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PROPERTIES OF ORGANIC

MOLECULES

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General properties

Most organic compounds are liquids and

gases at room temperature. The melting and

boiling points of substances is determined by

the intermolecular forces existing betweenmolecules.

Most organic molecules are non-polar. They

are slightly soluble or insoluble in water.

Hydrocarbons dissolve in non-polar 

solvents like tetrachloro methane or xylene.

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 ALKANES

� Hydrocarbon molecules are usuallyunreactive molecules. They release

less energy during combustion than

other organic molecules.

� Less dense than water.

� Immiscible with water ± due to their 

non-polar nature.� During combustion they form mainly

carbon dioxide and water.

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� The melting and boiling points increase

with the increase in carbon chain length.

Methane is a gas at room temperaturewhile pentane and hexane are liquids.

This is because the size of the

intermolecular forces increase withincrease in carbon chain length.

Hydrocarbons are environmental pollutants

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 ALKENES

� Unsaturated hydrocarbon molecules

� Less dense than water.

� Immiscible with water ± due to their non-polar nature.

�  As with alkanes, the boiling and

melting points increase with increasein molecular size.

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� Alkenes are formed by the cracking of an

alkane. When a long-chain alkane is heated

in the presence of a catalyst, it breaks intosmaller molecules.

�Alkenes undergo polymerisation reactions

in which alkene molecules add tothemselves to form long chains.

CH2=CH2 momomer 

CH3CH2CH=CH2 dimer 

CH3CH2 (CH2CH2)nCH=CH2 polymer 

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 Alkenes form GEOMETRIC ISOMERS.

These are molecules with the samemolecular and structural formulae, but differ 

in spatial orientation.

This arises because of the double bond inthe molecule. VSEPR theory predicts that

the bonding electrons spread as far apart as

possible. The bond angles around the

double bond are 120o No rotation about thedouble bond is possible. A cis- and a trans-

form of the molecule forms.

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Geometric Isomers

These two isomers have the same molecular 

and condensed structural formulae.

They differ only in the directions taken by thetwo CH3 groups attached to the double bond.

The two isomers have different boiling points.

cis -2 -butene trans -2 -butene

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 ALKYNES

�Unsaturated hydrocarbon molecules.

The triple bond makes these molecules

very reactive.

� Less dense than water.

� Immiscible with water ± due to their 

non-polar nature.

� As with alkanes, the boiling and melting

points increase with increase in molecular 

size.

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 Alcohols contain the hydroxide group (OH)

substituted for hydrogen.

The ±OH group is very polar and causesalcohol molecules to attract each other by

hydrogen bonding.

- O - H O -H

 Alcohols have higher boiling points because

of the hydrogen bonds.

HYDROCARBON DERIVATIVES:

 ALCOHOLS

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 Alcohol molecules have a non-polar 

hydrocarbon end and a polar ±OH section.

 Alcohols are solvents for polar and non-polar 

solutes.

Polar end

Non -polar 

end

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Methanol CH3OH

Prepared by the dry distillation of wood.

Methanol is very poisonous.

Ethanol CH3CH2OH

Ethanol is present in alcoholic beverages and

is formed during the fermentation of sugars.

C6H12O6 2CH3CH2OH + H2O

Sugar ethanol

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Glycerol

1,2,3 propantriol Commonly calledglycerine

Used as a lubricant.

Has a high viscosity (like syrup) and boiling

point due to the strong hydrogen bonds

between the molecules.

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HYDROCARBON DERIVATIVES:

CARBOXYLIC ACIDS

Carboxylic acids contain the ±COOH or 

carboxyl group.

methanoic acid ethanoic acid

These acids ionise when dissolved in water to a small extent. They are thus weak acids.

CH3COOH + H2O CH3COO- + H3O+

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REVISION of INTERMOLECULAR FORCES

There are three types of intermolecular forces -

Hydrogen bonds

 An electrostatic attraction between a lonepair of electron on an oxygen, nitrogen or 

fluorine atom covalently bonded to a

hydrogen atom in a molecule and a hydrogenin an adjacent molecule.

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London Forces

Very weak electrostatic attraction caused by

momentary dipoles set up due to theasymmetric distribution of electrons in atoms

or non-polar molecules.

Dipole-dipole forces

 An electrostatic force between oppositelycharged ends of polar molecules.