1. www.soran.edu.iq Organic chemistry Assistance Lecturer Amjad Ahmed Jumaa  Effect of substituents on acidity.  Basicity of aliphatic and aromatic amines. 1

2. www.soran.edu.iq 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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4.  The value of ionization constant (K a ) for the electron- withdrawing substituents is greater than the ionization constant (K a ) 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. k a of benzoic acid = 6.3 x 10 -5.  The value of ionization constant (K a ) for the electron- withdrawing substituents is greater than the ionization constant (K a ) 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. k a of benzoic acid = 6.3 x 10 -5.

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6. 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, 6- trichlorobenzoic 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. 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.

7. www.soran.edu.iq Basicity of aliphatic and aromatic amines: Amines are organic compounds which have the general formula RNH 2, R 2 NH, or R 3 N, 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

8. www.soran.edu.iq  Like ammonia (NH 3 ) 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:

9. www.soran.edu.iq  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, K b.  The larger the K b. the stronger the base.

10. www.soran.edu.iq Aliphatic amines are more basic than ammonia and they consequently more basic than aromatic amines. RNH 2, R 2 NH, or R 3 N > > ArNH 2.  In aliphatic amines alkyl group tends to disperse the positive charge of the substituted ammonium ion and therefore stabilize it.

11. www.soran.edu.iq For aromatic amines, we see the electron-releasing substituent like- CH 3 group increases the basicity of the aniline, but an electron- withdrawing group substituent like- X and – NO 2 groups decreases the basicity.  Look at the following table: basicity constant of substituted aniline. K b. of aniline = 4.2 x 10 -10