How can you identify a ring structure from an alkene, given only the molecular formula?

Question

For example:

C3H6
--> comes from the formula CnH2n

This can indicate that it is a ring structure or an alkene. It can be either cyclopropane or propene. How do you know which one it is, given only the molecular formula (C3H6)?

Answer 1

Cyclopropane and propene are isomers. So if we have only the formula C3H6 we can not know the compound

Answer 2

the picture I saw was not butene, but something much more complex: I counted 8 carbons and 16 hydrogens, C8H16 (with subscripts). Remember, each line is a bond, the end of each bond is a carbon atom and its hydrogens (unless something else is shown, like another atom). So you have (from left to right) 2 x CH3, a carbon (with 1 H attached to them), this is bonded to another C-H, which has a CH3 attached to it, and another CH (part of a double bond). If you keep going you get another CH, and a CH3. All together they give you the formula I mentioned.

Answer 3

Normal propene is C3H6, A straight chain, double bond molecule, 'n-propene'. If cyclic, as cyclo-propane,it should be indicated as 'cy-C3H6'.
Cyclopropene is C3H4, cyclic, double bond molecule.
'..Ane' means Saturated, ..'Ene' means Unsaturated and the difference is obvious.
Normal and Iso Alkenes are CnH2n with a double bond.
Cyclo alkenes have 2 Hydrogens less CnH2n-2 with a double bond.

Answer 4

You cannot tell which isomer a substance is by empirical formula alone; you have to measure its physical properties, i.e. melting and boiling points. These are usually different for isomers. Optical stereoisomers (where the structures are mirror-images of each other) have the same melting and boiling points, but affect polarised light differently. Reaction energies will also be different, since the C=C and C-C bonds have different energies.

Answer 5

you cant know anything from that

but normally it turns out to be propene , cyclopropentene is strange one because it would snap open immediately from large ring strain.

Answer 6

You can use Infrared spectroscopy graphs that would tell you if a ring is present. Other spectroscopy techniques can be used as well.

Answer 7

I think you have answered your own question.

You cannot possibly tell whether the molecular formula C3H6 refers to cyclopropane or propene, without more information.

In general, substances in the same molecular formula are called "isomers", and if all you know is the molecular formula, you cannot possibly tell which isomer it is.