1,3-Dibromopropane is a compound with the molecular formula C3H6Br2. When it reacts with metallic zinc, it undergoes a substitution reaction resulting in the formation of cyclopropane.

Explanation:
1. Substitution Reaction:
- Substitution reactions occur when one or more atoms or groups in a molecule are replaced by another atom or group.
- In this case, the bromine atoms (Br) in 1,3-dibromopropane are replaced by the zinc (Zn) atoms.

2. Zinc as a Reducing Agent:
- Metallic zinc (Zn) acts as a reducing agent in this reaction.
- A reducing agent donates electrons and gets oxidized in the process.
- In the reaction, zinc reduces the bromine atoms in 1,3-dibromopropane by donating electrons.

3. Formation of Cyclopropane:
- The reaction between 1,3-dibromopropane and zinc leads to the formation of cyclopropane.
- Cyclopropane is a three-membered ring compound with the molecular formula C3H6.
- The reaction involves the removal of the bromine atoms and the formation of the cyclopropane ring.

4. Mechanism of the Reaction:
- The reaction proceeds through a free radical mechanism.
- Initially, the zinc activates one of the bromine atoms in 1,3-dibromopropane by forming a zinc-bromine bond.
- The activated bromine atom then undergoes homolytic cleavage, resulting in the formation of a bromine radical.
- The bromine radical then abstracts a hydrogen atom from another molecule of 1,3-dibromopropane, forming a propyl radical.
- The propyl radical undergoes ring closure to form cyclopropane.

Overall, the reaction can be represented as follows:
1,3-Dibromopropane + Zinc → Cyclopropane + Zinc Bromide

In conclusion, when 1,3-dibromopropane reacts with metallic zinc, it undergoes a substitution reaction resulting in the formation of cyclopropane. The reaction proceeds through a free radical mechanism, with zinc acting as a reducing agent.