Question

Compound A, \(\mathrm{C}_{8} \mathrm{H}_{10}\), yields three substitution products, \(\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}\), on reaction with \(\mathrm{Br}_{2}\). Propose two possible structures for A. The \({ }^{1} \mathrm{H}\) NMR spectrum of A shows a complex four-proton multiplet at \(7.0 \delta\) and a sixproton singlet at \(2.30 \delta .\) What is the structure of \(\mathrm{A} ?\)

Short answer

Compound A is tert-butylbenzene.

Step-by-step solution

Analyze Substitution Products

The molecule \( \mathrm{C}_{8} \mathrm{H}_{10} \) produces three substitution products \( \mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br} \) when reacted with \( \mathrm{Br}_{2} \). This suggests that \( \mathrm{A} \) is aromatic with three distinguishable positions for substitution. Consider possible aromatic rings that can give three monosubstituted products; likely candidates include toluene (methylbenzene) derivatives.

Evaluate NMR Data

The \(^1\mathrm{H}\) NMR spectrum presents a complex four-proton multiplet at \(7.0 \, \delta\), typical for aromatic protons, indicating an aromatic ring. The six-proton singlet at \(2.30 \, \delta\) suggests equivalent protons attached to a carbon group, likely an isopropyl or tert-butyl group attached to a benzene ring.

Propose Possible Structures

Considering the structure must be \( \mathrm{C}_{8} \mathrm{H}_{10} \), two possibilities arise:1. Isopropylbenzene (cumene): an aromatic ring with an isopropyl group, giving three equivalent positions around the ring for substitution.2. tert-Butylbenzene: another aromatic compound with a tert-butyl group that allows similar substitution behavior.

Determine the Structure of A

The NMR data indicates a complex multiplet for four aromatic hydrogens and a singlet for six hydrogens. Tert-butylbenzene has a tert-butyl group causing the singlet (six protons) and four protons on the aromatic ring that can produce a multiplet. Therefore, the most fitting structure is tert-butylbenzene.

Key concepts

Substitution Reactions

In aromatic compounds, substitution reactions play a crucial role in determining the structure and reactivity of molecules. These reactions typically involve an aromatic ring, such as benzene, which can undergo substitution reactions depending on the kind of substituent attached.
A well-known example of substitution reaction is halogenation, where a hydrogen atom on the aromatic ring is replaced with a halogen, like bromine.
In our case, compound A, \( \mathrm{C}_{8} \mathrm{H}_{10} \), produced three substitution products when reacting with \( \mathrm{Br}_{2} \). This indicates that A is likely a mono-substituted aromatic compound. The molecule should have positions where three distinguishable substitution products can be formed.

• For aromatic rings like toluene derivatives, such as isopropylbenzene or tert-butylbenzene, specific positions enable different substitution patterns, like ortho, meta, and para, depending on the nature of the substituent.

Understanding substitution reactions is essential to predict which positions on the aromatic ring will be most reactive in the presence of substituents.

Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) spectroscopy is an advanced analytical technique used to determine the structure of organic compounds. It provides detailed information about the type and number of hydrogen atoms in a molecule.
The NMR spectrum of compound A provided valuable clues about its structure. It showed a four-proton multiplet at \(7.0\, \delta\), which is characteristic of aromatic protons.

• The peak indicates the presence of an aromatic ring, suggesting that the compound contains ring-bound protons that are magnetically different.
• The chemical shift at \(7.0\, \delta\) is typical for protons bonded to a benzene ring.

In addition to the multiplet, a six-proton singlet at \(2.30\, \delta\) indicates protons that are equivalent and attached to a side chain, likely an isopropyl or tert-butyl group. Such details allow us to deduce how groups are attached to the aromatic ring.

Aromatic Proton Multiplet

The concept of aromatic proton multiplet is crucial in understanding NMR spectra of aromatic compounds. Protons attached to an aromatic ring exhibit distinct multiplet patterns due to the complex environment created by adjacent protons and substituents.

• The term "multiplet" refers to a group of closely spaced NMR signals that result from the coupling of protons in an aromatic ring.
• For compound A, the four-proton multiplet observed at \(7.0\, \delta\) is indicative of protons that are part of a larger conjugated system.

These patterns provide insight into the number and arrangement of protons in relation to each other, which is fundamental for structural elucidation. Understanding how to read these signals in NMR spectra can clarify the overall geometry and substituent positioning on the aromatic ring.