1,3butadiene when treated with HBr of 1eq gives

1,3-Butadiene is a conjugated diene. When it reacts with 1 equivalent of HBr, it undergoes an electrophilic addition reaction. Here are the steps:

1. The HBr molecule approaches the 1,3-butadiene. The π electrons of the double bond are attracted to the hydrogen atom of HBr, which is electrophilic due to the polar bond between hydrogen and bromine.

2. The π electrons form a bond with the hydrogen atom, breaking the H-Br bond. This results in the formation of a carbocation at the second carbon atom (since it's more stable due to resonance) and a bromide ion.

3. The bromide ion, which is a nucleophile, attacks the carbocation, forming a bond with the carbon atom.

4. The final product is 3-bromo-1-butene.

This reaction follows Markovnikov's rule, where the hydrogen atom adds to the least substituted carbon of the double bond (the carbon with the most hydrogen atoms), and the bromine atom adds to the most substituted carbon.