Need help on some multiple choice questions!

For the 1st one, do we need to think about the intermolecular forces involved for each option?

If so then I think butanal only has permanent dipole - dipole forces in place. Boiling points also rise up as the molecule get's bigger and bigger but I think this cannot be used as all the options are practically around the same size.

For the 2nd one, not too sure what to do. If it's asking for the number of structural isomers but the carbon chain has to stay straight, so chain isomerism might be ignored here. So where left with only thinking about positional and functional isomerism.

For the last MCQ, if the species is to act as a nucleophile then it has to have a lone pair on it. Does it also have to a negative charge too. Would I have to draw out the whole molecule itself for each option and see if it has a lone pair or not to behave as a nucleophile?

Any help will be really appreciated! Thanks

yes

Butanal also has induced dipole-dipole forces. Anyway, consider all options; not just one. What are the forces in the other molecules?

Yes. Drawing the structures might be a good start

Only needs a lone pair. Negative charge not needed.

Thanks for replying!

Okay so this is what I got:

For the first one it has to be answer B, this is because butan-2-ol has hydrogen bondning in it which is known to be the strongest intermolecular forces out of them all. Whereas with butanal there is permanent dipole dipole forces, butene with VDW's and lastly 1-fluorobutane with VDW's. Would 1-fluorobutane also have dipole-dipole forces acting on them becuause it has the polar bond C-F since F is a very electronegative atom?

With the 3rd one I got an answer of C, as I see this is the only molecule that appears to have a lone pair of electrons when drawing them out and looking closely at the valence electrons. This may be a bit off topic but I see that BH3 only has 3 covalent bonds in place, leaving with B a outer shell of 6 electrons. Doesn't it always have to be that atoms have to have a outer shell that is an octet apart from hydrogens only where having 2 electrons is acceptable.

With the 2nd MCQ, I'm it took me a bit of time drawing out all the possible outcomes that could be made with C4H8Br2, but I think that there are 6 structural isomers that could be made which gives an answer of C

1-fluorobutane has permanent dipole-dipole forces + induced dipole-dipole

I agree with the answers

Thank you!

Could you let me know also why in BH3, b only has 6 electrons and not 8 electrons when covalently bonding to 3 H atom? Shouldn't this not be possible because atoms always strive to achieve a full electron configuration i.e. have an octet?

If BH3 was ionic, B would form B³⁺ and H would form H-. The total energy required to form B³⁺ is too high, and the lattice energy (+ other exothermic changes) is not exothermic enough to compensate for the energy needed. So B primarily forms covalent compounds

Since it has 3 electrons, it bonds to 3 H atoms and has a sexlet of electrons. It has an incomplete octet.