structural isomer questions :( (1 Viewer)

[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 but it might be straight-chain alkene. 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.

edited: Additionally, cis-2-butene and trans-2-butene are also stereoisomers, as their 3D structure are spatially different, but as their double bond is between the same two carbons, C2 and C3, they are defined as the same structural isomer.

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.

edited: They are structurally different spatially (see screenshot image from Wikimedia Commons), so may need to consider possible different properties.

 

[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.

View attachment 46339

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). I have edited my previous comments so that they are consistent with this definition.

 

[quarkkkk]

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). I have edited my previous comments so that they are consistent with this definition.

Ok thank you so much !!!!!

 

[wizzkids]

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

They are all good. Diethyl ether CH₃-CH₂-O-CH₂-CH₃ and methyl propyl ether CH₃-O-CH₂-CH₂-CH₃ are also structural isomers of n-butanol

 

[jazz519]

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)

No they won't ask you to do those kinds of isomers (unless it is specifically stated which I doubt they will because the isomer number could go into the double digits if the carbon amount becomes 6+).

The questions in HSC are very careful in how they word questions to avoid confusion like that so don't worry about that since they want to make sure they are actually testing the students knowledge rather than interpretation of a question. If you look at all the HSC questions, the wording is very specific. For example they will state "How many chain isomers", "how many position isomers" and "how many functional group isomers" as shown in the screenshots below:

HSC 2023:



In terms of way to ensure you don't miss any. Not really. You just have to move groups in a logical manner (i.e. make a methyl group, move the methyl group one carbon down or in case of double bond move the double bond down until the options are exhausted) and then check the name of compound if it's different. When you keep getting same ones means its finished.

 

[jazz519]

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?

outside of scope of the HSC, cis and trans isomers. That's something called a geometric isomer. In the syllabus they only want you to know about positional, chain and functional isomers.