Question

The compound with molecular formula C\(_6\)H\(_6\), which gives only one monobromo derivative and takes up four moles of hydrogen per mole for complete hydrogenation has ___ \(\pi\) electrons.

Hint:

Aromatic compounds like benzene (C\(_6\)H\(_6\)) have a conjugated system of \( \pi \)-electrons that satisfy Hückel's rule, which states that aromatic compounds have 4n + 2 \( \pi \)-electrons (where n is a whole number).

Answer 1

Correct Answer: 6

Benzene, with the molecular formula C\(_6\)H\(_6\), features a six-carbon ring structure characterized by alternating single and double bonds. This arrangement facilitates the delocalization of π-electrons throughout the ring. Due to resonance, all carbon-carbon bonds in benzene possess equal length, contributing to its stability and aromaticity.

The number of π-electrons is calculated as follows:

1. Benzene's cyclic structure (C\(_6\)H\(_6\)) includes alternating double bonds, which contribute π-electrons.
2. Each double bond involves π-electrons shared between two carbon atoms. Benzene contains three double bonds.
3. With three double bonds, each contributing two π-electrons, benzene has a total of 3 double bonds × 2 π-electrons/double bond = 6 π-electrons.

The fact that benzene produces only one monobromo derivative confirms the equivalence of all hydrogen atoms on the ring, reinforcing its structural identity as benzene.

Complete hydrogenation of benzene consumes 4 moles of hydrogen. This process converts each double bond to a single bond, aligning with the presence of three double bonds.

The calculated π-electron count is 6, which falls within the expected range of 6 to 6.