structural isomer questions :(

[quarkkkk]

for questions along the lines of "how many structural isomers does [insert organic compound here] have?" is there a way to ensure you don't miss any? like how are we supposed to think of the more difficult ones (e.g. methylcyclopropane for C4H8)

 

[wizzkids]

for questions along the lines of "how many structural isomers does [insert organic compound here] have?" is there a way to ensure you don't miss any? like how are we supposed to think of the more difficult ones (e.g. methylcyclopropane for C4H8)

Yeah, I agree these sorts of questions are a bit daunting. You won't be asked about really large molecules with dozens of potential structural isomers, but in the example you gave C₄H₈ it is pretty obvious that it is not a straight-chain alkane or a straight-chain alkene. Therefore you can dispense with straight chain, then go straight to branched chain then maybe consider cyclo-compounds. The most difficult example you can expect to be asked is an oxy-compound like an alcohol, aldehyde or ketone.
Can you write out all the structural isomers of n-butanol?

 

[quarkkkk]

or a straight-chain alkene

Wait i'm slightly confused aren't but-1-ene and but-2-ene structural isomers of C4H8?

Can you write out all the structural isomers of n-butanol?

The ones I got were:
2-butanol, 2-methyl-2-propanol, 2-methyl-1-propanol
Idk if there are any more though...

 

[Eagle Mum]

Wait i'm slightly confused aren't but-1-ene and but-2-ene structural isomers of C4H8?



The ones I got were:
2-butanol, 2-methyl-2-propanol, 2-methyl-1-propanol
Idk if there are any more though...

Yes, 1-butene and 2-butene are both isomers of C4H8.
Technically, cis-2-butene and trans-2-butene are different structural isomers, so there are six structural isomers of C4H8.

I otherwise agree with wizzkids about first considering straight chains with possible combinations of multiple bonds, then branched chains with multiple bonds, then cyclic molecules, then possible cis/trans different structures for each 2D chemical formula.

 

[quarkkkk]

cis-2-butene and trans-2-butene

wait are these in the hsc syllabus I've never heard of cis and trans isomers before haha

I otherwise agree with wizzkids about first considering straight chains with possible combinations of multiple bonds, then branched chains with multiple bonds, then cyclic molecules, then possible cis/trans different structures for each 2D chemical formula.

Thank you so much!!!!

 

[Eagle Mum]

wait are these in the hsc syllabus I've never heard of cis and trans isomers before haha

I don’t know if it’s in the HSC syllabus or not. They are structurally different spatially (see screenshot image from Wikimedia Commons), so I would describe them as structural isomers to anyone asking me this question, whether in an HSC exam or other situation. Cis or trans isomers are chemically relevant considerations in my professional experience.

 

[quarkkkk]

I don’t know if it’s in the HSC syllabus or not. They are structurally different spatially (see screenshot image from Wikimedia Commons), so I would describe them as structural isomers to anyone asking me this question, whether in an HSC exam or other situation. Cis or trans isomers are chemically relevant considerations in my professional experience.

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Oh wait one more question. from my understanding of the diagram the difference between cis and trans isomers are the different sides of the carbons which the hydrogens are bonded to which changes the shape of the molecule (i'm assuming by vsepr or something) so would they be considered positional isomers of each other?? like what type of isomer would it be categorised under?

 

[Eagle Mum]

Oh wait one more question. from my understanding of the diagram the difference between cis and trans isomers are the different sides of the carbons which the hydrogens are bonded to which changes the shape of the molecule (i'm assuming by vsepr or something) so would they be considered positional isomers of each other?? like what type of isomer would it be categorised under?

I stand corrected - by IUPAC definition, cis/trans stereoisomers or spatial isomers (molecules with the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space) are not classified as structural isomers (compounds with logically distinct order of bonds between them).