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.

combustion of alkynes

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.

formation of alkane from alkyne

For example, ethyne gives ethane in two steps.

formation of ethane from ethyne

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,

formation of propane from propyne

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

formation of dihaloalkenes from alkynes

RCX=CHX

formation of tetrahaloalkanes from alkynes

RCX2CX2

alkyne dihaloalkene tetrahaloalkane

For example, ethyne (acetylene) with chlorine gives,

formation of tetrachloroethane from ethyne

ethyne dichloroethene tetrachloroethane

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

formation of tetrabromoethane from alkyne

tetrabromoethane ethyne 1,2-dibromoethene(acetylene)

propyne gives,

formation of tetrabromopropane  from propyne

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.

formation of dihaloalkane from alkyne

For example, ethyne (acetylene) with HBr gives,

formation of dibromoethane from ethyne

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

ethyne gives vinyl chloride

vinyl chloride

Propyne gives

formation of dibromopropane from propyne

propyne 2-bromopropene 2,2-dibromopropane

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


Mechanism

mechanism of conversion of alkynes to alkenes

mechanism of conversion of alkenes to alkanes

Addition of hypochlorous acid

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

formation of dihaloketone from alkyne

For example, ethyne with HOCl gives,

formation of dichloroethanal from ethyne
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

formation of ethylidene hydrogen sulphte from ethyne

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,

formation of ketone from alkyne

alkyne ketone

Ethyne gives ethanal and propyne gives acetone.

Ethyne gives ethanal

ethyne (acetylene) ethanal (acetaldehyde)

propyne gives acetone


Addition of HCN

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

reaction of alkynes with HCN addition

Ethyne gives

ethyne gives vinyl cyanide

ethyne vinyl cyanide


Addition of ozone

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

addition of ozone with alkynes

Ethyne gives glyoxal and formic acid,

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,

alkynes form metallic salts called alkynides

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

formation of dicopper acetylide from ethyne

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 polymerizes to benzene

ethyne benzene benzene

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

Oxidation

Oxidation of alkynes gives mono or dicarboxylic acids.

For example,

  • Alkaline KMnO4 oxidises ethyne to oxalic acid.
ethyne oxidises to oxalic acid

oxalic acid(ethanedioic acid)

  • With chromic acid, ethyne gives acetic acid.
formation of acetic acid from ethyne

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.

ethyne on oxidation give mixture of acids

Dienes

Dienes are unsaturated hydrocarbons containing two carbon-carbon double bonds per molecule. Their general formula is CnH2n-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:

example for isolated dienes

Cumulated dienes

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

example for cumulated dienes

Conjugated dienes

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

conjugated dienes 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

maximum stability of conjugated dienes

maximum stability of conjugated dienes

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.

1 comment:

Rohit G said...

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