So this question wanted me to find two cyclical compounds that are 'structural' isomers of the molecular formula given in the question. The two isomers I came up with are: 1,2-dichlorocyclopropane and 1,1-dichloropropane since I thought that structural isomers are molecules with the same molecular formula but different arrangements of atoms. However, the mark scheme to this question states that the answer is: '(cis or trans) 1,2-dichloropopane', which is confusing to me since I thought that cis/trans isomers are stereoisomers and not structural isomers where stereoisomers are molecules with the same structural formula but different arrangements of atoms in space.

So is the mark scheme correct in it's solution?

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  • $\begingroup$ Structural isomers have the same molecular formula (1-butene and 2-butene). Stereoisomers are a subset of isomers in that they have the same atom connectivity but not the same spatial arrangement, e.g., (E)-2-butene and (Z)-2-butene. $\endgroup$ – user55119 Apr 11 '19 at 15:06
  • $\begingroup$ Alright! But the question wanted me to write down two structural isomers which i assumed i did, but the mark scheme mentions stereoisomers i.e. cis/trans isomers which is confusing to me $\endgroup$ – Selena Carlos Apr 11 '19 at 15:13
  • $\begingroup$ For a ring it must be cyclopropane. Now figure out where to place the H's and Cl's. $\endgroup$ – user55119 Apr 11 '19 at 15:18

I agree with you but in a slightly different way. The question is correct, i.e. there are two structural isomers, but as given in your answer, the two isomers are not cis and trans dichlorocyclopropane, because as you mentioned correctly, cis and trans isomers are definitely stereoisomers and not structural isomers.

To answer your question about two different structural isomers, the only two possibilities are two $\ce{Cl}$ either on same $\ce{C}$ atom or on two different $\ce{C}$ atoms, and that's how you get the two structural isomers. So, one should be $\text{1,1-dichlorocyclopropane}$ and the other one should be $\ce{1,2-dichlorocyclopropane}$.

Now in the compound $\ce{1,2-dichlorocyclopropane}$ you can have stereoisomerism (basically diastereomerism) and therefore have cis and trans 1,2-dichlorocyclopropane, but the correct answers are not them, but the two compounds mentioned in the paragraph above, i.e. the two correct structural isomers should be $\ce{1,1-dichlorocyclopropane}$ and $\ce{1,2-dichlorocyclopropane}$.

Here is a picture to demonstrate that..

enter image description here

The last two compounds i.e. $\text{(1R,2S)-1,2-dichlorocyclopropane}$ and $\text{(1S,2S)-1,2-dichlorocyclopropane}$ are basically cis$\text{1,2-dichlorocyclopropane}$ and trans $\text{1,2-dichlorocyclopropane}$ respectively.

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