[kilian22]

Hi,

I have a problem finding out an answer to a pretty simple question.

At the last exam that I failed, the question was which intermolecular forces are in alkanes, alkenes, alkines, ketones and carboxylic acids. I`m still not sure what is the answer, so if someone could help me I would be very grateful.

[Dan]

Which types of intermolecular forces do you know?

[kilian22]

I know Van der Walls/London, hydrogen bonding, polar bond. English is not my first language, so excuse me if i wrote something wrong.

In what I did find on the internet I would say :

Alkane, alkene, alkine - London dispersion forces

Ketone - polar bond

Carboxylic acids - hydrogen bond

Is this correct?

[Dan]

What you have written is correct, but it is incomplete - you do not have to choose only one for each molecule, two or three types of intermolecular force can operate together.

[kilian22]

Yes, I understand that. But the question was to name one, so I guess the professor meant which one is dominant or something like that. 

[Babcock_Hall]

Dominant?  That might mean strongest, or it might mean universal (I suspect we should focus on the latter).  I agree, for example, that carboxylic acids have the ability to form hydrogen bonds with other molecules.  What other forces do they have?

[kilian22]

Well, as I said earlier the question was to name one specifically for each compound. I guess he meant the strongest one and to explain difference between them because of a different intermolecular force.

As I understand, every compound has Van der Waals forces, but the compounds sometimes also have stronger and more relevant forces regarding this question. For example hydrogen bond in carboxylic acids, because of which they have higher boiling points or something like that.

[Babcock_Hall]

I think it might be worth distinguishing various subtypes of Van der Waals attractive forces.  There are (1) dipole-dipole, (2) dipole-induced dipole, and (3) induced dipole-induced dipole.  All molecules have (3).  To have (1) a molecule must have a permanent dipole.

It might also be worth distinguishing molecules that are hydrogen bond donors, H-bond acceptors, or both H-bond donors and acceptors (only a few chemical species fall into the category of being only H-bond donors).  Carboxylic acids are both H-bond donors and acceptors.    How about ketones?  Besides whatever H-bond forces that a ketone might be able to undergo, what other forces are present?  You called it a "polar bond" above.  Do you mean that it has a permanent dipole moment?