Alkynes

ALKYNES

 

Like alkenes, alkynes are also unsaturated hydrocarbons. They contain at least one triple bond between two carbon atoms. The number of hydrogen atoms is still less in alkynes as compared to alkenes or alkanes. Their general formula is CnH2n–2.

The first stable member of alkyne series is ethyne which is popularly known as acetylene. Acetylene is used for arc welding purposes in the form of oxyacetylene flame obtained by mixing acetylene with oxygen gas. Alkynes are starting materials for a large number of organic compounds. Hence, it is interesting to study this class of organic compounds.

 

Nomenclature and Isomerism of alkynes

Isomerism of alkynes

1.     Structural isomerism: Alkynes can have structural isomers, where the number and arrangement of carbon-carbon triple bonds differ.

2.     Stereoisomerism: Alkynes can also exhibit stereoisomerism if they have two different substituents attached to the same carbon atom in the triple bond. In such cases, cis-trans isomerism is possible. For example, 2-butyne  can exist as two isomers - the cis isomer, where the two ethyl groups are on the same side of the triple bond, and the trans isomer, where they are on opposite sides of the triple bond.

Structure of triple bond

Each carbon atom of ethyne has two sp hybridised orbitals. Carbon-carbon sigma (σ) bond is obtained by the head-on overlapping of the two sp hybridised orbitals of the two carbon atoms. The remaining sp hybridized orbital of each carbon atom undergoes overlapping along the internuclear axis with the 1s orbital of each of the two hydrogen atoms forming two C-H sigma bonds. H-C-C bond angle is of 180°. Each carbon has two unhybridized p orbitals which are perpendicular to each other as well as to the plane of the C-C sigma bond. The 2p orbitals of one carbon atom are parallel to the 2p orbitals of the other carbon atom, which undergo lateral or sideways overlapping to form two pi (π) bonds between two carbon atoms. Thus ethyne molecule consists of one C–C σ bond, two C–H σ bonds and two C–C π bonds. The strength of CC bond (bond enthalpy 823 kJ/mol) is more than those of C=C bond (bond enthalpy 681 kJ mol–1) and C–C bond (bond enthalpy 348 kJ/mol). The CC bond length is shorter (120 pm) than those of C=C (133 pm) and 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 alkyne

From calcium carbide to alkyne

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:

 

 

From vicinal dihalide to alkyne

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.

Properties of alkyne

Physical properties of alkynes

(1.)  Acetylene is a colourless gas with ethereal smell in pure state; b. pt.

(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 bonds increases from 25 percent to 33.3 percent  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

Chemical properties of alkynes

Acidic character of alkyne

An alkyne molecule contains at least one triple bond between two carbon atoms. For example ethyne (CH≡CH). Ethyne reacts with strong bases such as sodium metal and sodamide (NaNH2)to form sodium acetylide along with the liberation of dihydrogen gas. This reaction of alkynes with bases to liberate dihydrogen gas indicates the acidity of alkynes.

HC ≡ CH + Na → HC ≡ C Na+ + 1/2H2

Acidity of alkynes is due to their ability to lose hydrogen atom to form alkynideions. Thus, alkynes act as Brønsted-Lowry acids. The triple bonded carbon atom in alkynes is “sp” hybridized. Due to the high percentage of “s” character (50%) in alkynes, the “sp” hybridized orbitals of carbon atom in alkynes exhibit high electronegativity. These attract the C-H bond of alkynes to a great extent. Thus, alkyne molecules can easily lose hydrogen atom forming alkynide ions. Hence, we can say that the hydrogen atom attached to the triply bonded carbon atom is acidic in nature.

The acidity of alkynes is greater than the acidity of alkanes and alkenes as the carbon atom in alkanes and alkenes are “sp3” and “sp2” hybridized respectively. Hence, these molecules contain a smaller percentage of “s” character in comparison to alkynes. Thus, the electronegativity of the carbon atom in these cases is lesser than alkynes. Hence, alkanes and alkenes don’t show the reactions with bases to liberate hydrogen gas. It can further be noted that only hydrogen atom attached to a triply bonded carbon atom are acidic not the other hydrogen atoms in the alkyne chain. The general trend in acidity is seen as:

HC≡CH > H2C=CH2> CH3–CH3

HC≡CH>CH3–C≡CH>>CH3–C≡C–CH3

 

Addition reactions of alkynes

Addition of dihydrogens to alkynes

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.

   

 

Addition of halogens to alkynes

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

   

The order of reactivity with halogens is

          

Addition of hydrogen halides to alkynes

The addition is in accordance of markovnikov’s rule.

Because of I effect of the bromine atom, the availability of the π - 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.

Addition of water to alkynes

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.

Polymerisation of alkynes

linear polymerisation of alkynes

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 (CH = CH – CH = CH ) and can be represented as —(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.

 

Cyclic polymerisation of alkynes

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: