Concept Explanation: Catalytic Hydrogenation of Alkenes to Iso-Pentane

Catalytic hydrogenation is a reaction where hydrogen (H₂) adds across the double bond of an alkene in the presence of a catalyst (e.g., Ni, Pd, Pt), converting it to an alkane. The alkane produced has the same carbon skeleton as the alkene. Here, we need to find how many different alkene structures (including stereoisomers) upon hydrogenation give iso-pentane (common name for 2-methylbutane).

Step 1: Understand the Structure of Iso-Pentane

Iso-pentane is 2-methylbutane. Its structure is:

$CH 3 -CH(CH 3)-CH 2 -CH 3$ or

$C 5 H 12$ with a branched chain: a central carbon bonded to H, CH₃, CH₃, and CH<2CH₃.

Step 2: Identify Possible Alkene Precursors

Since hydrogenation saturates the double bond without altering the carbon skeleton, the alkene must have the same carbon framework as iso-pentane. We need to consider all positions where a double bond can be placed in this skeleton, and also stereoisomers if applicable (for alkenes with two different groups on each carbon of the double bond).

Step 3: List All Possible Alkenes

The carbon skeleton of iso-pentane has 5 carbons with types: 1° (terminal), 2° (chain), 3° (branched), and another 1°. Possible double bond positions:

Between C1 and C2 (terminal double bond): This gives 1 possible alkene (no stereoisomers).
Between C2 and C3 (internal double bond): The carbons are C2 (3° carbon) and C3 (2° carbon). This alkene can have E/Z stereoisomers because each carbon has two different groups. So, 2 stereoisomers.
Between C3 and C4 (terminal-like, but symmetric): This is similar to C1-C2, so 1 alkene.

Also, check for rearranged skeletons? No, because hydrogenation does not change skeleton.

Step 4: Count the Total Number

So, we have:

1 alkene from C1-C2
2 alkenes (E and Z) from C2-C3
1 alkene from C3-C4

Total = 4 alkenes.

Final Answer

There are 4 alkenes that upon catalytic hydrogenation give iso-pentane.

Related Topics

Alkene Structure and Isomerism (structural and stereoisomers)
Catalytic Hydrogenation Mechanism
IUPAC Naming of Alkanes and Alkenes
Degree of Carbon Atoms (1°, 2°, 3°)

Important Formulae and Theory

General reaction for catalytic hydrogenation:

$R-CH=CH-R' + H 2 \overset{catalyst}{\to} R-CH 2 -CH 2 -R'$

Key points:

Addition is syn (both H add from same side).
Does not change carbon skeleton; only saturates double bond.
For unsymmetrical alkenes, stereoisomers (E/Z) are possible if each carbon of the double bond has two different substituents.

To find number of alkenes giving same alkane, consider all possible double bond positions and stereochemistry.