[afridi2482]

If someone ask me to make possible isomers of C4H8 (Butene), how would i know that butene has 4 or 6 isomers? I mean how do i know about the number of possible isomers? Is there a general formula or something for that? Please help me with that.

[UG]

The key word here is 'make', you need to draw out all the possible isomers. I don't think there is any other way around this.

[afridi2482]

Ohh ya i mean to say that, How do i draw these isomers? infact how would i know that suppose butene has 6 isomers?

[UG]

Well you need to consider all the possibilities. Since butene has one double bond you can try placing the double bond into different positions to make different isomers, you can also consider a side group as well as cyclic molecules. See if you can try and draw out the isomers.

[afridi2482]

ahan thanks alot. i guess i am able to do that but there is no specific rule or something that you can tell straight up about the number of isomers. its just that you have to draw them manually one by one.

[UG]

I don't know of any rules or formulas which allows you to work out the number of isomers for each compound.

[fledarmus]

The only helpful rule I know of is the hydrogen deficiency for hydrocarbons. If you have the formula C_nH_2n+2, you have a fully saturated alkane - no double bonds or rings. For every loss of two hydrogens, you will have either a ring or a pi bond.

So if you have the formula C_nH_2n-2, you will know that your structures need to have either two rings, a ring and a double bond, two double bonds, or a triple bond. Then you just work your way through all the possibilities, making sure to include cis/trans double bonds and cis/trans substituents on the rings.

[afridi2482]

thank you sir for your example. Can you explain it with an example if you have time. regards

[Sophia7X]

One thing to do when drawing the isomers is to name each one to make sure you don't make the common mistake of repeating isomers.



For example, two structures you drew may look different on paper; but they could be the same (let's say, you accidentally drew 1-butene two times. For butene, the double bond being at the first carbon of butene is the same as being at the last carbon).
Think of cyclo-, cis-, trans-, etc structures. 

Also, double checking formulas will prevent error. Isomers of C4H8 must also have a formula of C4H8.