The nomenclature of hydrogen bonding can be a little bit confusing, so let's work through this problem to bring a little more clarity to the situation. So let's start by defining hydrogen bond donors and acceptors. Every hydrogen bond includes two components, a polarized XH bond in which the hydrogen end is the positive end and the X or heteroatom end is the negative end, and a second heteroatom that has a lone pair.
which is partially negatively charged. The hydrogen bond itself is the interaction between the positively charged hydrogen atom, at least partially positive, and the partially negative X heteroatom. Even though the black heteroatom is donating electron density towards the hydrogen, this is referred to as the hydrogen bond acceptor, in a sense because it's accepting the XH covalent bond from the blue portion.
Meanwhile, the XH bond or the molecule containing the XH bond is known as the hydrogen bond donor. So we see that in order for a molecule to act as a hydrogen bond donor, it must have this XH bond. And in the vast majority of cases, molecules that contain XH bonds also contain lone pairs on the heteroatom and so these can also act as acceptors.
though that's not a universal requirement for XH bonds. Hydrogen bond acceptors must contain a heteroatom bearing alone pair and there are many cases, many of which we'll see in these examples, where a molecule can serve as a hydrogen bond acceptor but not as a hydrogen bond donor because all of its heteroatoms are bonded to either other heteroatoms or to carbon. So let's go through...
each of these molecules and determine whether each can act as a hydrogen bond acceptor or hydrogen bond donor. So in the case of A, we see a nitrogen atom. This nitrogen has a lone pair on it, and so it can certainly act as a hydrogen bond acceptor. However, it does not include any nitrogen-hydrogen bonds, and so it cannot act as a hydrogen bond donor.
Molecule B has an oxygen which bears lone pairs, and so it, like A, can act as an acceptor. and it also includes an OH bond and so it can act as a hydrogen bond donor as well. Molecule C includes two heteroatoms, two oxygens, each of which has two lone pairs, and so C can, like A and B, act as a hydrogen bond acceptor. However, like A, the heteroatoms in C are not bonded to hydrogens, and so this molecule cannot act as a hydrogen bond donor.
Molecule D... is hydrogen fluoride and the fluorine has three lone pairs on it and so this molecule can act as a hydrogen bond acceptor. It also includes an HX bond, here the heteroatom is fluorine and so this can act as a hydrogen bond donor as well.
Now if we look at molecule E, molecule E includes no heteroatoms and as a result it can neither serve as a hydrogen bond donor or a hydrogen bond acceptor. So this will not participate in hydrogen bonding. at all.
Molecule F has an oxygen atom with two lone pairs and so can serve as a hydrogen bond acceptor. But again a lot like molecule A, the oxygen is not bonded to any hydrogens and so this can't serve as a hydrogen bond donor. Molecule G includes a carbonyl oxygen, again with two lone pairs. We're seeing this pattern over and over again.
And so this can act as a hydrogen bond acceptor. It can engage in a hydrogen bond with XH on something else, but it doesn't include any oxygen-hydrogen bonds itself, and so it cannot serve as a hydrogen bond donor. Molecule H includes two oxygen atoms as part of a carboxylic acid, and one of those is bonded to a hydrogen.
So molecule H can serve both as a hydrogen bond acceptor and a hydrogen bond donor. Molecule I includes no heteroatoms, and so A lot like case E, this molecule cannot engage in hydrogen bonding at all. Molecule J includes a nitrogen with one lone pair, and so it can serve as a hydrogen bond acceptor. It also includes nitrogen-hydrogen bonds, which can act as hydrogen bond donors.
So molecule J needs to go on our list of hydrogen bond donors. Molecule K includes two heteroatoms, an oxygen and a nitrogen. However, neither of these is bonded to a hydrogen atom, and so while molecule K can act as a hydrogen bond acceptor, it cannot act as a hydrogen bond donor.
And finally, molecule L includes this central oxygen atom with two lone pairs, but that oxygen is bonded to two phenyl rings and includes no bonds to hydrogen, and so molecule L can act as a hydrogen bond acceptor, but not a hydrogen bond donor. So hopefully running quickly through these examples, has shown you that as long as we can identify lone pairs and XH bonds within molecules, it's fairly straightforward to identify molecules that can hydrogen bond and can serve as either hydrogen bond acceptors with lone pairs or hydrogen bond donors with XH bonds.