Question

Which isomerism relates $2 - methyl - 1 - butene$ and $2 - methyl - 2 - butene$ ?
A. Optical isomerism
B. Geometrical isomerism
C. Positional isomerism
D. Metamerism

Answer 1

Hint: Isomerism is the process in which a compound has more than one structure with the same molecular formula. The two different structures are called isomers. In the given two isomers the carbon skeleton is the same but the functional groups are attached at different positions.

Complete answer:
In the case of $2 - methyl - 1 - butene$ and $2 - methyl - 2 - butene$ ;
The structure of $2 - methyl - 1 - butene$ is:
$C{H_2} = \mathop C\limits_{\mathop {C{H_3}}\limits_{}^I } - C{H_2} - C{H_3}$
And the structure of $2 - methyl - 2 - butene$ is:
$C{H_3} - \mathop C\limits_{\mathop {C{H_3}}\limits_{}^I } = C{H_2} - C{H_3}$
Here we can see that the molecular formula is the same of the both compounds but the functional group that is a double bond is at a different position. These both are isomers of each other. In more precise words they both are positional isomers of each other. Because they have the same carbon skeleton but double bonds at different positions. So they are called positional isomerism.

Hence, option C is correct.

Note:
Metamerism is that type of isomerism which occurs due to the presence of different alkyl chains on each side of the functional group. This type of isomerism is very rare and it is only limited to molecules that contain divalent atoms such as oxygen and sulphur. Geometrical isomers are the cis and trans form of a compound. The compounds which show cis and trans forms are only show geometrical isomers. And lastly in the case of optical isomerism, the compounds who have different mirror images which are non-superimposable to each other are called optical active compounds. And these mirror images are called optical isomers of each other. The compounds which show optical isomers are called chiral compounds. And this property is called chirality. The mixture of these two mirror images are called enantiomers and have zero specific rotation.