Intra-molecular Hydrogen Bond

You have seen that Hydrogen bond is formed between Hydrogen bond donor and Hydrogen bond accepter. If both of them are present in the same molecule and at a close distance, they can form Hydrogen bond. In this case it is called Intra-molecular Hydrogen Bond.

Let’s study it with a few examples, in acetylacetone molecule when you draw kekulé structure you will be able to distinguish Hydrogen bond donor and Hydrogen bond accepter. Hydrogen bond accepter C=O^δ- shares its lone pair with ^δ+H-O and makes Hydrogen bond.

Other example is salicylaldehyde, it also has Hydrogen bond accepter C=O^δ- which shares its lone pair with Hydrogen bond donor ^δ+H-O and makes Hydrogen bond. You have seen that if Hydrogen bond acceptor and donor both are present in the same molecule they are likely to form intra-molecular hydrogen bond. But in some molecules geometry doesn’t allow them to make hydrogen bond.

Let’s compare two different molecules which have similar groups at different position Ortho- nitro phenol and Para-nitro phenol. Both molecules have N-O^δ- and ^δ+H-O but in the latter molecule (para) the two groups are far apart, so they cannot form Hydrogen bond.

Hydrogen bonds are important for molecules as they affect a number of properties of molecules and they are important for our lives too. You know DNA and Proteins are crucial for our life. Hydrogen bonding gives them their unique structure and properties.

You may wonder how Hydrogen bonding affects the properties of molecules. Let’s try to understand it, Intermolecular hydrogen bonding binds together a number of molecules so it affects properties that depend on number of molecules like boiling point, melting point, viscosity etc. Intramolecular hydrogen bonding affects the property of that particular molecule like acidity, reactivity, stability.

You have learnt about elements and there relations and you have become familiar with molecules. In the coming posts we will study how molecules react and form a new molecule. We can neither count the number nor can we weigh as molecules are too small to see. So we will need a measuring unit that suits for electrons, atoms as well as molecules. In the next post we will learn about this measuring unit and see how it suits all of them.

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