Question

3) Write notes on 1. Markovnikoy's rule 3. Sandineyer Gattermann reaction​

Answer 1

Answer:

When a protic acid (HX) is added to an asymmetric alkene, the acidic hydrogen attaches itself to the carbon having a greater number of hydrogen substituents whereas the halide group attaches itself to the carbon atom which has a greater number of alkyl substituents.

Explanation:

To understand this mechanism, let us consider the same example illustrated earlier, i.e. the addition reaction of hydrobromic acid with propene. The Mechanism of Markovnikov’s rule can be broken down into the following two steps.

Step 1

The alkene is protonated and it gives rise to the more stable carbocation as shown below.

From the illustration shown above, we can see that there are two types of carbocations that can be formed from the protonation of the alkene, one is a primary carbocation and the other is a secondary carbocation. However, the secondary carbocation is far more stable and therefore, its formation is preferred over the formation of a primary carbocation.

Step 2



The halide ion nucleophile now attacks the carbocation. This reaction yields the alkyl halide. Since the formation of the secondary carbocation is preferred, the major product of this reaction would be 2-bromopropane as illustrated below.

It is important to note that the Markovnikov’s rule was developed specifically for its application in the addition reaction of hydrogen halides to alkenes. The opposite of  ‘Markovnikov’ addition reactions can be described as Anti-Markovnikov based on the regioselectivity of the reaction.

Examples of Markovnikov and Anti-Marknovnikov Addition Reactions

The Hydration of Alkenes

When alkenes are treated with certain aqueous acids (usually sulfuric acid), the resulting electrophilic addition reaction yields an alcohol as the product. The regioselectivity of such reactions can be predicted by Markownikoff’s rule. Therefore, these reactions can be classified as Markovnikov reactions. In the hydration of alkenes, the H+ ion acts as an electrophile and attacks the alkene to generate a carbocation intermediate (the intermediate with greater stability is protonated). The subsequent nucleophilic attack on the carbocation by water molecules forms an oxonium ion, which is deprotonated to afford the required alcohol product.

Gattermann reaction:

Formylation of aromatic compounds with HCN in presence of AlCl3 is carried out. We can also use Zn(CN)2 instead of HCl/AlCl3.

C6H6(i)HCN/AlCl3(ii)H2O,−NH4ClC6H5−CHO

(b) Sandmeyer reaction:

It converts aryl diazonium salts to aryl halides. An aromatic amino group is substitued with a nucleophile (such as halide anions, cyanide, thiols, water, and others) via diazonium salt intermediate. The reaction is  catalyzed by cuprous compounds.  

PhN2+Cl−CuCl−N2Ph−Cl

Answer 2

1 Markovnikov rule, in organic chemistry, a generalization, formulated by Vladimir Vasilyevich Markovnikov in 1869, stating that in addition reactions to unsymmetrical alkenes, the electron-rich component of the reagent adds to the carbon atom with fewer hydrogen atoms bonded to it, while the electron-deficient component

3 the key difference between Sandmeyer reaction and Gattermann reaction is that the Sandmeyer reaction refers to the synthesis of aryl halides from aryl diazonium salts in the presence of copper salts as a catalyst, whereas Gattermann reaction refers to the formylation of aromatic compounds in the presence of a Lewis