ALKANE
Hydrocarbons The organic compounds which are made of only carbon and hydrogen are known as hydrocarbons. The number of carbon, hydrogen, and type of bond between the two elements are immaterial. Alkanes The hydrocarbons which have only single bond present between carbon-carbon and carbon-hydrogen atoms are called alkanes. The general formula for alkanes is CnH2n+2 where ‘n’ represent number of carbons in the molecule. Alkyl group The alkanes containing one hydrogen atom less than corresponding alkanes is known as alkyl. It is generally denoted by ‘R’ The general formula for alkyl is CnH2n+1 Nomenclature Alkanes and alkyl are named depending upon the number of carbon atoms present in the compound.
List of name of alkanes and alkyl group
Classification of alkanes Alkanes are classified as straight, branched and cyclic chain. Alkanes are also classified as methyl, primary,( 10) secondary,( 20) and tertiary.(30) depending upon number of carbon or alkyl group attached to the particular carbon.
Straight chain The compound has no branch
Branched chain
Types of Carbon
Cyclic Alkanes
Physical Properties Physical properties of alkanes follow a general trend. We will discuss few general properties such as boiling point, melting point, density, solubility. Boiling point Boiling point of a compound depends upon inter-molecular forces. As inter-molecular forces increases boiling point increases and vice versa. Inter molecular forces are directly proportional to molecular mass and inversely proportional to the number of branches.
The boiling point increases as molecular mass increases. Also boiling point deceases as number of braches increases. First four alkanes (listed in the table of names) are gases at room temperature, next twelve members (all not listed) are liquid, and rest of the members are solid. Melting Point It follows the same trend as boiling point but not so smoothly. It increases with the increase in molecular mass. However melting point does not follow any particular trend in branched alkanes. Density Density follows molecular mass i.e. as molecular mass increases density also increases. However alkanes have lowest density among all organic compounds. Solubility Alkanes are almost non-polar compounds. Also they cannot form a hydrogen bond and hence alkanes are insoluble in water. Liquid alkanes and cycloalkanes are soluble in one another. Good solvents for them are benzene, carbon tetrachloride, chloroform and other hydrocarbons. Chemical properties Alkanes are quite un-reactive compounds. That is why they are also called paraffins which means ’less affinity ’. In crude oil the alkane molecules have remained non reactive for millions of years. However alkanes react under vigorous condition with oxygen and show affinity with halogens (F, Cl, Br, I). Reaction with oxygen and halogens are called oxidation and halogenation respectively. Oxidation (Combustion)
The reaction takes place at high temperature and inside the flame. The reaction does not start on its own. It needs high energy but once started it gets its energy from the heat developed in the reaction and is self propagating. The complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide, water and heat.
Incomplete combustion Incomplete combustion (where there is no enough oxygen present) can lead to the formation of carbon or carbon monoxide.
Combustion and halogenation both follow free radical reaction mechenism .The