Question

Propose structures for compounds that fit the following descriptions: (a) \(\mathrm{C}_{10} \mathrm{H}_{14}\) \(1 \mathrm{H} \mathrm{NMR}: 7.18 \delta(4 \mathrm{H},\) broad singlet \() ; 2.70 \delta(4 \mathrm{H},\) quartet, \(J=7 \mathrm{~Hz})\) \(1.20 \delta(6 \mathrm{H},\) triplet \(, J=7 \mathrm{~Hz})\) \(\mathrm{IR}: 745 \mathrm{~cm}^{-1}\) (b) \(\mathrm{C}_{10} \mathrm{H}_{14}\) \({ }^{1} \mathrm{H} \mathrm{NMR}: 7.0 \delta(4 \mathrm{H},\) broad singlet \() ; 2.85 \delta(1 \mathrm{H},\) septet, \(J=8 \mathrm{~Hz})\) \(2.28 \delta(3 \mathrm{H},\) singlet \() ; 1.20 \delta(6 \mathrm{H},\) doublet, \(J=8 \mathrm{~Hz})\) \(\mathrm{IR}: 825 \mathrm{~cm}^{-1}\)

Short answer

(a) Ethylbenzene; (b) Cumene (isopropylbenzene).

Step-by-step solution

Analyze the Molecular Formula and NMR for Compound (a)

For compound (a), we have the formula \( \text{C}_{10} \text{H}_{14} \). The \(^1\text{H NMR}\) spectrum shows signals at: * \(7.18 \delta\) (4H, broad singlet) - likely aromatic protons.* \(2.70 \delta\) (4H, quartet, \(J = 7 \text{Hz}\)) - likely protons adjacent to a methylene group.* \(1.20 \delta\) (6H, triplet, \(J = 7 \text{Hz}\)) - likely protons attached to methyl groups.The \(\text{IR}\) spectrum with a band at \(745 \text{~cm}^{-1}\) is consistent with monosubstituted benzene.

Propose Structure for Compound (a)

Given the NMR data, compound (a) is likely an ethylbenzene. The aromatic protons fit the 4H singlet, indicating symmetry, supported by the IR band for a monosubstituted benzene. The quartet at \(2.70 \delta\) and triplet at \(1.20 \delta\) correspond to the ethyl group \((\text{-CH}_2\text{-CH}_3)\) connected to the benzene ring.

Analyze the Molecular Formula and NMR for Compound (b)

For compound (b), we have the same formula \( \text{C}_{10} \text{H}_{14} \), indicating isomerism with compound (a). The \(^1\text{H NMR}\) spectrum shows signals at:* \(7.0 \delta\) (4H, broad singlet) - indicating symmetry, likely aromatic protons.* \(2.85 \delta\) (1H, septet, \(J = 8 \text{Hz}\)) - possibly a proton attached to a branching point.* \(2.28 \delta\) (3H, singlet) - likely a methyl group.* \(1.20 \delta\) (6H, doublet, \(J = 8 \text{Hz}\)) - likely two isopropyl groups. IR signal at \(825 \text{~cm}^{-1}\) matches para-substitution in benzene.

Propose Structure for Compound (b)

Based on the NMR data, compound (b) is likely isopropylbenzene (cumene). The septet and doublet pattern are characteristic of an isopropyl group \((\text{-CH(CH}_3)_2)\) attached to a benzene ring. The singlet at \(2.28 \delta\) is consistent with the methyl group in this arrangement.

Key concepts

Aromatic Compounds

Aromatic compounds are a unique class of compounds characterized by the presence of a benzene ring. Benzene, the simplest aromatic compound, consists of six carbon atoms arranged in a ring, with each carbon bonded to a single hydrogen atom. The special feature of aromatic compounds is their stable ring system, which is a consequence of a concept known as conjugation. Conjugated systems allow the electrons in these bonds to be delocalized around the ring, which gives aromatic compounds significant stability.

Furthermore, benzene rings manifest characteristic signal patterns when analyzed using spectroscopic techniques such as NMR spectroscopy. Typically, aromatic hydrogen atoms (protons) display signals in the region of 6-8 ppm, which helps in their identification. The stability and particular spectroscopic signatures make aromatic compounds crucial in organic chemistry and molecular structure studies.

• In the context of the exercise, both ethylbenzene and cumene contain benzene rings that provide the aromatic characteristics.

Molecular Structure Determination

Molecular structure determination is the process of identifying the specific arrangement of atoms within a molecule. This is key in fields like organic chemistry, where knowing the exact structure can influence understanding and prediction of chemical properties and reactivity.

The combination of techniques like Nuclear Magnetic Resonance (NMR) spectroscopy and Infrared (IR) spectroscopy is common practice in this determination. NMR is particularly powerful for revealing the environment around hydrogen atoms (protons) in a compound. Various signal patterns (like singlets, doublets, triplets, etc.) are tied to the number of neighboring protons, providing clues to the structure.

• For ethylbenzene, the noteworthy aspect was the quartet and triplet pattern in the NMR, suggesting the presence of an ethyl group.
• In cumene, the septet and doublet pattern were key, indicating an isopropyl group attachment.

IR spectroscopy complements this by providing information on functional groups through characteristic absorption bands. For example, the observed bands for ethylbenzene and cumene help confirm the presence of benzene rings and the type of substitution on those rings.

Ethylbenzene

Ethylbenzene is a compound that consists of a benzene ring with an ethyl group (-CH extsubscript{2}CH extsubscript{3}) attached to it. It is one of the many derivatives of benzene, making it an aromatic compound. The ethyl group is connected directly to the benzene core, which influences the spectroscopic data observed.

In NMR spectroscopy, ethylbenzene can be identified by a sequence of signals:

• Aromatic protons generally resonate around 7 ppm indicating the aromatic nature of the compound.
• The quartet feature around 2.7 ppm suggests that these protons are near a methylene group (-CH extsubscript{2}-), which is part of the ethyl attachment.
• The triplet around 1.2 ppm is indicative of the methyl group (-CH extsubscript{3}) completing the ethyl side chain.

The IR spectra of ethylbenzene typically show a strong absorption band around 745 cm extsuperscript{-1}, supporting the conclusion of a monosubstituted benzene configuration. Understanding these properties is crucial when distinguishing ethylbenzene from its isomers.

Cumene

Cumene, also known as isopropylbenzene, has a benzene ring with an isopropyl group (-CH(CH extsubscript{3}) extsubscript{2}) attached. This configuration uniquely distinguishes cumene from other isomers of C extsubscript{10}H extsubscript{14}.

From the perspective of NMR spectroscopy, cumene is characterized by:

• A signal around 7 ppm indicates the presence of aromatic protons similar to ethylbenzene.
• The septet near 2.85 ppm suggests a proton attached to a branching (tertiary) carbon, a hallmark of the isopropyl structure.
• A doublet around 1.20 ppm represents the methyl groups of the isopropyl component.

Additionally, the IR spectrum of cumene typically shows an absorption band around 825 cm extsuperscript{-1}, supporting the hypothesis of parasubstitution on the benzene ring. Such distinct spectroscopic features are invaluable for scientists in accurately identifying cumene during molecular structure determination.