I synthesized a compound which could be either a cis or trans. How do I know determine which isomer was formed?
cis or trans what? alkenes (like cis vs trans stilbene), fused rings (like cis vs trans decalin), or coordination compounds (cis vs trans platin)... In all cases, the most definitive analysis would be single-crystal x-ray crystallography (but this requires growing a crystal, collecting the reflections and analyzing the data)
As Enrico mentioned, there are also several NMR methods that can give valuable (though not always unambiguous) information:
• Looking for NOE between protons will let you see which groups are close to oneanother, but if you don't see an NOE signal between two protons, that does not conclusively mean that they are not close as there are many reasons that a signal would not be observed.
• Coupling constants of nearby NMR-active nuclei can also give information on their relative position and bonding orientation. For instance the two vinylic protons in disubstituted alkenes often have characteristic coupling constants (< 5 Hz for geminal, 5–12 Hz for cis and >10 Hz for trans). These are not rigorous proof of geometry, especially for values between 10 and 12 Hz, but if you have a mixture of two isomers, the one with the greater coupling constant can be confidently assigned as trans. NOTE: this method does not help for completely symmetric disubstituted alkenes because the two protons are chemically equivalent, and will appear together as a singlet (unless coupled to another nucleus). Coupling constants of other nuclei (especially 31P) can also be useful for determining the geometry of coordination compounds.
• IR stretches can also be useful for this (especially for carbonyl complexes), but I would only resort to this for very quick analysis by comparison to known spectra, or for very simple structures.
cis or trans what? alkenes (like cis vs trans stilbene), fused rings (like cis vs trans decalin), or coordination compounds (cis vs trans platin)... In all cases, the most definitive analysis would be single-crystal x-ray crystallography (but this requires growing a crystal, collecting the reflections and analyzing the data)
As Enrico mentioned, there are also several NMR methods that can give valuable (though not always unambiguous) information:
• Looking for NOE between protons will let you see which groups are close to oneanother, but if you don't see an NOE signal between two protons, that does not conclusively mean that they are not close as there are many reasons that a signal would not be observed.
• Coupling constants of nearby NMR-active nuclei can also give information on their relative position and bonding orientation. For instance the two vinylic protons in disubstituted alkenes often have characteristic coupling constants (< 5 Hz for geminal, 5–12 Hz for cis and >10 Hz for trans). These are not rigorous proof of geometry, especially for values between 10 and 12 Hz, but if you have a mixture of two isomers, the one with the greater coupling constant can be confidently assigned as trans. NOTE: this method does not help for completely symmetric disubstituted alkenes because the two protons are chemically equivalent, and will appear together as a singlet (unless coupled to another nucleus). Coupling constants of other nuclei (especially 31P) can also be useful for determining the geometry of coordination compounds.
• IR stretches can also be useful for this (especially for carbonyl complexes), but I would only resort to this for very quick analysis by comparison to known spectra, or for very simple structures.
Hi Rajiv,
Its very simple to determine the difference between Cis and Trans isomers by the help of Coupling constants in the 1H NMR spectra. If it is a trans the J-Coupling constant of the unsaturated protons in the spectrum shows nearly 13-16 Hz, and it is nearly 10-12 in case of Cis isomers.
But Dr Rajesh, What if both are present together? What is your take on this?
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