why do heteroatoms make such a difference in the physical and chemical properties of an organic molecule?

Answer 1

The ordinary chemistry that we are always involved with is all about electrons and electron movement. Normal hydrocarbons protect electrons quite well. That is a reason they do not interact strongly with other compound, London forces are very weak. The heteroatoms have two things going for them. Since they have more protons in their nuclei, they do not need protons to be attached to the electrons to balance thier charge. Secondly, because they have more protons in their nuclei, they also polarize bonds more by pulling them away from the atoms they are attached to, thus making them more reactive.

Amines are basic as the electrons of nitrogen are able to interact with protons from water, alcohols, and carboxylic acids. Alcohols can be converted into leaving groups because they can be protonated and make water a good leaving group. They are electron donors for hydrogen bonding and their protons are good hydrogen bond donors. The greater the interaction in compounds they greater their mp and bp may be.

These examples illustrate a pattern of reactivity and properties that your question asked about.

Answer 2

they're different, aren't they? example, replacing a CH by N in a hydrocarbon turns it into an amine, related to ammonia, and introduces bonds that are more polar than C-H..