L6 [Stereochemical Aspects of SN] Class 12 NCERT Chemistry Chapter 10 HALOALKANES AND HALOARENES

In this video you will learn about Stereochemical Aspects of SN (nucleophilic substitution) reactions i.e. stereochemistry of sn1 and sn2 reactions. For discussion we use Class 12 NCERT Chemistry Chapter 10 - HALOALKANES AND HALOARENES.
THEORY
(c) Stereochemical aspects of nucleophilic substitution reactions
A SN2 reaction proceeds with complete stereochemical inversion while
a SN1 reaction proceeds with racemisation.
In order to understand this concept, we need to learn some basic
stereochemical principles and notations (optical activity, chirality,
retention, inversion, racemisation, etc.).
(i) Plane polarised light and optical activity: Certain compounds
rotate the plane polarised light (produced by passing ordinary
light through Nicol prism) when it is passed through their
solutions. Such compounds are called optically active
compounds. The angle by which the plane polarised light is
rotated is measured by an instrument called polarimeter. If the
compound rotates the plane polarised light to the right, i.e.,
clockwise direction, it is called dextrorotatory (Greek for right
rotating) or the d-form and is indicated by placing a positive (+)
sign before the degree of rotation. If the light is rotated towards
left (anticlockwise direction), the compound is said to be laevorotatory
or the l-form and a negative (-) sign is placed before
the degree of rotation. Such (+) and (-) isomers of a compound
are called optical isomers and the phenomenon is termed as
optical isomerism.
(ii) Molecular asymmetry, chirality and enantiomers:
The spatial arrangement of four groups (valencies) around a central carbon
is tetrahedral and if all the substituents attached to that carbon
are different, such a carbon is called asymmetric carbon or
stereocentre. The resulting molecule would lack symmetry and
is referred to as asymmetric molecule. The asymmetry of the
molecule is responsible for the optical activity in such organic
compounds.
The symmetry and asymmetry are also observed in many day to day
objects: a sphere, a cube, a cone, are all identical to their mirror images
and can be superimposed. However, many objects are non superimposable on their mirror images. For example, your left and right hand look similar but if you put your left hand on your right hand, they do not coincide. The objects which are nonsuperimposable on their mirror image (like
a pair of hands) are said to be chiral and this property is known as chirality. While the objects, which are, superimposable on their mirror images are called achiral. The above test of molecular chirality can be applied to organic molecules by constructing models and its mirror images or by drawing three dimensional structures and attempting to superimpose them in our minds. There are other aids, however, that can assist us in ecognising chiral molecules. One such aid is the presence of a single asymmetric carbon atom. Let us consider two simple molecules propan-2-ol and butan-2-ol and their mirror images.
Enantiomers possess identical physical properties namely, melting
point, boiling point, solubility, refractive index, etc. They only differ
with respect to the rotation of plane polarised light. If one of the
enantiomer is dextro rotatory, the other will be laevo rotatory.
A mixture containing two enantiomers in equal proportions will
have zero optical rotation, as the rotation due to one isomer will be
cancelled by the rotation due to the other isomer. Such a mixture is
known as racemic mixture or racemic modification. A racemic mixture
is represented by prefixing dl or (±) before the name, for example
(±) butan-2-ol. The process of conversion of enantiomer into a racemic
mixture is known as racemisation.
Now let us have a fresh look at SN1 and SN2 mechanisms by
taking examples of optically active alkyl halides.
In case of optically active alkyl halides, the product formed as a
result of SN2 mechanism has the inverted configuration as compared
to the reactant. This is because the nucleophile attaches itself on the
side opposite to the one where the halogen atom is present. When
(-)-2-bromooctane is allowed to react with sodium hydroxide,
(+)-octan-2-ol is formed with the -OH group occupying the position
opposite to what bromide had occupied.
Thus, SN2 reactions of optically active halides are accompanied by
inversion of configuration.
In case of optically active alkyl halides, SN1 reactions are
accompanied by racemisation. Can you think of the reason why it
happens? Actually the carbocation formed in the slow step being sp2
hybridised is planar (achiral). The attack of the nucleophile may be
accomplished from either side resulting in a mixture of products, one
having the same configuration (the -OH attaching on the same position
as halide ion) and the other having opposite configuration (the -OH
attaching on the side opposite to halide ion). This may be illustrated
by hydrolysis of optically active 2-bromobutane, which results in the
formation of (±)-butan-2-ol.