Chemical properties of alkynes

Alkynes contain a triple bond (). A triple bond has one and two bonds.

Some characteristic reactions of alkynes are,

Combustion

Alkynes burns in air or oxygen with smoky flame.

Electrophilic addition reactions

Carbon-carbon triple bond, C=C, is a combination of one and two bonds. Alkynes give electrophilic addition reactions as they show reactivity due to the presence of bonds. This property is similar to alkenes but alkynes are less reactive than alkenes towards electrophilic addition reactions due to the compact CC electron cloud. Some typical electrophilic addition reactions given by alkynes are:

Addition of hydrogen

An alkyne reacts with hydrogen in the presence of catalyst (Pt or Ni) at 250°C, first forming alkenes and finally alkane.

For example, ethyne gives ethane in two steps.

ethyne etheneethane

Ethane is obtained in good yields if hydrogenation is done with a calculated amount of hydrogen in the presence of palladium or barium sulphate.

Propyne gives,

Addition of halogens

Alkynes react with halogens (Cl₂ or Br₂) in the dark, forming dihaloalkenes first and finally tetrahaloalkanes. The reaction gets accelerated in the presence of light or halogen carriers.

RCCH

RCX=CHX

RCX2CX2

alkyne dihaloalkene tetrahaloalkane

For example, ethyne (acetylene) with chlorine gives,

ethyne dichloroethene tetrachloroethane

Dilute bromine water with ethyne gives dibromo, while liquid bromine gives tetrabromo derivative.

tetrabromoethane ethyne 1,2-dibromoethene(acetylene)

propyne gives,

The order of reactivity is Cl₂ > Br₂ > I₂.

Addition of halogen acids

Alkynes reacts with halogen acids according to the Markownikoff's rule i.e. the carbon atom carrying the least number of hydrogen atoms will have the negative part of the addendum attached to it.

For example, ethyne (acetylene) with HBr gives,

With diluted HCl at 65°C and in the presence of Hg²⁺ (mercuric ion) ethyne gives vinyl chloride.

vinyl chloride

Propyne gives

propyne 2-bromopropene 2,2-dibromopropane

The rate of addition of halogen acids follows the order, HI > HBr > HCl

Mechanism

Addition of hypochlorous acid

Alkynes react with hypochlorous acid according to the Markownikoff's rule.

For example, ethyne with HOCl gives,

dichloroethanal

In the presence of peroxides the addition of HBr takes place according to the anti-MarkowniKoff's rule.

Addition of sulphuric acid

Alkynes add up two molecules of sulphuric acid. For example, ethyne gives

Nucleophilic addition reactions

Alkynes also give the following nucleophilic addition reactions.

Addition of water

In the presence of sulphuric acid (42%) and 1 % mercuric sulphate at 60°C, alkynes add on one water molecule to give aldehydes or ketones. For example,

alkyne ketone

Ethyne gives ethanal and propyne gives acetone.

ethyne (acetylene) ethanal (acetaldehyde)

Addition of HCN

Alkynes add one molecule of HCN in the presence of Ba(CN)₂. For example,

Ethyne gives

ethyne vinyl cyanide

Addition of ozone

Ozone adds up across the triple bond to give ozonides. After hydrolysis, ozonides give diketones and carboxylic acids.

Ethyne gives glyoxal and formic acid,

glyoxal formic acid

Substitution reactions

Due to their acidic nature, alkynes form metallic salts called alkynides e.g., sodium, silver and copper(ous) salts. Examples are,

Ethyne (acetylene) has two acidic hydrogen atoms, hence it finally gives dimetal salts.

Acidic hydrogen in 1-alkynes

Hydrogen atoms in ethyne and 1-alkynes, linked to the carbon atom having a triple bond on it, are acidic in nature. For example, ethyne (acetylene) is a weak acid: weaker than water but stronger than ammonia. This may be explained as follows:

The -electrons are more weakly bound than electrons. Thus, in those compounds containing carbon-carbon double or triple bonds, the electron density around such carbon atoms will be lesser than the carbon atoms linked only through bonds. Thus, electronegativity of differently hybridized carbon atoms will follow the order,sp > sp² > sp³

i.e., the electronegativity will increase with the s character in the hybrid orbitals. This increase in the electronegativity of an alkyne carbon, (relative to the carbon atoms in alkenes and alkanes) will polarize the C-H electron bond towards carbon and facilitate the release of proton(s). Accordingly the acid strength of hydrogens will follow the order,Alkynes > Alkenes > Alkanes.

The stabilities of the anion left after the removal of proton, i.e. carbanions follow the order,

RCC^- > RCH = CH^- > R-CH₂-CH₂^-

Thus, the acid strength follows the order, HCCH > H₂C = CH₂ > H₃C-CH₃

Compared to the organic acids e.g.. ethanoic acid (CH₃OOH), ethyne is about 10²⁰ time less acidic, while ethane is 10⁴⁰ times less acidic.

Polymerization

On heating alkynes undergo polymerization in the presence of catalyst. The nature of products depends upon the conditions. For example,

• When ethyne (acetylene) is passed through a hot copper tube, it polymerizes to benzene.

ethyne benzene benzene

• When passed through a solution of cuprous chloride in ammonium chloride, ethyne undergoes linear polymerization.

• Vinyl acetylene with hydrogen chloride gives chloroprene (2-chlorobuta-1,3-diene), which readily polymerizes to give neoprene (a synthetic rubber).

Oxidation

Oxidation of alkynes gives mono or dicarboxylic acids.

For example,

• Alkaline KMnO4 oxidises ethyne to oxalic acid.

oxalic acid(ethanedioic acid)

• With chromic acid, ethyne gives acetic acid.

ethyneethanoic acid(acetylene)(acetic acid)

Homologues of ethyne on oxidation with alkaline KMnO₄ give mixture of acids. During oxidation, rupture takes place at the triple bond.

Dienes

Dienes are unsaturated hydrocarbons containing two carbon-carbon double bonds per molecule. Their general formula is C_nH_2n-2. Dienes and alkynes are functional isomers.

Classification of dienes

Depending upon the relative positions of the two double bonds dienes are classified as:

Isolated dienes

In these dienes the double bonds are separated by more than one single bond and are called isolated dienes. For example:

Cumulated dienes

When the double bonds are present between successive carbon atoms such dienes are called cumulated dienes or allenes. For example:

Conjugated dienes

When the double bonds are separated by a single bond, dienes are termed as conjugated dienes. For example:

Relative stabilities of dienes

A conjugated diene is more stable as compared with non-conjugated dienes. The relative order of stability of dienes is:Conjugated diene > Isolated diene > Cumulated diene

The evidence in favour of maximum stability of conjugated dienes can be obtained from the values of heats of hydrogenation

As more heat is given out during hydrogenation of 1,4-pentadiene (an isolated diene) as compared with 1,3-pentadiene (a conjugated diene), it indicates that 1,4-pentadiene has more energy than 1,3-pentadiene and hence is less stable than it.