24. Organic Chemistry

       Organic Chemistry

Carbon is unique in the variety of molecules it can form.

Three Special features of covalent bonding involving Carbon:

1. Carbon atoms can join each other to form long chain. Atom of each element can attach to each to form     long chains.

2. The carbon atoms in the chain can be linked by single, double, triple Covalent bond.

3. Carbon atoms can also arrange themselves in ring.

Atoms of other elements (for example sulphur) can imitate parts of this versatility. But only carbon can achieve all these different bonding arrangements which can produce different types of molecules.      

The chemistry of these molecules forms a separate branch of subjects known as organic chemistry.

Another definitions of Organic chemistry:

Organic chemistry is the subject of chemicals derived , directly or indirectly from living organisms.

Organic compounds :

+ Form from living things

+ Found in living organisms(fat,sugar,urea etc)

Classification of Organic chemistry :

Homologous series  is a family of organic compound that :

1. Have the same general formula.

2. Have similar chemical properties.

3. Show a gradual increase in physical properties(for eg. melting and boiling point).

Functional Group are specific group of atom or bonds within molecules that are responsible for the characteristic chemical reaction of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecules it is a part of. However, its relativity can be modified by nearby functional groups.  

Saturated Hydrocarbons are compounds consisting of single covalent bonds between carbon atoms. No double bond is present.

Unsaturated Hydrocarbons are compounds consisting of double bonds between carbon atoms which can break down to form individual bonds.

                                                                                           Alkanes
Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from burning or combustion, which is their reaction with oxygen in the air.

Physical Properties  

+ Made of carbon and hydrogen only, i.e. are hydrocarbons.   

+ Saturated hydrocarbons
+ General formula C_nH_2n+2 ("n" represents the number of carbon atoms present).

+ Mainly unreactive (The single molecualr bonds (C-C, C-H) are very stable and hard to break, therefore unreactive).  .

+ As the length of the chain increases the opportunity for weak interaction between the molecules (van der Waals force) is increased.
+ Can form long chains starting with CH₄, Methane.

Note that the name of this series of hydrocarbon all end with ........ane

Chemical Properties

+ 1 chemical property that these alkanes have in common is that they burn very exothermically.

+ Compared to other hydrocarbons alkanes are less reactive because:

   1. They have no functional group.

   2. They are saturated(only carbon to carbon single bonds).

+ Combustion

As like all hydrocarbons, alkanes take part in combustion reaction to produce -

    1. Burns in the presence of oxygen to give off carbon dioxide and water.

    2. Forms carbon monoxide in absence of oxygen.

+The reaction between alkanes and fluorine

This reaction is explosive even in the cold and dark, and you tend to get carbon and hydrogen fluoride produced. It is of no particular interest. For example:

+The reaction between alkanes and iodine

Iodine doesn't react with the alkanes to any extent - at least, under normal lab conditions.

+The reactions between alkanes and chlorine or bromine

There is no reaction in the dark.

In the presence of a flame, the reactions are rather like the fluorine one - producing a mixture of carbon and the hydrogen halide. The violence of the reaction drops considerably as you go from fluorine to chlorine to bromine.

The interesting reactions happen in the presence of ultra-violet light (sunlight will do). These are Photochemistry  reactions, and happen at room temperature.

We'll look at the reactions with chlorine. The reactions with bromine are similar, but rather slower.

+Methane and chlorine

Substitution reactions happen in which hydrogen atoms in the methane are replaced one at a time by chlorine atoms. You end up with a mixture of Chloromethane, Dichloromethane, Trichloromethane (Chloroform) and Tetrachloromethane(Carbon tetrachloride).

The original mixture of a colourless and a green gas would produce steamy fumes of hydrogen chloride and a mist of organic liquids. All of the organic products are liquid at room temperature with the exception of the chloromethane which is a gas.

If you were using bromine, you could either mix methane with bromine vapour, or bubble the methane through liquid bromine - in either case, exposed to UV light. The original mixture of gases would, of course, be red-brown rather than green.

You wouldn't choose to use these reactions as a means of preparing these organic compounds in the lab because the mixture of products would be too tedious to separate.

The mechanisms for the reactions are explained on separate pages.

                                                                                                                              to be continued.....