Question

The negative part of the addendum (the molecule to be added) adds on the carbon atom of the double bond containing the least number of hydrogen atoms. This rule is known as?

Answer 1

Markaunikov rule

• The rule states that the negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms.

Example

Mechanism

• When propene reacts with hydrogen bromide, two products are possible as shown in the above diagram.
• When ${\text{H}}^{+}$ attack on alkene, the formation of two carbocation takes place, that is (a) ${\mathrm{CH}}_3-\mathrm{CH}-\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2$, and(b) ${\mathrm{CH}}_3-\stackrel{+}{\mathrm{C}}\mathrm{H}-{\mathrm{CH}}_3$.
• The secondary carbocation (b) is more stable than the primary carbocation (a).
• Therefore, the secondary carbocation predominates because it is formed at a faster rate.
• The carbocation (b) is attacked by ${\text{Br}}^{–}$ ion to form the product as follows :

So, the given rule is known as the Markaunikov rule.