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