Basics of Carboxylic acids

• Organic compounds containing –COOH group.
• Based on number of carboxyl groups; acids can be
monocarboxylic acids, dicarboxylic acids or tricarboxyic
acids.
Nomenclature of carboxylic acids:
• Many carboxylic acids are known by their common names.
• These common names are often formed by adding the suffix
“ic” + acid to the common names for aldehydes and ketones.
OR
OH

• In IUPAC Nomenclature system, suffix –oic acid is used after
parent hydrocarbon chain name.
Structure of carboxylic acids:
Carboxylic acid is a resonance hybrid of structure I and II
OR
O
OR
O
H
I II
H
OR
OH
resonance hybrid

• After removal of hydrogen, carboxylate ion is formed that is
more resonance stabilized.
• Due to the formation of more stable carboxylate ion,
carboxylic compounds exhibit acidic behaviour.
OR
OH
-H+
OR
O
OR
O
OR
O

Method of Preparation
1. Oxidation of primary alcohols:
2. Oxidation of aldehydes:
3. Hydrolysis of nitriles:
R OH
K2Cr2O7/H+
R OH
O
R H
KMnO4/H+
R OH
OO
N
2H2O+
(i) OH-
(ii) H+ OH
O
+ NH3

4. From Grignard reagent:
5. Arndt Eistert Reaction: increases no. of c-atoms
Mg
Br
+ O C O
O
O
MgBr
H3O+
OH
O
R OH
O
R Cl
O
R CHN2
O
R
OH
O
SOCl2
CH2N2
Ag2O / H2O

Physical properties
• Aliphatic acids up to C4 are water soluble and higher acids are
generally insoluble in water due to larger alkyl chains.
• Acids generally have high boiling points than corresponding
alcohols due to their ability to make stronger hydrogen bonds.
• Carboxylic acids mainly exists as cyclic dimer by forming H-
bond in solutions.
R O
O
H
RO
O
H

Chemical Properties
1. Acidic Character:
Carboxylic acids make salts with bases.
The acid strength mainly depends on following factors:
– conjugate acid base pairs
– Polarity of O-H bond
– Relative stabilities of conjugate bases
The greater is the stability of acid anion (conjugate base),
greater will be the acidity of corresponding carboxylic acid.
OH
O
+ NaOH
O
O
Na + H2O

• The relative stability of acid anion depends on the following
factors:
– Resonance effect
– Inductive effect
– Intramolecular hydrogen bonding
– Solvation effect
– Ortho effect
1. Resonance Effect
• The acidity depends on number of contributing resonance
structures.
• Greater is the number of contributing structures, more is the
stability of resonance hybrid and greater is the delocalization
of electrons

2. Inductive Effect
In general, electron withdrawing groups (-I) increase the
acidity and electron releasing groups (+I) decrease the acidity
of carboxylic acids.
Electron releasing (+I) groups tend to increase the negative
charge on carboxylate ion and hence destabilize it and decrease
acidity of corresponding acid.
OR
OH
-H+
OR
O
OR
O
OR
O

Electron withdrawing (-I) groups tend to decrease and neutralize
the negative charge on carboxylate ion and hence stabilize it
and increase acidity of corresponding acid.
H OH
O
OH
O
pKa : 3.77 4.74
H3C OH
O
C
H2
OH
O
pKa : 4.74 2.86 1.25
Cl
CH OH
O
Cl
Cl

3. Intramolecular H-bonding
Intramolecular hydrogen bonding stabilizes the acids and
decrease the dissociation of proton and hence decrease acidity.
In acid anion, intramolecular hydrogen bonding stabilize it,
and hence increase the acidity of corresponding acid.
O
H
O
O
Intramolecular H bond in acid anion

4. Solvation Effect
If the acid anion is more solvated than parent acid, its stability
is increased and hence acidity is also increased.
Example; acetic acid is stronger acid than propionic acid in
aqueous solution because acetate ion is more solvated than
propionate ion due to small size. The propionic acid is more
stronger acid in gas phase where solvation is not observed.

5. Ortho Effect
The ortho substituent to carboxylic acid always increase the
acidity of acid. e.g., ortho substituted benzoic acids are
stronger acid than benzoic acid whether the ortho group is
electron withdrawing or electron releasing in nature. This
effect is known as ortho effect.
This is observed due to combined application of inductive
effect, steric effect and intramolecular hydrogen bonding.
This effect is more prominent with OH, Cl or NO2

2. Reaction with metals:
3. Conversion to acid derivatives:
Acid chlorides:
+ Zn (CH3COO)2Zn + H2
2 CH3COOH
R
O
OH
SoCl2 / PCl5 / PCl3
R
O
Cl
acid chloride
R
O
OH R
O
NH2
amide
NH3
-H2O
R
O
NH2
amide
NH3
-HCl
R
O
OH
SoCl2
R
O
Cl
acid chloride
Amides:

R
O
OH
R'-OH + H+
R
O
O
Ester
H2OR' +
R'-OH
R
O
O
Ester
R'-HCl
R
O
OH
SoCl2
R
O
Cl
acid chloride
R
O
OH
+
R'
O
Cl
acid chloride
R O
O
R'
O
acid anhydridecarboxylic acid
pyridine
-HCl
Acid Anhydrides:
Esters:
R
O
ONa
+
R'
O
Cl
acid chloride
R O
O
R'
O
acid anhydridecarboxylic acid salt
-NaCl

4. Reduction to alcohols:
5. Hunsdiecker Reaction: to get alkyl halides
6. Curtius Reaction: to get primary amine with one carbon less
R
C
OH
O
LiAlH4
R
H2
C
OH
R
C
OAg
O
Br2 + CCl4
R-Br + AgBr + CO2
R
O
N3
acyl azide
NaN3
R
O
OH
SoCl2
R
O
Cl
acid chloride
-N2
N C OR
isocyanate
R-NH2 + CO2
hydrolysis

7. Reaction with grignard reagent:
8. Decarboxylation Reaction:
CH3COONa + NaOH (CaO) CH4 + Na2CO3 (CaO)
sodalime
+ R' - MgX R' - H + RCOOMgXRCOOH
alkane

Carboxylic Acid Derivatives
1. Amides (RCONH2)
Method of Preparations:

2. Esters (RCOOR’)
Preparation:

Qualitative tests of carboxylic acids
a) Litmus Test: Place a drop of solution of carboxylic acid
sample on a blue litmus paper. If blue litmus turns red, it
indicates the presence of acid.
b) Sodium Bicarbonate test: carboxylic acids reacts with
sodium bicarbonate to produce salt of acid with the
evolution of carbon dioxide gas in the form of
effervescence.
Add saturated sodium bicarbonate solution to aq. solution
of sample, brisk effervescence indicates the presence of
acid.
R OH
O
NaHCO3
R ONa
O
+ CO2+H2O

c) Ester Formation Test: Heat gently 0.5g of acid sample with 1
ml of ethanol i.p.o. few drops of conc. Sulphuric acid for about
one minute. Cool and pour into water and note the odour. The
fruit like odor indicates the formation of ester and hence
presence of carboxylic acid in sample.
R
O
OH
R'-OH + H+
R
O
O
Ester
H2OR' +
Fruity odor

Qualitative tests of amides
a) Alkali Test: primary amides can be decomposed by boiling with
alkali and ammonia is evolved.
Boil 0.5g of sample with 5 ml of 10% NaOH solution and
observe the odor. If ammonia like odor is observed, the sample is
contains amide group.
b) Biuret Reaction: When aliphatic diamide is heated at temperature
above its m.p., ammonia is evolved and crystalline biuret is
formed which in alkaline medium gives a violet color with
CuSO4 solution.
R
O
NH2 R
O
O
NH3Na +
10% NaOH
2 NH2CONH2
H2N N
H
NH2
O O
+ NH3
Biuret
CuSO4
violet color complex

c) Hydroxamic acid Test: to identify primary aromatic amines.
Hydrogen peroxide reacts with aromatic primary amides to
form the hydroxamic acid, which then reacts with ferric
chloride to form ferric hydroxamate complex having violet
color.

Qualitative tests of Esters
a) Hydroxamic acid Test: Esters upon reaction with
hydroxylamine yields hydroxamic acid, which upon treatment
with ferric chloride forms ferric hydroxamate complex with
bluish red / violet color.
b) Hydrolysis Test: alkaline hydrolysis of esters converts ester into
acid salt and alcohol.

Structures and Uses
1. Acetic Acid:
Structure:
Uses:
 Used as polar protic solvent.
 Used in the synthesis of vinyl acetate, which is used to prepare poly vinyl
acetate (PVA) used in paints and adhesives.
 Used in the preparation of acetic anhydride.
 Used to preserve food e.g. vinegar is an 4-8% solution.
 Used to prepare esters such as ethyl acetate, butyl acetate.
H3C OH
O

2. Lactic Acid:
Structure:
Uses:
 Used in topical preparations and cosmetics to adjust acidity and for its
disinfectant properties.
 Used in milk products.
 Lactic acid along with ammonium bicarbonate is used in mosquito attractant.
3. Tartaric Acid:
Structure:
Uses:
 Potassium hydrogen tartarate is a componant of baking powder.
 Sodium potassium tartarate (Rochelle salt) is an important componant of
fehling’s solution used to detect aldehydes.
 Tartar emetic is used as emetic.
CH OH
O
H3C
OH
CH OH
O
CH
OH
OH
HO
O

4. Citric Acid:
Structure:
Uses:
 Used in confectionary and food items.
 Used in benedict’s reagent.
 Its magnesium salt is used as laxative.
 Used as preservative in food and beverages.
5. Succinic Acid:
Structure:
Uses:
 Used as precursor to polyesters.
 Food additive and dietary supplement.
 Used as excipient in pharmaceutical preparations to control acidity.
 Used in food and beverages as acidity regulator.
CH2COOH
CH2COOH
COOHHO
HO
O
OH
O

6. Oxalic Acid:
Structure:
Uses:
 Used in cleaning and bleaching for removal of rust by forming ferric oxalate with Fe3+.
 Used in preparation of formic acid.
 Used in redox tritrations as primary standard solution.
 Used to remove ink stains
7. Salicylic Acid:
Structure:
Uses:
 Used to treat warts, psoriasis, dandruff and acne.
 Used in the synthesis of aspirin.
 Methyl salicylate is used in muscle and joint pain removing ointments and gels.
 Used in preparation of azo dye.
 Used in preparation of salol, an intestinal antiseptic.
HO
O
O
OH
COOH
OH

8. Benzoic Acid:
Structure:
Uses:
 Food preservative
 Used in antifungal skin preparations.
 Used as main component of gum benzoin, tincture of benzoin.
 Used to prepare benzoyl chloride.
 Preservative
9. Benzyl Benzoate:
Structure:
Uses:
 Used topically to treat scabies.
 Used in some asthma and whooping cough preparations due to vasodilating effects..
 Used as mosquito repellent.
 Used as solvent for cellulose derivatives, plasticizer.
COOH
O
O

10. Dimethyl phthalate:
Structure:
Uses:
 Used as ectoparasiticide.
 Used as mosquito and flies repellent.
11. Methyl Salicylate:
Structure:
Uses:
 Used as rubefacient and analgesic.
 Flavouring agent
 Used in antiseptic mouthwashes.
 Used to mask odor of organophosphate pesticides.
O
O
O
O
CH3
CH3
OH
O
O
CH3

12. Acetyl salicylic acid (Aspirin):
Structure:
Uses:
 Analgesic
 Antipyretic
 Anti-inflammatory
 Used as platlet aggregation inhibitor
 In myocardial infarction
O
OH
O
CH3O

Basics of Carboxylic acids

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Basics of Carboxylic acids