When another atom or group of atoms is substituted for one...

When another atom or group of atoms is substituted for one of the hydrogen atoms in benzene, $\mathrm{C}_{6} \mathrm{H}_{6},$ the boiling point changes. Explain the order of the following boiling points: $\mathrm{C}_{6} \mathrm{H}_{6}, 80^{\circ} \mathrm{C} ; \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}, 132^{\circ} \mathrm{C}$ $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}, 156^{\circ} \mathrm{C} ; \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}, 182^{\circ} \mathrm{C}$.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractive interactions between molecules that determine many physical properties, including boiling points. In comparing benzene and its derivatives, differences in dispersion forces, dipole–dipole interactions, and hydrogen bonding can explain why some compounds have higher boiling points than others. Stronger intermolecular interactions require more energy to overcome during the phase change from liquid to gas, leading to a higher boiling point.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole–dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) and is attracted to a lone pair of electrons on another electronegative atom. In aromatic compounds like phenol, the O–H group forms hydrogen bonds, significantly increasing the boiling point compared to molecules that lack this capability.

Substituent Effects on Physical Properties

Substituent effects refer to how adding different atoms or groups to a molecule, such as replacing a hydrogen in benzene with halogens or hydroxyl groups, alters the physical properties. These effects include changes in polarity, the ability to form specific intermolecular interactions, and modifications to the overall electron distribution, all of which can influence boiling points. For instance, polar substituents can lead to increased dipole interactions, thereby raising the boiling point.

Molecular Weight and Polarizability

Molecular weight and polarizability are important factors in determining the strength of London dispersion forces, a type of van der Waals force. Heavier atoms such as bromine, which are more polarizable, result in stronger dispersion forces, contributing to a higher boiling point. In benzene derivatives, as the substituent increases in size and polarizability, the boiling point tends to increase due to enhanced intermolecular attractions.

Transcript

00:01 So in this question, we've basically given a series of molecules and their boiling points.

00:05 Yeah, and they're boiling points.

00:07 So we have to explain why this happens.

00:09 So we have, pardon me, c6, h6 has a smaller boiling point than c6 h5cl.

00:19 So basically one of the hydrogens has been replaced with chlorine, which has a smaller boiling point than c6 h5br, which is the same thing.

00:30 But instead of chlorine we have bromine, which has a smaller boiling point than c6 h5 -o -h, which is to say that this chlorine or bromine group over here got replaced with an oh branch, that sort of thing.

00:47 And so we have to explain why.

00:51 They were given numbers for these, but it doesn't really matter.

00:53 And so now the reason c6h6 is the weakest is because all these bonds are non -polar.

01:01 And so the interactions like the dipole dipole interactions or the hydrogen bonding interactions aren't going to happen for c6h6 the benzene molecule.

01:11 And so the only force that's playing a role here is the london dispersion forces.

01:15 And as you'll see later, all these other ones have other forces in addition to that.

01:20 And so those are going to play a stronger role.

01:23 The next one, c6h5cl.

01:26 Now we have a polar bond between the chlorine and the carbon that's next to it.

01:31 And so what that means is we're going to have dipole -dipole interactions, and we're also still going to have significant light and dispersion forces.

01:41 In fact, probably more here than we did for the benzene because chlorine is a larger atom than hydrogen, significantly larger.

01:50 And so with many more electrons...

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