Question

Classify the following reactions in one of the reaction type studied in this unit
a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}+\mathrm{Br}^{-}\)
b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{HCl} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{ClC}-\mathrm{CH}_{3}\)
c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HO}^{-} \rightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{Br}^{-}\)
d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}+\mathrm{HBr} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CBrCH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O}\)

Answer 1

(a) It is an example of substitution reaction as in this reaction the bromine group in bromoethane is substituted by the -SH group. 

(b) It is an example of addition reaction as in this reaction two reactant molecules combine to form a single product. 

(c) It is an example of elimination reaction as in this reaction hydrogen and bromine are removed from bromoethane to give ethene. 

(d) In this reaction, substitution takes place, followed by a rearrangement of atoms and groups of atoms.

Answer 2

(a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH} + \mathrm{Br}^{-}\)
This is a Nucleophilic Substitution Reaction. The \(\mathrm{HS}^{-}\) ion (nucleophile) attacks the \(\mathrm{Br}\) atom in bromoethane, leading to the \(\mathrm{Br}^{-}\) leaving and forming ethanethiol \((\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH})\).

(b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2} + \mathrm{HCl} \rightarrow \left(\mathrm{CH}_{3}\right)_{2} \mathrm{ClC}-\mathrm{CH}_{3}\)
This is an Electrophilic Addition Reaction. The \(\pi\) bond in the alkene (2-methylpropene) is broken, and both \(\mathrm{H}\) and \(\mathrm{Cl}\) from \(\mathrm{HCl}\) add across the double bond, forming 2-chloro-2-methylpropane.

(c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{HO}^{-} \rightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2} + \mathrm{H}_{2} \mathrm{O} + \mathrm{Br}^{-}\)

This is an Elimination Reaction. The \(\mathrm{Br}^{-}\) and a hydrogen atom is removed from adjacent carbon atoms in bromoethane, resulting in a double bond formation (ethene) along with \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{Br}^{-}\).

(d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH} + \mathrm{HBr} \rightarrow \left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr} + \mathrm{H}_{2} \mathrm{O}\)

This is a Nucleophilic Substitution Reaction. The \(\mathrm{Br}^{-}\) from \(\mathrm{HBr}\) attacks the hydroxyl group \((\mathrm{OH})\) in \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}\), leading to the substitution of \(\mathrm{OH}\) by \(\mathrm{Br}\), resulting in the formation of \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}\) and \(\mathrm{H}_{2} \mathrm{O}\).