R3.4 Electron-pair sharing reactions

A Lewis acid is an electron-pair acceptor. The ${\text{Cu}}^{2+}$ ion has an empty orbital and accepts lone pairs from the surrounding ${\text{NH}}_3$ ligands, so it is acting as a Lewis acid. Statement I is correct.

Ammonia (${\text{NH}}_3$) has a lone pair on nitrogen, making it a Lewis base (electron-pair donor). Each ${\text{NH}}_3$ donates its lone pair to the ${\text{Cu}}^{2+}$ ion to form a coordinate (dative) bond. Statement II is correct.

The coordination number is defined as the number of ligands directly bonded to the central metal ion. The formula $[{\text{Cu(NH}}_3{)}_4{]}^{2+}$ shows 4 ammonia ligands, so the coordination number is 4. Statement III is correct.

All three statements are correct, so the answer is I, II and III.

The question asks which statements correctly describe the complex $[{\text{Cu(NH}}_3{)}_4{]}^{2+}$ — including Lewis acid/base roles and coordination number.

This option excludes Statement III. However, the coordination number of 4 is simply the count of ${\text{NH}}_3$ ligands in the formula, which is clearly 4. Statement III is correct, so this option is wrong.

This option excludes Statement II. But ${\text{NH}}_3$ has a lone pair on nitrogen and donates it to ${\text{Cu}}^{2+}$, acting as a Lewis base (electron-pair donor). Statement II is correct, so this option is wrong.

This option excludes Statement I. ${\text{Cu}}^{2+}$ accepts electron pairs from ammonia ligands — it is clearly a Lewis acid. Statement I is correct, so this option is also wrong.

All three statements are correct. The answer is I, II and III.

Unlike alkenes, benzene does not readily decolourise bromine water at room temperature. Alkenes undergo electrophilic addition with bromine, but benzene's delocalised $\pi$ system makes it thermodynamically stable and resistant to addition reactions.

Benzene undergoes electrophilic substitution (not addition) to preserve its stable aromatic ring. A catalyst such as ${\text{FeBr}}_3$ is needed for benzene to react with bromine — bromine water alone does not react with benzene.

The other three options are all correct: (1) benzene undergoes substitution to preserve aromaticity; (2) all C–C bonds are equivalent at ~140 pm due to delocalisation; (3) each carbon is $s{p}^2$ hybridised with $120°$ bond angles.

The incorrect statement is that benzene readily decolourises bromine water through electrophilic addition. This is a property of alkenes, not benzene.

The question asks for the incorrect statement about benzene. Three options should be true; one should be false.

This is correct — benzene undergoes substitution (not addition) to maintain the stability of its delocalized $\pi$ system. This cannot be the incorrect statement.

This is correct — the delocalisation of $\pi$ electrons means all C–C bonds in benzene are equivalent (~140 pm, intermediate between single and double bonds). This is not the incorrect statement.

This is correct — all carbons in benzene are $s{p}^2$ hybridised, with one unhybridised p-orbital contributing to the $\pi$ system, and trigonal planar geometry giving $120°$ angles.

The claim that benzene readily decolourises bromine water through electrophilic addition is incorrect. Benzene does not react with bromine water under normal conditions. This is the answer.