Q.Explain why thionyl chloride ($SOCl_2$) method is preferred for preparing alkyl chlorides from alcohols.

Thionyl chloride has the formula SOCl2 and is the preferred reagent for converting alcohols into alkyl chlorides.

SOCl2 (thionyl chloride) reacts with an alcohol to give the corresponding chloroalkane, with sulphur dioxide and hydrogen chloride as the only by-products, both of which escape as gases:

R-OH + SOCl2 --(pyridine)--> R-Cl + SO2(g) + HCl(g)

…

SOCl$_2$ replaces the –OH of cyclohexanol by –Cl, giving chlorocyclohexane; SO$_2$ and HCl are the by-products.

Cyclohexanol reacts with thionyl chloride (best carried out in the presence of pyridine, which absorbs the HCl formed) by the following overall equation:

$C_6H_{11}OH + SOCl_2 \xrightarrow{\text{pyridine}} C_6H_{11}Cl + SO_2\uparrow + HCl\uparrow$

Mechanism (briefly): the oxygen of the alcohol attacks the electrophilic sulfur of SOCl$_2$, displacing Cl$^-$ and forming an alkyl chlorosulfite ester, $C_6H_{11}-O-S(=O)-Cl$, with loss of HCl. This chlorosulfite is a good leaving group; the chloride ion (released in the same step, or delivered by pyridinium chloride) then displaces it, either from the front side (SN$i$, retention of configuration, in the absence of a base) or the back side (SN2, inversion, when pyridine is present to remove HCl and free Cl$^-$), giving the monochloro produc …

Thionyl chloride converts an alcohol directly into the corresponding chloroalkane, with both by-products escaping as gases.

The missing reagent is thionyl chloride, SOCl2. This reaction (called Darzens' process) converts an alcohol into the corresponding chloroalkane:

R-OH + SOCl2 → R-Cl + SO2↑ + HCl↑

…

Butan-1-ol is converted to 1-bromobutane using PBr3 (3 CH3CH2CH2CH2OH + PBr3 → 3 CH3CH2CH2CH2Br + H3PO3).

Alcohols are converted to alkyl bromides by phosphorus tribromide (PBr3), a standard reagent that replaces the -OH group by -Br:

3 R-OH + PBr3 → 3 R-Br + H3PO3

So for butan-1-ol:

3 CH3CH2CH2CH2OH + PBr3 → 3 CH3CH2CH2CH2Br + H3PO3

• Br2 alone does not convert an alcohol to an alkyl bromide. …

Replacing Cl/Br by F using a metal fluoride is named the Swarts reaction.

In the Swarts reaction, an alkyl chloride or bromide is heated with a metallic fluoride such as $AgF$, $Hg_2F_2$, $CoF_2$ or $SbF_3$; the halogen is exchanged for fluorine because the M–F bond formed is thermodynamically very stable:

$CH_3Cl + AgF \rightarrow CH_3F + AgCl\downarrow$ …

Thionyl chloride converts alcohols into alkyl chlorides while producing only gaseous by-products, so the product needs no further purification — this is why it (the Darzens procedure) is the preferred laboratory method.

The reaction of an alcohol with thionyl chloride proceeds as:

$R{-}OH + SOCl_2 \xrightarrow{\text{pyridine}} R{-}Cl + SO_2\uparrow + HCl\uparrow$

Both by-products, sulphur dioxide ($SO_2$) and hydrogen chloride ($HCl$), are gases at room temperature, so they escape from the reaction mixture spontaneously, leaving behind only the desired alkyl chloride (with no other solid or liquid contaminant to remove).

This is in contrast to alternative methods:

• $R{-}OH + PCl_5 \to R{-}Cl + POCl_3 + HCl$ — the by-product $POCl_3$ (phosphorus oxychloride) is a liquid that must be separated from the product by distillation. …

Alkyl chloride + metallic fluoride $\rightarrow$ alkyl fluoride is the Swarts reaction.

The Swarts reaction is the method by which an alkyl fluoride is prepared by heating an alkyl chloride (or bromide) with a metallic fluoride such as $AgF$, $Hg_2F_2$, $CoF_2$, or $SbF_3$; the halogen exchange occurs because these fluoride …