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Chapter 11: Problem 32

The generic structural formula for a 1 -alkyl-3-methylimidazolium cation is where \(\mathrm{R}\) is \(\mathrm{a}-\mathrm{CH}_{2}\left(\mathrm{CH}_{2}\right)_{n} \mathrm{CH}_{3}\) alkyl group. The melt- ing points of the salts that form between the 1 -alkyl3-methylimidazolium cation and the \(\mathrm{PF}_{6}^{-}\) anion are as follows: $\mathrm{R}=\mathrm{CH}_{2} \mathrm{CH}_{3}\left(\mathrm{~m} \cdot \mathrm{p} .=60^{\circ} \mathrm{C}\right), \mathrm{R}=\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ $\left(\mathrm{m} \cdot \mathrm{p} .=40^{\circ} \mathrm{C}\right), \mathrm{R}=\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\left(\mathrm{~m} \cdot \mathrm{p} .=10^{\circ} \mathrm{C}\right),$ and $\mathrm{R}=\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\left(\mathrm{~m} \cdot \mathrm{p} .=-61^{\circ} \mathrm{C}\right) .$ Why does the melting point decrease as the length of alkyl group increases?

Short Answer

Expert verified
The melting point of salts formed between the 1-alkyl-3-methylimidazolium cation and the PF6^- anion decreases as the length of the alkyl group increases because the increase in van der Waals forces between the alkyl chains is not sufficient to compensate for the decrease in electrostatic forces between the cations and anions due to the larger separation of charges. This leads to weaker overall intermolecular forces, requiring a lower temperature to break these forces and melt the substance.

Step by step solution

01

Identify the given information

We are given the melting points of salts formed between the 1-alkyl-3-methylimidazolium cation and the PF6^- anion for different alkyl groups R: - R = CH2CH3 (m.p. = 60°C) - R = CH2CH2CH3 (m.p. = 40°C) - R = CH2CH2CH2CH3 (m.p. = 10°C) - R = CH2CH2CH2CH2CH2CH3 (m.p. = -61°C) Here, the melting point decreases as the length of the alkyl group increases.
02

Examine the structure of the cation and anion

The 1-alkyl-3-methylimidazolium cation has a positively charged nitrogen atom in the imidazolium ring and a long alkyl chain on one side, while the PF6^- anion contains a central phosphorus atom surrounded by six fluorine atoms with a negative charge, making it quite symmetric.
03

Analyze the forces affecting the melting point

The melting point of a substance is influenced by the strength of the intermolecular forces between its molecules. In this case, those forces come from two main sources: electrostatic forces between the cations and anions, and van der Waals forces between the alkyl chains.
04

Evaluate the effect of increasing alkyl chain length

As the length of the alkyl chain (R) increases, the van der Waals forces between the alkyl chains will also increase. These forces arise from momentary fluctuations in electron distribution within the molecules, leading to temporary dipoles that attract the neighboring molecules. On the other hand, the overall strength of the electrostatic forces between the cations and anions will decrease. This happens due to the larger separation of charges as the bulky alkyl chains push the charged nitrogen atom farther away from the anion.
05

Conclude why the melting point decreases

The melting point decreases as the length of the alkyl group increases because the increase in van der Waals forces between the alkyl chains is not sufficient to overcome the decrease in electrostatic forces between the cations and anions due to the larger separation of charges. The weaker overall intermolecular forces result in a lower temperature needed to break these forces and cause the substance to melt.

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Most popular questions from this chapter

Freon, \(\mathrm{CCl}_{2} \mathrm{~F}_{2},\) and dichloromethane, \(\mathrm{CH}_{2} \mathrm{Cl}_{2},\) are common organic substances. Freon is a gas with a normal boiling point of \(-29.8^{\circ} \mathrm{C}\); dichloromethane's normal boiling point is \(39.6^{\circ} \mathrm{C}\). Which statement is the best explanation of these data? (a) Dichloromethane can form hydrogen bonds, but freon cannot. (b) Dichloromethane has a larger dipole moment than freon. (c) Freon is more polarizable than dichloromethane.

Indicate whether each statement is true or false: (a) The liquid crystal state is another phase of matter, just like solid, liquid, and gas. (b) Liquid crystalline molecules are generally spherical in shape. (c) Molecules that exhibit a liquid crystalline phase do so at well-defined temperatures and pressures. (d) Molecules that exhibit a liquid crystalline phase show weaker-than- expected intermolecular forces. (e) Molecules containing only carbon and hydrogen are likely to form liquid crystalline phases. (f) Molecules can exhibit more than one liquid crystalline phase.

Ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) and pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\) are both liquids at room temperature and room pressure, and have about the same molecular weight. (a) One of these liquids is much more viscous than the other. Which one do you predict is more viscous? (b) One of these liquids has a much lower normal boiling point \(\left(36.1^{\circ} \mathrm{C}\right)\) compared to the other one \(\left(198^{\circ} \mathrm{C}\right)\). Which liquid has the lower normal boiling point? (c) One of these liquids is the major component in antifreeze in automobile engines. Which liquid would you expect to be used as antifreeze? (d) One of these liquids is used as a "blowing agent" in the manufacture of polystyrene foam because it is so volatile. Which liquid would you expect to be used as a blowing agent?

(a) What phase change is represented by the "heat of vaporization" of a substance? (b) Is the process of vaporization endothermic or exothermic? (c) If you compare a substance's heat of vaporization to the amount of heat released during condensation, which one is generally larger (consider the numerical value only)?

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