Question

Which of the following is hydrogenation reaction for ethene?
(A) ${{C}_{3}}{{H}_{6}}+{{H}_{2}}\xrightarrow{Ni/Pd}{{C}_{2}}{{H}_{6}}$
(B) ${{C}_{2}}{{H}_{4}}+{{H}_{2}}\xrightarrow{Ni/Pd}{{C}_{2}}{{H}_{6}}$
(C) $2{{C}_{2}}{{H}_{4}}+{{H}_{2}}\xrightarrow{Ni/Pd}{{C}_{2}}{{H}_{6}}$
(D) ${{C}_{2}}{{H}_{4}}+{{H}_{2}}\xrightarrow{Ni/Pd}2{{C}_{2}}{{H}_{6}}$

Answer 1

Ethene is an unsaturated hydrocarbon. Its degree of saturation is 1 and has 1 double bond. It is an alkene and can be easily converted into an alkane by hydrogenation. Hydrogenation is the process of breaking the double bond of an unsaturated compound and replacing it with 2 hydrogen atoms.

Complete step by step solution:
-There are 2 types of compounds- saturated and unsaturated compounds. Alkanes are termed as saturated compounds while alkenes and alkynes are unsaturated compounds.
-Alkanes are the open-chain aliphatic hydrocarbons. They are also called paraffin. Their general formula is ${{C}_{n}}{{H}_{2n+2}}$ where n is the natural number set.
-Alkenes are open-chain unsaturated aliphatic compounds. They are also called olefins. Their general formula is of the form ${{C}_{n}}{{H}_{2n}}$ where n is the natural number set.
-Alkynes are also open-chain unsaturated hydrocarbons which are less reactive than alkenes. They contain carbon-carbon triple bonds. Their general formula is denoted as ${{C}_{n}}{{H}_{2n}}_{-2}$ where n is the natural number set.
-The root word in ethene is eth. So it has 2 carbon atoms connected in a form of a chain. As it is an alkene, it has 1 degree of unsaturation and so 1 pi bond between the 2 carbon atoms. All the other valencies of carbon atoms are satisfied by hydrogen atoms.
-Hydrogenation comes from the word hydrogen. So, it involves the addition of hydrogen atoms to the carbon chain. The hydrogen atom has the least value of electronegativity and so it cannot replace any other atom in a compound directly. Hydrogenation actually refers to the attachment of hydrogen atoms by breaking the double or triple bonds of an alkene or alkyne.
-In the given question, we need to find the correct reaction for the hydrogenation of ethene. For that, we first need to check if all the reactions involve ethene or not. There should be 2 carbon atoms in ethene and 1 double bond as it has a degree of unsaturation to be 1.
-In the first option, we see that there are 3 carbon atoms and 6 hydrogen atoms. So we can directly eliminate this option as it cannot be ethene. There can only be 2 carbon atoms in ethene.
-All the other options have 2 carbon atoms only. They have the same formula. They are of the type ${{C}_{n}}{{H}_{2n}}$.
So all other options are ethene only. Now we need to check for hydrogenation reactions.
- Hydrogenation involves the addition of 1 mole of hydrogen molecule to 1 mole of ethane to form ethane. In the second option, this condition is being satisfied. In the third option, there are 2 moles of ethane and 1 mole of hydrogen. So all the bonds will not be able to break to form hydrogenation products. In the fourth option, 1 mole of ethene is present in the reactant side but 2 moles of ethene is present on the product side which is not possible. So this is also wrong.

Therefore the correct option is (B).

Note: For hydrogenation to occur, the hydrogen changes its valency from 0 to +1 which needs strong reducing agents. For this nickel and palladium are used as catalysts. They can easily convert the alkene to alkanes to undergo hydrogenation reactions. So they are always used in hydrogenation reactions and act as strong reducing agents.