On the basis of presence of single and multiple covalent bonds in the aliphatic hydrocarbon, they can be classified as alkanes, alkene and alkynes. Alkanes are saturated hydrocarbons in which all carbon atoms are bonded with single covalent bond. Due to saturation, they are usually giving substitution reactions and less reactive compare to other aliphatic hydrocarbons. Another type of hydrocarbons is unsaturated hydrocarbons which contain either double or triple covalent bond in between at least two carbon atoms. Alkynes have at least one triple bond between two carbon atoms such as ethyne (CH=CH). Alkenes contain at least one double covalent bond between two carbon atoms.
We know in multiple bonds like in a triple bond, there is one pi bond with two sigma bond. Similarly in a double bond, there is one pi bond with one sigma bond. Pi bonds are weaker compared to sigma bond and readily cleave than sigma bond. That is the reason; alkenes and alkynes are more reactive comparable to alkanes which are saturated hydrocarbons.
Alkenes are mainly involved in additional reactions such as hydrogenation, halogenations etc. They can also give some substitution reactions but with less speed compare to addition reactions. Addition reactions play an important role in our daily life also such as hydrogenation of vegetable oil, hydrogenation of alkenes etc. Let’s discuss some common addition reactions of alkenes with their mechanism.
Hydrogenation of alkyne can be done in two ways.
1. Complete reduction or hydrogenation
On reacting with strong reducing agents like palladium, platinum, nickel or rhodium alkynes are reduced to alkanes. For example propyne is reduced to propane.
CH3-C = CH + 2H2 $\to $ CH3-CH2-CH3
The heats of hydrogenation is double that of corresponding alkenes. Like alkenes the heats of hydrogenation depends on nature of branching in alkynes. Due to +I effect the alkyl groups release electrons to carbon atom and hence the heats of hydrogenation is reduced. So the heats of hydrogenation of 1-pentyne will be more than that of 2-pentyne.
CH3-CH2-CH2-C = CH > CH3-CH2-C = C-CH3
2. Partial reduction or hydrogenation
Alkynes on partial reduction with poor catalysts give alkenes. The partial reduction of alkynes can be done in two ways.
- By reacting with Palladium in Calcium carbonate / Palladium in barium sulphate. Alkynes on treating with palladium in barium sulphate give cis alkenes. For example 2-Butyne on reacting with palladium in barium sulphate give cis 2-Butene.
CH3-C = C-CH3 + H2
- By reacting with alkali metal in liquid ammonia. Alkynes on treating with alkali metals in liquid ammonia give trans alkenes. For example 2-butyne on reacting with sodium in liquid ammonia give trans-2-butene.
CH3-C = C-CH3 + H2
Alkynes undergo addition with halogen to give tetra halo derivatives. For example acetylene on reacting with chlorine give 1,1,2,2 tetra chloro ethane (Westron) which on further reacting with alcoholic KOH give westrol ( 1,1,2 trichloro ethene). Westron and westrol are used as solvents for organic compounds.
CH ≡ CH + 2Cl2 → CHCl2-CHCl2Similarly alkynes also add with bromine water in carbon disulphide. This is the characteristic test to determine unsaturated compounds as the color of the bromine water will be decolorized.
CHCl2-CHCl2 + KOH → CHCl=CCl2
Alkynes on reacting with hydrogen chloride give vinyl chloride which may undergo polymerization to give polyvinyl chloride or it can undergo addition with HCl again to give ethyledene chloride. For example the reaction of acetylene with hydrogen chloride is given below.
CH ≡ CH + HCl → CH2 = CH-Cl
CH2 = CH-Cl → [CH2-CH(Cl)-]n or
CH2 = CH-Cl + HCl → CH3-CH-Cl2
Alkynes react with water in the presence of HgSO4
to give a alcohol with double bond. This alcohol is called as enol (en for double bond and ol for alcohol). This reaction is called as Kocharov's reaction. For example acetylene on hydrolysis with mercuric sulphate gives an enol as given below.
CH ≡ CH + H2O → CH2=CH-OH
This enols are highly unstable and undergo migration to give carbonyl compounds ( aldehydes and ketones). The migration of enol to carbonyl compound and vice versa is called tautomerism, a type of isomerism. Some type of enols are stable and acidic and can be isolated by adding suitable base. For example the enol formed due to hydrolysis of acetylene changes to acetaldehyde as given below.
CH2 = CH-OH ↔ CH3-CHOSimilarly 2-Butyne on hydrolysis followed by tautomerism gives butanone.
CH3-C ≡ C-CH3 + H2O → CH3-CH = C(OH)-CH3
CH3-CH = C(OH)-CH3 ↔ CH3-CH2-CO-CH3
Alkynes on ozonolysis gives ozonides which on further hydrolysis give carboxylate ions. From the nature of carboxylate ions the position and nature of triple bond in an organic compound can be determined. For example the ozonolysis of propyne is given below.
In the first step the ozone adds with alkyne to give ozonide which on further hydrolysis gives carboxylic acids.
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Alkynes undergo two type of polymerization reaction.
1. Linear polymerization
Acetylene on linear polymerization give vinyl acetylene which on further polymerization give polyvinyl acetylene. This is linear polymerization.
CH = CH $\to$ [-CH = CH-]n
2. Cyclic polymerization
Acetylene when passed through red hot tube undergoes cyclic polymerization to give benzene.
3 CH=CH ? C6H6
Similarly propylene on cyclic polymerization gives mesitylene.
The triple bond between carbon atoms can be easily oxidized by oxidizing agents like alkaline potassium permanganate to give carboxylic acids. The alkaline potassium permanganate is called Bayer's reagent and it is decolorized when acetylene gas is bubbled through the Bayer's solution.
CH ≡ CH → COOH-COOH
The reaction of alkynes can be summarized below.
- Alkynes on adding with hydrogen undergo hydrogenation to give alkanes. The heats of hydrogenation is more than alkenes. Similarly the partial hydrogenation give alkenes.
- Alkynes can undergo addition reaction with 2 moles of chlorine and hydrogen halides to give addition product.
- Similarly they can undergo oxidation reaction with alkaline potassium permanganate to give carboxylic acids.
- Alkynes can undergo linear and cyclic polymerization to give polymers.
- Alkynes can react with ozone to give ozonides which on hydrolysis give carboxlic acids. This reaction can be used to identify the position and nature of triple bond.
- Alkynes add with water in the presence of mercuric sulphate to give enols (an organic compound containing both alcohol group and double bond). This enols are unstable and undergo tautomerism to give carbonyl compounds (ketones or aldehydes). The presence of enol form can be proved by separating it with the help of a strong base.
In an addition reaction atom/groups are added across an unsaturated bond (double/triple) in an organic compound. Alkynes have a triple bond in their structure and can add 2 moles of compound to become saturated. This addition reaction can be of following types.
- Addition of hydrogen (hydrogenation)
- Addition of halogen (halogenation)
- Addition of hydrogen halides
- Addition of water (hydration)
As alkynes are rich source of π electrons, all the addition reactions stated above are electrophilic addition reactions.