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Alkanes: Nomenclature And Isomerism

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Alkanes: Nomenclature And Isomerism - Lesson Summary

Alkanes are the simplest form of hydrocarbons which contains Carbon-Carbon single bond. As each of the Carbon atom is surrounded by single bonds they are called saturated hydrocarbons.
General formula of alkanes is C nH 2n+2. Where n = number of Carbon atoms, 2n+2 = number of Hydrogen atoms.
The structure of alkanes becomes complicated with the increase in the number of Carbon atoms in alkanes.

Carbon atoms of different molecules were categorized into different types. They are primary carbon atom, secondary carbon atom, tertiary carbon atom and quaternary carbon atom.






 
 
 
The molecules which posses more than one structural arrangement of carbon atoms are called isomers and the phenomenon is called isomerism. First three members of alkanes does not show isomerism.
Ex: Isomers of Butane: Isomers of butane differ in their structures thus called as chain isomers. Such structures have difference in their properties.


Isomers possible with the general formula C 5H 12: Chain isomers.



The table gives number of isomers present for different number of carbon atoms.     Number of
  Carbon Atoms
  Number of
   Isomers
            C 4       2             C 5       3             C 6       5             C 12     355

Alkyl group:
The groups derived by removing one hydrogen atom from alkanes are called alkyl groups.
Ex: -CH 3, -C 2H 5, -C 3H 7…etc
General formula of alkyl groups is C nH 2n+1.

In order to differentiate each molecule of isomer International Union of Pure and Applied Chemistry (IUPAC) has introduced some rules.
Key steps to label a structure:
  1. Identify the longest continuous chain of carbon atoms.
  2. Name this longest parent chain using standard naming rules.
  3. Add the name of each side chain before the name of the parent carbon chain. Number the parent carbon chain so that the sum of the numbers assigned to each substituent will be a least number.
  4. Write the name of side chain with its number before as prefix to the name of the root word of the parent chain.
  5. If there are multiple side chains of the same type attached to the parent carbon chain, use prefixes such as “di” and “tri” to indicate it with its assigned number to the parent chain.
          Ex:
             
 
            

     Key steps to draw a structure from a given name:
  1. Draw the chain of carbon atoms from the molecular formula.
  2. Number the carbon atoms within the parent chain.
  3. Attach the substituent alkyl groups given as prefix in the name to the carbon atoms as specified in the numbering.
  4. Satisfy the remaining valence of each carbon atom by putting the requisite number of hydrogen atoms.
          Ex: 3-Ethyl 2,2-dimethyl pentane

             

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