What is the difference between the terms constitutional isomers, stereoisomers, geometric isomers? I know cis and trans-1,2-dichloroethane shows cis trans isomerism and 1,1 dichloroethene, 1,1,2 trichloroethene do not show cis trans isomerism rather show stereoisomerism.

In both the cases the compounds are not superimposable. So what is the difference between cis trans isomerism and stereoisomerism?

  • $\begingroup$ You'd better not do that again. Instead, just delete the old question and ask a new one. $\endgroup$ – Ivan Neretin Aug 28 '20 at 20:22
  • $\begingroup$ @Anusha I guess because OP edited the old question to make a completely new one instead of asking it as a new question.. Also, formatting was poor, $\endgroup$ – Safdar Faisal Aug 29 '20 at 7:13
  • $\begingroup$ Does this answer your question? Difference between conformational, constitutional, and structural isomers and the same and different molecules $\endgroup$ – Safdar Faisal Aug 29 '20 at 7:16
  • $\begingroup$ It is preferred if you can post separate questions instead of combining your questions into one. That way, it helps the people answering your question and also others hunting for at least one of your questions. Thanks! $\endgroup$ – Safdar Faisal Aug 29 '20 at 7:30

Among different isomer classes, cis- and trans-isomerism falls under the category of diastereomers where two compounds are neither superimposable nor mirror images of each other. I believe the other type of isomerism you're referring to here is structural isomers (constitutional isomers), which are in fact not stereoisomers, but are defined by compounds that have the same molecular formula, but different bonding patterns. Stereoisomers differ from this in that they have the same bonds between the same atoms, but they are arranged differently in space.

Types of isomers

In the example given for structural isomers, 2-fluoropropane has the fluorine atom bonded to the 2nd carbon on the parent chain, while 1-fluoropropane has it bound to the 1st. Neither of these compounds have stereoisomers, because they do not have special properties such as chiral centers or double bonds that can create 2 or more forms of the same compound that have differing chemical properties. The examples that you gave in your question all have cis- and trans-isomerism, because the double bond present prevents it from rotating freely, and thus the full accurate model of the compound cannot be fully explained without including a "cis" or "trans" that indicates whether not the groups on either side of the double bond are on the same side or opposing sides.

Hope this clears things up a bit.

Edit: Looks like I made a mistake in reading your examples. 1,2-dichloroethane does not have cis- or trans- (geometric) isomerism, as it doesn't have a double bond that restricts rotation. 1,1-dichloroethene and 1,1,2-trichloroethene do not have isomers either. Although they both have a double bond, there are no two forms of the compounds with the same bonding patterns that are non-superimposable.

  • $\begingroup$ 1,1 dichloroethene, 1,1,2 trichloroethene dont show cis trans isomerism rather stereoisomerism but what is the difference $\endgroup$ – Saniya Aug 29 '20 at 5:23
  • $\begingroup$ @Saniya None of them have isomerism. If you're looking for an explanation of enantiomers, one of the other subclasses of stereoisomers, they are created by swapping any two substituents of a chiral center such that the end result is a mirror image of the original. Conformational isomers, a subclass of diastereomers, do not apply in the case of double bonds as they deal with single bonds. $\endgroup$ – NickC64 Aug 31 '20 at 14:43

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