Structural formulae of cyclic alkanes/alkenes?

Factual Questions
1

I’m currently doing a module on organic chemstry in my chemistry As level, and I’m trying to work out how to write the structural formulae of cyclic alkanes/alkenes.

For example, the structural formula of 2,3 dibromobut-2-ene is CH[sub]3[/sub]C(Br)=C(Br)CH[sub]3[/sub]. Or the strucutral formula of 2-chlorobutane is CH[sub]3[/sub]CHBrCH[sub]2[/sub]CH[sub]3[/sub].

However, I dont know how I’d write the structural formula of a cyclic alkane/alkene. I can draw the given formula:

Cyclohexane:



         CH[sub]2[/sub]
        / \
       /   \
    H[sub]2[/sub]C     CH[sub]2[/sub]
      ¦     ¦
      ¦     ¦
    H[sub]2[/sub]C     CH[sub]2[/sub]
       \   /
        \ /
         CH[sub]2[/sub]

Oooh, that took a long time, and it still looks kinda crappy but you get the idea.

I can’t think how you’d indicate that it’s a cyclic alkane when you write the structural formula.

The structure is (CH[sub]2[/sub])[sub]6[/sub] (obviously it would be more complicated if it were a cycloalkene or a halocycloalkene), but I can’t think how to indicate that it’s cyclic.

Is there a way to do this? Or should I just write the given formula instead?

Someone please help me!

2

Well, obviously !

Ummm…if ever there was a question I have absolutely NO idea about, this is it.

Move on to aromatic hydrocarbon bonding, then come back to this…

3

C[sub]6[/sub]H[sub]12[/sub] is good enough. Try drawing another cyclic or non-cyclic structure with the same formula.

4

It isn’t usually needed - most lists will have the full name (think Aldrich ordering catalogue) as well as the structural formula. Being able to write this out is a good way to begin to understand the basics of the structures, and about isomers, etc, but it isn’t really fundamental to organic chemistry in terms of the chemistry, you know? I think I might have seen it shown as c-C6H12 or something, but that doesn’t really work either, because a methyl-cyclopentane has the same formula, and besides, it isn’t accepted nomenclature (neither common nor IUPAC). The ONLY way to indicate for certain what the true formula is would be to either draw it out (with stereochemistry where appropriate), or use a completely proper IUPAC name, although most people haven’t bothered to learn all of the notation for that.

5

Well, I’d drawn out the given formula, and went on to write what I thought the structural formula might be, and realised that I’d picked a crappy alkane, as it has a really simple structural formula!

Ok, it’s for writing out a reaction, and showing the isomers. I’ve just realised that with such a formula, I wouldn’t be able to differentiate between cis and trans isomers anyway.

I’ll just draw the given formula, which I was trying to avoid since I’m doing this in word and hate wordart!

Thanks everyone, I’ll get on with this now then…

6

Actually, I drew C[sub]6[/sub]H[sub]12[/sub] because it was easy to do in ascii art. The one I wanted it for has the formula would be [start ring]CH[sub]2[/sub]CH[sub]2[/sub]CH[sub]2[/sub]CH[sub]2[/sub]C(CH[sub]3[/sub])=CH[end ring], the chemical formula for which would be C[sub]7[/sub]H[sub]12[/sub] which is a bit ambiguous.

7

damnit, forgot the name of it, it’s called 1-methyl hex-1-ene, which you all knew of course, but I just thought I’d put it in anway.

Another thing, which is more important, to give the side chain the lowest carbon number or the double bond the lowest number? It doens’t matter in this case, and I’m sure it’s in my notes somewhere, I just can’t find it.

8

Grrr, preview…remember to preview.

That should read:

Another thing. Is it more important to give…

Otherwise it sounds totally wrong, like I’ve got some heirarchy of question importance.

9

I’ve used a program called ISIS/Draw in the past and have found it very useful for the sort of chemistry graphics you wish to use. It’s no fee for academic and personal use.

ISIS/Draw: Chemically Intelligent Drawing Package

Hope that helps. Good luck with your studies.

10

Well, just plain formulae are always a bit ambiguous. You don’t know whether a molecule is cyclical, or branched, or where double bonds are, or any of that just from the formula. That’s why they also use IUPAC nomenclature.

11

Man, either I’ve forgotten everything in the last year, or your work is much harder than mine ever was!

12

I’ve seen something along these lines before
CH2CH2CH2CH2CH2
|____________|
Or even

CH2CH2CH2CH2CH2
|O|

Is that what your looking for?

13

At school we used something like cyclo-C[sub]6[/sub]H[sub]12[/sub], which I was never happy with at the time, but is clear and short.

Do you have to use the right answer, or is it ok if it’s clear what chemical you mean?

When explaining the chemical to us you said:

[start ring]CH2CH2CH2CH2C(CH3)=CH[end ring],

Maybe something like that?

14

That seems to make sense, thanks Sigene

Excellent, I just downloaded the program and it looks really helpful! Thanks for point it out Mr. Matthew

15

You’re welcome. My karma is now balanced out from the unfortunate incident last week where I told my baby girl to turn the page in the book we were reading and she got a paper cut.

Oh the horror!