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Haloalkanes and Haloarenes — Chemistry STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceChemistryHaloalkanes and Haloarenes2 Marks
Question
Elimination reactions (especially β-elimination) are as common as the nucleophilic substitution reaction in case of alkyl halides. Specify the reagents used in both cases.
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Answer

Alkyl halides undergo nucleophilic substitution as well as elimination (Beta-elimination) reaction. However, by proper choice of reagents and reaction conditions, a particular product can be obtained. Usually strong and bulky bases and high temperature favour elimination reactions while weaker and smaller bases and lower temperature favour substitution reactions. For example, ethyl bromide on heating with alcoholic KOH (which contain stronger base, C2H5O ion) at about 473-523 K undergoes elimination to give ethene. But with aqueous KOH at about 373 K, it gives ethanol.
$\text{CH}_3\text{CH}_2\text{Br}\xrightarrow[473-523\text{K}]{ \ \text{alc. KOH} \ \ }\text{CH}_2=\text{CH}_2(\text{Elimination})$
$\text{CH}_3\text{CH}_2\text{Br}\xrightarrow[373\text{K}]{ \ \text{alc. KOH} \ \ }\text{CH}_3\text{CH}_2\text{OH}(\text{Substitution})$
Nucleophilic substitution: Reagents used are nucleophilies like $-\bar{\text{O}}\text{H},\text{NH}_3,\bar{\text{C}}\cong\text{N}:,\text{AgCN}:,\text{O = N}-\text{O},\bar{\text{O}}\text{R}'$ etc. Also, alc. KOH at lower tempereture (373 k) undergose substitution reaction.

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