Question

Following are structural formulas, dipole moments, and \({ }^{1}\) H-NMR chemical shifts for acetonitrile, fluoromethane, and chloromethane. $$ \begin{array}{|ccc|} \hline \mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{N} & \mathrm{CH}_{3} \mathrm{~F} & \mathrm{CH}_{3} \mathrm{Cl} \\ \hline \text { Acetonitrile } & \text { Fluoromethane } & \text { Chloromethane } \\ \hline 3.92 \mathrm{D} & 1.85 \mathrm{D} & 1.87 \mathrm{D} \\ \hline \delta 1.97 & \delta 4.26 & \delta 3.05 \\ \hline \end{array} $$ (a) How do you account for the fact that the dipole moments of fluoromethane and chloromethane are almost identical even though fluorine is considerably more electronegative than chlorine? (b) How do you account for the fact that the dipole moment of acetonitrile is considerably greater than that of either fluoromethane or chloromethane? (c) How do you account for the fact that the chemical shift of the methyl hydrogens in acetonitrile is considerably less than that for either fluoromethane or chloromethane?

Short answer

Question: Explain why the dipole moments of fluoromethane and chloromethane are almost identical, and why the dipole moment of acetonitrile is considerably larger. Additionally, explain why the chemical shift of the methyl hydrogen atoms in acetonitrile is less than in fluoromethane or chloromethane. Answer: The dipole moments of fluoromethane and chloromethane are almost identical because the increase in electronegativity difference between fluorine and carbon is compensated by the increase in bond length between carbon and chlorine atoms. Acetonitrile has a larger dipole moment due to its triple bond between carbon and nitrogen, resulting in a large difference in electronegativity and a shorter bond length. The chemical shift of the methyl hydrogen atoms in acetonitrile is less than in fluoromethane or chloromethane because the electron shielding effect in acetonitrile is more significant due to the shorter C≡N bond and the lower electronegativity of nitrogen compared to fluorine or chlorine.

Step-by-step solution

(a) Dipole moments of fluoromethane and chloromethane

We know that the dipole moment (\(\mu\)) can be calculated using the formula \(\mu = Qr\), where \(Q\) is the charge or electronegativity difference between the bonded atoms, and \(r\) is the bond length. Fluorine is more electronegative than chlorine, but the bond in chloromethane (C-Cl) is longer than the bond in fluoromethane (C-F). As a result, the increase in electronegativity difference is compensated by the increase in bond length, making the dipole moments of these compounds almost identical.

(b) Dipole moment of acetonitrile

Acetonitrile has a triple bond between carbon and nitrogen, resulting in a large difference in electronegativity between the carbon and nitrogen atoms. This causes a significant accumulation of negative charge on the nitrogen atom and positive charge on the carbon atom. Furthermore, the triple bond between carbon and nitrogen is shorter than the bonds in fluoromethane and chloromethane, making the charge separation more significant. This results in a larger dipole moment for acetonitrile compared to fluoromethane or chloromethane.

(c) Chemical shift of methyl hydrogens in acetonitrile

The chemical shift of a hydrogen nucleus in a molecule is influenced by the electron shielding effect caused by surrounding electron clouds. In acetonitrile, the electron cloud surrounding the methyl hydrogen atom is relatively far away from the nitrogen atom since the C≡N bond is shorter than C-F or C-Cl bonds in fluoromethane or chloromethane. Additionally, the electronegativity of nitrogen is lower than that of fluorine or chlorine. As a result, the electron shielding effect is more significant for the methyl hydrogens in acetonitrile than in fluoromethane or chloromethane, leading to a lower chemical shift value.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in organic chemistry that reflects how strongly an atom attracts electrons in a chemical bond. It's crucial to predict how molecules will behave in reactions and how they are structured. Fluorine, for instance, is the most electronegative element, which means in compounds like fluoromethane, it pulls electron density towards itself, creating a significant charge difference across the C-F bond. However, compared to chlorine, which is less electronegative, the expected higher dipole moment does not follow due to compensating factors like bond length, which we will explore in more detail in the context of bond length and dipole moment.

Understanding electronegativity teaches us why certain atoms in molecules attract more electrons and hence aids in understanding the behavior of molecules like acetonitrile where carbon and nitrogen exhibit a distinct electronegativity difference, affecting the molecule's physical properties.

Chemical Shift in NMR Spectroscopy

The chemical shift in Nuclear Magnetic Resonance (NMR) spectroscopy is an insightful indicator of the electronic environment surrounding a nucleus, often a hydrogen in organic compounds. When comparing the chemical shifts of the methyl hydrogens in acetonitrile, fluoromethane, and chloromethane, differences arise from varying electron cloud densities. The methyl groups in fluoromethane and chloromethane experience a greater deshielding effect due to the more electronegative atoms (fluorine and chlorine) drawing electron density away from the hydrogen atoms. Acetonitrile's lower chemical shift reflects the fact that its methyl hydrogens are better shielded, attributable to the triple bond's shortening effect and nitrogen's lower electronegativity.

Grasping the chemical shift concept helps students interpret NMR spectra and understand molecular structures, providing deep insights into a molecule's composition and conformation.

Bond Length and Dipole Moment

The bond length is a pivotal factor in determining a molecule's dipole moment. This vector quantity, representing the bond's polarity, is the product of the charge difference and the distance between the atoms. A longer bond length, like that in chloromethane, can diminish the dipole moment despite a significant electronegativity difference. This counterintuitive effect occurs because the increased distance between atoms gives more space for charge distribution, reducing the overall polarity even when the electronegativity difference is high.

Through the lens of bond length and dipole moment, we understand why the dipole moments of fluoromethane and chloromethane are nearly identical despite the difference in their atoms' electronegativity. It also helps elucidate why acetonitrile, with its triple bond and associated short bond length, exhibits a greater dipole moment.

Electron Shielding Effect

The electron shielding effect arises from the repulsion between electrons in different shells of an atom. In molecules like acetonitrile, the shielding effect has a substantial impact on the chemical shift observed in NMR spectroscopy. The electrons in inner shells repel the outer electrons, reducing the effective nuclear charge felt by said outer electrons. Thus, in acetonitrile, the shielding effect is greater around the methyl hydrogens than in fluoromethane or chloromethane, leading to the observed lower chemical shift due to the electron clouds being less pulled towards the more electronegative atoms.

Understanding electron shielding is crucial for interpreting the effects on chemical shift. It also explains variations in reactivity and physicochemical properties among organic molecules, which hinges on the arrangement and density of electrons around the atoms.