Question

write isomerism of C5H8 .write the structure
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Answer 1

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C₅H₈ is a hydrocarbon with 4 hydrogen atoms less than those in the corresponding saturated hydrocarbon, C₅H₁₂ (pentane).

Therefore, the molecule either has two double bonds, or one triple bond or is cyclic.

Two double bonds:

There are three penta-di-enes and one methylbuta-di-ene:

• CH₂=C=CH-CH₂-CH₃ (penta-1,2-diene)
• CH₂=CH-CH=CH-CH₃ (penta-1,3-diene). This exists in two cis-trans isomers:
• CH₂=CH-CH₂-CH=CH₂ (penta-1,4-diene)
• CH₂=C(CH₃)-CH=CH₂ (methylbutadiene)

One triple bond:

There are two pentynes and one methylbutyne.

• CH≡C-CH₂-CH₂-CH₃ (penta-1-yne)
• CH₃-C≡C-CH₂-CH₃ (penta-2-yne)
• CH≡C-CH(CH₃)-CH₃ (3-methyl-1-butyne)

Cyclic compounds:

There is one cyclopentene and two methylcyclobutenes.

• Cyclopentene
• The two methylcyclobutenes

If this was homework, it was a rather complicated one. Check for possible mistakes.

Edit:

Mr Keshab Kabra suggested the addition of the remark “There are more”.

There probably are, for example with 3-membered rings, but I don’t have the patience to find them all.