Hydrogen Bonding

Dr.Alhaiqa FarooqiPharm-DRIPS

Hydrogen Bonding:Definition:

A hydrogen bond is a dynamic attraction between a pair of typically electronegative atoms involving a hydrogen atom located between them. Hydrogen bonds are responsible for the life-giving properties of water.

Hydrogen Bonds:Polar molecules, such as water

molecules, have a weak, partial negative charge at one region of the molecule (the oxygen atom in water) and a partial positive charge elsewhere (the hydrogen atoms in water).The water molecule (O-H) bond is highly polar bond.

Hydrogen Bonding:

EFFECT OF HYDROGEN BONDING ON BOILING POINT: (ALCOHOL)

An alcohol is an organic molecule containing an -OH group. Any molecule which has a hydrogen atom attached directly to an oxygen or a nitrogen is capable of hydrogen bonding. Such molecules will always have higher boiling points than similarly sized molecules which don't have an -O-H or an -N-H group. The hydrogen bonding makes the molecules "stickier", and more heat is necessary to separate them.

Boiling point:Ethanol, CH3CH2−O−H, and methoxy methane, CH3−O−CH3, both have the same molecular formula, C2H6O.

Ethanol and Methoxy methane both have same no. of electrons.

Similar length of molecules.Vander Waals attraction and dipole-dipole interactions are same in both cases.

BOILING POINT: (Continued…)

ETHANOL: It has H atom attached

directly to O and O has exactly the same 2 lone pairs as in H2O molecule.

Hydrogen bonding occurs in Ethanol but not as effective as in water.

Ethanol(continued…)

Hydrogen bonding in ethanol is limited by the fact that there is only one H in each ethanol molecule with sufficient Partial + charge.

In Methoxy Methane:CH3OCH3

The lone pair on O are still there, but H are not sufficient Partial + charge for hydrogen bonds to form.

For its case Hydrogen must have attach to some highly electronegative atom for hydrogen bonding to occur.

BOILING POINT OF ETHANOL AND METHOXY METHANE:

Ethanol (with hydrogen bonding) 78.5°C

Methoxy Methane (without hydrogen bonding)

-24.8°C

BOILING POINTS COMPARING Alkanes, Ether and Alcohol:

Propane: Molecular Weight: 44Boiling point: -45°C

Dimethyl ether: Molecular weight: 46Boiling point: -25°C

Ethanol: Molecular Weight: 46 Boiling Point: +78°C

Why Ethanol has high boiling point?

Effect Of Hydrogen bonding on Water Solubility:

A hydrogen bonded substance can usually soluble in another hydrogen bonded substance.For-example: Alcohols are soluble in water, but alkanes are not.

Effect on Water Solubility:

WHY?Because non-polar substances can not break the hydrogen bonding present in the water molecule as in the case with alkenes.

----O H---------O H-----O

H

H H H

R HR H

NON-POLAR ALKANE MOLECULES CANNOT SLIP INTO THE HYDROGEN-BONDED SEQUENCE:

IN CASE OF ALCOHOL MOLECULE:AN Alcohol molecule, it is capable of hydrogen bonding. It can slip into the hydrogen bonded sequence in water. It can replace the hydrogen bonds that must be broken to let it in.

NON POLAR SOLIDS:

Non-Polar solids cannot be dissolved in water, otherwise they would break the hydrogen bonds between the water molecules. Therefore, it does not happen.

IMPORTANCE OF HYDROGEN BONDING