Intermolecular and Intramolecular Hydrogen Bonding — Definition

Definition

Imagine a tiny magnet. When you bring two magnets close, they attract each other. Hydrogen bonding is a bit like that, but on a molecular level, and it's not as strong as a full magnet, more like a weak, specific attraction. For a hydrogen bond to form, we need three key players:

1
A Hydrogen Atom (H): — This hydrogen atom must be directly attached (covalently bonded) to a very greedy atom. We call these greedy atoms 'highly electronegative'. The most common ones are Fluorine (F), Oxygen (O), and Nitrogen (N). Think of F, O, N as the 'FON' atoms.
2
A Highly Electronegative Atom (Donor): — This is the F, O, or N atom that is *covalently bonded* to the hydrogen atom. Because it's so greedy, it pulls the shared electrons in the H-X bond (where X is F, O, or N) very strongly towards itself. This makes the hydrogen atom slightly positive (we call this a partial positive charge, denoted as $\delta^+$) and the electronegative atom slightly negative (partial negative charge, $\delta^-$).
3
Another Highly Electronegative Atom (Acceptor): — This is a second F, O, or N atom, which can be in the *same molecule* or a *different molecule*. This atom has a lone pair of electrons and a partial negative charge ($\delta^-$). The slightly positive hydrogen atom from the first molecule is then attracted to this slightly negative, electron-rich F, O, or N atom.

So, a hydrogen bond is essentially an electrostatic attraction between a partially positive hydrogen atom (bonded to F, O, or N) and a partially negative F, O, or N atom (which has a lone pair). It's represented by a dotted line (e.g., X-H$\cdots$Y, where X and Y are F, O, or N).

Now, depending on *where* this attraction happens, we classify hydrogen bonding into two main types:

Intermolecular Hydrogen Bonding: — 'Inter' means 'between'. So, this type of hydrogen bonding occurs *between two different molecules* of the same or different compounds. For example, water molecules (H$_2$O) form strong intermolecular hydrogen bonds with each other. This is why water has a relatively high boiling point compared to other similar-sized molecules. Alcohols and carboxylic acids also exhibit strong intermolecular hydrogen bonding.

Intramolecular Hydrogen Bonding: — 'Intra' means 'within'. This type of hydrogen bonding occurs *within the same molecule*. For this to happen, the molecule must have both the hydrogen donor (H bonded to F, O, or N) and the hydrogen acceptor (F, O, or N with a lone pair) present in suitable positions to form a stable five- or six-membered ring structure. A classic example is o-nitrophenol, where the hydrogen of the -OH group forms a bond with the oxygen of the -NO$_2$ group within the same molecule. This type of bonding often leads to different physical properties compared to intermolecular bonding, such as lower boiling points and increased volatility, because the molecule's ability to interact with other molecules is reduced.