Lecture 10
Carboxylic Acids
1.Introduction:
Carboxylic acids are characterized by the presence of the carboxyl
group –C=O .That gives an acid the general formula RCOOH.typically
|
O-H
These molecules are weak acids in aqueous solution.
2. Nomenclature:
Organic acids are named from the parent alkane by dropping the final-e
and adding –oic acid .thus CH3COOH commonly called acetic acid,has
the systematic name (IUPAC) ethanoic acid.other examples of carboxylic
acids are shown in the following examples:
O
O
O
║
║
║
H-C-OH
CH3-C-OH
CH3CH2C-OH
Systematic(IUPAC):methanoicacid ethanoic acid
Common name:
Formic acid
acetic acid
O
║
CH3CH2CH2C-OH
Butanoic acid
Butyric acid
propanoic acid
propionic acid
Carboxylic acids containing six or fewer carbons are frequently called
by their common names.
In the systematic nomenclature the position of substituent is designated
by a number. The carbonyl of carboxylic acid always the C-1 carbon .in
common nomenclature,the position of a substituent is designated by a
lower case Greek letter,and the carbonyl carbon is not given a
designation.The carbon adjacent to the carbonyl carbon is the αcarbon,the carbon adjacent to the α-carbon is the ß-carbon and so on.
O
O
║
║
EX(1) : CH3CH2CH2CH2CH2C-OH
CH3CH2CH2CH2CH2C-OH
6
5 4 3 2 1
є б
γ ß α
Systematic
Common
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CL
O
|
║
EX(2): CH3CH2CHCH2CH2C-OH
Systematic:4-chlorohexanoic acid
Common: γ-chlorocaproic acid
3- Preparation of carboxylic acids:
These are several methods for preparing carboxylic acids:
(1) primary alcohols can be oxidized to give carboxylic acids:
O
CrO3
║
CH3(CH2)8CH2OH
CH3(CH2)8COH
H2O,H2SO4
Decan-1-ol
Decanoic acid
(2) Aldehydes can be oxidized to give carboxylic acids:
O
O
║
AgNO3
║
CH3CH2CH2CH2CH2CH
CH3CH2CH2CH2CH2COH
Hexanal
NH4OH
Hexanoic acid
(Tollens′ reagent)
(3) oxidation of alkenes:
KMno4
a- CH3-CH=CH2
CH3COOH+CO2+H2O
Tollen′s reagent
b- RCH=CHR′+O3
RCOH+R′COH
dil.acid
Aldehyde
RCOOH+R′COOH
Carboxylic acids
(4) Hydrolysis of esters:
RCOOR′
H3O,∆
RCOOH+R′OH
4- Physical properties:
Like alcohols ,carboxylic acids form strong intermolecular hydrogen
bonds,which has notice value effect on boiling points.
Carboxylic acids normally boil at much higher than alkenes or alkyl
halide of similar molecular weight.(acetic acid at 118◦c,chloropropane at
46.6◦c).Carboxylic acids are often foul in odor ,and miscible in water (till
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4 carbon atoms),and higher than (5)are insoluble .solubility like alcohols;
in less polar solvent(ether, alcohol).although the carboxylic acids are
much weaker than mineral acids ,but much stronger acids than alcohols.
5-Chemical properties
(1) Decarboxylation (loss of CO2)
RCOOH
∆
RH+CO2
(2) Formation of acid chlorides
3RCOOH+PCL3
3RCOCl
Acid chloride
The net effect is substitution of the -OH group by -Cl
(3) Esterification :
RCOOH+R′OH
RCOOR′+H2O
O
║
CH3C-OH +H-OCH2CH3
O
║
CH3C-OCH2CH3 +H2O
Acetic acid+ ethanol
ethyl acetate+ water
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