Question

How can alkenes be made from alkanes?

Answer 1

Hint As we know that in the classification of the hydrocarbons, alkenes and alkanes basically come under unsaturated and saturated hydrocarbons. We classified these hydrocarbons in order to understand these compounds. It is found that the general chemical formula of alkane is ${{C}_{n}}{{H}_{2n+2}}$ and the general chemical formula of alkene is ${{C}_{n}}{{H}_{2n}}$

Complete Step by step solution:
- It is found that the most common method of preparation of alkenes from alkanes is by halogenation – dehydrohalogenation
- Let us discuss this halogenation – dehydrohalogenation method in brief:
- The free radical halogenation of an alkene is the first step. We can see the reaction that takes place as:
${{\left( C{{H}_{3}} \right)}_{3}}CH+B{{r}_{2}}\xrightarrow{hv}{{\left( C{{H}_{3}} \right)}_{3}}CBr+HBr$
- Now, further the alkyl halide is dehydrohalogenation by reacting it with a strong base.
${{\left( C{{H}_{3}} \right)}_{3}}CBr+KOH\xrightarrow{ethanol/\Delta }{{\left( C{{H}_{3}} \right)}_{3}}C=C{{H}_{2}}+KBr+{{H}_{2}}O$
- Hence we can say that alkenes can be made from alkanes by halogenation – dehydrohalogenation method.

Note:
- We should not get confused in between the terms alkane and alkene. We can see the basic difference in between these two as:

Alkane	Alkene
1. these are the saturated hydrocarbons	1. these are the unsaturated hydrocarbons
2. the general chemical formula is ${{C}_{n}}{{H}_{2n+2}}$	2. the general chemical formula is${{C}_{n}}{{H}_{2n}}$
3. the bond length is basically longer than alkenes	3. the bond length is basically smaller than alkanes
4. These are found to contain only single bonds that are sigma bonds.	4. These are found to contain single or double bonds that basically comprises both sigma as well as pi bonds.