(ii) Intramolecular hydrogen bonding. When hydrogen bonding exists within the molecule it is called intramolecular hydrogen bonding. In such type of hydrogen bonding two groups of the same molecule link through hydrogen bond, forming a stable five or six membered ring structure e.g., salicylaldehyde, o-chlorophenol, acetylacetone, ethylacetoacetate etc.
This intramolecular hydrogen bonding was first called chelation (after the Greek word "Chela" meaning, claw) because in the same molecule the formation of a ring hydrogen bonding is a pincer like action resembling the closing of a Crab's claw. Some more examples of intramolecular hydrogen bonding are :
(iii) 
-Hydrogen bonding. Sometimes -electrons of an olefinic or aromatic system act as proton acceptor in hydrogen bonding. Such type of hydrogen bonding is called -Hydrogen bonding. These are again of two types :
(a) Intermolecular -hydrogen bonding. It occurs between two or more molecules of the same or different compounds. For example,
(b) Intramolecular -hydrogen bonding. This occurs between the groups of the same molecule. Hydrogen of a group (e.g., -- OH) forms hydrogen bond with -electrons of an aromatic ring or an olefinic bond. For example,
Properties of Hydrogen Bond
(i) Hydrogen bond is a bond of hydrogen between two electronegative atoms only. It never involves more than two atoms.
(ii) Bond energy of hydrogen bond is in the range of 3-10 kcal/mole. Thus hydrogen bond is a weaker bond than a covalent bond (bond energy of a covalent bond is 50-100 kcal/mole). But it is stronger than Van der Waal's forces (1 kcal/mol).
(iii) In the formation of hydrogen bond electron pair is not shared. In this respect it is different from the covalent bond.
(iv) The strength of hydrogen bond depends upon the electronegativity of the atom A to which hydrogen atom is attached with a covalent bond. As the electronegativity of A increases, strength of hydrogen bond increases. Thus HF will form most strong hydrogen bond as fluorine is the most electronegative atom.
(v) Typical hydrogen bond is linear. Angular hydrogen bonds exist in solids or in intramolecular hydrogen bonding.
(vi) The bond length A -- H and B -- H are generally different except HF2-(F -- H -- F) which is a symmetrical ion.
Effect of Hydrogen Bonding on Physical Properties of the Molecules
Unusual physical properties of H2O, HF, NH3, and alcohols can easily be explained on the basis of hydrogen bonding.
(1) Physical States
i) H2O and H2S. As we have already stated that the ease of formation of a hydrogen bond decreases as the electronegativity of the atom attached to the hydrogen decreases. Oxygen is more electronegative than sulphur. There is a considerable hydrogen bonding in H2O while in H2S the same is absent. H2O molecules are associated together in which hydrogen atom acts as bridge between two oxygen atoms and the intermolecualr distance decreases, therefore, H2O exists as liquid.
(ii) HF and HCl. In HF, molecules are associated through hydrogen bonding and it is a liquid at ordinary temperature.
In HCl, due to less electronegativity of chlorine atom and its large size, hydrogen bond does not exist hence molecules of HCl are not associated as in HF. Therefore, HCl is a gas at ordinary temperature.
(2) Melting Points and Boiling Points
(i) M.P. and B.P. of Hydrides of Oxygen, Fluorine and Nitrogen
The melting points and boiling points of compounds in a group of the periodic table increase with the increasing molecular weights. This is evident from the melting point and boiling point curves of IV A group hydrides. [(a) and (b)].
The melting point and boiling point of CH4, SiH4, GeH4, and SnH4 decrease with decreasing molecular weights. But in case of VA, VIA and VIIA groups the melting point and boiling point of H2O, NH3 and HF are exceptionally high than the hydrides of other members of their groups.
The melting point and boiling point of the hydrides of the elements of IVA, VA, VIA and VIIA groups can be represented as below :
IVA CH4 < SiH4 < GeH4 < SnH4
VA NH3 > PH3 < AsH3 < SbH3
VIA H2O > H2S < H2Se < H2Te
VIIA HF > HCl < HBr < HI
It is clear from these plots that there is a sudden increase in melting point and boiling point of HF, H2O and NH3. The existence of hydrogen bond in these molecules exceptionally increases their melting point and boiling point. Boiling point of water is higher than that of hydrogen fluoride because the extent of association through hydrogen bonding in water is more than hydrogen fluoride. Since CH4 cannot form hydrogen bond, its melting point and boiling point are the lowest among the hydrides of carbon family.
(ii) Melting point of ortho- nitrophenol and para- nitrophenol
Intramolecular hydrogen bonds do not involve molecular association and in these, change in physical properties is quite different than in case of intermolecular hydrogen bonds. For example the ortho isomers have lower melting point (Intramolecular hydrogen bonding) than respective para isomers (Intermolecular hydrogen bonding) as shown above.
Melting points of substituted Nitro compounds
