Question

when does chlorine form hydrogen bonding, there are two cases explain both​

Answer 1

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Answer 2

Answer:

The chlorine atom is too large. Despite its electronegativity, the size of the atom is such that its electron density is too low to form hydrogen bonds. This is why Chlorine does not display hydrogen bonding while nitrogen does.

Explanation:

Both cases explanation:-

Why can chlorine atoms not form hydrogen bonds even though they have very similar electronegativity to nitrogen, which can?

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This is a great question.

The answer is a little complicated.

I think the place to start with this question is to establish the difference between a dipole-dipole interaction and a hydrogen bond.

A dipole-dipole interaction is an electrostatic interaction. Hydrogen chloride, for example, has a negative dipole on the chlorine atom and a positive dipole on the hydrogen atom. A group of hydrogen chloride molecules align such that the negative chlorine atoms of one molecule of hydrogen chloride will be proximal to the positive dipole of another hydrogen chloride molecule. Thus, there is an interaction between a chlorine atom and a hydrogen atom in a group of hydrogen chloride molecules, but it is not a hydrogen bond.

A hydrogen bond has an electrostatic component, but what differentiates it from a dipole-dipole interaction is that it also has a covalent component. The HOMO (highest occupied molecular orbital), the nonbonding electrons, of the donor mixes with the LUMO (lowest unoccupied molecular orbital) of the hydrogen atom, which would be the sigma antibonding orbital. Unlike dipole-dipole interactions, orbital interactions are very directional, which is why groups of water molecules form into well-defined tetrahedra whereas groups of hydrogen chloride molecules do not have a well-defined geometry.

I was having a conversation about hydrogen bonds with a friend over beers , and he pointed out that a hydrogen bond can be thought of as an incomplete acid-base reaction where the base has partially reacted with the acid. It's almost like the reaction stopped in the transition state.

With the difference between a hydrogen bond and a dipole-dipole interaction established, let's get back to the question. Why don't chlorine atoms make good hydrogen bond acceptors?

There is no (or very little) orbital mixing between the HOMO of chlorine and the LUMO of hydrogen. This is because chlorine is in row 3. As you move down the rows, the difference in the size between the hydrogen LUMO and the acceptor orbitals becomes larger resulting in poor overlap and less mixing (all the good acceptors are in period 2). Without orbital mixing, there is no hydrogen bond - only a dipole-dipole interaction.