H2 Chemistry 17 - Carbonyl Compounds

1^o ROH + [O] -> RCHO + H₂O

Oxidation

r&c: K₂Cr₂O₇(aq), H₂SO₄(aq), heat with immediate distillation

obs: orange K₂Cr₂O₇(aq) turns green due to Cr³⁺ formed

**KMnO4 is not used as it is too strong of a O.A, 1^o ROH -> RCOOH

RCHO + [H] -> 1^o ROH

Reduction

1. r&c: LiAlH₄, dry ether OR NaBH₄, methanol

RCHO + 2[H] -> 1^o ROH

2. r&c: H₂(g), Ni catalyst, heat

RCHO + H₂ -> 1^o ROH

2^o ROH + [O] -> RCOR' + H₂O

Oxidation

r&c: K₂Cr₂O₇/KMnO₄(aq), H₂SO₄(aq), heat

obs: orange K₂Cr₂O₇ turns green OR

purple KMnO₄ turns colourless

RCOR' + [H] -> 2^o ROH

Reduction

1. r&c: LiAlH₄, dry ether OR NaBH₄, methanol

RCOR' + 2[H] -> 2^o ROH

2. r&c: H₂(g), Ni catalyst, heat

RCOR' + H₂ -> 2^o ROH

nucleophilic addition

r&c: HCN, trace amounts of NaCN/trace amounts of NaOH, cold

3 steps(GNA)

1. Generate Nu

trace amt NaCN; NaCN->Na⁺+CN^-

trace amt NaOH; HCN <->H⁺+CN^-

2. Nu attack(slow)

Nu attacks e^- deficient C of C=O, pi e- pair of C=O bond shifts to O -> forms

-vely charged intermediate

3. Acid-base

-vely charged intermediate protonated by HCN

HCN is a weak acid, provides low [CN^-].

NaCN acts as catalyst by providing CN^-, since CN^- regenerated only trace amount is required.

NaOH reacts w/ H⁺, decreases [H⁺] such that POE of rxn shifts right in dissociation of HCN, [CN^-] increases

acts as Bronsted-Lowry acid in step 2 by protonating the intermediate anion

there is equal probability for nucleophile to attack either side of the trigonal planar C of C=O, producing racemic mixture

1. steric hindrance

ketones have 1 more bulky alkyl/aryl group that hinders attack of nucleophile on carbonyl C

2. electronic factor

ketones have 1 more e^- donating alkyl group, makes the carbonyl C less e^- deficient and less susceptible to attack by nucleophiles

> energy required to overcome stronger pd-pd attractions bw molecules of carbonyl compounds than weaker id-id attractions bw molecules of alkanes/alkenes

< energy required to overcome weaker pd-pd attractions bw molecules of carbonyl compounds than stronger H bonds bw ROH/RCOOH molecules

aldehydes, ketones can form H bonds w/ H₂O molecules >

solubility decreases as no. of C atoms increases >

non-polar alkyl groups become larger, hinders formation of effective H bonds w/ H₂O molecules

ketone/aldehyde + 2,4-DNPH(hydrazine:-NH-NH₂) -> orange ppt(carbonyl + 2,4-DNPH w/ removal of 2 -H, 1 =O) + H₂O

reaction: condensation

r&c: 2,4-DNPH

obs: orange ppt

Silver mirror Test w/ Tollens' reagent

reaction: oxidation

r&c: Tollens' reagent/[Ag(NH₃)₂]⁺(aq), heat

RCHO -> RC(O^-NH4⁺)O + Ag(s) ( silver mirror)

balanced eqn:

RCHO + 2Ag(NH₃)₂⁺ + 3OH^- -> RC(O^-)O + 2Ag + 4NH₃ + 2H₂O

reaction w/ Fehling's solution

reaction: oxidation

r&c: Fehling's solution(alkaline Cu²⁺ complex), heat

obs: red-brown ppt of Cu₂O

balanced eqn:

RCHO + 2Cu²⁺ + 5OH^- -> RCOO^- + Cu₂O + 3H₂O

reaction: positive triiodomethane/iodoform test

r&c: alkaline I₂(aq), heat

obs: pale yellow ppt of CHI₃

balanced eqn: RCOCH₃ + 4OH^- + 3I₂ -> RCOO^-Na⁺ + 3NaI(aq) + 3H₂O(aq) + CHI₃(s)