Question

Draw the three alkenes, each of formula C5H10, that will form 2-methylbutane upon hydrogenation?

Answer 1

Concepts and reason

Hydrogenation is the reaction in which hydrogen is added to the unsaturated molecules. This reaction is carry out through addition of molecular hydrogen in the presence of metal catalyst to unsaturated molecules.

Fundamentals

Hydrogenation is a process which explains the reaction between unsaturated molecules and molecular hydrogen. H₂ with any unsaturated compound in the presence of a catalyst such as nickel, palladium or platinum to give the corresponding saturated compound.

Finding the unsaturated reactant using the given saturated product can arrive infour steps:

1.Apply DBE formulae for given molecular formula to find degree of unsaturation.

2.Drawing isomers

3.the obtained unsaturated isomers to be hydrogenated

4.isolation of unsaturated isomers which are leading to 2-methylbutane

By using DBE (Double bond equivalence) formulae, the number of unsaturation can be found for a molecule.

The formula to calculate DBE is given below:

${\rm{DBE = C - }}\frac{{\rm{H}}}{{\rm{2}}}{\rm{ - }}\frac{{\rm{X}}}{{\rm{2}}}{\rm{ + }}\frac{{\rm{N}}}{{\rm{2}}}{\rm{ + 1}}$

Where the number of hydrogen atoms is H

The number of halogenatoms is X

The number of nitrogenatoms is N

The number of carbonatoms is C

Isomers are compounds which have same chemical formulae but differ in their order of arrangements. Using double bond equivalence, various isomers of the given chemical formulae of unsaturated reactants can be drawn.

Hydrogenation is used for reduction of alkenes and other higher unsaturated compounds to give their corresponding alkanes. Processing hydrogenation reaction to the isomers gives their respective products. The isomer which gives 2-methyl butane is the target of interest.

The given chemical formula is ${{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{10}}}}$ .

$\begin{array}{l}\\{\rm{Number of carbons,\;\;\;\;}}\,\,\,\,\,{\rm{C}}\,\,{\rm{ = }}\,\,{\rm{5}}\\\\{\rm{Number of hydrogens,\;\;H }}\,{\rm{ = 10}}\\\\{\rm{Number of halogens,\;\;\;}}\,\,{\rm{X}}\,\,\,{\rm{ = }}\,\,{\rm{0}}\\\\{\rm{Number of nitrogens, }}\,\,{\rm{N = 0}}\\\end{array}$

The double bond equivalence is:

$\begin{array}{l}\\{\rm{DBU = C - }}\frac{{\rm{H}}}{{\rm{2}}}{\rm{ - }}\frac{{\rm{X}}}{{\rm{2}}}{\rm{ + }}\frac{{\rm{N}}}{{\rm{2}}}{\rm{ + 1}}\\\\{\rm{ = 5 - }}\frac{{{\rm{10}}}}{{\rm{2}}}{\rm{ - }}\frac{{\rm{0}}}{{\rm{2}}}{\rm{ + }}\frac{{\rm{0}}}{{\rm{2}}}{\rm{ + 1}}\\\\{\rm{ = 1}}\\\end{array}$

All the possible isomers of ${{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{10}}}}$ with one double bond are given below.

Hydrogenation of 1-pentene:

Hydrogenation of 2-pentene:

Hydrogenation of 2-methyl-1-butene:

Hydrogenation of 2-methyl-2-butene:

Hydrogenation of 3-methyl-1-butene:

The isomers which give 2-methylbutane are:

2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene.

Ans:

The three alkenes which give 2-methyl butane upon hydrogenation are: