Stepwise Mechanism of Hydration and Other Alkynes Addition Reactions
First, make sure you know what you are reading and pronounce it correctly. Alkynes are different from an alkane, and Alkene. Now coming to the main point, what are Alkynes? Well, in chemistry there are ways one can form a bond with carbon but when two carbon molecules form a triple bond in any of the unbranched sites with the given carbon chain of an unsaturated hydrocarbon are called alkynes. In the old times, alkynes used to be called acetylenes. On the other hand, when it comes to naming ethyne using IUPAC nomenclature we call it acetylene. The empirical formula of carbon-carbon triple bonds of alkynes are CnH2n-2. All the alkenes naming has to end with -ene and, and -yne in the case of alkynes
The addition of hydrogen takes place in alkynes because pi electrons that are present in them are loosely held. Also, with the help of triple bonds, we can add different halogens, and water to alkynes with the help of addition reactions. Today we are going to learn about different alkyne addition reactions, some of the popular ones are the addition of hydrogen and the addition of halogens to alkenes.
Addition of Halogens to Alkenes
Halogens are quite reactive, and it is not feasible to find a halogen element in nature. The most abundant combined form of halogens is fluorine. Likewise, the Astatine and tennessine both of these halogens don’t occur in nature in any form possible, because they contain short-lived radioactive isotopes.
When we check the properties of different halogens, we can find out that each of them has a great resemblance to each other in terms of their chemical behaviour. But still, there are some changes in their chemical properties which can be noted as we go from fluorine to iodine.
When it comes to alkenes and their addition with halogens, as they consist of pi bonds we can add two halogen molecules in it. Due to the fact that di-halogen Alkene is much less reactive during the addition reaction. Individual alkyne molecule gets a halogen molecule when we apply a halogen equivalent to one molar. When we apply two or more than two equivalents of halogens, we form tetra-halogen alkane.
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Now, you must have a question in mind about the reactivity of di-halogen in addition reaction, why is it less than alkynes? Well, the answer is if we compare the two alkenes and alkynes. The first one is certainly more reactive for the reaction that uses electrophilic halogenations.
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Furthermore, the product of given alkyne dehalogenation gives out di-halogen Alkene. As a result, we can perform the selective dehalogenation of alkynes. The electron deficiency of pi-electron clouds is more in a di-halogen alkene. Therefore, these di-halogen alkenes have -I effect on their halogen substituents and become less nucleophilic. This is your addition of halogen to alkynes.
Hydration Reaction of Alkynes
When you see the word hydration, you know water is being added in the system. A classic example of hydration is when a given salt such as sodium chloride gets dissolved in water the hydration of sodium and chloride ions take place. In the same way, the hydration reaction of alkynes is one of the most common alkyne reactions to date.
If you look at the hydration of alkynes and hydration of alkenes, both have the same steps at the very beginning of the reaction. In both these reactions, you need to add water molecules first. But after that alkynes form enol, an alcohol bond with vinyl carbon as the hydration reaction proceeds, the enol present in the reaction quickly from carbonyl groups such as aldehydes and ketones. The formation of carbonyl groups takes place due to the quick tautomerization of enols. In simple terms, this whole reaction with the hydration of alkynes is “called enol-keto.”
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One more thing you should know, in order to add water in the alkynes you need to have strong acid, some of the most common acids used for the hydration reaction of alkynes are sulfuric acid along with usage of mercuric sulfate which works as a catalyst.
FAQs on Addition Reaction of Alkynes: Detailed Mechanism & Study Guide
1. What is an addition reaction of alkynes?
An addition reaction of alkynes is a chemical process where atoms are added across the carbon-carbon triple bond. Alkynes are unsaturated hydrocarbons containing at least one triple bond (one strong sigma bond and two weak pi bonds). During this reaction, the weaker pi bonds break, allowing new atoms or groups to attach to the carbon atoms, eventually converting the alkyne into an alkene and then an alkane.
2. What are the most common types of addition reactions that alkynes undergo?
Alkynes can undergo several types of addition reactions, depending on the reagent used. The most common ones covered in the CBSE syllabus are:
- Addition of Dihydrogen (H₂): This reaction, also called hydrogenation, converts alkynes to alkenes and then to alkanes.
- Addition of Halogens (Cl₂, Br₂): Halogenation results in the formation of dihaloalkenes and then tetrahaloalkanes.
- Addition of Hydrogen Halides (HCl, HBr, HI): This reaction follows Markovnikov's rule for unsymmetrical alkynes.
- Addition of Water (H₂O): Hydration of alkynes leads to the formation of carbonyl compounds like aldehydes or ketones.
3. Can you explain the addition of bromine to ethyne with an example?
Certainly. When ethyne (CH≡CH) reacts with bromine (Br₂), the reddish-brown color of bromine disappears, indicating a reaction has occurred. In the first step, one molecule of bromine adds across the triple bond to form 1,2-dibromoethene (CHBr=CHBr). If more bromine is available, a second molecule can add across the double bond to form 1,1,2,2-tetrabromoethane (CHBr₂-CHBr₂).
4. Why is Markovnikov's rule important when adding hydrogen halides to unsymmetrical alkynes?
Markovnikov's rule is crucial because it helps predict the major product formed in the reaction. The rule states that when a hydrogen halide (like HBr) adds to an unsymmetrical alkyne (like propyne), the hydrogen atom attaches to the carbon atom of the triple bond that already has more hydrogen atoms. This is because this pathway forms a more stable carbocation intermediate, which leads to the formation of the primary, or major, product.
5. What is the key difference between the addition reactions in alkenes and alkynes?
The key difference lies in the number of pi bonds available for reaction. Alkenes have one pi bond and undergo addition in a single stage to become saturated. Alkynes have two pi bonds, so their addition reactions can occur in two successive steps. The first addition breaks one pi bond to form an alkene derivative, which can then undergo a second addition reaction to become fully saturated.
6. How does the addition of water to an alkyne result in a carbonyl compound instead of an alcohol?
This is a great question that involves a process called tautomerism. When water adds to an alkyne (in the presence of an acid and mercuric sulphate catalyst), it initially forms an unstable intermediate called an enol, which has a hydroxyl (-OH) group attached to a double-bonded carbon. This enol immediately rearranges itself into a more stable form called a keto form. This rapid rearrangement, or tautomerism, results in the final product being a carbonyl compound (an aldehyde or a ketone) rather than an alcohol.
