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Alkynes are organic compounds that are associated with the carbon-carbon triple bond in any unbranched sites with a given chain of unsaturated hydrocarbons. The number of hydrogen bonds in alkynes is comparatively less to alkanes and alkenes. The general Empirical formula for alkynes is Cn H2n-2. The first stable member of this particular series is Ethylene. The addition of hydrogen takes place in the case of alkynes due to the presence of pi electrons that are loosely held in the alkyne. The presence of a triple bond helps in the addition of different halogens, hydrogen, water, etc by the process of “Addition reaction”.
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Key terms: Alkynes, Halogens, Hydrogen, Water, Addition reaction, Halogenation, Hydrogenation, Hydration, Hydrohalogenation
Addition reaction of alkyne with Halogen (halogenation)
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Alkynes and halogens react together to form halogenated alkene which further reacts with the halogen to form halogenated alkane.
Reaction: Halogenation
The addition of halogen to an alkyne occurs in the same manner as it would occur for an alkene. The stability of the vinylic cation plays an important role in this process as this determines the stability of the addition reaction’s products. The halogenation of alkynes is an example of an electrophilic addition reaction.
The halogenation of alkynes results in the formation of tetrahalogenated alkanes.
Most halogenation processes can be stopped in the intermediate stage by lowering the temperature to 0° C, unlike hydrogenation mentioned below.
Read more: haloalkanes
Addition reaction of alkynes with Hydrogen (Hydrogenation)
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Hydrogenation of alkynes occurs in the presence of catalysts like Platinum, Palladium, or Nickle to form alkenes which further react with dihydrogen to form alkanes.
Reaction: Hydrogenation
Slowing this process of hydrogenation in the intermediate stage is considered to be impossible since the reaction is very smooth. The alkene formed in the intermediate stage can although, be isolated using poisoned catalysts such as the Lindlar catalyst.
Addition reaction of alkynes with H2O (Hydration)
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Alkynes do not react with water under normal conditions just like alkanes and alkenes (immiscible). Hence hydration or the addition reaction of alkynes with water occurs in the presence of dilute sulphuric acid and mercuric sulphate at the temperature of 333K.
Reaction: Hydration
Hydration of carbonyl compounds results in the formation of carbonyl compounds (aldehydes or ketone). Addition reaction of terminal alkyne results in the formation of aldehyde whereas the addition reaction of internal alkyne with water results in the formation of ketones.
Read more: uses of ethanol and methanol
Addition reaction of hydrogen halides (hydrohalogenation)
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Hydrogen halide when reacted with an alkyne, results in the formation of a gem halide. Gem halides are basically compounds, wherein two halogen atoms are attached to the same carbon atom in the molecule. This is one of the simplest electrophilic addition reactions. This process follows Markovnikov’s rule.
Hydrohalogenation
The reactivity order of hydrogen halide is HI > HBr > HCl > HF
Markovnikov’s rule
This rule dictates that the addition of a hydrogen halide (HX, in the case of HBr) to an alkene will lead to the formation of a product where the hydrogen is attached to the carbon with fewer alkyl substituents, while the halide group is attached to the carbon with more alkyl substituents.
Read more: Ester hydrolysis
Things to remember
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- Alkynes are a class of organic compounds where there is a triple bond exists between two carbons.
- Alkynes undergo reactions due to the loosely held pi electrons
- The empirical formula of alkynes is CnH2n-n.
- The electrophilic additions to alkynes are slow and generally display Markovnikov Rule, regioselectivity, and anti-stereoselectivity.
- Gem halides are formed in the addition reaction of alkynes with hydrogen halide.
Sample Questions
Ques. Why do alkynes exhibit addition reactions? (2 marks)
Ans. The presence of loosely held pi-electrons enables alkynes to undergo addition reactions. Also, the presence of a triple bond contributes to the fact that addition reactions can occur in alkynes.
Ques. Explain the Markovnikov rule. (3 marks)
Ans. This rule dictates that the addition of a hydrogen halide (HX) to an alkene will lead to the formation of a product where the hydrogen is attached to the carbon with fewer alkyl substituents, while the halide group is attached to the carbon with more alkyl substituents.
Ques. What is an electrophilic addition reaction with an example? (3 marks)
Ans. Electrophiles are the electron deficiency compound. In the alkyne, only one sigma bond and two pi bonds between two carbon atoms. These pi bonds are electron-rich species; they form bonds with electrophiles to form electrophilic addition reactions.
Example: halogenation, hydration reaction.
Ques. How is hydration of alkyne done? (4 marks)
Ans. Alkynes do not react with water under normal conditions just like alkanes and alkenes (immiscible). Hence hydration or the addition reaction of alkynes with water occurs in the presence of dilute sulphuric acid and mercuric sulphate at the temperature of 333K. They react with water under the above-mentioned conditions to form enols that spontaneously tautomerize to ketones.
Ques. What is hydrogenation? (2 marks)
Ans. Dihydrogen gas gets added to alkenes/alkynes in the presence of finely divided catalysts like Pt, Pd, or Ni to form alkanes. This process is called hydrogenation.
Ques. What is a Lindlars’ catalyst? (1 mark)
Ans. Partially deactivated palletized charcoal is known as Lindlar’s catalyst.
Ques. What is the use of an addition reaction? (2 marks)
Ans. In the case of an 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 oil is an unsaturated compound added by hydrogen presence of a catalyst to form saturated compounds of vegetable ghee.
Ques. Write the general formula for alkynes, alkenes, and alkanes. What are the suffixes that are used for these? (3 marks)
Ans. Cn H2n-n is the general formula of alkynes and the suffix used for these is “-yne”. CnH2n is the general formula for alkenes and the suffix used for these is “-ene”. CnH2n+n is the general formula for alkanes and the suffix used for these is “-ane”.
Ques. What is the difference between addition and elimination reactions? (2 marks)
Ans. 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.
Ques. Ethyne is acidic in nature in comparison to ethene and ethane. Why is it so? (3 marks)
Ans. Hydrogen atoms in ethyne are attached to the sp hybridized carbon atoms whereas they are attached to sp2 hybridized carbon atoms in ethene and sp3 hybridized carbons in ethane. Due to the maximum percentage of s – character (50%), the sp hybridized orbital’s of carbon atoms in ethyne molecules have the highest electronegativity; Which attracts the shared pair of the C-H bond of ethyne to a greater extent than that of the sp2 hybridized orbital’s of carbon in ethene and the sp3 hybridized orbital of carbon in ethane. Thus, in an ethyne molecule, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane.
Ques. Explain the bond structure in Alkynes. (2 mark)
Ans. An alkyne is an unsaturated hydrocarbon with at least one carbon-carbon triple bond in organic chemistry. The simplest acyclic alkynes form a homologous series with the generic chemical formula CnH2n-2.since they only have one triple bond and no other functional groups.
Ques. What are polyhalogenated compounds? (3 marks)
Ans. Compounds that contain multiple halogen atoms are called polyhalogenated compounds. These compounds are mostly used in industries and are only created for industrial purposes. These compounds are generally toxic and bioaccumulated in humans. DDT is the most common polyhalogenated compound. It is dichloro diphenyl trichloroethane and is a very famous insecticide. Some other examples are - Freons(CFC), CCl4, Iodoform, etc.
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