CBSE 2025, Set 2

Question

In the following pair of halogen compounds, which compound undergoes S$_N$1 reaction faster and why?

![SN1 pair: tertiary and secondary chloroalkanes](https://www.edvaya.com/images/chemistry/cbse/2025/set2/q17a-sn1-pair-chloroalkanes.webp)

Edvaya · Accepted Answer

The correct answer is Subjective. The **tertiary chloroalkane** (2-chloro-2-methylpropane / tert-butyl chloride, $(	ext{CH}_3)_3	ext{CCl}$) undergoes S$_N$1 reaction **faster**. **Reason:** S$_N$1 reactions proceed via formation of a carbocation intermediate. The rate-determining step is the ionisation of the C–X bond. The ease of this step depends on the **stability of the carbocation** formed. A **tertiary carbocation** is more stable than a secondary or primary carbocation due to hyperconjugation and inductive effects of three alkyl groups donating electron density to the positively charged carbon. More stable carbocation → lower activation energy → faster S$_N$1 rate. $$(	ext{CH}_3)_3	ext{C–Cl} ightarrow \underbrace{(	ext{CH}_3)_3	ext{C}^+}_{	ext{stable tertiary}} + 	ext{Cl}^-$$

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Chemistry > Organic Chemistry > Haloalkanes And Haloarenes

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