JEE Main & Advanced Chemistry Haloalkanes and Haloarenes / हैलोएल्केन तथा हैलोएरीन्स Preparations And Properties Of Dihalides

(1) Methods of preparation of dihalides

(i) Methods of preparation of gemdihalide

(a) From alkyne (Hydrohalogenation) :

\[R-C\equiv C-H+HX\xrightarrow{{}}R-\underset{X}{\mathop{\underset{|}{\mathop{C}}\,}}\,=\underset{H}{\mathop{\underset{|}{\mathop{C}}\,}}\,-H\] \[\xrightarrow{+HX}R-\underset{X\,\,\,}{\overset{X\,\,\,}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,-}}}\,C{{H}_{3}}\]

(b) From carbonyl compound :

\[RCHO+PC{{l}_{5}}\xrightarrow{{}}\underset{\text{ }\!\![\!\!\text{ Terminal}\,\text{dihalide }\!\!]\!\!\text{ }}{\mathop{RCHC{{l}_{2}}}}\,+POC{{l}_{3}}\]

• If ketone is taken internal dihalide formed.

(ii) Methods of preparation of vicinal dihalide

(a) From alkene [By halogenation] :

\[R-CH=C{{H}_{2}}+C{{l}_{2}}\xrightarrow{{}}R-\underset{Cl\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,H}}\,-\underset{Cl\,\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,{{H}_{2}}}}\,\]

(b) From vicinal glycol :

\[\overset{R-CH-OH\,\,\,}{\mathop{\underset{\,\,\,\,\,\,C{{H}_{2}}-OH}{\mathop{|\,\,\,\,\,\,\,\,\,\,\,\,\,}}\,}}\,+2PC{{l}_{5}}\xrightarrow{{}}\overset{R-CH-Cl}{\mathop{\underset{\,\,\,\,\,\,\,\,\,\,C{{H}_{2}}-Cl}{\mathop{|\,\,\,\,\,}}\,}}\,\]\[\,\,\,\,+\,\,2HCl+2POC{{l}_{3}}\]

(2) Properties of dihalides

(i) Physical properties

(a) Dihalide are colourless with pleasant smell liquid. Insoluble in water, soluble in organic solvent.

(b) M.P and B.P µ -molecular mass.

(c) Reactivity of vicinal dihalides > Gem dihalide.

(ii) Chemical properties of dihalide

(a) Reaction with aqueous KOH :

\[RCH{{X}_{2}}+2KOH(aq.)\xrightarrow[-KX]{}RCH{{(OH)}_{2}}\]\[\xrightarrow{-{{H}_{2}}O}RCHO\]

(b) Reaction with alcoholic KOH :

\[RC{{H}_{2}}-CH{{X}_{2}}\underset{-(KX+{{H}_{2}}O)}{\mathop{\xrightarrow{\text{Alc}\text{.}\,KOH}}}\,R-\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,=\overset{X}{\mathop{\overset{|}{\mathop{C}}\,}}\,-H\]\[\underset{-(NaX+N{{H}_{3}})}{\mathop{\xrightarrow{NaN{{H}_{2}}}}}\,R-C\equiv CH\]

\[R-\underset{X\ \ }{\mathop{\underset{|\ \ \ }{\mathop{CH}}\,}}\,-\underset{X\ \ \ \ }{\mathop{\underset{|\ \ \ \ \ \ \ }{\mathop{C{{H}_{2}}}}\,}}\,\xrightarrow{Alc.KOH}R-C\equiv C-H+2KX+2{{H}_{2}}O\]

(c) Reaction with Zn dust

• Gem halide (di) form higer symmetrical alkene.

• Vicinal dihalide form respective alkene.

(d) Reaction with KCN :

\[R-CH{{X}_{2}}+2KCN\xrightarrow[-2KX]{}RCH{{(CN)}_{2}}\]\[\underset{\text{Hydrolysis}}{\mathop{\xrightarrow{{{H}_{3}}{{O}^{\oplus }}}}}\,RCH{{(COOH)}_{2}}\]

(e) Other substitution reaction

• \[\underset{C{{H}_{2}}-X}{\overset{C{{H}_{2}}-X}{\mathop{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,\xrightarrow{N{{H}_{3}}/373K}\underset{\text{Ethylene diamine}}{\mathop{\underset{C{{H}_{2}}-N{{H}_{2}}}{\overset{C{{H}_{2}}-N{{H}_{2}}}{\mathop{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,}}\,\]

• \[\underset{C{{H}_{2}}-X}{\overset{C{{H}_{2}}-X}{\mathop{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,\xrightarrow{2C{{H}_{3}}COONa}\underset{C{{H}_{2}}-OCOC{{H}_{3}}}{\overset{C{{H}_{2}}-OCOC{{H}_{3}}}{\mathop{|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,+2NaX.\]