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