Haloalkanes and Haloarenes 4

Plane of plane polarised light produced by passing ordinary light through Nicol prism is rotated when it is passed through the solutions of certain compounds. Such compounds are called optically active compounds. Nicol prism is called polariser. it is made up of calcite

The angle by which the plane polarised light is rotated is measured by an instrument called polarimeter

If the compound rotates the plane of 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 laevo-rotatory 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

If all substituents attached to a carbon are different, then the carbon is said to be chiral or asymmetric or stereocentre. The resulting molecule would lack symmetry( asymmetric or chiral molecule) The asymmetry of the molecule along with non superimposability of mirror images is responsible for the optical activity in such organic compounds. 2-chlorobutane, 2, 3-dihyroxypropanal, (OHC–CHOH–CH2OH), bromochloro-iodomethane (BrClCHI), 2-bromopropanoic acid (H3C–CHBr–COOH), etc

The stereoisomers related to each other as nonsuperimposable mirror images are called enantiomers Enantiomers possess identical physical properties namely, melting point, boiling point, 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.

Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation. In general, 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 retention of the configuration.

the reaction that takes place when (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid. It is important to note that configuration at a symmetric centre in the reactant and product is same but the sign of optical rotation has changed in the product. This is so because two different compounds with same configuration at asymmetric centre may have different optical rotation. One may be dextrorotatory (plus sign of optical rotation) while other may be laevorotatory (negative sign of optical rotation).

There are three outcomes for a reaction at an asymmetric carbon atom, when a bond directly linked to an asymmetric carbon atom is broken. If (A) is the only compound obtained, the process is called retention of configuration. Note that configuration has been rotated in A. If (B) is the only compound obtained, the process is called inversion of configuration. Configuration has been inverted in B. If a 50:50 mixture of A and B is obtained then the process is called racemisation and the product is optically inactive, as one isomer will rotate the plane polarised light in the direction opposite to another.

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, SN 2 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 sp 2 hybridised is planar (achiral). The attack of the nucleophile may be accomplished from either side of the plane of carbocation 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.

When a haloalkane with b-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from b-carbon and a halogen atom from the a-carbon atom. As a result, an alkene is formed as a product. Since b-hydrogen atom is involved in elimination, it is often called b-elimination.

“in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. 2 bromopentane can give 1 pentene (19%) 2 pentene (81%)

An alkyl halide with b-hydrogen atoms when reacted with a base or a nucleophile has two competing routes: substitution (SN 1 and SN 2) and elimination. Which route will be taken up depends upon the nature of alkyl halide, strength and size of base/nucleophile and reaction conditions. Thus, a bulkier nucleophile will prefer to act as a base and abstracts a proton rather than approach a tetravalent carbon atom (steric reasons) and vice versa. Similarly, a primary alkyl halide will prefer a SN 2 reaction, a secondary halide- SN 2 or elimination depending upon the strength of base/nucleophile and a tertiary halide- SN 1 or elimination depending upon the stability of carbocation or the more substituted alkene Elimination: (CH3)3CO- + (CH3)2CHBr ——> CH3-CH=CH2 + (CH3)3OH Substitution OH + (CH3)2CHBr ———>(CH3)2CHOH + Br-