`=>` A `color{red}(S_N 2)` reaction proceeds with complete stereochemical inversion while a `color{red}(S_N 1)` 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.).

`=>` 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 `color{green}("optically active compounds")`.

● The angle by which the plane polarised light is rotated is measured by an instrument called `color{green}("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 `color{red}(d)`-form and is indicated by placing a positive (`color{red}(+)`) sign before the degree of rotation.

● If the light is rotated towards left (anticlockwise direction), the compound is said to be laevorotatory or the `color{red}(l)`-form and a negative (`color{red}(–)`) sign is placed before the degree of rotation.

● Such (`color{red}(+)`) and (`color{red}(–)`) isomers of a compound are called `color{green}("optical isomers")` and the phenomenon is termed as `color{green}("optical isomerism")`.

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 arrangements of atoms (configurations) in two types of crystals. Dutch scientist, J. Van’t Hoff and French scientist, C. Le Bel in the same year (1874), independently argued that the spatial arrangement of four groups (valencies) around a central carbon is tetrahedral and

`color{green}("Asymmetric Carbon" )` : 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.

`color{green}("Chirality ")` : The objects which are non-superimposable on their mirror image (like a pair of hands) are said to be `color{green}("chiral")` and this property is known as chirality. For example, your left and right hand look similar but if you put your left hand on your right hand, they do not coincide.

`color{green}("Achiral Objects ")` : 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. The aids that can assist us in recognising chiral molecules 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.

● Propan-2-ol does not contain an asymmetric carbon, as all the four groups attached to the tetrahedral carbon are not different. Thus it is an achiral molecule. See fig.1.

● Butan-2-ol has four different groups attached to the tetrahedral carbon and as expected is chiral. See fig.2.

● Some common examples of chiral molecules such as 2-chlorobutane, 2, 3-dihyroxypropanal, `color{red}((OHC–CHOH–CH_2OH))`, bromochloro-iodomethane `color{red}((BrClCHI))`, 2-bromopropanoic acid `color{red}((H_3C–CHBr–COOH))`, etc.

`color{green}("Enantiomers ")` : The stereoisomers related to each other as nonsuperimposable mirror images are called enantiomers (Fig. 10.5).

● 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.

`color{green}("Racemic Mixture ") ` : 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 `color{red}(dl)` or (`color{red}(±)`) before the name, for example (`color{red}(±)`) butan-2-ol.

● The process of conversion of enantiomer into a racemic mixture is known as `text(racemisation)`.

`=>` Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.

● It is also the configurational correlation when a chemical species `color{red}(XCabc)` is converted into the chemical species `color{red}(YCabc)` having the same `color{green}("relative configuration")`. See fig.1.

`=>` If during a reaction, no bond to the stereocentre is broken, the product will have the same general configuration of groups around the stereocentre as that of reactant. Such a reaction is said to proceed with `text(retention of the configuration)`.

● Consider as an example, the reaction that takes place when (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid.

`=>` There are three outcomes for a reaction at an asymmetric carbon atom.

`=>` Consider the replacement of a group `color{red}(X)` by `color{red}(Y)` in the reaction given in fig.

`=>` If (A) is the only compound obtained, the process is called retention of configuration.

`=>` If (B) is the only compound obtained, the process is called inversion of configuration.

`=>` If a `50:50` mixture of the above two is obtained then the process is called racemisation and the product is optically inactive, as one isomer will rotate light in the direction opposite to another.

`=>` In case of optically active alkyl halides, the product formed as a result of `color{red}(S_N 2)` 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 `color{red}(–OH)` group occupying the position opposite to what bromide had occupied. See fig.1.

● Thus, `color{red}(S_N 2)` reactions of optically active halides are accompanied by inversion of configuration.

`=>` In case of optically active alkyl halides, `color{red}(S_N 1)` reactions are accompanied by racemisation.

● Actually the carbocation formed in the slow step being `color{red}(sp^2)` 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 `color{red}(–OH)` attaching on the same position as halide ion) and the other having opposite configuration (the `color{red}(–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. See fig.2.