
1,4 - dibromobutane ( ie. a dibromoderivative of an alkane , butane) when reacted withl in ethereal solution produces alicyclic hydrocarbon, Cyclobutane as per following reaction.

Therefore , the dibromo-derivative of alkane is identified as 1 , 4-dibromobutane.
It is because
(i) radius of anion ( anionic radius ) is greater , and
(ii) radius of cation ( cationic radius ) is lesser
than that of its parent neutral atom.