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Physical properties of acids and acid derivatives :
1. Boiling point :
The high boiling points of carboxylic acids is the result of formation of a stable hydrogen-bonded dimer.
Hydrogen bonded acid dimer
2 Solubility :
Carboxylic acids form hydrogen bonds with water and the lower molecular –weight carboxylic acids (upto 4 carbon atoms) are miscible with water.
Acid derivatives (esters, acid chlorides, anhydride, nitriles and amides) are soluble in organic solvents such as alcohols, ethers, chlorinated alkanes and aromatic hydrocarbons.
Methods of preparation of carboxylic acids:
1. Synthesis of carboxylic acids by the carboxylation of Grignard's reagent:
2- Synthesis of Carboxylic acids by the hydrolysis of nitriles:
Nitrile carboxylic acid
1. Acidic strength %
Acidity of carboxylic acids :-
Ex. HCOOH > CH3COOH > CH3-CH2-COOH
Ex. Relative acid strength is:-
RCOOH > HOH > ROH > HC = CH > NH3 > RH
Note:- Acidity of acids is compared by comparing stability of conjugate base.
2- Reaction involving removal of proton from –OH group.
1.Action with blue litmus : All carboxylic acids turn blue litmus red.
2.Reaction with metals :
2 CH3 COOH + 2Na → 2CH3COONa + H2
2CH3 COOH + Zn → (CH3COO)2 Zn + H2
3.Reaction with alkalies :
CH3 COOH + NaOH → CH3 COONa + H2O
4.Reaction with carbonates and bicarbonates :
2CH3COOH + Na2CO3 → 2CH3 COONa + CO2 + H2O
CH3COOH + NaHCO3 → CH3COONa + CO2 + H2O
Reaction of carboxylic acid with aqueous sodium carbonate solution produces brisk effervescence. However, most phenols do not produce effervescence. Therefore, the reaction may be used to distinguish between carboxylic acids and phenols.
5.Reaction with Grignard's reagent :
Note: A stronger acid displaces a weaker acid from salt of the weaker acid.
Ex. CH3COOH (Stronger acid) + CH3ONa → CH3 COONa + CH3 —OH (WeakerAcid)
Ex. CH3COOH (stronger acid) + NaHCO3 → CH3 COONa + H2 CO3 (Weaker acid) → H2O + CO2
3. Reaction involving replacement of –OH group
(i) Formation of acid chlorides :
(2) Fisher Esterification:
Carboxylic acid react with alcohol to form esters through a condensation reaction known as esterification.
General Reaction :
Acid catalysed esterfication:
If we follow the forward path in this mechanism, we have the mechanism for the acid catalysed esterification for an acid. If however, we follow the reverse reactions, we have the mechanism for the acid catalysed hydrolysis of an ester. Acid catalysed ester hydrolysis gives:
(3) Formation of amides:
(4) Formation of acid anhydride:
(4) Decarboxylation reactions:
1 Soda-lime decarboxylation :
2. Decarboxylation of β- keto carboxylic acids :
- keto acid
3) Kolbe's electrolysis:
(II) R. + R. → R - R
If n is the number of carbon atoms in the salt of carboxylic acid, the alkane formed has 2(n-1) carbon atoms.
4) Hunsdiecker Reaction (Brome-decarboxylation):
R-COOAg + Br2 → R-Br + CO2 + AgBr
5. HVZ Reaction (Halogenation of aliphatic acids and Substituted acids):
Carboxylic Acid Derivatives
Closely related to the carboxylic acids and to each other are a number of chemical families known as functional derivatives of carboxylic acids : acid chloride, anhydrides, amides, and esters. These derivatives are compounds in which the -OH of a carboxyl group has been replaced by–Cl, -OOCR, -NR2 or –OR.
They all contain the acyl group ,
(A) Acid halides:
Methods of preparation of Acyl halides:
(i) RCOOH + PCl5 → RCOCl + POCl3 + HCl
(ii) 3RCOOH + PCl3 → 3RCOCl + H3PO3
(iii) RCOOH + SOCl2 RCOCl +SO2 + HCl
1. Reaction with carboxylic acids:
2. Reaction with alcohols:
Acyl chlorides react with alcohols to form esters. The reaction is typically carried out in the presence of pyridine.
4) Reaction of acid halide with organometallic compounds:
(a) with Grignard's reagent-
(b) Reaction with Gilmann's reagent-
5) Reduction of acid halide:
(a) Reduction LiAlH4
(b) Reduction with H2/Pd/BaSO4 Rosenmund reduction-
(B) Acid amides
Methods of preparation of acids amides:
1. By reaction of esters with ammonia and amines.
Ammonia is more nucleophilic than water, making it possible to carry out this reaction using aqueous ammonia.
2. From acid halides:
RCOCl + 2NH3 → RCONH2 + NH4Cl
3. From anhydride:
(RCO)2O + 2NH3 → RCONH2 + RCOO NH4
4. From esters:
RCOOR + NH3 → RCONH2 + R’OH
5. From ammonium salt of carboxylic acid:
RCOONH4 RCONH2 + H2O
CH3 COONH4 CH3CONH2
6. From cyanides:
(1) Hoffmann rearrangement:
(2) Hydrolysis of amides-
In acid, however, the amine is protonated, giving an ammonium ion, R2’
Summary of Reaction of Amide:
Methods of Preparation:
(i) CH3 COOH + C2H5OH CH3COOC2 H5 + H2O
C6H5COOH + CH3OH C6H5 COOCH3 + H2O
(ii) CH3 COCl + C2H5OH CH3COOC2H5 + HCl
Alcohols react with acyl chlorides by nucleophilic acyl substitution to yield esters. These reactions are typically performed in the presence of a weak base such as pyridine.
Summary of reaction of esters :
(D) Acid anhydrides
Methods of Preparation of acid anhydrides:
1- From carboxylic acids
CH3COOH + HOOCCH3 CH3CO.O.COCH3 + H2O
Acetic acid Acetic anhydride
2. From acid and acid halide
Ex. CH3COOH + CH3COCl CH3 CO.O.COCH3 + HCl
Ex. CH3COCl + CH3COONa CH3CO.O.COCH3 + NaCl
1. Reaction with aromatic compounds (Friedel crafts acylation)-
2. Reaction with alcohols-
3. Reaction with ammonia and amines-
Acid anhydrides react with water to yield two carboxylic acids. Cyclic anhydrides yield dicarboxylic acids.
5. Heating Effects:
a. Heating effect on monocarboxylic acid
b. Heating effect on dicarboxylic acid
c. Heating effect on Hydroxy acids
1- Hydroxy acid
2. Hyroxy acid
3- Hydroxy acid
Since 4 or 8 membered rings are less stable therefore β-Hydroxy acids on heating produce α,β unsaturated carboxylic acid.
4- An α -Hyroxy acid
5. Heating effect on esters
This reaction follows syn elimination & Hoffman product is formed.