Order of Reactivity towards E2 Elimination with a Strong Base

To determine the correct order of reactivity towards E2 elimination with a strong base, we need to consider the stability of the carbocations formed during the reaction. The more stable the carbocation, the faster the reaction will proceed.

The reactivity of alkyl halides towards E2 elimination can be influenced by factors such as the nature of the halogen, the position of the halogen substituent, and the presence of other substituents on the carbon chain.

Let's analyze each option and determine the correct order of reactivity:

a) 1-chlorobutane vs 2-chlorobutane
- In 1-chlorobutane, the chlorine atom is attached to a primary carbon, whereas in 2-chlorobutane, the chlorine atom is attached to a secondary carbon.
- The primary carbocation formed in 1-chlorobutane is less stable compared to the secondary carbocation formed in 2-chlorobutane.
- Therefore, 2-chlorobutane will be more reactive towards E2 elimination compared to 1-chlorobutane.

b) 4-chloro-1-butene vs 2-chlorobutane
- In 4-chloro-1-butene, the chlorine atom is attached to a terminal carbon, whereas in 2-chlorobutane, the chlorine atom is attached to an internal carbon.
- The terminal carbocation formed in 4-chloro-1-butene is less stable compared to the internal carbocation formed in 2-chlorobutane.
- Therefore, 2-chlorobutane will be more reactive towards E2 elimination compared to 4-chloro-1-butene.

c) 2-iodobutane vs 2-bromobutane
- Both 2-iodobutane and 2-bromobutane have halogens attached to secondary carbons.
- Iodine is a larger atom compared to bromine, resulting in weaker carbon-halogen bond in 2-iodobutane.
- The weaker carbon-iodine bond in 2-iodobutane makes it more susceptible to nucleophilic attack and, therefore, more reactive towards E2 elimination compared to 2-bromobutane.

d) chloroethane vs penta deuterated chloroethane
- Penta deuterated chloroethane is chloroethane where all five hydrogen atoms are replaced by deuterium (heavy hydrogen).
- The presence of deuterium atoms in penta deuterated chloroethane does not affect the reactivity towards E2 elimination.
- Therefore, chloroethane and penta deuterated chloroethane will have similar reactivity towards E2 elimination.

Correct Order of Reactivity:
The correct order of reactivity towards E2 elimination with a strong base is:
1. 2-iodobutane > 2-bromobutane
2. 2-chlorobutane > 1-chlorobutane
3. 2-chlorobutane > 4-chloro-1-butene
4. chloroethane = penta deuterated