Hydrogenation reactions are those reactions in which Hydrogen molecule is added to the double bond or triple bonds of unsaturated compounds to convert then into saturated compounds with all single bonds in the presence of catalysts like Nickle, Platinum, Palladium e.t.c.
Below three different reactions are given in which the starting materials (Alkenes) have molecular formula C₆H₁₂ and the final product in all three reactions is the formation of same Alkane with the formula of C₆H₁₄.
Remember that the three starting materials are isomers of each other. Such isomers are called as position isomers because they only differ in the position of double bond across the carbon chain.