Question

The \(^{13} \mathrm{C}\) NMR spectrum of 2 -methylpropan- 2 -ol (14.16) has signals at \(\delta 31.2\) and 68.9 ppm. Suggest assignments for these signals. What other feature of the spectrum would confirm the assignments?

Short answer

Assign 68.9 ppm to the quaternary carbon and 31.2 ppm to the methyl groups; use DEPT NMR to confirm.

Step-by-step solution

Identify the Carbons in the Molecule

2-methylpropan-2-ol has the chemical structure: \[ \text{C(CH}_3\text{)}_3\text{OH} \]. It has a total of four carbon atoms: one quaternary carbon bonded to three methyl groups and a hydroxyl group, and three equivalent methyl groups.

Assign the Carbons to the NMR Signals

In an NMR spectrum, carbons bonded to more electronegative atoms like oxygen will resonate at higher chemical shifts (downfield). The signal at 68.9 ppm is likely from the quaternary carbon, which is bonded to the oxygen and thus experiences a deshielding effect. The signal at 31.2 ppm is from the three equivalent methyl carbons, which are surrounded by the same environment.

Confirm the Assignment with Signal Count

Since all three methyl carbons are equivalent, they will produce one single signal in the NMR spectrum, and the quaternary carbon will produce its own signal. The presence of two signals in the NMR spectrum — one for the quaternary carbon and one for the equivalent methyl groups — confirms this assignment.

Additional Feature to Confirm

A DEPT (Distortionless Enhancement by Polarization Transfer) 13C NMR could be used to further confirm. This technique distinguishes between CH, CH2, and CH3 groups. The signal at 31.2 ppm should show up in the DEPT-135 spectrum (indicating it's a CH3 group), while the quaternary carbon signal at 68.9 ppm would not appear in such a spectrum (since it has no attached hydrogens).

Key concepts

Chemical Shift

In the world of 13C NMR spectroscopy, the term 'chemical shift' is a key concept. This term refers to the position on the spectrum where the signal for a given carbon atom appears. It's measured in parts per million (ppm). The chemical shift provides essential information about the electronic environment surrounding a carbon atom.

A carbon atom's chemical shift is influenced by the electronegativity of nearby atoms and the molecular structure. If a carbon is near electronegative atoms, such as oxygen, it tends to shift the signal downfield (to higher ppm values), indicating that the carbon is less shielded by electrons. For instance, in 2-methylpropan-2-ol, the quaternary carbon bonded to an oxygen shows a chemical shift at 68.9 ppm due to this deshielding effect.

Understanding chemical shifts is like reading a map of the molecule, allowing chemists to deduce structural information, make assignments, and confirm the atoms' positions in the molecular structure.

Carbon Assignment

Carbon assignment in NMR spectroscopy involves linking each signal in the spectrum to a specific carbon atom in the molecule. This can be challenging but is essential for understanding the molecule's structure.

In the case of 2-methylpropan-2-ol, there are four carbon atoms total, consisting of one quaternary carbon and three equivalent methyl groups. The assignment is made by understanding which carbons are in similar environments and recognizing unique structural features.

Here, the signal at 68.9 ppm is assigned to the quaternary carbon because it's connected to a more electronegative atom (oxygen), while the 31.2 ppm signal belongs to the equivalent methyl groups. Successful carbon assignment allows for accurate interpretation of the molecule's 3D structure and helps verify the chemical identity of the compound.

DEPT (Distortionless Enhancement by Polarization Transfer)

The DEPT technique in 13C NMR stands out for its ability to distinguish different types of carbon environments. By utilizing polarization transfer, DEPT separates signals into CH, CH2, and CH3 groups, enhancing the analysis capability of the NMR spectrum.

In a DEPT-135 spectrum, signals from CH and CH3 appear positive, while CH2 signals appear negative, and quaternary carbons, like the one in 2-methylpropan-2-ol, remain absent because they have no hydrogens.

Using DEPT allows chemists to differentiate these groups effectively and confirm which carbons correspond to which parts of the structure, making it a powerful ally in structure elucidation. For example, in the exercise, the methyl groups show up in DEPT-135 due to attached hydrogens, affirming their identity in the molecule.

Deshielding Effect

The deshielding effect is a central concept in NMR spectroscopy and is often crucial for interpreting chemical shifts. It occurs when electron density around a carbon nucleus decreases, typically due to the electronegativity of nearby atoms or groups, causing the nucleus to experience a stronger external magnetic field. This results in signals that appear at higher ppm values in the spectrum, termed as 'downfield'.

In 2-methylpropan-2-ol, the quaternary carbon bonded to a highly electronegative oxygen atom undergoes such deshielding. The oxygen pulls electron density away from the carbon, reducing its shielding and causing the carbon's signal to shift downfield to 68.9 ppm.

Understanding the deshielding effect helps in identifying the position of electronegative atoms in a molecule, thus offering insights into the molecular structure and aiding in the precise interpretation of NMR data.