8.2 Replacements

  Alkyl Halides and Aryl Halides

Replacement by hydroxyl group:

Alcohols are formed when haloalkanes are treated with boiling aqueous alkali solution. Haloalkanes are hydrolysed to the corresponding alcohols by moist $Ag_2O$.

$$\begin{equation} \begin{aligned} R - X + KOH \to R - OH + KX \\ A{g_2}O + {H_2}O \to 2AgOH \\ {C_2}{H_5}Cl + AgOH \to {C_2}{H_5}OH + AgCl \\\end{aligned} \end{equation} $$

Replacement by cyano group:

Alkane nitriles are the major product formed when haloalkanes are treated with alcoholic $KCN$. The minor product is isocyanide. $$R - X + KCN \to R - CN + KX$$

Replacement by isocyanide group:

Alkyl isocyanide is the major product when haloalkanes react with aqueous ethanolic silver cyanide. The minor product of this replacemenent reaction is alkyl cyanide. $$R - X + AgCN \to R - NC + AgX$$

Replacement by nitrile group:

Alkyl nitrite is the major product formed when alkyl halides react with potassium or sodium nitrite. With dimethyl formamide as a solvent, $30\%$ of alkyl nitrite is formed. $$R - X + KN{O_2} \to R - O - N = O + KX$$

Replacement by nitro group:

Treatment of ethanolic solutions of haloalkanes with silver nitrite form nitro alkanes as the major product and alkyl nitrites as the minor product.

Replacement by alkoxy group:

Treatment of haloalkanes with alcoholic potassium or sodium alkoxide form ethers. This reaction is called Williamson synthesis. Ethers can also be formed from haloalkanes using dry silver oxide.

$$\begin{equation} \begin{aligned} R - X + NaOR' \to R - O - R' + NaX \\ 2R - X + Ag - O - Ag \to R - O - R + 2AgX \\\end{aligned} \end{equation} $$

Replacement by amino group:

A mixture of amines and quaternary ammonium salt is obtained when haloalkanes react with aqueous or alcoholic solution of ammonia. The reaction takes place in a sealed tube at ${100^ \circ }C$.

$$R - X + N{H_3} \to R - N{H_2} + HX$$

• If there is an excess of haloalkane, secondary and tertiary amines are formed. This is because the other two hydrogen atoms of the amino group are replaced by alkyl groups. $$\begin{equation} \begin{aligned} R - X + R - N{H_2} \to {R_2}NH\left( {{\text{secondary amine}}} \right) \\ R - X + {R_2} - NH \to {R_3}N\left( {{\text{tertiary amine}}} \right) \\\end{aligned} \end{equation} $$

• Tertiary amines can combine with another molecule to give quaternary ammonium salt. $$R - X + {R_3}N \to {\left[ {{R_4}N} \right]^ + }C{l^ - }$$ This reaction is also known as Hofmann ammonolysis of alkyl halides.

Replacement by hydrosulphide group:

Haloalkanes react with $KSH$ or $NaSH$ (potassium or sodium hydrogen sulphide) to form thioalcohols. $$R - X + NaSH \to R - SH + NaX$$

Replacement by carboxylate:

Esters are obtained when haloalkanes react with silver acetate in alcoholic solution. $$R'COOAg + R - X \to R'COOR + AgX$$

Replacement by alkynyl group:

Higher alkynes are obtained when haloalkanes react with sodium acetylide.