IIt JEE Revision Ch.26 Alkyl and Aryl Halides - Core Points

syllabus


METHODS OF PREPARATION
PHYSICAL PROPERTIES
CHEMICAL REACTIONS
charateristic reactions

Specially highlighed topics

Rearrangement reactions of alkyl carbocation,
Grignard reactions,
Nucleophilic substitution reactions;
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1. When hydrogen atom or atoms of alkanes are replaced by the corresponding number of halogen atoms, the compounds are called halogen derivatives of alkanes.



2. Methods of preparation

1. From hydrocarbons
a) from alkanes: halogens react with alkanes in the presence of uv light to form haloalkanes.
b) from alkenes: by the electrophylic addition of halogen acids (HBr, HCl, or HI)

3. The only methyl halide which is a liquid is iodomethane.  chloroethane is a gas.

4. Nucleophilic substitution in primary halogenoalkanes
The nucleophilic substitution reaction - an SN2 reaction - S stands for substitution, N for nucleophilic, and the 2 is order of reaction. It is because the initial stage of the reaction involves two species - the bromoethane and the Nucleophilic (Nu-) ion.

5. Nucleophilic substitution in tertiary halogenoalkanes - The nucleophilic substitution reaction - an SN1 reaction (1 denotes 1st order)

6.


2020 Presentation

Haloalkanes and Haloarenes

1. The groups which possess two nucleophilic centres are called ambident nucleophiles.

Explanation: An ambident nucleophile has "teeth" on two sides.

It can attack from two different places and form two different products.

For example, the thiocyanate ion, SCN⁻, is a ambident nucleophile. It is a resonance hybrid.

Both the S  and the N  atoms can act as nucleophiles.

So, the SN2  reaction of an alkyl halide with SCN⁻  often leads to a mixture of an alkyl thiocyanate and an alkyl isothiocyanate.

R-X+SCN⁻→X⁻   + R-SCN          + R-NCS
                alkyl            alkyl
                thiocyanate      isothiocyanate
https://socratic.org/questions/what-is-an-ambident-nucleophile


2. The rate of reaction depends on the concentration of both alkyl halid and base, the reation is known as SN2 reactions.

3. The rate of reaction depends on the concentration of alkyl halides, the reaction is known as SN1 reaction.

4. The order of reactivity of alkyl halides for SN1 reactions

3 degree halide > 2 degree halide > 1 degree halide > CH3X
(tertiary)              (secondary)           (primary)

5. The order of reactivity of alkyl halides for SN2 reactions

3 degree halide < 2 degree halide < 1 degree halide < CH3X

6.  In SN1 and SN2 mechanism, the reactivity of halides follows the order

R-I > R-Br > R-Cl > R-F

7. If all the substituents attached to a carbon are different, that carbon is called asymmetric (or) Chiral carbon or stereocentre.

8. The stereo isomers related to each other as non-super imposable images are called enantiomers.

9. A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be exactly cancelled by the rotation due to the other isomer. Such a mixture is known as racemic mixture or racemic modification.

10. The process of conversion of enantiomer into a racemic mixture is known as racemisation.


Key words:  ambident nucleophiles, SN2 reactions, SN1 reaction, The order of reactivity of alkyl halides for SN1 reactions (degree of halides),  The order of reactivity of alkyl halides for SN2 reactions(degree of halides), order of  reactivity of halides (based on halogen), asymmetric (or) Chiral carbon or stereocentre, enantiomers, racemic mixture or racemic modification, racemisation.


Updated on 12 January 2020.
2 May 2008