Chemistry 201 – Boiling Points

Question:

Why does CH3C(O)CH2CH3 have a higher boiling point than CH3OCH2CH2CH3? They both have D-D bonds and I thought since the ether is more linear it would have a higher boiling point.

Answer:

Ketones have a MUCH higher dipole than ethers.

Depending on the class, ethers are often considered to have NO dipole. Mind you, in Chem 201, ethers are considered to have a weak dipole.

Assuming very similar molecular weights, the boiling points of

Carboxylic Acids > Alcohols > Ketones > Aldehydes > Esters > Ethers > Alkanes

Ketones have a stronger dipole than aldehydes (the C-H dipole on an aldehyde is insignificant).

Here are some actual values:

propanoic acid 141.2 °C
butanol 117.7 C
butanone 79.64 C
butanal 74.8 C
methyl ethanoate 57.1 C
diethyl ether 34.6 C

*1 less carbon was used in esters/carboxylic acids to account for the extra oxygen present. Hence all compounds above have similar molecular weight/number of electrons.

Stacking and hence a linear molecule is more important when you’re comparing the SAME functional group.

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