Lowest Boiling Point

[emminent]

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Which has the lowest boiling point:

heptane
2,3-dimethylpentane
2,2,3-trimethylbutane
3-methylhexane
octane

So answer is 2,2,3-trimethylbutane but I recalled that Chad said the longest chain has the lowest boiling point and you only consider branching if there is an "equal" number of C's. I thought it would be octane even though 2,2,3-trimethylbutane has a total of 7 C's.

 

[jwan14]

Branching raises melting point and lowers boiling point. That's what I recall Chad saying. Branched molecules fit within a crystal better so it takes a higher temperature to melt. In liquid the molecules remain further apart because of the branching and therefore boil at lower temperatures. That's what I remember but I may be wrong about the reasons!

 

[jwan14]

And branching significantly changes the boiling point while adding 1 carbon and 3 hydrogens wouldn't as much. Heptane boils at a higher boiling point than octane generally but only because of London dispersion forces. Octane is a bit heavier than heptane thus higher London dispersion forces. Jumbled I know but I'm fried haha

 

[discensdentibus]

Octanes got more VDW forces. for instance, BP of ethanol>BP of methanol
straight chain alkanes methane thru butane are gases at stp and then liquid from pentane on. Why are they liquid? Greater IMFs come with longer chain length. What he said^