Question

Give some important methods of preparation of haloalkanes and haloarenes

Answer 1

Answer:

halognation is the method by whichhalo alkens are formed

Answer 2

Methods of preparation

of Haloalkanes and Haloarenes

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1) From Alcohol

a) Using Halogen Acid

$\sf{R-OH\:+\:H-X\:\xrightarrow{Anhy.\:AlCl_3}R-X\:+\:H_2O}$

Mechanism

• $\sf{AlCl_3\:+\:H^+-Cl^-\:\rightarrow\:AlCl_4^-\:+\:H^+}$

(Generation of Proton)

• $\sf{CH_3CH_2OH\:+\:H^+\:\rightarrow\:CH_3CH_2O(H)H\:\rightarrow\:CH3CH2^+\:+\:H_2O}$

(Generation is carbocation)

• $\sf{CH_3CH_2^+\:+\:AlCl_4^-\:\rightarrow\:CH_3CH_2Cl\:+\:AlCl_3}$

(Generation of catalyst)

• Reactivity Order

3° > 2° > 1° Alcohol

Because of the stability of carbocation and +I effect.

b) $\sf{3CH_3-CH_2-CH2-OH\:+\:PCl_3\:\longrightarrow\:3CH_3CH_2CH_2Cl\:+\:P(OH)_3}$

c) $\sf{CH_3-CH_2-CH2-OH\:+\:PCl_5\:\longrightarrow\:CH_3CH_2CH_2Cl\:+\:POCl_3\:+\:HCl}$

d) $\sf{3CH_3-CH_2-CH2-OH\:+\:SOCl_2\:\longrightarrow\:3CH_3CH_2CH_2Cl\:+\:SO_2\:+\:HCl}$

It is a best method because in it bi-products are formed in gasesous state and escape easily.

2) From Hydrocarbon

a) Markoniv's Rule

$\sf{CH_3-CH=CH2\:+\:H-Br\:\longrightarrow\:CH_3-CH_2-CH_2-Br}$

Product formed is less symmetrical and less stable.

$\sf{CH_3-CH=CH2\:+\:H-Br\:\longrightarrow\:CH_3-CH(Cl)-CH_3}$

Product formed is more symmetrical and more stable.

According to it, negative part of reagent goes to that carbon which contains less number of hydrogen atom because it will provide more symmetrical and stable product.

b) Anti-Markonikov's Rule

$\sf{CH_3-CH=CH_2\:+\:H-Br\:\xrightarrow{Peroxide}CH_3-CH_2-CH_2-Br}$

According to it, negative part of reagent goes to that carbon which contains more number of hydrogen atom.

c) Allylic Halogenation

$\sf{CH_3-CH=CH2\:+\:Cl\:\xrightarrow{h\nu}Cl-CH_2-CH=CH_2\:+\:HCl}$

$\sf{CH_3-CH=CH2\:+\:Cl\:\xrightarrow{dark}CH_3-CH(Cl)-CH_2(Cl)}$

d) Free Radical Halogenation

$\sf{CH_3-CH=CH2\:+\:Cl_2\:\xrightarrow{h\nu}CH_3-CH_2CH_2Cl\:+\:CH_3CH(Cl)CH_3}$

In the presence of dark, it gives no reaction.

3) From Diazonium Salt

a) Diazotization Reaction

b) Sandmeyer's Reaction

c) Gattermann's Reaction

d) Balz-Schimann Reaction

$\sf{\red{Refer\:the\:attachment\:for\:reactions}}$

4) Swartz Reaction

$\sf{CH_3-Br\:+\:Ag-F\:\longrightarrow\:CH_3-F\:+\:Ag-Br}$

5) Frinkelstein Reaction

$\sf{CH_3-Br\:+\:Na-I\:\xrightarrow{Acetone}\:CH_3-I\:+\:Na-Br}$

6) Borodiene Hunsdicker's Reaction

$\sf{CH_3-C(O)-O-Ag\:+\:Br_2\:\xrightarrow{h\nu}CH_3-Br\:+\:CO_2\:+\:AgBr}$