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# Alkyne

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 Sub Topics We are familiar with most common organic compounds that are known as hydrocarbons, are mainly found in crude oil. They compose of carbon and hydrogen atoms. The carbon atoms are linked with each other through single, double or triple covalent bonds. The presence of multiple bonds (double or triple) makes the molecule unsaturated. The unsaturation in the hydrocarbon affects the physical and chemical properties of the compounds such as alkenes show addition reactions whereas alkanes show substitution reactions. Alkynes are highly unsaturated molecules with triple covalent bonds.  The general formula of alkynes is CnH2n-2 which shows the presence of less number of hydrogen atoms. The less number of hydrogen atoms makes the alkyne unsaturated in nature and prone to addition reactions. Let’s discuss some other important points related to alkynes.

## Definition

Acetylene is the simplest alkyne that have a triple bond between two carbon atoms, with the formula CnH2n-2. Under standard conditions in the laboratory it a colorless but unstable gas. Alkynes are unsaturated hydrocarbons. The characteristic functional group of alkynes are triple bonds.

Examples of some of the alkynes are shown below.

## Formula

The smallest alkyne contains two carbon atoms which are bonded by triple covalent bond. The name of smallest alkyne is ethyne with CH$\equiv$CH structure formula. Some common alkynes with their IUPAC names, molecular formulas and structural formulas of some of the alkynes are listed below.

 Name Structural formula Molecular Formula Ethyne $CH\equiv CH$ $C_{2}H_{2}$ Propyne $CH_{3}C\equiv CH$ $C_{3}H_{4}$ 1-Butyne $CH_{3}CH_{2}C\equiv CH$ $C_{4}H_{6}$ 1-Pentyne $CH_{3}CH_{2}CH_{2}C\equiv CH$ $C_{5}H_{8}$ 1-Hexyne $CH_{3}CH_{2}CH_{2}CH_{2}C\equiv CH$ $C_{6}H_{10}$ 1-Heptyne $CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}C\equiv CH$ $C_{7}H_{12}$ 1-Octyne $CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}C\equiv CH$ $C_{8}H_{14}$ 1-Nonyne $CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}C\equiv CH$ $C_{9}H_{16}$ 1-Decyne $CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}C\equiv CH$ $C_{10}H_{18}$

## Naming

The naming of alkynes has been just like alkanes and also follows the IUPAC nomenclature rules. The name of alkyne can be identified with the suffix –yne. Like other hydrocarbons, in the naming of alkynes, first we have to find out the longest parent chain which includes the triple bonded carbon atoms. Then we have to number the parent chain in such a way that the triple bond will get the lowest number. On the basis of position of triple bonds, alkynes can be classified as terminal and internal alkynes. In a terminal alkyne, the triple bond is placed at one of the terminal points of the parent chain whereas triple bond is located at any other position in internal alkynes.

For example, 4-chloro-6-diiodo-7-methyl-2-nonyne is an internal alkyne as the triple bond is placed between C-2 and C-3 atom of chain. After naming and numbering of parent chain, now check the position of substitute bonded on the parent chain. The names of substitute must be arranged in the alphabetical order. Prefix di, tri, and tetra indicate more than one of the substitute.

For example 1-triiodo-4-dimethyl-2-nonyne indicates that there are three iodo-groups and two methyl groups in the molecule. In the presence of two triple bonds in alkyne molecule, the longest carbon chain must include both the triple bonds of the molecule. The numbering of parent chain will start from that end which is close to triple bond. The suffix –diyne indicates two triple bonds. For example in 4-methyl-1, 5-octadiyne molecule there are two triple bonds at 1 and 5 position of the parent chain.  If there is any substitute contains a triple bond, it is named as alkynyl such as in this molecule.

The cyclic ring becomes the part of parent chain and IUPAC name of the molecule would be 1-chloro-1-ethynyl-4-bromocyclohexane.  Alkenyne indicates the presence of both double and triple bonds in the parent chain of the molecule.

## Structure and Bonding

We know that alkyne molecules contain at least one triple covalent bond between the carbon atoms of the parent chain of hydrocarbon. The triple bonded carbon atoms are sp-hybridised and show linear geometry. For understanding the structure of alkyne, let’s take the example of simple alkyne with two carbon atoms. The name of this alkyne is ethyne with structural formula CH$\equiv$CH. Each carbon atom is bonded with one hydrogen atom and one other carbon atom in the molecule. Each carbon atom has to make four covalent bonds; two sigma and two pi-bonds.

The atomic number of carbon is 6 with electronic configuration of 1s2, 2s2, 2p2. It has four electrons in its valence shell with 2s2, 2p2 electronic configuration.  Since each carbon atom of ethyne has to make four bonds, it requires four unpaired electrons. Hence the excited state of carbon is 2s1, 2px1, 2py1, 2pz1. For the formation of two sigma bonds, two orbitals; 2s and 2p involve in hybridisation to form two sp-hybrid orbitals. These hybrid orbitals are arranged in a linear manner at 180 bond angle. One of the sp-hybrid orbital gets overlap with another sp-hybrid orbital to form sigma bond. Remaining sp-hybrid orbitals on each of the carbon atom involve in overlapping with 1s orbital of each hydrogen atom to form a C-H bond.

Now for the formation of pi-bonds, the un-hybridized 2py1 and 2pz1 orbitals involve in side-way overlapping with their counter part of another carbon atom and form pi-bonds. Both of these pi-bonds are aligned perpendicular to the plane of carbon atoms. The electron density of pi-bonds lies above and below the plane of sigma bond. Therefore pi-bonds are weaker compared to sigma bond and can easily cleave. The orbital structure of alkyne with their linear geometry is as given below.

## Synthesis

The preparation methods of alkynes are quite similar to alkenes and only requires one step ahead to convert the double bond to triple through elimination reaction.  Let’s have a look on some elimination reactions which are widely used for the preparation of alkynes.

1. Dehydrohalogenation

This reaction involves the elimination of hydrogen and halogen from a dihaloalkane in two steps to form alkyne. The first step leads to the formation of haloalkane which further involves in elimination reaction (dehydrohalogenation) to form alkyne. The reaction can be preceded with germinal dihalide or vicinal dihalide. The reaction requires high temperatures and presence of strong basic solution. For example, 2, 2- dichlorobutane involves in dehydrohalogenation to form 2-butyne.

2. Dehalogenation

The dehalogenation of vicinal tetrahaloalkanes in the presence of zinc metal in an organometallic reaction also forms alkynes.

3. Substitution

In this method the lower alkynes form higher alkynes through an acetylide ion. Since acetylide ions are bases therefore elimination reactions leads to the formation of an alkene from haloalkane. It has limited use due to limitation of side reaction.

## Properties

Like other hydrocarbons, alkynes have low polarity and insoluble in water. They are also soluble in organic solvents like alkanes and alkenes. They are less dense compared to water and usually float on the water surface.  The boiling points of alkynes increase with increasing molecular mass. The boiling points of alkynes are almost same as of corresponding alkane and alkenes.

The sp-hybridization in alkynes allows the uniqueness to the molecule such as non-polar bonding strength, acidity of alkyne and linear geometry. The terminal hydrogen atoms of terminal alkynes are acidic in nature due to presence of triple bond. The sp hybridization makes the hydrogen atom most acidic, hence the terminal alkynes can deprotonate in the presence of stronger bases. High s-character of the triple bonded carbon atom contributes to the electronegativity of the molecule.

These triple bonded hydrocarbons are colourless. Lower the members of alkynes exist in gaseous state such as ethyne, allylene, and crotonylene. For C-5 to C15 alkynes exist in liquid state and higher members of alkynes exist in solid state. Ethyne possesses a pleasant odour in its pure form whereas impure form smells bad due to the presence of phosphine and hydrogen sulphide. Ethyne can liquify at 0$^{\circ}$C at high pressure. It boils at -83.4$^{\circ}$C and melts at -81.8$^{\circ}$C. Like other alkynes, it is slightly soluble in water and highly soluble in organic solvents. It decomposes with increment in volume and liberation of heat.

## Chemical Properties

The chemical properties of both the unsaturated hydrocarbons; alkene and alkyne are almost similar due to the presence of the loosely held pi-electrons in the molecule. Compare to >C=C< bond, the carbon-carbon triple bond is less reactive towards electrophilic reagents. The addition reactions of alkyne furnish in two steps compare to reaction alkenes which occur in one step only to form saturated hydrocarbons. The bond strength of C$\equiv$C is 123 kcal /mole that is two pi-bonds and one sigma bond whereas the bond strength of C=C is 100 kcal / mol.

• High bond strength contributes to less reactivity of the molecule. Let’s discuss some common chemical reactions of alkynes. The most unique chemical reaction of alkyne is the substitution reactions which cannot be shown by alkene and alkanes. For example reactions of ethyne with metal to form metal acetylide in the presence of liquid ammonia. The reaction also involves the liberation of hydrogen gas as given below.
$HC\equiv CH + 2Na \overset{liq. NH_3}{\rightarrow} 2HC\equiv C-Na + H_2$
• In the next step, acidic hydrogen can also replace by metal. Another example is the reaction ethyne with the solution of cuprous chloride in ammonia that leads the formation of cuprous acetylide.
$HC\equiv CH + Cu_{2}Cl_{2} + NH4OH \rightarrow CuC\equiv CCu$  (Copper acetylide)
• We can also prepare silver acetylide with the reaction of ethyne and ammonical solution of silver nitrate.
$HC\equiv CH + NH_{4}OH + AgNO_{3} \rightarrow AgC\equiv C-Ag + 2 HNO_{3}$

These substitution reactions are generally used to distinguish between ethene and ethyne because precipitate of silver acetylide is white and cuprous acetylide is red in color. Another unique chemical reaction of alkynes is polymerisation reaction due to unsaturation in the molecule.

For example the ethyne molecules get polymerise on the red hot iron tube to form benzene. It is one of the preparation methods of benzene. Similarly propyne also polymerises to form aromatic compounds. Usually bulky alkynes cannot show this reaction due to steric hindrance in the polymerisation of molecules.

## Reactions

1. Hydration of alkynes

Like alkenes, alkyne also gives addition reactions such as halogenations, hydrogenation, hydration etc. Since there is one triple covalent bond with two pi-bonds therefore the addition of reagent takes place two times on the same carbon atoms to form saturated compounds. Usually addition reactions of alkyne stop after the first step to form derivatives of alkenes but they can be continued to form alkane derivatives.

The hydration reaction of alkynes occurs in the gaseous state of alkyne and also in the presence of dilute sulphuric acid at 60$^{\circ}$C temperatures. It is a catalytic reaction that forms carbonyl compounds in the presence of mercuric sulphate as a catalyst. In the absence of catalyst, the rate of reaction becomes very slow.  For example hydration of ethyne leads to the formation of ethanal.

It is an electrophilic addition reaction in which alkynes hydrates and form enols as intermediate. Enols further Tautomerism to form carbonyl compounds.  The regioselectivity of the reaction is predicted by the Markovnikov's rule. In the mechanism of the reaction, the protonation of alkyne occurs with the attack of water molecule and forms an oxonium ion. In the second step, the deprotonation occurs in the presence of a base to form an alcohol and the acid catalyst. The acid catalyst re-protonate and form again an oxonium ion. The deprotonation of the oxonium ion forms the carbonyl group.

2. Hydroboration of alkynes

The hydroboration-oxidation reaction of alkynes is an electrophilic addition reaction to form carbonyl compounds. The reaction involves the formation of enols by the reaction of alkyne with boron trihydride followed by reaction with $H_{2}O_{2}$ and NaOH. Enol further reacts with a base and rearranges to carbonyl compound.

The hydroboration of terminal alkynes form aldehydes whereas internal alkynes lead to form ketones. The reaction is an example of oxidation of multiple bonds that follows anti-Markovnikov’s rule to form carbonyl compounds.