(ii) Molecular asymmetry, chirality and enantiomers: The
observation of Louis Pasteur (1848) that crystals of certain
compounds exist in the form of mirror images laid the
foundation of modern stereochemistry He demonstrated that
aqueous solutions of both types of crystals showed optical
rotation, equal in magnitude (for solution of equal concentration)
but opposite in direction 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