Hydrogen Bonding

Definition

(i) It is an electrostatic attractive force between covalently, bonded hydrogen atom of one molecule and an electronegative atom (F, O, N & sp carbon)

(ii) It is not formed in ionic compounds.

(iii) H-bond forms in polar covalent compounds, (not in non-polar).

(iv) It is very weak bond (strength 8-42 KJ/mol) but stronger than vander waal’s force.

(v) It is also known as dipole-dipole attraction.

Main condition for H-bonding

1. H-should be covalently bonded with high electro –ve element like F,O,N.

2. Electronegative element should be small in size.

Decreasing order of atomic size is –

N     >             O             >             F

Decreasing order of electromagenticvity is –

F      >             O             >             N

(4.0)               (3.5)                       (3.0)

3. Strength of H-bond Electronegativity of Z (element)

4. Hydrogen bonding occurs in HCN, due to (- ) triple bond (sp hybridisation), electronegativities of carbon and nitrogen increases.

Types of Hydrogen Bonding

Intermoleuclar H-Bond

H-bond formation between two or more molecules of either the same or different compounds known as Intermolecular H-bonding.

These are of two types:-

(i) Homo intermolecular:-

H-bond between molecules of same compounds

(ii) Hetro intermolecular:-

H-bond between molecules of different compounds

Intramolecular H-bond

It takes place within the molecule.

1. H-bonded with electronegative element of a functional group, form H-bond with another electronegative element present on nearest position on the same molecule.

2. This type of H-bond is mostly occurred in organic compounds.

3. It results in ring formation (Chelation).

Effect of H-bond on Physical Properties

(A)  Solubility

(a)  Inter molecular H-bonding

(I)     Few organic compounds (Non-polar) are soluble in water (Polar solvent) due to H-bonding.

(II)    Ketone, either, alkaline etc. are insoluble (no H-bond). Dimethyl ether is soluble in water

while diethyl ether is partially soluble, due to bulky ethyl groups H-bonding interrupts.

(III)   Solubility order- CH₃OCH₃ < CH₃OH

(IV)   It can form H-bond with water molecule so it can dissolve.

(b)  Intra molecular H-bonding

(I)   It decreases solubility as it form chelate by H-bonding, so H- is not free for other molecule.

(II)   It can not form H-bond with water molecule so it can not dissolve.

(B) Viscocity 

H-bond associates molecules together so viscosity increases.

(C) Surface Tension

Surface Tension of a liquid ………. extent of H-bonding.

(D) Melting point and boiling point

(I) Due to intermolecular H-bond M.P. & B.P. of compounds increases.

H₂O > CH₃OH > CH₃ – O – CH₃

(II) Boiling points of VA, VIA, VIIA hybrids are as shown below:

(Group 15 Hydrides)          SbH₃ > NH₃ > AsH₃ > PH₃

(Group 16 Hydrides)           H₂O > TeH₂ > SeH₂ > H₂S

(Group 17 Hydrides)           HF > Hl > HBr > HCl

(III)   But sudden increase in boiling point of NH₃ , H₂O and HF is due to H-bonding.

(IV)  H₂O > HF > NH₃ (BP comparison due to combined effect of strength and number of H-bond)

Intramolecular H-bonding gives rise to ring formation, so the force of attraction among these

molecules are vander waal force. So M.P. and B.P. are low.

(E)  Molecular weight

Molecular wt. of CH₃COOH is double of its molecular formula, due to dimer formation occur by H-bonding.

(F) Physical states

H₂O is liquid while H₂S is gas.

Water and Ice:- Both have H-bonding even then density of ice is less than water.

Volume of ice is more because of open cage like crystal structure, form by association of water

molecules with the help of H-bond.

H₂O becomes solid due to four hydrogen bond among water molecule are formed in tetrahedral

manner.