Question

Among the given reactions, which is faster and why?
i. \[Me-CHCl-Me\xrightarrow{alc.KOH}C{{H}_{3}}-CH=C{{H}_{\begin{smallmatrix}2 \\

\end{smallmatrix}}}\]
ii. \[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-Cl\xrightarrow{alc.KOH}C{{H}_{3}}-CH=C{{H}_{2}}\]

Answer 1

Hint: Use the basics of elimination reaction. Alcoholic KOH is a strong base. So, it will favor the E2 elimination mechanism. In E2 elimination, there is no formation of a carbocation, there’s only a transition state. The reaction occurs in a single step.

Complete step by step answer:
To solve any given reaction, we must first look at the reagent given to us and then the substrate. It is because these two factors determine the fate of the reaction.
Let us understand the term ‘Elimination reaction’. “An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either one or two-step mechanism.” It is categorized as –

E1
Unimolecular reaction
Needs a weak base as a reagent
Occurs in two steps, with formation of a carbocation
Requires a polar protic solvent

E2
Bimolecular reaction
Needs a strong base as a reagent
Occurs in a single step, with formation of a transition state
Requires a polar aprotic solvent
Now let us take a look at both the given reactions (i) and (ii). As we can see, the reagent is alcoholic KOH (Potassium hydroxide). It is a strong base. Therefore, the preferred mechanism will be E2.
In E2, the priority order for reaction of compound is – tertiary > secondary > primary. It is because the transition state formed will be more stable in this order.
In compound (i) chlorine is attached to a secondary carbon, whereas in compound (ii) chlorine is attached to a primary carbon.
Applying the priority order rule, we can say that reaction (i) will be faster.
Therefore, the answer is – option (i)

Additional Information:
Rate of any reaction is proportional to its stability. It is because all compounds have a tendency to achieve stability. The closer a compound is to achieving stability, the faster it acts.

Note: In organic chemistry, we majorly use two types of solvents.
Polar protic
It contains a free proton and has a tendency to solvate the nucleophile
For example - water
Polar aprotic
It is inert and does not contain proton
For example – Carbon tetrachloride