleophile

Question 6

Upon nucleophilic addition, if product has chiral C, then a racemic mixture would have resulted

Answer 6

Geometry about carbonyl C is trigonal planar. The electron deficient carbonyl C is likely to be attacked by nucleophile from both top and bottom which results in an equal mixture of 2 enantiomers which rotate plane polarised light in the opposite directions to an equal extent

Question 7

Why aldehydes are more likely to undergo nu: add than ketones?

Answer 7

Electronic factor
Ketones are attached to two electron donating R groups whereas aldehydes are attached to only one electron donating R group. Hence the carbonyl C in ketones is less electron deficient hence less susceptible to attack by electron rich nucleophile
Steric factor
The two R groups attached to ketones are of greater steric hindrance. Hence making it harder for the nucleophile to attack the carbonyl C.

Question 8

Why C in CN acts as the nucleophile rather than N?

Answer 8

C in CN acquires negative charge and has a lone pair of electrons on it
The lone pair in N delocalises into the electron rich CN bond due to pi orbitals overlap of C and N hence lone pair on N is not available for nucleophilic attack.

Question 9

Test for carbonyl compds

Answer 9

1. For both aldehydes and ketones
   Condensation rxn with 2,4-dinitrophenylhydrazine
   Positive test produces orange ppt
2. Aliphatic and aromatic aldehydes
   Oxidation with tollen’s reagent (Ag(NH3)2))+
   Positive test produces silver mirror (Ag)
   Products involve also, carboxylate salt, Ag, NH3, H2O
3. Aliphatic aldehydes
   Oxidation with Fehling’s solution (copper II complex)
   Positive test gives brick red ppt
   Products involve also, carboxylate salt, Cu2O(brick red), H2O
4. Test for carbonyl cmpds with (CRCH3O) also can CH2I, CHI2,CI3
   NaOH, I2(aq), warm
   Postive test gives yellow ppt
   Products involve : CHI3 (yellow), carboxylate salt, I-