R1 or R2 must, however, be either hydrogen or {HEAD ~ an unsubstituted alkyl group} or {TAIL ~ the reaction} will proceed at a very slow rate 
R1 or R2 must , however , be either hydrogen or { HEAD ~ an unsubstituted alkyl group } or { TAIL ~ the reaction } will proceed at a very slow rate
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH