Carboxylic Acid
And their Derivatives
Formation of Carboxylic acids:
Hydrolysis of Nitriles
Hydrolysis of α-hydroxynitriles :
OH OH
H3O+
Ketones/Aldehydes + HCN R — C — CN R — C — COOH
heat
R/H R/H
Acid hydrolysis :
SN1 H3O+
RX + CN- RCN RCOOH + NH4+
reflux
Alkali hydrolysis :
SN1 OH-/H2O
RX + CN- RCN RCOO- + NH3
reflux
Oxidation of Primary Alcohols and Aldehydes
(1)KMnO4
RCH2OH RCOOH
(2)H3O+
(1)K2Cr2O7
RCHO RCOOH
(2)H3O+
Oxidation of Alkylbenzenes
KMnO4/H+(aq)
R R’COOH
heat
If R contains more than one carbon skeleton , oxidation only occurs at the carbon bonding to the ring to form benzoic acid and CO2(g). but 2-methyl-2-phenylpropane is resistant to side chain oxidation.
Oxidation of Methyl Ketones and some Alcohol
O OH O
∥ ∥
R — C — CH3 /R — C — CH3 + 3I2 + 3OH- R — C — CH3/CI3 + 3I- + 3H2O
OH-
O
∥
— C —O- + CHI3
(one carbon less )
Reactions of Carboxylic Acids
Acidity of carboxylic acids
Order of acidity :
RCOOH > —OH > ROH
We can think about the following factors :
1. The strength of the H-A bond
2. The electronegativity of A and ,
3. Factors stabilizing its conjugate base/acid anion with respect to HA.
Influence of substituents on acidity
Electron withdrawing groups can increase acid strength by weakening the OH bond and stabilizing the acid anion. The positive inductive effect of E-groups is very small through more than two or three carbon-carbon bonds.
Electron donating groups reduce the partially positive charge of carboxyl carbon atom , thus strengthening the O—H bond => not easily break. There will be intensified negative charge of O- and destabilized acid anion when dissociation of acids occurs.
Formation of Salts
Most carboxylic acids displace CO2(g) from carbonates and hydrogencarbonates but phenols do not, so we can use this test to distinguish them.
Reduction with Lithium Aluminiumhydrate
(1)LiAlH4/ ether
RCOOH RCH2OH(Primary alcohol )
(2)H3O+
The C═C,C≡C, and phenyl parts in unsturated acids are unaffected.
Conversion to Acid Derivatives
(1) Conversion to acyl chlorides
SOCl2 RCOCl(intermediate b.p.)+SO2+HCl
or
RCOOH + PCl5 RCOCl(high b.p.)+POCl3+HCl
or
PCl3 RCOCl(low b.p.)+H3PO3
(2) Conversion to acid anhydrides
O O
pyridine ∥ ∥
RCOOH + R’COCl RC — O — CR’ + HCl
(3) Conversion to amides
O
excess RCOOH ∥
RCOOH + NH3 RCOO-NH4+ RC — NH2 + H2O
? , reflux
(4) Conversion to esters
H3O+
RCOOH + R’OH RCOOR’ + H2O
reflux
The yield of ester can be enhanced by :
(1) increasing the amount of carboxylic acid or alcohol or
(2) increasing the amount of inorganic acid used for catalysis.
Reaction of Acyl Chlorides:
(1) Hydrolysis with water
O
∥ rapid
CH3— C — Cl + H2O(cold) CH3COOH + HCl
(2) Ester formation with alcohol
O O
∥ ∥
R — C — Cl + R’OH RC — OR’ + HCl
If R is aromatic alkane , a base catalyst is required, e.g. OH- .
(3) Amide formation with ammonia and amines
O O
∥ low temp. ∥
R— C — Cl + 2NH3(in excess) RC — NH2 + NH4Cl
O O
∥ NaOH ∥
R— C — Cl + RNH2 RC — NH2R + NaCl +H2O
(4) Anhydride formation
O O O
∥ ∥ ∥
R— C — Cl + R’COO-Na+ RC — O — CR’ + NaCl
Reaction of Acid Anhydrides
(1) Hydrolysis with water
slowly
(RCO)2O + H2O RCOOH
(2)Ester formation with alcohol
(RCO)2O + R’OH RCOOR’ + RCOOH
Δ
If phenol is used , alkaline medium is required to provide a powerful
nucleophile — phenoxide ion.
(2) Amide formation with ammonia and amines
(RCO)2O + 2NH3 RCONH2 + RCOO-NH4+
(RCO)2O + 2R’NH2 RCONHR’ + RCOO-R’NH3+
Reaction of Esters
(1) Hydrolysis of ester
OH-/reflux RCOO- + R’OH
RCOOR’ + H2O
H3O+/reflux
RCOOH + R’OH
Alkaline hydrolysis is much faster than acid catalyzed hydrolysis as the equilibrium in alkaline hydrolysis lies almost to the right.
(2) Reduction to alcohol with lithium aluminiumhydrate
(1)LiAlH4/ether
RCOOR’ RCH2OH + R’OH
(2)H3O+
Reaction of amides
(1) Hydrolysis
reflux
RCONH2 + OH- RCOO- + NH3
reflux
RCONH2 + H3O+ RCOOH + NH4+
(2) Dehydration — nitrile formation
O
∥ P2O5 , Δ
—C—NH2 C≡N + H2O
-H2O
(1) Hofmann degradation
CH3CH2CONH2 + 4KOH + Br2 CH3CH2NH2 + K2CO3 + 2KBr + 2H2O
one carbon less
(2) Reduction to amines with lithium aluminiumhydrate
O
∥ LiAlH4
R — C — NH2 RCH2NH2
in ether
Some specimen results of the investigation of the reactions of ethanoic acid
Reaction
/ ObservationspH of aqueous solution / Orange-red-pH3-4
Reaction with sodium
hydrogencarbonate solution / Gas evolved which turns limewater milky—CO2(g)
Reaction with sodium / Gas evolved which popped with a lighted splint—H2(g)
Reaction with phosphorus pentachloride(PCl5) / Steamy gas evolved which gave white fumes with ammonia—HCl(g)
Reaction with 2,4-dinitrophenylhydrazine / No change
Triodomethane reaction / No change