Molecules and Spectral Data Matching

This question asks us to match specific organic molecules with characteristic spectral data obtained from techniques like Infrared (IR) spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy, and Electron Ionization Mass Spectrometry (EI-MS).

Spectral Analysis of Molecules

Ethyl Acetate (A)

Ethyl acetate is an ester with the structure CH\(_3\)COOCH\(_2\)CH\(_3\). Let's consider the given spectral data:

• i. Two singlets in \({^1}\)H NMR: Ethyl acetate has three distinct types of protons: the methyl protons next to the carbonyl (CH\(_3\)CO-), the methylene protons next to the oxygen (-OCH\(_2\)-), and the methyl protons at the end (-CH\(_3\)). These environments are different, and the methylene protons will be a quartet due to coupling with the terminal methyl protons, and the terminal methyl protons will be a triplet due to coupling with the methylene protons. The CH\(_3\)CO- protons will be a singlet. Thus, it will show a singlet, a quartet, and a triplet, not just two singlets.
• ii. Peak intensity at M:(M+2) is 3:1 in EI-MS: This isotopic pattern is characteristic of a molecule containing one chlorine (\(^{35}\)Cl/\(^{37}\)Cl ratio is approx. 3:1). Ethyl acetate contains only C, H, and O.
• iii. Absorption band at 1740 cm\(\textsuperscript{-1}\) in IR: Carbonyl groups (\(C=O\)) in esters typically show a strong absorption band in the IR spectrum around 1735-1750 cm\(\textsuperscript{-1}\). Ethyl acetate has an ester carbonyl group.

Based on this analysis, Ethyl acetate (A) matches the IR data (iii).

2-Chloropentane (B)

2-Chloropentane is CH\(_3\)CHClCH\(_2\)CH\(_2\)CH\(_3\). Let's consider the remaining spectral data:

• i. Two singlets in \({^1}\)H NMR: 2-Chloropentane has protons in several different chemical environments (CH\(_3\) at C1, CH at C2, CH\(_2\) at C3, CH\(_2\) at C4, CH\(_3\) at C5). Due to coupling, the \({^1}\)H NMR spectrum will be complex, with multiple peaks (doublet, multiplet, sextet, triplet, etc.), not just two singlets.
• ii. Peak intensity at M:(M+2) is 3:1 in EI-MS: 2-Chloropentane contains one chlorine atom. Chlorine has two major isotopes, \(^{35}\)Cl and \(^{37}\)Cl, with a natural abundance ratio of approximately 3:1. When a molecule contains one chlorine atom, its mass spectrum will show a molecular ion peak (M) and a peak two mass units higher (M+2) with an intensity ratio of approximately 3:1. This matches the description.

Based on this analysis, 2-chloropentane (B) matches the EI-MS data (ii).

1,2-Dibromo-2-methylpropane (C)

1,2-Dibromo-2-methylpropane is (CH\(_3\))\(_2\)CBrCH\(_2\)Br. Let's consider the remaining spectral data:

• i. Two singlets in \({^1}\)H NMR: The molecule has two types of protons: the six equivalent methyl protons ((CH\(_3\))\(_2\)CBr-) and the two methylene protons (-CH\(_2\)Br). The methyl protons are attached to a carbon that has no attached hydrogens, so they will appear as a singlet. The methylene protons are attached to a carbon that has no attached hydrogens (the quaternary carbon), so they will also appear as a singlet. Therefore, the \({^1}\)H NMR spectrum will show two singlets.
• ii. Peak intensity at M:(M+2) is 3:1 in EI-MS: This ratio is for one chlorine atom. This molecule contains two bromine atoms. Bromine isotopes \(^{79}\)Br and \(^{81}\)Br are present in approximately a 1:1 ratio. A molecule with two bromine atoms will show M:(M+2):(M+4) peaks in a ratio of approximately 1:2:1.

Based on this analysis, 1,2-dibromo-2-methylpropane (C) matches the \({^1}\)H NMR data (i).

Summary of Matches

Based on the spectral analysis, the correct matches are:

Thus, the correct match is A - iii, B - ii, C - i.