Rank each group of alkanes in order of increasing boiling...

Rank each group of alkanes in order of increasing boiling point. Explain your choice of order.
a. CH$_3$CH$_2$CH$_2$CH$_2$CH$_3$, CH$_3$CH$_2$CH$_2$CH$_3$, CH$_3$CH$_2$CH$_3$
b. CH$_3$CH$_2$CH$_2$CH(CH$_3)_2$, CH$_3$(CH$_2)_4$CH$_3$, (CH$_3)_2$CHCH(CH$_3)_2$

Key Concepts

London Dispersion Forces

These are the primary intermolecular forces in nonpolar molecules like alkanes. They increase in strength with increasing electron cloud size and polarizability, which generally means that molecules with larger molecular weights or greater surface areas will have higher boiling points due to stronger attractions between molecules.

Molecular Size and Weight

The size and weight of an alkane largely dictate its boiling point since larger molecules typically have more electrons and a greater surface area to interact via London dispersion forces. As the chain length increases, the boiling point generally increases because more extensive intermolecular attractions require more energy to overcome.

Branching Effect

The degree of branching in an alkane affects its boiling point by altering its shape and the effective surface area available for intermolecular interactions. More highly branched alkanes tend to have lower boiling points compared to their straight-chain isomers because the compact structure reduces the surface area, thereby decreasing the strength of London dispersion forces.

Transcript

00:02 This is the answer to chapter 4, problem number 45 from the smith organic chemistry textbook.

00:09 And in this problem, we're given two sets of three molecules each.

00:13 And we're asked to arrange each in order of, just make sure i get it right, in order of increasing boiling point.

00:22 So what i'm going to do is i'm going to just write one through three under them.

00:26 So one will be the lowest boiling point.

00:30 Three will be the highest boiling point.

00:32 Okay.

00:34 And we need to remember the characteristics that we use to decide order of boiling points.

00:41 So the more carbons that a molecule has, everything else being equal, the more carbons that a molecule has, the heavier the molecule is going to be.

00:50 And so the higher its boiling point is going to be.

00:53 It's going to take more energy to get a heavier molecule to take off into the gas phase.

01:00 And so the other rule to keep in mind is that the more that a molecule branches, the less service area it's going to have.

01:13 And so more branching is correlated with a lower boiling point.

01:18 So more carbons is going to make for a higher boiling point.

01:21 More branching is going to make for a lower boiling point.

01:24 And so each of the parts of this problem actually addresses one of those two characteristics.

01:31 So when we look at a, there's no branching at all in any of these molecules, and we can make a direct comparison based on the number of carbons in each...

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