Organic Chemistry - Part IV

4. Physical and chemical properties of hydrocarbons
In this section we will see various reactions, physical properties, preparation, etc. of a few alkanes, alkene and alkynes.  

Alkanes
Alkanes, as we have discussed earlier, are saturated hydrocarbons. The main source of alkanes is petroleum products and natural gas. Methane is one of the chief constituents of natural gas. Other alkanes are obtained from fractional distillation of crude petroleum.  

Properties of alkanes
1. Physical state : Lower molecular weight alkanes are gases. Methane, ethane, propane and butane are gases at ordinary room temperature.  Higher alkanes up to those having 17 carbon atoms are liquids; higher alkanes are solids at room temperature.
2. Melting and boiling points : Homologous alkanes show increase in melting and boiling points. Similar to the behavior of elements in the same group in a periodic table.
3. Solubility : Alkanes, like all other organic chemicals are insoluble in water. They are however soluble in organic liquids. Alkanes are non-polar and are hence soluble in other non-polar liquids and not in water, as water is a polar molecule.
4. Combustion : Alkanes are inflammable and are easy to catch fire. Complete combustion of an alkane leads to carbon dioxide and water. During combustion, the supply of oxygen has to be sufficient. Insufficient oxygen leads to carbon monoxide reaction and the heat generated is less than when sufficient oxygen is available.
The reactions are given below.

5. Reactivity : Alkanes have saturated covalent bonds. Hence their reactivity with other chemicals  is relatively low.
6. Substitution reaction :  We have seen in the chapter on chemical reactions how substitution reactions take place. In alkanes, substitution reactions take place easily by replacement of hydrogen atoms with more reactive atoms. Chlorine is one such example.  

Next we will see in detail about methane, which is the simplest of the alkanes. Methane is the first of the homologous series in alkanes.  

Methane   

Methane (CH₄) occurs as a natural gas in the underground petroleum wells deep inside the earth. Methane gas is also known as marsh gas as it is emitted by bacterial decomposition of dead plants and animals. Methane is found in coal mine gases, gobar gas, sewage gas and bio-gas.  

Laboratory preparation of methane gas
In the lab methane is prepared using the following chemical reaction.  

Sodium acetate is the sodium salt of acetic acid. Sodium hydroxide (NaOH) is obtained from soda lime which is a combination of NaOH +CaO. Only the NaOH component of the soda lime reacts in the reaction.  

A mixture of sodium acetate and soda lime are taken in a glass tube and heated. Methane gas is evolved which is collected by downward displacement of water. The downward displacement method makes other soluble gases or salts evaporating from the tube to be absorbed. Also it must be remembered that methane is insoluble in water, hence the downward displacement technique can be usefully employed.

Physical properties of methane
Methane (CH₄) is an odourless, colourless, tasteless gas. It is lighter than air. It is a non-polar molecule and is insoluble in water. It dissolves in non-polar solvents like alcohol, carbon tetrachloride, etc.

Chemical properties of methane
1. Combustion : methane burns in air with a blue flame. We have seen earlier that in sufficient amount of oxygen, methane burns to give carbon dioxide and water. In insufficient oxygen it gives out carbon monoxide.  Methane produces a good amount of heat when it undergoes combustion. This is the reason why it is used as fuel.
2. Reactivity :  Methane is quite unreactive, except with fluorine, chlorine, etc. With these it undergoes substitution reactions.  

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