Question

Identify the compound with the lowest boiling point.

A) CH3OCH3 B) CH3CH2CH3 C) CH3CHO D) (CH3)2O E) CH3CN

* I need help understanding why the answer is B, when E has a lower molar mass. Also, I was under the impression that linear molecules such as B, have increased dispersion forces, and would ultimately have a higher boiling point.

Answer 1

we know that intermolecular forces strength order :

hydrogen bond > dipole -dipole > ion-dipole > dispersion forces

if intermolecular force strength increases boiling point increases .

now coming to that point.

A) CH3-O-CH3 . here C and O electronegaitivity is different . so there will be partial positive charge on C and partially negative charge on O will be formed . so it is dipole . intermolecular force is also dipole-dipole forces

like wise C) D) E ) all are also have dipole -dipole forces. except B)

B has only dispersion forces these are very weak forces. so readily evoporates into gas . so lesser boiling point for B)