He believed that this difference in
optical activity was associated with the three dimensional
arrangements of atoms in the molecules (configurations) of
(i) CH3CH2CH2CH2Br < (CH3)2CHCH2Br < CH3CH2CH(Br)CH3 < (CH3)3CBr  (SN1)
CH3CH2CH2CH2Br > (CH3)2CHCH2Br > CH3CH2CH(Br)CH3> (CH3)3CBr (SN2)
Of the two primary bromides, the carbocation intermediate derived from
(CH3)2CHCH2Br is more stable than derived from CH3CH2CH2CH2Br because
of greater electron donating inductive effect of (CH3)2CH- group Therefore,
(CH3)2CHCH2Br is more reactive than CH3CH2CH2CH2Br in SN1 reactions CH3CH2CH(Br)CH3 is a secondary bromide and (CH3)3CBr  is a tertiary
bromide Hence the above order is followed in SN1