What is the addition of alkynes?
The chemistry associated with carbon-carbon triple bonds can be called as alkyne chemistry. Alkynes undergo addition reactions due to the presence of loosely held pi-electrons. Due to the presence of a triple bond in alkynes, halogens, water etc.can be added to them by the process of the addition reaction. Addition products are formed via a set of steps. The stability of vinylic cations is responsible for the formation of addition products. Markovnikov’s rule is followed by asymmetric alkynes in order to undergo addition reaction. Few addition reactions of alkynes are explained below:
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Addition of alkynes with halogens (halogenation)
Alkynes and halogens undergo addition reaction to form halogenated alkenes which further react with halogens to give halogen substituted alkanes. The reddish orange coloured solution of bromine and carbon tetrachloride gets decolorized as a result of the addition reaction. This is used as a test for unsaturation.
Unlike the hydrogentaion reaction mentioned above, most halogenation reactions can be stopped in the intermediate stage by lowering the temperature below 0oC.
Addition of alkynes with hydrogens (hydrogenation)
Alkynes react with dihydrogen in the presence of catalysts such as Pt/Pd/Ni in order to form alkenes. The alkenes formed, further react with dihydrogen to form alkanes. It has been observed that in most reactions triple bond is converted into double bond and double bond is then converted into a single bond due to the addition reaction with dihydrogen.
The catalyst that are involved in the case of rhodium, nickel, palladium, and platinum. Hydrogenation is a step-by-step process in which initially an alkene is formed. After which it undergoes further hydrogenation to form an alkane.
Slowing down the reaction in the intermediate stage is actually quite impossible as the whole reaction is really smooth. But some alkenes are isolated with the use of poisoned catalysts. One such example of a poisoned catalyst is Lindlar catalyst.
Lindlar catalyst
Lindlar catalyst is combined form of palladium which is coated with quinoline and absorbed on calcium carbonate.
Addition of alkynes with water (hydration)
Alkynes are immiscible in water. They do not react with water under normal conditions. Alkynes may react with water in the presence of dilute sulphuric acid and mercuric sulphate at a temperature of 333K. This results in the formation of carbonyl compounds.
Addition of alkynes with hydrogen halides (hydrohalogenation)
When hydrogen halide is treated with alkynes (triple bond compounds) it results in the formation of gem halides. Gem halides are the compounds in which two halogens are attached to the same carbon atoms in a molecule.
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Frequently Asked Questions – FAQs
Why do alkynes show addition reactions ?
Alkynes undergo addition reactions due to the presence of loosely held pi-electrons. Due to the presence of a triple bond in alkynes, halogens, water etc.can be added to them by the process of the addition reaction.
What is electrophilic addition reaction with example?
Electrophiles are the electron deficiency compound. In the alkyne there are one sigma bond and two pi bonds between two carbon atoms. These pi bonds are electron rich species; they form bonds with electrophile to form electrophilic addition reactions.
Example: halogenation, hydration reaction.
What is the use of addition reaction?
In case of addition reaction unsaturated compounds are added by a group of atoms or molecules to form saturated compounds. It is widely used to synthesize all kinds of natural products and drugs. In industrial application vegetable oils is an unsaturated compound added by hydrogen presence of catalyst to form saturated compounds of vegetable ghee.
What is the difference between addition and elimination reactions?
An addition occurs when two or more compounds combine to form a single product. In this case unsaturated compounds convert to saturated compounds. An elimination reaction occurs when a reactant is broken up into two products. In this case generally saturated compounds convert to unsaturated compounds.
How is hydration of alkyne done?
Alkynes are immiscible in water. They do not react with water under normal conditions. So in the presence of dilute sulphuric acid and mercuric sulphate catalyst alkynes react with water to form enols that spontaneously tautomerize to ketones.