Pyridostigmine bromide

a ch3

Fig. 2.22

CI b

Drug type: anti-cholinergic.

Functional groups:

• A Salt of strongly basic quaternary ammonium ion. Quaternary ammonium ions are charged at all pH values.

• B Carbamate group, the nitrogen is neutral as in an amide but the carbamate group is less stable than an amide having a stability similar to that of a phenolic ester.

Compounds containing a quaternary ammonium group: atracarium besylate, bretylium tosylate, clindium bromide, glycopyrronium bromide.

Predominant form of guanethidine at pH 7.0; charge on A is delocalised over all three nitrogens

^^^ Additional problems

1. Calculate the pH of the following solutions assuming that the concentration and the activity of the solutions are the same:

(ii) 0.1 M chloroacetic acid (Ka = 1.4 x 10~3). (in) 0.1 M phosphoric acid (First Ka 7.5 x 10 3).

(iv) 0.1 M fumaric acid {Ka 9.3 x 10" and 3.4 x 105).

(v) 0.1 M di-isopropylamine base (Kb = 9.09 x 10").

101 (|a) '0zl (a) -'wl (a!) '-9S'l (!!!) i£6'l (!!) ie'l (!) -SJ3MSUV

2. Calculate the pH of the following salt solutions:

(i) 0.1 M sodium formate (Ka formic acid = 1.77 x 10")

(ii) 0.1 M sodium fusidate (Ka fusidic acid = 4.0 x 10-«).

(iii) 0.1 M ephedrine hydrochloride (Ka of ephedrine = 2.5 x 10"°).

3. Calculate what volumes of the salt solutions specified which would be required to prepare 1 I of the following buffers:

(i) 0.1 M phosphoric acid/0.1 M sodium dihydrogen phosphate pH 2.5 (pKa H3P042.13)

(ii) 0.1 M sodium dihydrogen phosphate/0.1 M disodium hydrogen phosphate pH 8.0 (pKa H2P04 7.21).

(iii) 0.1 M sodium bicarbonate/0.1 M sodium carbonate pH 9.5 (pKa HC03 10.32). ■|W S UEL/ILU S'898 (!!!) :|W S098/|w S'6EL (!!) -'|w 10£/|W 66Z (!) -SJaMSuy

4. Indicate the percentage of ionisation of the functional groups specified in the following drugs at pH 7.0 (Fig. 2.23).

cr n

n ch3


B pKa 9.2 chch2ch2n(ch3)2


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5. Calculate the percentage of the following compounds that would be extracted under the conditions specified:

(i) A solution of basic drug pKa 9.2 in an oral liquid is mixed with a buffer having a pH of 7.2 and is extracted with an equal volume of chloroform (the partition coefficient of the un-ionised base into chloroform is 500).

(ii) an acidic drug with a pKa 4.2 is extracted from a solution of pH 4.5 with an equal volume of chloroform (the partition coefficient of the un-ionised acid into chloroform is 300).

6. Extracts containing benzylpenicillin were prepared for analysis in buffer at pH 6.5 at 25°C, the rate constant for the hydrolysis of benzylpenicillin under these conditions is 1.7 x 10~7 s. What is the maximum length of time the solutions can be stored before analysis so that no more than 1% decomposition occurs.

7. Determine the absolute configurations of the chiral centres in menthol and phenbutrazate. List the configurations of the pairs of enantiomeric diastereoisomers of menthol (Fig. 2.24).



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1. J.B. Stenlake. Foundations of Molecular Pharmacology. Athlone Press (1979).

2. A.T. Florence and D. Attwood. Physicochemical Principals of Pharmacy. 2nd Edn, Macmillan Press (1988).

3. E.J. Ariens. Trends Pharmacol. Sei. 7, 200-205 (1986).

4. P.A. Lehmann. Trends Pharmacol. Sei. 7, 281-285 (1986).

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