trans-2-fluoro-3-methylpent-2-ene this compound in also known as (Z)2-fluoro-3-methylpent-2-ene. I have given my possible structure in the image but it is wrong, Can anyone explain this ?
Answer
The structure you drew
is ‒ according to the CIP-rules ‒ (E)-configured. These rules are applied on each side of the double bond; namely
- C of the ethyl group has a higher priority than C of the methyl group
- F has a higher priority than C
- C(ethyl group) opposes F, as seen across the double bond
This is not in contradiction that the two methyl groups are opposing each other, are in (trans)-relationship with each other.
In instances with only two subsitutents around a double bond, like (E)-2-pentene, you may use (E), derived from German "entgegen", literally "opposing", as synonym to (cis). For the opposite instance, (Z), derived from German "zusammen", like "together", to describe a (trans)-configuration.
In instances with three or four substitutions around a double bond, and instances of several conjugated double bonds, follow IUPAC and adhere to the (E/Z) notation; which is absolute and not ‒ as (cis/trans) ‒ relative and potentially ambiguous. See the example E/Z notation, for example. (Or, as one teacher told me, "stay on the E-Z pass".)
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