E/Z Isomers
My question is that why don't we use E/Z in all situations even when there is a hydrogen on each carbon of the C=C ?
It will always work correct with the E and Z.
It will always work correct with the E and Z.
Pardon?
Original post by Daniel Atieh
My question is that why don't we use E/Z in all situations even when there is a hydrogen on each carbon of the C=C ?
It will always work correct with the E and Z.
It will always work correct with the E and Z.
Because it's meaningless. If you have a hydrogen on each carbon of the C=C (i.e. ethene), there are no E/Z isomers because there's only one way of arranging the groups around the C=C.
Original post by Chlorophile
Because it's meaningless. If you have a hydrogen on each carbon of the C=C (i.e. ethene), there are no E/Z isomers because there's only one way of arranging the groups around the C=C.
This is not what I mean really. I mean for those structures exhibiting geometric isomers, why not to exclude the cis/trans thingy and just use E/Z? Got my point?
Edit: my first post wasn't clear, sorry.
(edited 9 years ago)
Original post by Infraspecies
Pardon?
Read my reply to Chlorophile. Like say you have cis 2-butene. Why just always use the rule of priorities to all molecules, as it works well with all those exhibiting geometric isomers?
Original post by Dentist95
The carbon double bond prevents free rotation in space.
A single bond can rotate though.
A single bond can rotate though.
Yes. A double bond/ring will restrict the rotation, and that give rise to geometric isomers, but that's not my question here.
If I understand you correctly, you seem to be asking why there is the E/Z naming AND the cis/trans naming, when they are often describing the same thing.
My understanding is that cis/trans is an older system and a type of E/Z, which is a more generalistic system.
Cis/trans describes E/Z, when both ends have at least one group in common, e.g. 1,2-dichloroprop-1-ene has Cl at both ends of the C=C. OCR A would have you think that both ends must have a H.
My understanding is that cis/trans is an older system and a type of E/Z, which is a more generalistic system.
Cis/trans describes E/Z, when both ends have at least one group in common, e.g. 1,2-dichloroprop-1-ene has Cl at both ends of the C=C. OCR A would have you think that both ends must have a H.
To make my point clear:
If we will apply the E/Z rules regarding the priority, we will get Z-2-butene and E-2-butene. Well, why then we still use the cis/trans for the disubstituted alkenes when the E/Z works for the di/tri/tetra substitutes of alkenes.
If we will apply the E/Z rules regarding the priority, we will get Z-2-butene and E-2-butene. Well, why then we still use the cis/trans for the disubstituted alkenes when the E/Z works for the di/tri/tetra substitutes of alkenes.
Original post by Daniel Atieh
To make my point clear:
If we will apply the E/Z rules regarding the priority, we will get Z-2-butene and E-2-butene. Well, why then we still use the cis/trans for the disubstituted alkenes when the E/Z works for the di/tri/tetra substitutes of alkenes.
If we will apply the E/Z rules regarding the priority, we will get Z-2-butene and E-2-butene. Well, why then we still use the cis/trans for the disubstituted alkenes when the E/Z works for the di/tri/tetra substitutes of alkenes.
The EZ system (Cahn Ingold Prelog) is a more sophisticated system that works for all stereoisomers of this type (restricted rotation).
Cis-trans is the "old" notation used with geometric isomers and (as stated above) only works when two of the groups are the same.
There are many examples of more than one convention in science, such as the optical isomers, which have (AFAIK) at least four conventions, maybe more.
Yes, the CIP notation is better than cis-trans, but historically they both have existed and continue to do so. It is pointless to ask why...
Original post by charco
The EZ system (Cahn Ingold Prelog) is a more sophisticated system that works for all stereoisomers of this type (restricted rotation).
Cis-trans is the "old" notation used with geometric isomers and (as stated above) only works when two of the groups are the same.
There are many examples of more than one convention in science, such as the optical isomers, which have (AFAIK) at least four conventions, maybe more.
Yes, the CIP notation is better than cis-trans, but historically they both have existed and continue to do so. It is pointless to ask why...
Cis-trans is the "old" notation used with geometric isomers and (as stated above) only works when two of the groups are the same.
There are many examples of more than one convention in science, such as the optical isomers, which have (AFAIK) at least four conventions, maybe more.
Yes, the CIP notation is better than cis-trans, but historically they both have existed and continue to do so. It is pointless to ask why...
Brilliantly explained. Many thanks!
Do you think that they may just use the CIP notation for ALL geometric isomers in the future?
Posted from TSR Mobile
Original post by Daniel Atieh
Brilliantly explained. Many thanks!
Do you think that they may just use the CIP notation for ALL geometric isomers in the future?
Posted from TSR Mobile
Do you think that they may just use the CIP notation for ALL geometric isomers in the future?
Posted from TSR Mobile
For alkenes, E/Z is best practice and widely used. Cis/trans means something subtly different though - E/Z refers specifically to alkenes only, whereas cis/trans can refer to any face, like rings, or you'd describe a process as cis addition if both groups are added across a double bond to the same side.
Original post by Daniel Atieh
Brilliantly explained. Many thanks!
Do you think that they may just use the CIP notation for ALL geometric isomers in the future?
Posted from TSR Mobile
Do you think that they may just use the CIP notation for ALL geometric isomers in the future?
Posted from TSR Mobile
Quite possibly.
The International Baccalaureate dropped the term "geometric isomers" in the latest syllabus revision (2016 exams), but have called them cis/trans stereoisomers instead. However, they have also introduced the CIP system of priorities for the first time.
The problem is if you drop learning something altogether what happens when you read examples in the literature which used the "old" notation.
Nobody is going to revise and update the combined research of hundreds of years to fit modern standards.
Original post by charco
Quite possibly.
The International Baccalaureate dropped the term "geometric isomers" in the latest syllabus revision (2016 exams), but have called them cis/trans stereoisomers instead. However, they have also introduced the CIP system of priorities for the first time.
The problem is if you drop learning something altogether what happens when you read examples in the literature which used the "old" notation.
Nobody is going to revise and update the combined research of hundreds of years to fit modern standards.
The International Baccalaureate dropped the term "geometric isomers" in the latest syllabus revision (2016 exams), but have called them cis/trans stereoisomers instead. However, they have also introduced the CIP system of priorities for the first time.
The problem is if you drop learning something altogether what happens when you read examples in the literature which used the "old" notation.
Nobody is going to revise and update the combined research of hundreds of years to fit modern standards.
But that is what IUPAC are effectively doing, e.g. ethanoic acid instead of acetic acid.
Which IMO is a good thing.
(edited 9 years ago)
Original post by Pigster
But that is what IUPAC are effectively doing, e.g. ethanoic acid instead of acetic acid.
Which IMO is a good thing.
Which IMO is a good thing.
I meant that nobody is going to re-write past research papers to fit modern standards. You go into chemical abstracts/ JRCS etc. from 80 years ago and the terminology is noticeably different.
I agree that old papers won't (and shouldn't?) get re-written, but IUPAC have ruled that acetic et.al. are now called ethanoic etc. How are students of today meant to read chemistry papers from only a handful of years ago?
If they can re-name organic chemicals, why can't they be allowed to insist on CIP over cis/trans. IMO, we should drop cis/trans, even if it makes older papers harder to interpret.
But that is just my opinion.
If they can re-name organic chemicals, why can't they be allowed to insist on CIP over cis/trans. IMO, we should drop cis/trans, even if it makes older papers harder to interpret.
But that is just my opinion.
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