Question

Suggest explanations for the following. (a) The \(^{1} \mathrm{H}\) NMR spectrum of \(\mathrm{CF}_{3} \mathrm{CH}_{2} \mathrm{OH}\) contains a quartet \((J 9 \mathrm{Hz})\) at \(\delta+3.9 \mathrm{ppm}\) in addition to the signal assigned to the OH proton. (b) The addition of \(\mathrm{D}_{2} \mathrm{O}\left(\mathrm{D}=^{2} \mathrm{H}\right)\) to hexanol causes the disappearance of the signal assigned to the OH proton. (c) Whereas alcohols exhibit relatively high boiling points and enthalpies of vaporization, the same is not true of thiols, \(\mathrm{RSH}\), e.g. propan-1-ol, bp \(=370.2 \mathrm{K}\) \(\Delta_{\mathrm{vap}} H(\mathrm{bp})=41.4 \mathrm{kJ} \mathrm{mol}^{-1} ;\) propane-l-thiol \(\mathrm{bp}=340.8 \mathrm{K}, \Delta_{\mathrm{vap}} H(\mathrm{bp})=29.5 \mathrm{kJ} \mathrm{mol}^{-1}\)

Short answer

(a) Signal is a quartet due to CF3 coupling; (b) OH signal disappears due to D2O exchange; (c) Alcohols form H-bonds, thiols don't, leading to different boiling points.

Step-by-step solution

Analyzing the NMR Spectrum of CF3CH2OH

In the NMR spectrum of CF3CH2OH, the presence of a quartet at δ 3.9 ppm with a coupling constant J of 9 Hz is observed. This occurs due to the methylene protons (CH2) being adjacent to the three equivalent protons in the trifluoromethyl group (CF3). According to the n+1 rule, the methylene protons should give a quartet (n=3, thus n+1=4) when coupled with protons in CF3, and the coupling constant J=9 Hz is consistent with C-F coupling.

Effect of D2O on NMR Spectrum of Hexanol

The addition of D2O causes the disappearance of the OH proton signal in the NMR spectrum due to deuterium exchange. The OH proton (1H) can be easily exchanged with a deuterium (2H) from D2O, effectively changing the OH group to an OD group. Deuterium is NMR-invisible in most standard NMR experiments, leading to the disappearance of the original OH signal.

Comparing Boiling Points and Enthalpies of Vaporization of Alcohols and Thiols

Alcohols exhibit relatively high boiling points and enthalpies of vaporization because of hydrogen bonding between hydroxyl groups in different alcohol molecules. This intermolecular force is strong and requires significant energy to overcome. Conversely, thiols (RSH) do not form hydrogen bonds as effectively due to the lower electronegativity of sulfur compared to oxygen, resulting in weaker intermolecular forces and thus lower boiling points and enthalpies of vaporization.

Key concepts

Chemical Shift

Chemical shift is a fundamental concept in NMR spectroscopy that helps identify different types of hydrogen environments in a molecule. It is measured in parts per million (ppm) and indicates how the electronic environment around a proton affects its resonance frequency. In simple terms, chemical shifts help pinpoint where the hydrogen atoms are in relation to other groups in the molecule.

In the case of the compound CF3CH2OH, the chemical shift of the methylene protons (CH2) at 3.9 ppm reflects the influence of the adjacent trifluoromethyl group (CF3). Fluorine is highly electronegative and pulls electron density away from the methylene group. This deshielding effect raises the energy required for these protons to resonate, resulting in a shift downfield to 3.9 ppm. The nearby CF3 group's influence on the methylene protons, therefore, helps to identify the chemical environment and chemical shift location.

Methylene Protons

Methylene protons are two hydrogen atoms attached to the same carbon (CH2 group), frequently investigated in NMR spectroscopy for their distinctive splitting patterns. In an NMR spectrum, the splitting pattern of a group of equivalent protons is determined by adjacent non-equivalent protons through spin-spin coupling.
Methylene protons in the CF3CH2OH molecule are coupled with the protons of the trifluoromethyl group (CF3). According to the n+1 rule, where 'n' is the number of neighboring non-equivalent protons, the methylene protons in this molecule will appear as a quartet because they are coupled with three equivalent fluorine atoms in the adjacent CF3 group (n=3, thus n+1=4).
This coupling results in a quartet with a coupling constant of 9 Hz, indicative of the interaction strength between the methylene and trifluoromethyl protons. This key characteristic helps in distinguishing the structural environment of the molecule in an NMR analysis.

Deuterium Exchange

Deuterium exchange is a process where a hydrogen atom ( ^1H) in a molecule is replaced by a deuterium ( ^2H) atom, which is an isotope of hydrogen. This exchange can drastically influence NMR spectra, particularly in the context of hydroxyl groups.

When D2O (heavy water) is added to a compound like hexanol, it interacts with the hydroxyl (OH) group, swapping its proton for a deuterium atom. This transformation effectively changes the OH group into an OD group. Since ^2H is not generally detected in regular ^1H NMR experiments—because it resonates at a different frequency—the signal initially assigned to the OH proton disappears from the spectrum. This substitution is useful for identifying and confirming the presence of hydroxyl-bound hydrogens in a molecule by their absence.

Hydrogen Bonding

Hydrogen bonding is a type of strong intermolecular force that occurs between molecules containing electronegative elements, particularly oxygen, nitrogen, or fluorine, bonded to hydrogen. This type of bonding significantly impacts the physical properties of a substance, such as its boiling point and enthalpy of vaporization.
Alcohols, like hexanol, have hydroxyl groups that can form hydrogen bonds with one another due to the electronegative nature of oxygen. These bonds require more energy to break, which is why alcohols typically exhibit higher boiling points and greater enthalpies of vaporization compared to substances that cannot form hydrogen bonds.
Thiols, on the other hand, have sulfur in place of oxygen. Sulfur is less electronegative, which means the hydrogen bonds formed are weaker or virtually absent. This weakness in intermolecular forces results in lower boiling points and enthalpies of vaporization compared to alcohols, highlighting the critical role hydrogen bonding plays in molecular interactions and properties.