Explanation:

Vinyl chloride and chlorobenzene are organic compounds containing chlorine as a substituent. Chlorine is a highly electronegative atom and has a strong tendency to withdraw electron density from the adjacent carbon atom. This effect is responsible for the low reactivity of these compounds. However, the reason for this effect is different for both compounds.

Positive Mesomeric Effect in Vinyl Chloride:

In vinyl chloride, the carbon atom adjacent to the chlorine atom is sp2 hybridized and has a p orbital perpendicular to the plane of the molecule. This p orbital can overlap with the chlorine 3p orbital, resulting in the formation of a pi bond between the two atoms. The electron density from the pi bond is delocalized over the entire molecule, including the chlorine atom. This delocalization of electron density is called the mesomeric effect.

In the case of vinyl chloride, the mesomeric effect is positive, i.e., it increases the electron density on the adjacent carbon atom. This makes the carbon atom less reactive towards electrophiles. Thus, vinyl chloride is less reactive than an alkene towards electrophilic addition reactions.

Negative Mesomeric Effect in Chlorobenzene:

In chlorobenzene, the chlorine atom is directly attached to the benzene ring. The benzene ring is a highly conjugated system with delocalized pi electrons. When the chlorine atom is attached to the ring, it withdraws electron density from the ring through the mesomeric effect. This results in a decrease in electron density on the ring and makes it less reactive towards electrophiles.

In the case of chlorobenzene, the mesomeric effect is negative, i.e., it decreases the electron density on the ring. This makes the ring less reactive towards electrophiles. Thus, chlorobenzene is less reactive than benzene towards electrophilic substitution reactions.

Conclusion:

In conclusion, the low reactivity of vinyl chloride and chlorobenzene towards electrophiles is due to the mesomeric effect of the chlorine atom. In vinyl chloride, the positive mesomeric effect increases the electron density on the adjacent carbon atom, while in chlorobenzene, the negative mesomeric effect decreases the electron density on the benzene ring.

Both in vinyl chloride and chlorobenzene there is positive mesomeric effect or positive resonance effect in which chlorine is resonting with its lone pair and give electrons to molecule . resonance give stability so they are less reactive due to plus m and plus r effect