[Chemistry Class Notes] on Addition Reaction of Alkynes Pdf for Exam

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 C_nH_2n-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. 

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.  

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.” 

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.