Properties of Alkynes

PHYSICAL PROPERTIES

(1.) Acetylene is a colourless gas with ethereal smell in pure state; b. pt.$=-{84}^{o}C$

(2.) Liquified acetylene is explosive and burns with a luminous smoky flame due to high carbon content hence it is used for lighting purpose.

(3.) As the s – character of the orbitals involved in $\sigma -$bonds increases from 25 percent $(s{p}^3)$ to 33.3 percent $(s{p}^2)$ to 50 percent (sp), the electrons in those orbitals are on the average, close to carbon nucleus and this leads to a contraction in the internuclear distance.

Bond lengths are in the order

$C\equiv C<C=C<C-C$

CHEMICAL PROPERTIES

1. Electrophilic addition reactions:

Alkynes undergo electrophilic addition reactions but less readily because of the following reasons:

(a) C – atoms of alkynes being sp – hybridized are more electronegative and hence hold the $\pi -electrons$more tightly.

(b) $\pi -electrons$of alkynes are more delocalized and hence are less easily available for reaction.

(c) Vinyl carbocation formed after the addition of an electrophile to the alkyne is far less stable than the alkyl carbocations obtained by the addition of an electrophile to the alkene.

Some of the electrophilic addition reactions of alkynes are discussed below.

(i) Addition of halogens:

First trans – dihalides are formed which react further to form tetrahalides.

The order of reactivity with halogens is

$C{l}_2>B{r}_2>I_2$

(ii) Addition of Hydrogen

$C_2{H}_2\underset{Catalyst}{\overset{H_2}{\to }}{C}_2{H}_4\underset{Catalyst}{\overset{H_2}{\to }}{C}_2{H}_6$

If the triple bond is not present at the end of chain of the molecule then the dialkyl acetylene may be catalytically reduced to cis and trans alkanes.

(iii) Addition of halogen acids

The addition is in accordance of markovnikov's rule.

$CH\equiv CH\stackrel{HBr}{\to }C{H}_2=CHBr\stackrel{HBr}{\to }C{H}_{3}CHB{r}_2$

Because of $-$ I effect of the bromine atom, the availability of the $\pi$- electrons in the first step is decreased and the addition is much slower as compared to ethylene. The reactivity is HI > HBr > HCl. When passed through dilute HCl in the presence of Hg²⁺ as catalyst, acetylene forms vinyl chloride.

$CH\equiv CH+HCl\stackrel{H{g}^{2+}}{\to }C{H}_2=CHCl$

(iv) Addition of water

when passed into dilute H₂SO₄ at 60⁰C in the presence of HgSO₄ as catalyst acetylene adds on one molecule of water to form acetaldehyde.

The homologous of acetylene forms ketone when hydrated.

(v) Reaction with metals

A triply bonded carbon which is sp – hybridized has a high electronegativity and hydrogen atoms attached to it show appreciable acidity.

CH $\equiv$ C^-Na⁺ is also obtained by passing acetylene into a solution of sodium in liquid ammonia until the blue colour disappears.

Sodium acetylide can be used to prepare higher members of alkyne series.

$CH\equiv C^{-}N{a}^{+}\stackrel{RX}{\to }CH\equiv CR+NaX$

When acetylene is passed into an ammonical solution of Cu_­2Cl₂ or AgNO₃, cuprous acetylide Cu₂C₂ (red) or silver acetylide Ag₂C₂(white) is precipitated.

$CH\equiv CH+C{u}_{2}C{l}_2\stackrel{N{H}_{4}OH}{\to }C{u}^{+}{C}^{-}\equiv C^{-}C{u}^{+}+2N{H}_{4}Cl+2H_{2}O$

(vi) Ozonolysis

\underset{Butyne-2}{\mathop{C{{H}_{3}}C\equiv C-C{{H}_{3}}}}\,\xrightarrow[{{H}_{2}}{{O}_{2}}]{{{O}_{3}}/{{H}_{2}}O}2C{{H}_{3}}COOH

Acetylene is exceptional that it gives glyoxal as well as formic acid.

(vii) Polymerisation

When passed through a heated tube (Fe, Cu or Ni), acetylene changes to small extent of benzene.

1, 3 , 5 – trimethyl benzene (mesitylene)

When passed into a solution of Cu₂Cl₂ in NH₄Cl, acetylene gives vinyl acetylene.

$CH\equiv CH+CH\equiv CH\underset{N{H}_{4}Cl}{\overset{C{u}_{2}C{l}_2}{\to }}C{H}_2=CH-C\equiv CH$

But – 3 – en – 1 – yne

Compounds containing both double and triple bonds are named as alkenynes.

(viii) Isomerization

When alkynes are heated with NaNH₂ in an inert solvent triple bond moves towards the end of chain.

Acidity of Alkynes

$-$C $\equiv$ CH bond has considerable ionic character due to resonance.

The electronegativity of a carbon atom depends on the number of bonds by which it is attached to its neighbouring carbon. The carbon with electronegativity of an sp² hybridised carobn atoms is greater than that of sp³ hybridised carbon atom and the electronegativity of an sp hybridized carbon atom is further greater than that of sp² carbon atom. More the s – character a bond has more electronegative is the carbon atom. Thus the attraction for electrons by hybridized carbon will be

sp > sp² > sp³

Hence greater the electronegativity of an atom, more readily it accommodates a negative charge.

$\therefore$ Stabilities of the following carbanions follows the order.

CH $\equiv$ C^- > CH₂ = CH^- >$C{H}_{3}C{H}_2^{-}$

Illustration 1. 2-Butyne undergoes following reactions in steps as indicated. Identify A to H.

Solution: A is obtained by syn addition

is also obtained by syn addition

C is obtained by anti addition

D is mixture of d – and $\ell$ - (racemic mixture)

E is obtained by syn addition

E : as A

Addition of Br₂ is anti