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Alkyne Structure

Alkynes are hydrocarbons containing at least one triple bond between carbon atoms. Their general formula is CnH2n-2.

The hydrogen carbon ratio is less in alkynes comparing with alkenes and alkanes. Hence they burn with incomplete combustion to give carbon. As they contain triple bond their valence is not completed and hence they can undergo addition reaction to complete the valence.

Hence they are highly reactive. The alkynes in which the triple bond is in the end of the carbon chain is called as terminal alkyne. They are studied separately because the hydrogen attached with terminal carbon is acidic and can be released easily by bases like soda amide etc. Some of the alkynes with their formula are given below.

CH ≡ CH - Acetylene
CH3-C ≡ CH - Methyl acetylene
CH3-C ≡ C-CH3 - Dimethyl acetylene

 
There are various ways of representing the structural formula of alkynes. They are discussed below.

1. Molecular formula


Here the molecular formula of atoms present in the alkyne are represented. For example the molecular formula of butyne is represented as C4H6.

2. Condensed structural formula


In condensed structural formula the molecule is represented as a carbon chain along with the hydrogen atoms attached with the carbon. The triple bond in between carbon atom is represented in its position. For example the condensed structural formula for 2-Butyne is given below.

CH3-C≡C-CH3

3. Structural formula


Here the carbon chain is represented along with the hydrogen atoms attached are shown as bond line representation. For example 2-Butyne structural formula is represented below.

Structural Formula

4. Ball and stick model


It is the way of representing the three dimensional structure of alkynes. Different colored balls are used to describe different atoms and sticks are used to represent the bonds connecting the atoms. For example 2-Butyne ball and stick model is given below.

Ball and Stick Model

5. Space filling model


It is the model of representing the three dimensional structure of alkynes. Here the balls are alone shown without sticks. This is the actual structure of molecule. For example the space filling model of butane is given below.

Space Filling Model

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Lewis Dot Formula for Alkynes

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The carbon atom has four electrons in the valence shell. Three electrons are shared with carbon atom and one electron is shared with hydrogen. Hence the octet of all atoms are filled up. The Lewis dot formula of acetylene is given below.

H : C : : : C : H

Alkyne Hybridization

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In according to hybridization theory the different orbitals with nearly same energy hybridize to give hybridized orbitals with same energy and shape. In carbon atom the valence electron configuration is 2s2, 2p2. To accommodate the tetra valence of carbon one electron is 2s orbital is excited to 2p orbital. Hence the excited electronic configuration of carbon is 2s1, 2px1, 2py1, 2pz1.

Hybridization in Alkynes

Here the carbon atom undergoes sp hybridization to give two sp hybridized orbitals with equal shape and linear shape with bond angle 180o. Out of the two hybridized orbitals formed one is overlapped with another sp hybridized orbital of next carbon atom. The second hybridized orbital is overlapped with s orbital of hydrogen. Hence there are three sigma bonds in acetylene.

The 2p orbitals which do not take part in hybridization containing odd electrons overlap laterally to give π bonds. Hence there are three sigma bonds and 2 π bonds in acetylene. The structure of acetylene is linear with bond angle 180o.

Structure and Bonding Alkynes

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Similar to the structure the ethyne, the structure of alkynes will be straight line with respect to carbon atoms containing the triple bond. All other alkyl carbon atoms are sp3 hybridized with tetrahedron shape. Hence the structure of propyne will be as follows as a combination of tetrahedron carbon and two carbon atoms in linear structure.

Structure and Bonding Alkynes

Bond Strength in Alkynes

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1. Bond Length and Bond order


Bond order is the indication of number of bonds between atoms. In alkynes the bond order between two carbon atom is three. Bond order is inversely related to bond length. Hence alkynes with triple bond will have shortest bond. The bond length order is given below for reference

CH3-CH3 > CH2=CH2 > CH=CH

2. Bond strength and bond order


Bond strength is directly related with bond order. In alkynes there are three bonds between two carbon atoms hence the bond strength will be more than alkenes and alkynes. Bond dissociation energy is energy released when one mole of bond is formed from elements to give compounds. In other words it is the energy required to break one mole of bonds to give individual atoms. Bond dissociation energy can be considered as a direct measure of bond strength. The bond dissociation energy of various bonds are given below.

CH3-CH3 - 368 kJ/mol
CH2=CH2 - 682 kJ/mol
CH=CH - 962 kJ/mol

It is clearly shown that the bond dissociation energy of triple bond is approximately three times that of a single bond in between carbon atoms. Hence the triple bond is three times stronger than single bond.

Internal Alkyne

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  1. If the triple bond is present in the middle of a carbon chain in alkyne it is called as internal alkyne.
  2. In other words there are carbon substitutes in both sides of acetylene carbon atoms.
  3. Here the alkyl groups destabilize the triple bond by +I effect and hence the heats of hydrogenation will be more comparing with terminal alkynes.
  4. Similarly they are less acidic then terminal alkynes. 1-Butyne is an example for terminal alkyne while 2-Butyne is an example for internal alkyne.
CH3-CH2-C≡CH - 1-Butyne terminal alkyne
CH3-C≡C-CH3 - 2-Butyne internal alkyne

Terminal Alkyne

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Alkynes with triple bond in the terminal carbon are called as terminal alkynes. They are considered as special class because the hydrogen attached in the terminal carbon attains partial positive charge and an be easily removed as acidic hydrogen by bases. This is due to more s character in the hybridized orbitals and the more electronegative of carbon in sp hybridization.

Hence, the terminal alkynes are weak acids and react with metals like sodium in ammonia to deposit the salts.
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