IB Chemistry HL 3.4.13 Hl Electrophilic Substitution in Benzene Questions

Use real HL exam questions to identify the electrophile attacking benzene, represent the charged sigma-complex intermediate, and complete the electron-pair mechanism to the substituted aromatic product.

Exam points

Identify electrophilic substitution in benzene and the charged electrophile E+, and explain the electrophile/benzene π-electron interaction.
Represent the sigma-complex (arenium) carbocation intermediate with the correct ring connectivity and positive charge.
Complete the substitution mechanism with curly arrows from electron pairs, restore aromaticity by C–H bond electron movement, and show the substituted benzene product.

[Maximum number: 5]

Benzene is an aromatic hydrocarbon.

(a)

State the typical reactions that benzene and cyclohexene undergo with bromine.
Benzene:
Cyclohexene:

[ 1 ]

benzene: electrophilic substitution/ $\mathrm{S}_{\mathrm{E}}$
AND
cyclohexene: electrophilic addition/ $\mathrm{A}_{\mathrm{E}}$

(b)

Explain the mechanism for the nitration of benzene, using curly arrows to show the movement of electron pairs.

[ 4 ]

curly arrow going from benzene ring to N of ${ }^{+} \mathrm{NO}_{2} / \mathrm{NO}_{2}{ }^{+}$ carbocation with correct formula and positive charge on ring curly arrow going from C-H bond to benzene ring of cation formation of organic product AND $\mathrm{H}^{+}$

Marking guidance:

Accept mechanism with corresponding Kekulé structures.

Do not accept a circle in M2 or M3.

Accept first arrow starting either inside the circle or on the circle.

M2 may be awarded from correct diagram for M3.

M4: Accept $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}+\mathrm{H}_{2} \mathrm{SO}_{4}$ if $\mathrm{HSO}_{4}{ }^{-}$ used in M3.

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IB Chemistry HL 3.4.13 Hl Electrophilic Substitution in Benzene Questions question 1

Question (a)

Question (b)