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QUESTION

2-methylheptane , octane and 2,2,4-trimethylpentane in order of increasing boiling point?

Good question. The boiling points of the alkanes should increase in the order, "2,2,4-trimethylheptane", "2-methylheptane", "octane".

The longer, and straighter the chain (i.e. the less unbranched) the greater should be the interaction between chains, and thus the greater the boiling point.

Now of course I did not know these boiling points off the top of my head, but I can account for their order:

"2,2,4-trimethylheptane," 99 ""^@C

"2-methylheptane," 116 ""^@C

"octane", 125 ""^@C

Longer, less branched chains have greater opportunity for intermolecular dispersion forces, and this is certainly reflected in the boiling point. That the longest chain alkane, "octane", is the most involatile supports our analysis. And even though "2,2,4-trimethylheptane" is the most massive molecule, branching prevents as effective intermolecular interaction as exists in the other pair.

The boiling point of n-"heptane" is 98.5 ""^@C (this I did know off the top of my head, because I found it to be a highly useful laboratory solvent), which is comparable to the boiling point of "2,2,4-trimethylheptane". Once again, branching serves to decrease the boiling point.