The 2 chloride substitution on the aromatic ring of chloropro-caine is an electron-withdrawing functional group. Thus, it pulls the electron density from the carbonyl carbon into the ring. The carbonyl carbon is now a stronger electrophile and more susceptible to ester hydrolysis. Therefore, chloropro-caine has a more rapid metabolism than procaine. The in vitro plasma half-life is approximately 25 seconds. The 2-chloro-4-aminobenzoic acid metabolite precludes this from being used in patients allergic to PABA. The very short duration of action means that this drug can be used in large doses for conduction block (with rapid onset and short duration of action.) As with procaine, a decrease in the plasma cholinesterase activity will prolong the half-life. The pKa of the chloroprocaine alkyl amine is 9.0 and thus chloroprocaine is almost exclusively ionized at physiological pH. Chloroprocaine is formulated with a pH between 2.5 and 4.0 using hydrochloric acid. The acidic pH of the formulation is responsible for considerable irritation and pain on injection.
Chloroprocaine is used for cutaneous or mucous membrane infiltration for surgical procedures, epidural anesthesia (without preservatives) and for peripheral conduction block.
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