Topics to be covered

`=>` Alkynes
`=>` Nomenclature and isomerism
`=>` Structure of triple bond
`=>` Preparation of alkynes
`=>` Physical properties
`=>` Chemical properties


★ Alkynes are unsaturated hydrocarbons.
★ They contain at least one triple bond between two carbon atoms.
★ Their general formula is `color{red}(C_nH_(2n–2))`.
★ The first stable member of alkyne series is ethyne also known as acetylene.
★ Acetylene is used for are welding purposes in the form of oxyacetylene flame obtained by mixing acetylene with oxygen gas.

Nomenclature and Isomerism

★ In common system, alkynes are named as derivatives of acetylene.

★ In IUPAC system, they are named as derivatives of the corresponding alkanes replacing ‘ane’ by the suffix ‘yne’.

★ The position of the triple bond is indicated by the first triply bonded carbon.

★ There are two possible structures for butyne – (i) but-1-yne and (ii) but-2-yne. Since these two compounds differ in their structures due to the position of the triple bond, they are known as position isomers.

★ `color{green}("STRUCTURE OF" C_5H_8 )`

`tt(("𝐒𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐞" , "𝐈𝐔𝐏𝐀𝐂 𝐧𝐚𝐦𝐞" ) , ("I."\ \ \ \ Hoverset(1)C equiv overset(2)C - overset(3)CH_2 - overset(5)CH_3 , "Pent -1 - yne" ) , ("II."\ \ \ \ H_3 overset(1)C - overset(2)C equiv overset(3)C- overset(4)CH_2 - overset(5)CH_3 , "Pent - 2 - yne") , ( " III."\ \ \ \ H_3 overset(4)C - underset(underset(CH_3)(|))overset(3)CH - overset(2)C equiv overset(1)CH , " 3 - Methyl but - 1 - yne "))`

Structures I and II = position isomers and

Structures I and III or II and III = chain isomers.

Q 3274191056

Write structures of different isomers corresponding to the 5th member of alkyne series. Also write IUPAC names of all the isomers. What type of isomerism is exhibited by different pairs of isomers?


5th member of alkyne has the molecular formula `C_6H_(10)`. The possible isomers are:

(a) `underset("Hex -1 - yne")(HC equiv C - CH_2 - CH_2 - CH_2 - CH_3)`

(b) `underset("Hex - 2 - yne")(CH_3 - C equiv C - CH_2 - CH_2 - CH_3)`

(c) `underset("Hex - 3 - yne ")(CH_3 - CH_2 -C equiv C - CH_2 - CH_3)`

(d) `underset(" 3 - Methylpent - 1 - yne ")(HC equiv C - underset(underset(CH_3)(|))CH- CH_2 - CH_3)`

(e) `underset("4 - Methylpent - 1 - yne ")(HC equiv C - CH_2 - underset(underset(CH_3)(|))CH - CH_3)`

(f) `underset("4 - Methypent - 2 - yne ")(CH_3 - C equiv C - underset(underset(CH_3)(|))CH - CH_3)`

(g) `underset(" 3 , 3 - Dimethylbut - 1 - yne")(HC equiv C - underset(underset(CH_3)(|)) overset(overset(CH_3)(|))C - CH_3)`

Position and chain isomerism shown by different pairs.

Structure of Triple Bond:

Ethyne is the simplest molecule of alkyne series.

★ Each carbon atom of ethyne has two `color{red}(sp)` hybridised orbitals.

★ Carbon-carbon sigma `color{red}((σ)")` bond is obtained by the head-on overlapping of the two `color{red}(sp)` hybridised orbitals of the two carbon atoms.

★ The remaining `color{red}(sp)` hybridised orbital of each carbon atom undergoes overlapping along the internuclear axis with the `1s` orbital of each of the two hydrogen atoms forming two `color{red}(C-H sigma)` bonds. `color{red}(H-C-C)` bond angle is of `color{red}("180°")`.

★ Each carbon has two unhybridised `color{red}(p)` orbitals which are perpendicular to each other as well as to the plane of the `color{red}(C-C sigma)` bond.

★ The `color{red}(2p)` orbitals of one carbon atom are parallel to the `color{red}(2p)` orbitals of the other carbon atom, which undergo lateral or sideways overlapping to form two pi `color{red}((π))` bonds between two carbon atoms.

★ Thus ethyne molecule consists of one `color{red}(C–C σ)` bond, two `color{red}(C–H σ)` bonds and two `color{red}(C–C π)` bonds.

★ The strength of `color{red}(C equiv C)` bond (bond enthalpy `color{red}(823 kJ mol-1)` ) is more than those of `color{red}(C=C)` bond (bond enthalpy `color{red}(681 kJ mol^(–1)`) and `color{red}(C–C)` bond (bond enthalpy `color{red}(348 kJ mol–1)` ).

★ The `color{red}(C equiv C)` bond length is shorter (120 pm) than those of `color{red}(C=C)` (133 pm) and `color{red}(C–C)` (154 pm).

★ Electron cloud between two carbon atoms is cylindrically symmetrical about the internuclear axis.

★ Thus, ethyne is a linear molecule.

Preparation of alkynes

★ `color{green}("From calcium carbide :")`

On industrial scale, ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with coke. Quick lime can be obtained by heating limestone as shown in the following reactions:

`color{red}(CaCO_3 overset(Delta)→ CaO + CO_2)` ................(13.55)

`color{red}(CaO + 3C → underset("Calcium carbide")(CaC_2) + CO)` ............(13.56)

`color{red}(CaC_2 + 2H_2O → Ca(OH)_2 + C_2H_2)` ................(13.57)

★ `color{green}("From vicinal dihalides: ")`

Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halide is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne.

Physical properties

★ First three members are gases, the next eight are liquids and the higher ones are solids.

★ All alkynes are colourless.

★ Ethyene has characteristic odour. Other members are odourless.

★ Alkynes are weakly polar in nature.

★ They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene.

★ Their melting point, boiling point and density increase with increase in molar mass.

Chemical properties:

★ `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
Q 3284191057

How will you convert ethanoic acid into benzene?