please answer this question..............................................

Answer is R>P>Q>S ... See I m telling in very simple language u will better understand by this . So a nucleophile needs positive charge to attack so first see which will provide positive charge first .In case of R there is OH group which shows -I effect means it takes negative charge towards itself and provide positive charge . Now see alkyl chains ,, alkyl groups are electron releasing groups so less the no of alkyl chain less the electron it release ,more the positive charge it provide for substitution of Nucleophile . Hope u understand ! Thanks