Question

The order of increasing reactivity towards HCl of the following compounds will be
(1) CH₂=CH₂, (2) (CH₃)₂C=CH₂, (3) CH₃CH=CHCH₃
A. 1 < 2 < 3
B. 1 < 3 < 2
C. 3 < 2 < 1
D. 2 < 1 < 3

Answer 1

Hint: Inductive effect is an effect that is observed in organic compounds. Whenever any atom or a group is substituted on a carbon chain it results in making the whole carbon chain partially polarised because of the sigma electron transmission. This effect is called the Inductive effect.

Complete Step by Step Answer:
There are two types of inductive effects which are the +I effect and the -I effect. When the substituent shifts its shared pair of electrons (i.esigma electron transmission) towards the carbon chain it is called the +I effect and if it attracts the shared pair of electrons towards itself is called the -I effect.

As CH₃ substituent shows the +I effect during reaction when the formation of carbocation takes place +I effect helps in stabilising the carbocation because it shifts its electron towards the carbocation and helps in stabilising the positive charge on it. So, the compounds which form the least stable carbocation are highly reactive compared to the compounds which form a stable carbocation.

The increase in stability of the carbocations of the given alkenes will be-
 CH₃ — (+)CH₂ < (CH3)2C(+) — CH₃ < CH₃CH(+) — CH₂CH₃
So, their reactivity towards the HCl of the following compounds will be-
CH₃CH = CHCH₃ < (CH₃)₂C = CH₂ < CH₂ =CH₂
Which is 3 < 2 < 1.
Thus, Option (C) is correct

Note: Inductive effect is always measured with respect to the hydrogen atom because the hydrogen atom does not show an inductive effect. Its effect is zero. The stability of cis alkene is less than the stability of trans alkene.