An addition reaction over alkene causes

Options

A. Increase of unsaturation number in product W.r.t. reactant
B. Decrease of unsaturation number in product w.r.t reactant
C. Formation of new bonds without breaking any bond
D. Both (2) and (3)

Answer

To solve the question regarding the effect of an addition reaction on an alkene, let's break it down step by step.

### Step-by-Step Solution:

1. **Understanding Addition Reactions**:
An addition reaction involves the addition of atoms or groups to a molecule, typically across a double bond (π bond) in alkenes.

2. **Example of Addition Reaction**:
Consider the example of ethene (C2H4) reacting with HCl. In this reaction, the π bond between the two carbon atoms in ethene breaks, allowing new bonds to form.

3. **Breaking of the π Bond**:
When HCl is added to ethene, the π bond breaks. One carbon atom will bond with a hydrogen atom (from HCl), and the other carbon will bond with a chlorine atom (from HCl). This results in the formation of a new molecule, chloroethane (C2H5Cl).

4. **Unsaturation Number**:
The unsaturation number is defined as the total number of π bonds and cyclic rings in a molecule. In ethene, there is one π bond, so the unsaturation number is 1. After the addition of HCl, chloroethane has no π bonds, so its unsaturation number is 0.

5. **Change in Unsaturation Number**:
Since the π bond is broken during the addition reaction, the unsaturation number decreases from 1 (in ethene) to 0 (in chloroethane). Therefore, we can conclude that addition reactions decrease the unsaturation number.

6. **Formation of New Bonds**:
In the addition reaction, two new σ bonds are formed (one C-H bond and one C-Cl bond) as a result of breaking the π bond.

7. **Conclusion**:
The correct statement regarding the addition reaction over an alkene is that there is a decrease in the unsaturation number due to the breaking of the π bond and the formation of new σ bonds.

### Final Answer:
The addition reaction over an alkene causes a decrease in the unsaturation number.