Question

Assertion:$But - 1 - ene$ and $2 - methylprop - 1 - ene$ are position isomers.
Reason: Position isomers have the same molecular formula but differ in position of functional group of $C = C$ .
$A.$ Both Assertion and reason are correct and Reason is the correct explanation for Assertion.
$B.$ Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
$C.$ Assertion is correct but reason is incorrect.
$D.$ Assertion is incorrect but Reason is correct.

Answer 1

Substances having the same molecular formula but different structural formulas are known as isomers and this phenomenon is known as isomerism. Isomers not only show different structural formulas but also show different properties (Physical as well as chemical properties).

Complete answer:
$But - 1 - ene$ and $2 - methylprop - 1 - ene$ are not position isomers because as we know position isomers have the same molecular formula but differ in position of the functional group of $C = C$ . In both the compound position of the functional group $C = C$ same as indicated by a number $1$ in their $IUPAC$ name. That’s why they are not position isomers but they are $But - 1 - ene$ and $2 - methylprop - 1 - ene$ are chain isomer as $But - 1 - ene$ has a straight chain whereas $2 - methylprop - 1 - ene$ has a branched chain.
Therefore, the given assertion is incorrect and the correct assertion is $But - 1 - ene$ and $2 - methylprop - 1 - ene$ are not position isomers.

Hence, Assertion is incorrect but reason is correct. So, the correct option is $D.$

Additional information:
$But - 1 - ene$ is an organic compound with formula $\left( {{C_4}{H_8}} \right)$ . It is colourless, flammable easily condensed gas.
$2 - methylprop - 1 - ene$ is an organic compound with molecular formula $\left( {{C_4}{H_8}} \right)$ . Its molecular mass is $56g/mol$ .

Note:
The main difference between chain isomers and positional isomers;
Chain isomers: Chain isomers are compounds which have the same molecular formula but differ in their structure with respect to the length of the carbon chain. If any functional group is present in chain isomers it must be the same in all isomers.
Positional isomers: Those compounds which have the same molecular formula but differ in which the structures are different due to the functional group as a double bond. In positional isomers only the position of the functional group of the side chain must be different.