[Stupid.03]

Hello everyone, I'm new (both to the forum and to chemistry), I've been trying to draw every structural isomers of C₄H₉Br, and I found 5 of them. However, on my book it says there are maximum 4, and it gives me the third one I found wrong. How so? Doesn't it have different connections? Why can't it be an isomer?

[uploaded image on imgur]
http://imgur.com/a/JDyZSkk

[rolnor]

Is it possible that 2 of you isomeres are the same?

[Stupid.03]

Is it possible that 2 of you isomeres are the same?

It must be the case, I thought the third might be the same with the second one, however in the third CH₃ is in the same position of Br, in the second not. Is the reasoning correct?

[Orcio_87]

No, 3 and 5 are the same.

[Babcock_Hall]

When deciding whether or not two representations are the same molecule, there are a couple of approaches.  One is to name them.  Another is to think about rotating carbon-carbon single bonds to see whether or not they interconvert.

[rolnor]

Actually, if you include another type of isomerism, you are correct, its 5 isomeres.

[Stupid.03]

No, 3 and 5 are the same.

  you are very right sir, my bad, sorry for causing trouble

When deciding whether or not two representations are the same molecule, there are a couple of approaches.  One is to name them.  Another is to think about rotating carbon-carbon single bonds to see whether or not they interconvert.

I heard the last approach works only for conformational isomerism, is it right?

Actually, if you include another type of isomerism, you are correct, its 5 isomeres.

Unfortunately the exercise asked me only structural isomers so I'm guessing the other type of isomerism you are referring to is not included, and with it my chance to be right! Mmm, I'm going to guess, is it the diastereomers?

[rolnor]

Close, stereoisomer, they are R,S-isomeres.

[Babcock_Hall]

@OP
Conformational isomers may be interconverted by rotations around single bonds.  If I understand what you are asking, then you are correct.  My point is that drawings of two conformational isomers may look different, yet they are thought of as being one substance, provided that interconversion is rapid.  In introductory organic chemistry, that is a safe assumption.

How many centers of chirality are present in structure 3?

[Babcock_Hall]

Sometimes it helps to use plastic molecular model kits to construct representations and to rotate them in your hand.  They can be compared against drawings on paper.