Problem 5 - Basic Principles of Organic Chemistry

Question

Prominent peaks in the mass spectrum of a basic nitrogen compound have \(m / e\) values of \(87,72,57\), and 30 . The NMR spectrum shows only three proton resonances, having intensity ratios of \(9: 2: 2\) at \(0.9,1.3\), and \(2.3 \mathrm{ppm}\). Assign a structure to the compound and account for the fragment ions \(m / e 72,57\), and 30 .

Step-by-Step Solution

Answer

The compound is tert-butylamine (C4H11N).

Step 1: Determine the Molecular Structure

Based on the clue that it is a basic nitrogen compound, the presence of a simple amine structure should be considered first. With proton NMR having peaks in 9:2:2 ratios at the given ppm values, it suggests an isopropyl group might be present. Given that there is a peak with a ratio of 9, as seen with a tert-butyl group, a potential structure can be established with tert-butyl as the large part of the compound. These assigned groups account for the chemical shifts found in the NMR spectrum.

Step 2: Examine Fragmentation Patterns

Examine the prominent peaks of the mass spectrum: 87, 72, and 57. The molecular ion at m/e 87 is likely the entire compound's mass. The peak at 72 results from the loss of a methyl (CH3) group from the tert-butyl group, leading to a fragment with a mass of 72. The peak at 57 involves the loss of another -CH3 group, traditionally associated with the fragmentation pattern of tert-butyl cations. The 30 m/e fragment could result from a C2H6N+ ion, as associated often with the simplest alkylamino group.

Step 3: Assign and Confirm the Structure

Considering both the NMR and mass spectral data, a reasonable structure for the compound will be a tert-butylamine (C4H11N). This structure offers correct proton chemical shifts (approximately 0.9 for tert-butyl, given that three equivalent methyl groups produce a singlet, and 1.3 and 2.3 are attributed to the amine group hydrogens) and also supports the mass fragmentation pattern identified with ions at m/e 87, 72, and 57. The ion at m/e 30 supports the characteristic fragmentation away from the nitrogen atom.

Key Concepts

Mass Spectrometry

Mass spectrometry is a powerful analytical technique used to determine the mass-to-charge ratio of ions. In simpler terms, it helps us understand the mass of different fragments of a molecule. The basic idea is to ionize chemical compounds to generate charged particles and then measure their mass-to-charge ratio. This technique is used widely in organic chemistry to help identify unknown compounds by providing a molecular fingerprint.

• The value 'm/e' stands for mass-to-charge ratio. 'm' refers to mass, while 'e' represents charge.
• The peaks you observe in a mass spectrum correspond to ions formed when the molecule fragments.
• This pattern of peaks can unravel the entire molecular structure, indicating which parts of the molecule have been lost to form smaller fragments.

The peaks in the mass spectrum give hints about the structural elements and sometimes suggest possible molecular formulas for the compound being studied.

NMR Spectroscopy

NMR spectroscopy is a technique that allows us to observe the magnetic properties of certain nuclei. In chemistry, it is mostly used to determine the structure of organic compounds by analyzing different nuclei of atoms such as hydrogen, which is the most common target. NMR provides detailed information on the number of hydrogen atoms in different environments within the molecule, showing us a picture of its backbone.

• The measured chemical shifts (in ppm) indicate the electronic environment of particular hydrogens.
• Intensity ratios in the spectrum, like the 9:2:2 found here, reveal how many protons contribute to each resonance, helping identify functional groups.
• A resonance at a lower ppm usually points to protons in an alkyl environment, whereas higher ppm values suggest attachment to electronegative atoms or functional groups.

These shifts and intensities lead to insights about how atoms converse with each other through bonds.

Fragmentation Patterns

Fragmentation patterns in mass spectrometry are crucial for deducing the structure of a compound. When a molecule is ionized, it may break into smaller pieces, which helps trace back to the original molecular structure. Different structures yield varying fragmentation patterns, which serve as a fingerprint specific to that molecule.

• Terbutyl and other branched structures are predisposed to losing alkyl groups, which results in typical fragmentation peaks.
• Identifying common fragments, such as the loss of methyl groups here, is a standard practice to deduce the compound's parts.
• For instance, peaks at 72, 57, and 30 m/e indicate loss of small parts like -CH3 groups and can be linked back to specific sections of the molecule.

This helps chemists identify which part of the molecule the fragments are coming from, enabling us to piece together the larger molecular puzzle.

Molecular Structure Determination

Molecular structure determination involves constructing a molecular model that fits all spectroscopic data you have. It's akin to piecing together a puzzle, where each piece of data helps fit the next part of the structure.

• By combining NMR data which suggests specific chemical shifts and mass spectrometric data which suggests specific fragmentation patterns, chemists can hypothesize possible structures.
• It's crucial to ensure that derived structures align with the proton chemical shifts observed and the mass fragmentation data seen.
• For example, a tert-butylamine structure aligns well with the experimental data here, providing an accurate portrait of what's happening in the molecule.

The goal is to visualize the entire molecular framework, and each spectroscopic technique offers a piece to the mosaic that reveals the complete structure in a coherent manner.