Organic chemistry
 Effect of substituents on acidity.
 Basicity of aliphatic and
aromatic amines.
Assistance Lecturer Amjad Ahmed Jumaa
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Effect of substituents on acidity:
In general aromatic carboxylic acid is more acidic than
aliphatic carboxylic acid. Any factors that stabilize the
carboxylate anion (-RCOO-) more than it stabilizes the acid
should increase acidity. We see:
Electron-withdrawing substituents stabilize the anion, and
thus increase acidity.
Electron-releasing substituents destabilize the anion and
thus decrease acidity.
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The value of ionization constant (Ka) for the electronwithdrawing substituents is greater than the ionization
constant (Ka) for the carboxylic acid that substituent with
the electron-releasing groups thus more acidic. For
example:
P-chlorobenzoic acid is more acidic than benzoic acid.
Look at the following table which gives the acidity
constant of substituted benzoic acids.
ka of benzoic acid = 6.3 x 10-5.
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Problem: without referring to table, arrange the compounds of each set in
order of acidity:
Benzoic acid; p-chlorobenzoic acid; 2, 4-dichlorobenzoic acid; 2, 4, 6trichlorobenzoic acid.
Solution:
2, 4, 6-trichlorobenzoic acid. > 2, 4-dichlorobenzoic acid >
P-chlorobenzoic acid > benzoic acid.
Problem: arrange the following carboxylic acids according in increasing
acidity,
Benzoic acid; p-nitrobenzoic acid; p-toluic acid.
Solution:
P-nitrobenzoic acid > Benzoic acid > p-toluic acid.
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Basicity of aliphatic and aromatic amines:
Amines are organic compounds which have the general formula RNH2,
R2NH, or R3N, where R is any alkyl or aryl group.
Some aliphatic and aromatic amines:
Amines are classified as primary, secondary, or tertiary, according to the
number of groups attached to the nitrogen atom
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Like ammonia (NH3) the basicity of amines is due to the unshared pair of
electrons at the nitrogen atom. Therefore, amines are more basic than
water and less basic than hydroxide ion:
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It is convenient to compare basicities of amines by
measuring the extent to which they accept hydrogen ion
from water; the equilibrium constant for this reaction is
called basicity constant, Kb.
The larger the Kb. the stronger the base.
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Aliphatic amines are more basic than ammonia and they consequently
more basic than aromatic amines.
RNH2, R2NH, or R3N >
> ArNH2.
In aliphatic amines alkyl group tends to disperse the positive
charge of the substituted ammonium ion and therefore stabilize it.
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For aromatic amines, we see the electron-releasing substituent likeCH3 group increases the basicity of the aniline, but an electronwithdrawing group substituent like- X and –NO2 groups decreases
the basicity.
Look at the following table: basicity constant of substituted
aniline.
Kb. of aniline = 4.2 x 10-10
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