The boiling point of branched hydrocarbons is lower than the boiling point of straight chains hydrocarbons (I am talking about Isomers--> same number of Carbon atoms ) Why is this so? What does that have to do with intermolecular forces?

Branched chain hydrocarbons take up more space than straight chain ones. Straight chain hydrocarbons are able to pack more neatly, and have more weak interactions (hydrogen bonding, van der Waals, etc) that hold them together. This means they are have higher BP.

i think i have a better explanation :

 

Straight chain molecule has a elongate shape whereas brached chain has a slightly spherical shape .

 

electrons in the straight chain is more disperse whereas in branched chain the electrons are packed more compactly .

HENCE ,

 

the electron cloud of strainght chain is easy to be distorted .That is more polarisable .

Thus straight chain is more polar compared to brabched chain .

 

In addition .

 

the surface of contatct between straight chain molecules is larger compared to spherical shaped branched chain .

 

Hence the Van der Waals force between staight chain molecules is stronger then that between branched molecule !!!

 

As a result ,

the boiling point of the straight chain isomer is higher than the boiling point of the Branched chain isomer