Addtion of Halogens and Related Reagents

Halogens are the non-metallic elements from group 17 of the periodic table. Because of the presence of a triple bond in alkynes, halogens, water, and many others can be brought to them via the technique of the addition reaction. The steadiness of vinylic cations is responsible for the formation of addition products. Markovnikov’s rule is followed by means of asymmetric alkynes on the way to undergo addition response. Few additional reactions of alkynes are explained below-:

Addition of alkynes with halogens (halogenation)

Alkynes and halogens undergo additional reactions to form halogenated alkenes which similarly react with halogens to offer halogen-substituted alkanes. The solution of bromine and carbon tetrachloride gets decolourized. It is because of an addition reaction that is used as a check for unsaturation.

Addition of alkynes with hydrogens (hydrogenation)

Alkynes react with dihydrogen in the presence of catalysts which includes Pt/Pd/Ni in order to form alkenes. The alkenes formed, similarly react with dihydrogen to form alkanes. It had been found that in maximum reactions triple bond is converted into a double bond and the double bond is then converted into a single bond due to the addition reaction with dihydrogen. The catalyst is involved in the case of rhodium, nickel, palladium, and platinum. Hydrogenation is a step-through-step technique wherein to begin with an alkene is formed. And then it undergoes similar hydrogenation to form an alkane.

Functional groups 

Here we will discuss the topic, How Alkynes Form Other Functional Groups in detail. 

• Brominate form a tetrabromide -: 

Bromine reacts in alkynes to make the dibromide via the same mechanism as bromine addition to alkenes (suppose “bromonium ion”). Due to the fact that there are pi bonds in an alkyne, equivalents of bromine may be introduced to make a tetrabromide, as proven here.

• Saturate an alkyne to form an alkane -:

Alkynes can also be decreased to alkanes with the aid of effervescent two equivalents of hydrogen gas (H2) over the alkyne in the presence of a metal catalyst as shown within the next figure.. This catalyst is usually palladium on carbon (Pd/C), however, platinum (Pt) is also very often used.

Ralph Raphael

Ralph Alexander Raphael CBE FRS FRSE (1 January 1921 – 27 April 1998) was a British organic chemist, famous for his use of acetylene derivatives in the synthesis of natural products with biological activity.

• Synthesis of natural products -:

Raphael studied many natural products, in particular the type that had been biologically active and which would offer a venture for synthesis but might be the sensible goal of an unmarried Ph.D. student’s thesis. He and his college students published a synthesis of 2-deoxyribose, etc. Raphael also evaluated the composition of the wax coating of plant leaves and talked about the hydrocarbons of which they were composed. 

• Molecules of theoretical interest -: 

Raphael turned into interested in molecules of theoretical, in addition to realistic, interest. In 1951, co-labored and co-authored with J. W. cook dinner and A. I. Scott, he posted the first synthesis of the quasi-aromatic compound tropolone and the thujaplicin herbal products which contained this uncommon ring system. His interest in acetylenes led him to observe macrocyclic compounds containing this practical organization and bridged ring structures that might be derived from them. 

Haloalkanes and Haloarenes-

There are different ways to prepare the haloalkanes and haloarenes. These are prepared from alcohols, exchange reactions of halogen, hydrocarbon, and alkenes by adding hydrogen halides and halogens. 

• Preparing them from alkenes-

It is done by adding halogens in the double bond of the alkene. It can also be done by the addition of hydrogen halide. Here, a halogen can be any element of the periodic table under the category of halogens like bromine, iodine, etc.

By the process of an electrophilic addition reaction, alkenes can be converted into haloalkanes. Alkenes when reacted with HX form R-X. Here, we will obtain major as well as minor quantities. The reaction follows Markovnikov’s rule of the addition reaction in order to find the major product. It is done by the addition reaction along with the double bond of an alkene.

• Preparing from halogen exchange reaction-

Here, an alkyl chloride reacts with sodium iodide in acetone forming alkyl iodides. It is referred to as an equilibrium reaction. Alkyl halide in acetone is helpful in carrying out the reaction. 

• Preparing from hydrocarbons- 

It is done by different methods. It includes- halogenation of haloalkanes, electrophilic substitution reaction, and Sandmeyer reaction. 

• Preparation from alcohols–

The easiest way to prepare haloalkane. R-OH when reacted with reagents, the resultant will be the formation of R-X. We can use the reagents like concentrated halogens acids, phosphorus halides, and thionyl chloride.

Conclusion -:

Through their reactive ability, alkynes are in many ways similar to alkenes, but there are also some different specific reactions for them that are the only characteristics of compounds having a triple bond. Alkynes include the reactions of substitution, attachment, oxidation, and polymerization.