Haloalkanes and Haloarenes

Chemistry · Class 12

Simulation · Chemistry · Class 12

Where does the positive charge go?

"Where does the positive charge go?" is a free, interactive Class 12 Chemistry simulation for NEET and JEE, built on the NCERT section "SN1 and reactivity order" in Haloalkanes and Haloarenes. The carbocation each halide would form, with its positive charge spread by resonance (allyl, benzyl, diphenylmethyl) or not (1°, 2°, 3°), and no cation at all for vinyl and aryl halides; relative SN1 rates on a log bar. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 62 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson SN1 and reactivity order in Haloalkanes and Haloarenes. Change the values and watch what happens.

Where does the positive charge go?Chemistry · Class 12

What it shows

NCERT §6.1; §6.7.1

The carbocation each halide would form, with its positive charge spread by resonance (allyl, benzyl, diphenylmethyl) or not (1°, 2°, 3°), and no cation at all for vinyl and aryl halides; relative SN1 rates on a log bar.

Controls

3 to try

  • Selecthalide CH₃Br, 1°, 2°, 3°, allyl, benzyl, Ph₂CHBr, vinyl, chlorobenzene
  • Toggleshow resonance forms
  • Readoutssp³/sp² C–X, cation formed?, charge-bearing carbons, relative SN1 rate

Try this

Find the halide whose cation spreads its charge over the most carbons, and show why benzyl chloride is not an aryl halide.

Common mistake

Benzyl chloride is an aryl halide / vinyl and aryl halides undergo SN1.

In NEET

10 years of papers

Asked 2 times in 10 years of NEET.

See the past questions: Nucleophilic Substitution, Haloalkanes and Haloarenes (chapter)

In JEE Main

42 shifts analysed

62 questions from Haloalkanes and Haloarenes across the 42 JEE Main shifts Lumi has analysed (2017–2026).

See the past questions

The idea behind it

NCERT §6.7.1

  • SN1 (substitution nucleophilic unimolecular) reactions are usually run in polar protic solvents such as water, alcohol or acetic acid.
  • (CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻ follows first order kinetics: the rate depends on [tert-butyl bromide] only, not on [OH⁻].
  • Step I: the polarised C–Br bond breaks slowly and reversibly into a carbocation and Br⁻; the energy comes from solvation of the halide ion by the protons of the protic solvent. Step II: the nucleophile attacks the carbocation quickly. The slow step sets the rate.
  • The more stable the carbocation, the faster it forms and the faster the reaction. Tertiary carbocations are the most stable, so 3° halides react fastest: SN1 order 3° > 2° > 1° > CH₃X, the reverse of SN2.
  • Allylic and benzylic halides are very reactive in SN1 because their carbocations are stabilised by resonance.
  • For a given alkyl group, both mechanisms follow R–I > R–Br > R–Cl >> R–F; the large iodide ion leaves most readily.
  • Bromobutanes: SN1 order n-C₄H₉Br < (CH₃)₂CHCH₂Br < CH₃CH₂CH(Br)CH₃ < (CH₃)₃CBr, and SN2 the exact reverse. Of the two primary bromides, (CH₃)₂CHCH₂Br is faster in SN1 because (CH₃)₂CH– has a larger electron-donating inductive effect.
  • Benzylic bromides: SN1 order C₆H₅C(CH₃)(C₆H₅)Br > C₆H₅CH(C₆H₅)Br > C₆H₅CH(CH₃)Br > C₆H₅CH₂Br, and SN2 the reverse. Two phenyl groups stabilise a cation by resonance, but a phenyl group is bulkier than methyl, which slows SN2.

Questions

5 answers

What does the "Where does the positive charge go?" simulation show?
The carbocation each halide would form, with its positive charge spread by resonance (allyl, benzyl, diphenylmethyl) or not (1°, 2°, 3°), and no cation at all for vinyl and aryl halides; relative SN1 rates on a log bar.
What mistake does the "Where does the positive charge go?" simulation fix?
A common mistake it clears up: benzyl chloride is an aryl halide / vinyl and aryl halides undergo SN1.
Has NEET asked about this idea?
Yes. NEET has asked about this idea 2 times in 10 years of papers.
Does JEE Main ask questions from Haloalkanes and Haloarenes?
Yes. JEE Main asked 62 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
Is this simulation free?
Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "SN1 and reactivity order" in Haloalkanes and Haloarenes (Class 12 Chemistry).

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