Organic Chemistry question

[DNAngel]

This is a question from the Heinemann Chemistry 2 textbook:

Explain why there is only one compound corresponding to the formula C₃H₈ while there are over 70 compounds corresponding to the molecular formula C₁₀H₂₂.

[Greatness]

C3H8 only has 3 carbons, so the carbon backbone is limited to the 3 carbon structure therefore it can only produce 1 compund. However, C10H22 has a much larger carbon chain. This allows it to have many variations in the way that the chain can be formed i.e. there are numerous ways that branches can form. Which means it can form many different compounds

[Aurelian]

^ the technical term for which is isomerism =)

[jasoN-]

For interests sake: http://www.kentchemistry.com/links/organic/isomersofalkanes.htm (scroll down)

But yeah there are loads of isomers for higher numbers of carbons in a hydrocarbon chain.

[Greatness]

^ the technical term for which is isomerism =)

I didnt know that, that term even existed

[Chromeo33]

Yep, so there is only one possible arrangement of the compound C3H8 (i.e. one 'structural isomer').

[nacho]

Yep, so there is only one possible arrangement of the compound C3H8 (i.e. one 'structural isomer').

Question, what about: 1-methylethane?

[jasoN-]

1-methylethane is the same as propane

  |
 -C-
  |   |
 -C-C-
   |  |

[nacho]

1-methylethane is the same as propane

  |
 -C-
  |   |
 -C-C-
   |  |

Yea, so that would be considered a structural isomer of , yes?

[jasoN-]

it's not an isomer, it's the same molecule. You could easily rotate the methyl group back to a horizontal level to indicate the propane molecule

an isomer would be, for example:
butane --> 2-methylpropane
in this case you cannot rotate the substituents to form the same molecule


Edit: Also noteworthy, the structures don't appear as straight lines in real life.
They form a zig-zag shape
The atoms tend to repel away from each other as much as possible to maintain the lowest possible energy state, which is the most stable.

[kenhung123]

1-methylethane is the same as propane

  |
 -C-
  |   |
 -C-C-
   |  |

Yea, so that would be considered a structural isomer of , yes?

Consider naming that molecule again and you will see why you can't call that 1-methylpropane. The longest continuous carbon chain contains 3 carbons, all saturated.

[nacho]

it's not an isomer, it's the same molecule. You could easily rotate the methyl group back to a horizontal level to indicate the propane molecule

an isomer would be, for example:
butane --> 2-methylpropane
in this case you cannot rotate the substituents to form the same molecule

ah okay, thanks for the clearing up

Edit: Also noteworthy, the structures don't appear as straight lines in real life.
They form a zig-zag shape (Image removed from quote.)
The atoms tend to repel away from each other as much as possible to maintain the lowest possible energy state, which is the most stable.