Effects of Hydrogen Bonding on Physical Properties

(i) Boiling point and melting point

a. The boiling point of covalent compounds increases with increase in molecular weight and thus H20 (mol. wt. =18) should have low boiling point than H2S

b. Boiling point of HF (liquid) is greater than boiling . point of HCI (gas) .

c. NH3 has high boiling point than PH3

d. Alcohols: (R-OH) have high boiling point than those of corresponding thioalcohols (R-SH, mercaptans) and ethers (R-O-R).

e. b.pt. of water (100°C) is higher than ethyl alcohol (78°C) because greater number of water molecules are associated (through H-bonding) than the ethyl alcohol.

f. Polyhydric alcohols have higher boiling points than their ethers, inspite of increase in the molecular masses of ethers.

g. Boiling points of aldehydes and ketones are lower than alcohols.

h. The boiling points of primary (R-NH2) and secondary amines (R2NH) are higher than the isomeric tertiary amines (R3N) because like water and alcohols, the primary and secondary amines are capable of forming intermolecular hydrogen bonding but tertiary amine does not.

(ii). Solubility in water-

A hydrogen-bonded organic compound is usually soluble in another hydrogen-bonded substance. So, compounds whose molecule can form H-bonds with water molecule are soluble in water.

a. lower members of alcohols, acids, primary and secondary amines, phenols and carbonyl compounds ate more soluble in water, whereas alkanes, ethers and thiols are insoluble in water. 

(iii). States of matter-

H20 is liquid (high boiling point) and H2S is gas. Because in water molecule, hydrogen bonding is present and so become more compact and requires more energy to get them separated during evaporation, while sulphur is less electronegative nature than oxygen and give S-H bond, a smaller ionic character than the O-H bond and thus H2S forms very weak hydrogen bond

(iv). Stereo isomerism:  

the trans-isomer of indigo is so stabilised by H-bonding that it resists photochemical isomerisation to the cis-isomer,

(v). Adsorption: Intermolecular H-bonding plays a very important role in the process of dyeing of textiles.  

(vi). Hydrogen bonding In biological systems: H-bonding in biological systems also plays an important role and stabilizes usual structure of proteins and nucleic acids.

Surface chemistry - Complete ppt

States of matter

Thermodynamics -Complete ppt

HYDROGEN BONDING IN ORGANIC COMPOUNDS

In 1920 Latimer and Rodebush introduced the idea of  "hydrogen bond" to explain the nature of association in liquid state of substances like water, hydrogen fluoride, ammonia and formic acid, etc. In a hydrogen compound, when a hydrogen atom lies between two atoms having high electronegativities, it shows a unique property of forming a bond or a bridge between them, holding one of the atoms by a covalent bond and other by purely electrostatic forces (dipole-dipole attraction). This chemical combination between highly electronegative atoms of small atomic radius, such as F and N through a hydrogen bond is referred to as Hydrogen bond or Hydrogen bridge.

Hydrogen bond is defined as "an electrostatic attractive force between the. covalent bonded hydrogen atom of one molecule and an electronegative atom (such as F, 0, N) of the 'other molecule

Nature and Importance of Hydrogen Bonding 

 (i) Hydrogen bond is merely an electrostatic force rather than a chemical bond. 

  (ii) Hydrogen bond never involves more than two atoms. 

 (iii) Bond energy of hydrogen bond is in the range of 3 to 10 Kcal/mol or 10 to 40 kJ/moI, i. e., about 1/10th the energy of a covalent bond. . 

 (iv) With the increase of electronegativity of the atom to which hydrogen is covalently linked, the strength of' the hydrogen bond increases. 

 (v) All the three atoms in X -H- - -X lie in a straight line. 

Decreasing order of electronegativity of F>O>CI=N>Br>C>H

Electronegativity of an atom is a measure of its power to attract electrons that it is sharing in a covalent bond. 

The decreasing order of electronegativity of some common atoms which are generally encountered in organic chemistry is:

F >  O > CI= N > Br > C > H

Major sources of organic compounds

 There are two major sources of organic compounds: 

 (I) Natural and 

(II) Synthetic. 

 I) Natural

a).Plants- Carbohydrates (glucose, sucrose, starch, cellulose), acids, esters, vegetable oils, vitamins, gums, alkaloids, essential oils, perfumes, alcohol, acetone, etc. are some of the important organic compounds which are obtained from plants.

b).Animals-  Some of the important organic compounds which are derived from 'animals; are proteins, hormones, fats and urea,

 c).Fungi and microorganisms-This is the ·source responsible for the· production of hosts of useful substances like alcohols, acids, antibiotics (penicillin, streptomycin, tetracycline) 'and" vitamins by using the process of fermentation.

 c). Coal - Coal-tar obtained during destructive distillation of coal is the main source of aromatic hydrocarbons, phenols, heterocyclic compounds, dyes drugs; perfumes, etc. 

d). Natural gas and petroleum -  It is one of the major sources of organic compounds Gasoline, kerosene, lubricating oil, Vaseline, paraffin wax, etc., ate few examples which are obtained directly from petroleum by fractional distillation. Hundreds of useful organic compounds are prepared from the wide range of hydrocarbons which serve as starting materials for the synthetic reactions.

II). Synthetic Sources- More than 90% of the known organic compounds are synthetic, i.e., prepared by artificial methods. Simple organic ·compounds derived from petroleum and coal have been converted into useful products by synthetic methods.

Now-days, many of the natural products such as dyes, fibres, rubber, drugs, vitamins, etc., are prepared by synthetic reactions.