Preparation of Alkynes

METHODS OF PREPARATIONS

1. Industrial source:

$Ca{C}_2+H_{2}O\to Ca{(OH)}_2+HC\equiv CH$

Calcium Carbide Ethyn

The double negative carbide ion

which is strongly basic reacts with water to form acetylene

$M{g}_2{C}_3+H_{2}O\to Mg{(OH)}_2+C{H}_{3}C\equiv CH$

Propyne

$6C{H}_4+2O_2\underset{cracking}{\overset{{1500}^{o}C}{\to }}2CH\equiv CH+2C{O}_2+10H_2$

2. Kolbe's method:

Electrolysis of concentrated solution of sodium or potassium salt of maleic or fumaric acid gives acetylene at anode

3. Dehydrohalogenation of 1, 2 – dihalides:

1, 2 – dihalides on treatment with alcoholic KOH gives alkynes by loss of two molecules of hydrogen halide, the intermediate being vinyl halide.

Sodamide $(NaN{H}_2)$ in liquid $N{H}_3$can be used instead of alcoholic KOH.

Mechanism:

Step I:

Elimination of the first molecule of HX results in the formation of vinyl halide – an alkene with halogen bonded to one of the carbons of the double bond. Strong base $\stackrel{-}{N}{H}_2$is taken to make the step II easier for elimination.

4. Dehydrohalogenation of 1, 1 – dihalides:

1, 1 – dihalides (geminal) with alcoholic KOH or $NaN{H}_{2}inN{H}_3$ forms alkynes where intermediate is vinyl halide

.

5. Dehalogenation of tetrahalides or trihalides:

$CH{X}_3+6Ag+X_{3}CH\stackrel{\Delta }{\to }CH\equiv CH+6AgX$

6. Alkylation of acetylene and terminal alkynes:

i.e. an alkyne with $-C\equiv CH$ at the end of chain. This method is used to prepare higher alkynes.

$HC\equiv CH+Na\stackrel{LiquidN{H}_3}{\to }HC\equiv \stackrel{-}{C}\stackrel{+}{\mathrm{Na}}\stackrel{RX}{\to }HC\equiv CR$

$RC\equiv CH+Na\stackrel{LiquidN{H}_3}{\to }RC\equiv \stackrel{-}{C}\stackrel{+}{\mathrm{Na}}\stackrel{R^{\prime }X}{\to }RC\equiv C{R}^{\prime }$

Terminal alkyne

$C{H}_{3}C\equiv CH\underset{(ii)C{H}_{3}C{H}_{2}I}{\overset{(i)Na/liq.N{H}_3}{\to }}C{H}_{3}C\equiv CC{H}_{2}C{H}_3(2-pentyne)$

Note:

RX and $R^{\prime }Xshouldbe{1}^o$alkyl halides, since higher secondary and tertiary halides give mainly alkenes when they react with sodium salt of alkynes.

Alkylation can also be done by using Grignard's reagent.

$HC\equiv CH+C{H}_3-MgBr\to CH\equiv C-Mg-Br+C{H}_4$

$HC\equiv C-Mg-Br+C{H}_{3}Br\to CH\equiv CC{H}_3+MgB{r}_2$