★ `color{green}("𝐀𝐜𝐢𝐝𝐢𝐜 𝐜𝐡𝐚𝐫𝐚𝐜𝐭𝐞𝐫 𝐨𝐟 𝐚𝐥𝐤𝐲𝐧𝐞 :")` Sodium metal and sodamide `color{red}((NaNH_2))` are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas which indicates that ethyne is acidic in nature in comparison to ethene and ethane.
Atoms in ethyne are attached to the `color{red}(sp)` hybridised carbon atoms whereas they are attached to `color{red}(sp^2)` hybridised carbon atoms in ethene and `color{red}(sp^3)` hybridised carbons in ethane.
Due to the maximum percentage of `color{red}(s)` character (50%), the `color{red}(sp)` hybridised orbitals of carbon atoms in ethyne molecules have highest electronegativity; hence, these attract the shared electron pair of the `color{red}(C-H)` bond of ethyne to a greater extent than that of the `color{red}(sp^2)` hybridised orbitals of carbon in ethene and the `color{red}(sp^3)` hybridised orbital of carbon in ethane.
Thus in ethyne, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature.
i) `color{red}(HC equiv CH > H_2C = CH_2 > CH_3 - CH_3)`
ii) `color{red}(HC equiv CH > CH_3 - C equiv CH > > CH_3 - C equiv C - CH_3)`
`color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧𝐬:")` Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen,
hydrogen halides etc. Formation of the addition product takes place according to the following steps.
`color{red}(underset("Vinylic cation")(-C equiv C - + H - Z overset(H^(+))→ - overset(overset(H)(|))C = overset(⊕)C - + : barZ → - overset(overset(H)(|))C = overset(overset(Z)(|))C - ))`
The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. A few addition reactions are given below:
★ (𝐢) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐝𝐢𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧")`
★ (𝐢𝐢) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐡𝐚𝐥𝐨𝐠𝐞𝐧𝐬")`
Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a
test for unsaturation.
★ (𝐢𝐢𝐢) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧 𝐡𝐚𝐥𝐢𝐝𝐞𝐬")`
Two molecules of hydrogen halides `color{red}((HCl, HBr,HI))` add to alkynes to form gem dihalides (in
which two halogens are attached to the same carbon atom).
★ (𝐢𝐯) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐰𝐚𝐭𝐞𝐫")`
Alkynes do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form carbonyl compounds.
★ (𝐯) `color{green}("𝐏𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧")`
(a) `color{green}("𝐋𝐢𝐧𝐞𝐚𝐫 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧:")` Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne which is a high molecular weight polyene containing repeating units of `color{red}((CH = CH – CH = CH ))` and can be represented as `color{red}(—(- CH = CH – CH = CH-)_n —)` Under special conditions, this polymer conducts electricity. Thin film of polyacetylene can be used as electrodes in batteries. These films are good conductors, lighter and cheaper than the metal conductors.
(b) `color{green}("𝐂𝐲𝐜𝐥𝐢𝐜 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧 :")` Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization. Three molecules polymerise to form benzene, which is the starting molecule for the preparation of derivatives of benzene, dyes, drugs and large number of other organic compounds. This is the best route for entering from aliphatic to aromatic compounds as discussed
below:
★ `color{green}("𝐀𝐜𝐢𝐝𝐢𝐜 𝐜𝐡𝐚𝐫𝐚𝐜𝐭𝐞𝐫 𝐨𝐟 𝐚𝐥𝐤𝐲𝐧𝐞 :")` Sodium metal and sodamide `color{red}((NaNH_2))` are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas which indicates that ethyne is acidic in nature in comparison to ethene and ethane.
Atoms in ethyne are attached to the `color{red}(sp)` hybridised carbon atoms whereas they are attached to `color{red}(sp^2)` hybridised carbon atoms in ethene and `color{red}(sp^3)` hybridised carbons in ethane.
Due to the maximum percentage of `color{red}(s)` character (50%), the `color{red}(sp)` hybridised orbitals of carbon atoms in ethyne molecules have highest electronegativity; hence, these attract the shared electron pair of the `color{red}(C-H)` bond of ethyne to a greater extent than that of the `color{red}(sp^2)` hybridised orbitals of carbon in ethene and the `color{red}(sp^3)` hybridised orbital of carbon in ethane.
Thus in ethyne, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature.
i) `color{red}(HC equiv CH > H_2C = CH_2 > CH_3 - CH_3)`
ii) `color{red}(HC equiv CH > CH_3 - C equiv CH > > CH_3 - C equiv C - CH_3)`
`color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧𝐬:")` Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen,
hydrogen halides etc. Formation of the addition product takes place according to the following steps.
`color{red}(underset("Vinylic cation")(-C equiv C - + H - Z overset(H^(+))→ - overset(overset(H)(|))C = overset(⊕)C - + : barZ → - overset(overset(H)(|))C = overset(overset(Z)(|))C - ))`
The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. A few addition reactions are given below:
★ (𝐢) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐝𝐢𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧")`
★ (𝐢𝐢) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐡𝐚𝐥𝐨𝐠𝐞𝐧𝐬")`
Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a
test for unsaturation.
★ (𝐢𝐢𝐢) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧 𝐡𝐚𝐥𝐢𝐝𝐞𝐬")`
Two molecules of hydrogen halides `color{red}((HCl, HBr,HI))` add to alkynes to form gem dihalides (in
which two halogens are attached to the same carbon atom).
★ (𝐢𝐯) `color{green}("𝐀𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐰𝐚𝐭𝐞𝐫")`
Alkynes do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at 333 K to form carbonyl compounds.
★ (𝐯) `color{green}("𝐏𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧")`
(a) `color{green}("𝐋𝐢𝐧𝐞𝐚𝐫 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧:")` Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne which is a high molecular weight polyene containing repeating units of `color{red}((CH = CH – CH = CH ))` and can be represented as `color{red}(—(- CH = CH – CH = CH-)_n —)` Under special conditions, this polymer conducts electricity. Thin film of polyacetylene can be used as electrodes in batteries. These films are good conductors, lighter and cheaper than the metal conductors.
(b) `color{green}("𝐂𝐲𝐜𝐥𝐢𝐜 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧 :")` Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization. Three molecules polymerise to form benzene, which is the starting molecule for the preparation of derivatives of benzene, dyes, drugs and large number of other organic compounds. This is the best route for entering from aliphatic to aromatic compounds as discussed
below:
