Chemistry METHODS OF PREPARATION

From Alcohols :

(a) By using hydrogen halides

`R -OH overset(HX) rightarrow R-X+H_2O`

It must be noted that the `HX` used should be dry, which is produced in situ, as follows

`2NaCl +H_2SO_4 overset(heat) rightarrow 2HCl + Na_2SO_4`

`2NaBr + H_2SO_4 overset(heat) rightarrow 2HBr + Na_2SO_4`

`6NaI + 2H_3PO_4 overset(heat) rightarrow 6HI + 2Na_3PO_4`

It may be noted that `H_3PO_4` is used in place of `H_2SO_4` to prepare `HI`. This is because `HI` is a reducing agent and `H_2SO_4` being an oxidising agent can oxidize it.

The above conversion of alcohol to alkyl halides proceeds via `S_N 1` or `S_N2` mechanism. Both the mechanisms are operative during the reaction, having competition between them. The type of mechanism followed by an alcohol depends on the structure of alcohol and the type of solvent used for carrying out reaction. See fig.1.

(b) By using phosphorous halides

`R -OH + PCl_5 rightarrow R - Cl + POCl_3 + HCl`

`3R- OH + PCl_3 rightarrow 3R -Cl + H_3PO_3`

`3R -OH + PBr_3 rightarrow 3R -Br +H_3PO_3`

`3R - OH + PI_3 rightarrow 3R -I + H_3 PO_3`

Phosphorous halides are prepared by treating red phosphorous and halogen. The advantage of using phosphorous halides is that the reaction does not involve carbocation intermediate so, it is free from rearrangement.

(c) By using `SOCl_2` (thionyl chloride)

`R - OH + SOCl_2 oversettext(Pyridine)rightarrow R -Cl + SO_2 + HCl`

The usefulness of this method is that there is no side product, which has to be separated. The side products are gaseous `SO_2`, which escape from the reaction mixture and `HCl`, which forms a salt with the base (pyridine), named pyridinium chloride (`C_5H_5N^+Cl^-` ). The product alkyl chloride has a configuration inverted with respect to the reactant alcohol (if it is chiral) in the presence of pyridine base. In absence of a base and polar solvent, the chiral alcohol gives alkyl chloride with retention of configuration.

`text(Mechanism :)` Alcohol first reacts with `SOCl_2` to form an intermediate chlorosulphite ester, which gives alkyl chlorosulphite and `HCl`. In presence of pyridine, `HCl` reacts with it to give pyridinium `(PyH ^+)` ion and chloride (`Cl^ -`) ion. The `Cl^-` displaces the leaving group `ClSO _2` and chloro sulphite ester decomposes to `SO_2, Cl^ -` and `R - Cl` with inversion of configuration. See fig.2.

In the absence of a base and polar solvent, the chlorosulphite ester dissociates into an intimate ion - pair. The `Cl` of the anion of ion- pair attacks from the front side of `R^+` to give retention of configuration. The retention is observed because `Cl` cannot reach the rear of the `R^+` group but is close to its front side. See fig.3.

This is referred as `S_N1` (substitution nucleophilic internal) mechanism because a part of the leaving group detaches itself from the rest of the leaving group during the process and attacks the substrate.

By Direct Halogenation Of Hydrocarbons :

`R-H overset(X_2 //(hv)) rightarrow R-X+ HX`

Reactivity of above reaction with respect to type of hydrogen to be replaced follows following order

Tertiary hydrogen > Secondary hydrogen > Primary hydrogen

As far as the reactivity of halogen is concerned, `F_2` is most reactive while `I_2` is least reactive. Infact, reaction with `I_2` is reversible and is carried out in the presence of some oxidising agents like `H I O_3, HNO_3` etc. to oxidise `H I`. Mechanism of this reaction has already been discussed in the lesson "alkanes".

By Halide Exchange :

Alkyl iodides can be prepared from alkyl chlorides and alkyl bromides by nucleophilic substitution. This can be achieved by treating them with `NaI`, using acetone as a solvent. Feasibility of this reaction is due to the solubility of `NaI` in acetone and more nucleophilic character of `I^-` ion.

`R-Cl undersettext(Acetone) overset (NaI) (->) R-I + NaCl`

`R-Br undersettext(Acetone) overset (NaI) (->) R-I + NaBr`

The reaction proceeds by `S_N2` mechanism and is possible because `NaCl` and `NaBr` are precipitated in the reaction, as they are not soluble in weakly polar aprotic solvent.

By Addition Of H-X Alkenes :

Alkyl chlorides, bromides and iodides can be prepared by treating an alkene with corresponding hydrogen halide ( `HCl, HBr` or `HI`). The addition of these compounds to alkene takes place according to Markownikov's rule. The reaction proceeds by electrophilic addition of `H^+` to give more stable carbocation followed by attack of `X^-`. Anti - Markownikov addition of `HBr` can be achieved, if the reaction is carried out in presence of peroxides (`H_2O_2` or benzoyl peroxide or di - tert butyl peroxide). Addition of `HBr` to alkenes in the presence of peroxide follows free radical mechanism.

From Silver Salt Of Carboxylic Acid :

`RCOOAg + X_2 overset(C Cl_4)rightarrow R - X + AgX + C O_2`

(`X_2 =Cl_2 ` or `Br_2`)

This reaction is called Hunsdieker reaction.

`text(Mechanism :)` The mechanism is uncertain but probably in the first step acyl hypohalite is formed which then decomposes into free radicals.

Then, the propagation steps are repeated again. Chain can be terminated at any stage, when any two radicals will collide to form a neutral molecule.

In Hunsdieker reaction, the yield of `R- Br` is better than `R- Cl` because `O - Br` bond is weaker than `O - Cl` bond as well `AgBr` is less soluble than `AgCl`.

The yield and ease of formation of `R- X` is

`1^(circ) RX > 2^(circ) RX>3^(circ) RX`

Preparation Of Allylic Or Benzylic Halides :

(i) Direct halogenation of any aromatic hydrocarbon preferably gives benzylic halide. This is because benzyl radical is resonance stabilized.

The reaction follows free radical mechanism.

(ii) When alkenes are reacted with halogens at high temperature or in the presence of radiations or any reagent, which is able to provide halogen radicals in low concentrations, then allyl halides are produced.

`CH_3 -CH =CH_2 overset(X^(circ) )rightarrow X -CH_2 -CH =CH_2`

Preparation Of Polyhalides

(i) Addition of halogen to alkenes produces vicinal dihalides.

(ii) Addition of halogen (`Cl_2` or `Br_2`) to alkynes produces tetrahaloalkanes.

Mechanism of addition of halogen to alkenes and alkynes has been discussed in the lesson "alkenes".

Preparation Of Vinyl Halides :

The above reaction proceeds via electrophilic addition as follows