Chemistry Preparation of Haloalkanes and Haloarenes

Topics Covered :

● Nature of `C-X` Bond
● Methods of Preparation
● From Alcohols
● From Hydrocarbons
● By Free Radical Halogenation
● By Electrophilic Substitution
● Sandmeyer's Reaction
● From Alkenes
● Halogen Exchange

Nature of `C-X` Bond :

`=>` Since halogen atoms are more electronegative than carbon, the carbon-halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge as shown in fig.

`=>` Since the size of halogen atom increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest.

● Consequently the carbon-halogen bond length also increases from `color{red}(C—F)` to `color{red}(C—l)`.

`=>` Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2.

Methods of Preparation :

Haloalkanes and haloarenes are prepared as follow :

From Alcohols :

`=>` Alkyl halides are best prepared from alcohols, which are easily accessible.

`=>` The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride.

● Thionyl chloride is preferred because the other two products are escapable gases.

● Hence the reaction gives pure alkyl halides.

`=>` Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively.

`=>` The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a solution of alcohol in concentrated aqueous acid.

`color{red}(R - OH + HX overset(ZnCl_2)→ R- X + H_2O)`

`color{red}(R - OH +NaBr + H_2SO_4 → R - Br + NaHSO_4 +H_2O)`

`color{red}(3R - OH +PX_3 → 3R - X +H_3PO_3 \ \ \ \ \ (X = Cl , Br))`

`color{red}(R - OH +PCl_5 → R - Cl +POCl_3+HCl)`

`color{red}(R- OH underset(red P//X_3) overset(X_a = Br_2 , I_2)→ R- X)`

`color{red}(R - OH + SOCl_2 → R - Cl +SO_2+HCl)`

`=>` The reactions of primary and secondary alcohols with `color{red}(HX)` require the presence of a catalyst, `color{red}(ZnCl_2)`.

● With tertiary alcohols, the reaction is conducted by simply shaking with concentrated `color{red}(HCl)` at room temperature.

● Constant boiling with `color{red}(HBr)` (`48%`) is used for preparing alkyl bromide.

● Good yields of `color{red}(R—I)` may be obtained by heating alcohols with sodium or potassium iodide in `95%` phosphoric acid.

● The order of reactivity of alcohols with a given haloacid is `3^° > 2^° > 1^°`.

● The above method is not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond.

From Hydrocarbons :

From hydrocarbons, these are prepared by the following methods :

By free radical halogenation :

Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds. Consequently, the yield of any one compound is low.

`color{red}(CH_3CH_2CH_2CH_3 undersettext(or heat) overset(Cl_2//UV light)→ CH_3CH_2CH_2CH_2Cl + CH_3CH_2CHClCH_3)`

Q 3022156031

Identify all the possible monochloro structural isomers expected to be formed on free radical monochlorination of `(CH_3)_2CHCH_2CH_3`.

Solution:

In the given molecule, there are four different types of hydrogen atoms. Replacement of these hydrogen atoms will give the following

`(CH_3)_2CHCH_2CH_2Cl \ \ \ \ \ \ \ \ \ \ \ \ (CH_3)_2CHCH(Cl)CH_3`
`(CH_3)_2C(Cl)CH_2CH_3 \ \ \ \ \ \ \ \ \ \ \ \ \ CH_3CH(CH_2Cl)CH_2CH_3`

By electrophilic substitution :

`=>` Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride.

`=>` The ortho and para isomers can be easily separated due to large difference in their melting points.

`=>` Reactions with iodine are reversible in nature and require the presence of an oxidising agent (`color{red}(HNO_3, HIO_4)`) to oxidise the `color{red}(HI)` formed during iodination.

`=>` Fluoro compounds are not prepared by this method due to high reactivity of fluorine.

Sandmeyer’s reaction

`=>` When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed.

`=>` Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by `color{red}(–Cl)` or `color{red}(–Br)`. See fig.1.

`=>` Replacement of the diazonium group by iodine does not require the presence of cuprous halide and is done simply by shaking the diazonium salt with potassium iodide. See fig.2.

From alkenes :

(i) `color{green}("Addition of Hydrogen Halides ")` : An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide. See fig.1

● Propene yields two products, however only one predominates as per Markovnikov’s rule.

`color{red}(CH_3CH = CH_2+H- I → undersettext(minor)(CH_3CH_2CH_2I) +undersettext(major)(CH_3CHICH_3))`


(ii) `color{green}("Addition of Halogens ")` : In the laboratory, addition of bromine in `color{red}(C Cl_4)` to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule.

● The addition results in the synthesis of vic-dibromides, which are colourless. See fig.2.
Q 2616478379

Write the products of the following reactions:

Solution:


Halogen Exchange :

`=>` Alkyl iodides are often prepared by the reaction of alkyl chlorides/ bromides with `color{red}(NaI)` in dry acetone.

● This reaction is known as `text(Finkelstein reaction)`.

`color{red}(R - X + NaI → R - I + NaX)`

`color{red}(X = Cl , Br)`

● `color{red}(NaCl)` or `color{red}(NaBr)` thus formed is precipitated in dry acetone.

● It facilitates the forward reaction according to Le Chatelier’s Principle.

`color{green}("Swarts Reaction ")` : The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as `color{red}(AgF, Hg_2F_2, CoF_2)` or `color{red}(SbF_3)` is termed as Swarts reaction.

`color{red}(H_3C - Br +AgF → H_3 C - F + AgBr)`


 
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