Question

Which compound in each group would have the most intense infrared absorption band corresponding to stretching vibrations of the bonds indicated? Give your reasoning. a. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}\) (multiple bond) b. \(\mathrm{CH}_{3}-\mathrm{CH}_{3}, \mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}(\mathrm{C}-\mathrm{C}\) vs. \(\mathrm{C}-\mathrm{O})\) c. \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}, \mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}\) (multiple bond) d. \(\mathrm{H}-\mathrm{Cl}, \mathrm{Cl}-\mathrm{Cl}\)

Short answer

a: (CH₃)₂C=O; b: CH₃-O-CH₃; c: CH₃C≡CH; d: H-Cl.

Step-by-step solution

Analyze Group a

In group a, we have two compounds: \((\mathrm{CH}_{3})_{2}\mathrm{C}=\mathrm{O}\) (acetone) and \((\mathrm{CH}_{3})_{2}\mathrm{C}=\mathrm{CH}_{2}\). The stretching vibration of interest here involves the multiple bond, specifically the C=O bond in acetone and C=C bond in the alkene. The C=O bond stretching vibration generally appears in the infrared (IR) spectrum as a sharp and intense peak because of the large difference in electronegativity between carbon and oxygen, which causes a significant dipole moment change during the vibration. Therefore, the compound with the most intense IR band will be \((\mathrm{CH}_{3})_{2}\mathrm{C}=\mathrm{O}\) due to the strong dipole moment of the carbonyl group.

Analyze Group b

In group b, the compounds are \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (ethane) and \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\) (dimethyl ether). We are comparing the C-C bond in ethane to the C-O bond in dimethyl ether. The C-O bond is polar due to the difference in electronegativity between carbon and oxygen, which results in significant dipole moment change when stretching. Thus, the compound with the most intense IR band will be \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}\).

Analyze Group c

For group c, we have \(\mathrm{CH}_{3}\mathrm{C}\equiv\mathrm{CH}\) (propyne) and \(\mathrm{CH}_{3}\mathrm{C}\equiv\mathrm{CCH}_{3}\) (2-butyne). Both feature a triple bond, but propyne includes a terminal alkynyl hydrogen that increases the polarity of the C≡C bond compared to the internal alkyne bond in 2-butyne. The terminal alkyne carbon also increases the dipole moment change during the stretch. Hence, \(\mathrm{CH}_{3}\mathrm{C}\equiv\mathrm{CH}\) will have a more intense IR absorption band.

Analyze Group d

Group d consists of \(\mathrm{H}-\mathrm{Cl}\) (hydrogen chloride) and \(\mathrm{Cl}-\mathrm{Cl}\) (chlorine gas). The H-Cl bond is more polar because of the large difference in electronegativity between hydrogen and chlorine, leading to a significant dipole moment. The Cl-Cl bond is nonpolar with no change in dipole moment during vibration. Therefore, \(\mathrm{H}-\mathrm{Cl}\) will exhibit the most intense IR absorption band.

Key concepts

C=O Bond Stretching

C=O bond stretching is a crucial concept in infrared absorption spectroscopy. When we talk about the C=O bond, we are referring to the carbonyl group, which is a carbon atom double-bonded to an oxygen atom. This bond is particularly important because it exhibits a strong and distinct peak in the IR spectrum. The intensity of this peak is due to the significant difference in electronegativity between the carbon and oxygen atoms.

• The electronegativity difference causes the C=O bond to have a strong dipole moment.
• During stretching, this large dipole moment changes significantly, which leads to intense IR absorption.
• This characteristic is why C=O bonds are easily detectable in IR spectroscopy, especially compared to C=C bonds, which do not have such a pronounced effect.

In IR spectroscopy, the presence of a carbonyl group can be identified by a sharp peak usually found in the range of 1650-1750 cm^-1. Acetone is a classical example, where its C=O bond contributes to a strong band in the IR spectrum, much more pronounced than similar compounds without this bond.

Dipole Moment

The concept of a dipole moment is integral to understanding IR spectroscopy. A dipole moment occurs when there is an uneven distribution of electrons between two atoms, leading to areas of partial positive and negative charges. This typically happens when atoms of different electronegativities bond together, like in H-Cl or C=O.

• A larger difference in electronegativity between bonded atoms results in a more significant dipole moment.
• During a bond's vibration or stretching in infrared spectroscopy, changes in the dipole moment determine the intensity of absorption.
• More significant dipole moment changes correspond to stronger IR absorption bands, meaning these bonds are more easily detected.

When analyzing compounds, those like hydrogen chloride (H-Cl) or carbonyl-containing molecules exhibit substantial dipole changes upon stretching. This results in their IR spectra displaying intense bands, making dipole moments a critical predictor of IR absorption intensity.

Polarity in Bonds

Polarity in bonds plays a vital role in determining the strength and characteristics of IR absorption. Polarity occurs when differing electronegativities between bonded atoms cause a shift in electron density, resulting in a dipole moment.

• In IR spectroscopy, polar bonds, such as the C-O in dimethyl ether, show intense absorption due to dipole moment changes during vibration.
• Conversely, nonpolar bonds like Cl-Cl do not show significant IR absorption because they lack a dipole and therefore do not change dipole moment during stretching.
• Understanding bond polarity can help predict which bonds will appear strongly in an IR spectrum.

The presence of polar bonds not only influences the sharpness and position of IR spectral peaks but also aids in resolving structural details about organic compounds, enabling chemists to determine functional groups and molecular interactions.

Electronegativity Effect

Electronegativity effect explains how the tendency of an atom to attract shared electrons influences molecular properties and the resulting IR spectra. It hinges on the principle that more electronegative atoms hold bonded electrons closer, creating polar bonds.

• Greater electronegativity differences lead to more pronounced polarity and higher dipole moments in bond formations.
• For example, in compounds like hydrogen chloride (H-Cl), the large electronegativity difference results in a strong dipole moment and thus an intense IR absorption.
• In comparison, similar electronegativities, as in a Cl-Cl bond, result in no dipole moment, showing negligible IR activity.

In essence, electronegativity determines how shared electrons are distributed, affecting both the chemical behavior and the IR spectral features. By assessing the electronegativity differences in molecules, chemists can predict the presence and intensity of specific bonds in IR spectroscopy, aiding in compound identification and analysis.