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