Question

(c) 5-chloro-2-vinyl-cyclohexadine (d) (3R)-3-bromo-2-allyl-cyclohexene Br (e) Z-2-amino-3-Fluoro-2-pentene F NH2
just show me how to count them please and why is allyl and the other one is vinyl ?

Answer 1

(c) 5-Chloro-2-vinyl-cyclohexadiene the number of carbons in the ring is greater than the branch chain so, ring is the parent . -CH=CH2 is called vinyl group (both sp2 hybridised carbon ) which is attached to the ring that is why called vinyl group in the name . Counting in the ring starts from the double bond because double bond is given greater preference then other substituents , the ring has two double bonds the double bond nearer to the allyl substituent is given greater preference then nearer to the chlorine because allyl group is given greater preference than chlorine substituent.

С0 cе. 5 3 CHECH vingl gnoup LO

(d)(3R)-3-bromo-2-allyl-cyclohexane

Br CH2 CH2 allyl grop farther Flip Br H Br Ln

Same as the previous case, the ring contains more number of carbon atoms then the branch , therefore it is the parent. It has -CH2-CH=CH2 attached to it which contains two sp2 hybridised carbon attached to a sp3 hybridised carbon , and it is attached to the ring by the sp3 hybridised carbon , therefore it is called allyl group attached to the ring.

The counting starts, from the double bond because double bond is given higher priority than the substituents ,here substituents are are allyl group at second position and bromine group at third position

It has bromo substituent below the plane at third position to complete the valency there is a hydrogen at third position above the plane because the ring is shown on the plane. To give preference to assign the configuration, the lowest priority group has to be below the plane and here hydrogen is the lowest priority group we have to flip the ring by 180 degree and then and give preference to each group . bromine has highest atomic mass so it is given number 1, hydrogen has lowest atomic mass so it is given number 4, the ring carbons are numbered by checking which is nearest to the the substituent allyl group ,the one nearest to the allyl group is given number 2 and the left carbon is given number 3. As the the numbering goes clockwise, thus it has R configuration at third position.

(e)Z-2-Amino3-fluoro-2-pentene

5m 2 3 F NH2 1 OF

Here the longest carbon chain contains 5 carbon atoms ,double bond has to be given the lowest possible carbon number and substituents also should get low numbers .Thus , counting starts from right to left and NH2 group gets second position and fluorine group get third position while the double bond gets second position , according to IUPAC nomenclature .

For checking Z isomer , draw a line sectioning the double bond and give priority to the group with higher atomic number on each side . The priority is given to NH2 group and fluorine group to methyl and ethyl group respectively ,because of N and F have higher atomic number than carbon atom. Now as the higher priority groups on the same side of the double bond it is called Z isomer.

( if the higher priority groups were on the opposite sides of the double bond it would be E isomer)