Alkenes priority rules

These priority rules are also used to assign Eand Zto alkenes. (see p. 4B7) and are sometimes called the Cahn-lngold-Preiog (CIP) rules, after their devisors. 

For this, we use the same set of priority rules we used in Section 4.2C to assign an E,Z configuration to an alkene. 

Whether an alkene is cis or trans is determined by the orientation of the main carbon chain about the double bond. The configuration of a carbon-carbon double bond is specified more precisely by the E, Z system, which uses the same set of priority rules used for the R, S system. 

Alkenes are unsaturated molecules. Their lUPAC names are derived from alkanes, the longest chain incorporating the double bond serving as the stem. Double-bond isomers include terminal, internal, cis, and trans arrangements. Tri- and tetrasubstituted alkenes are named according to the system, in which the R,S priority rules apply. 

You might normally expect an E-alkene to be more stable than a Z-alkene—it just so happens here that Cl has a higher priority than Ph and the Z-alkene has the two largest groups (Ph and Me) trans. (See p. 4B5 for rules of nomenclature.)... 

Example 4 In Figure 13.30, there is a double bond, represented by the extra vertical line inside the ring structure. You must follow the same rules as for alkenes. That is, the double bond gets priority for the lowest number. This means that one of the carbon atoms, on either end of the double bond, must be carbon number 1. The carbon atom at the other end must be carbon number 2. Next you have to decide in which direction to count so that the branch gets the lowest possible position number. In this compound, the carbon atom on the bottom end of the double bond is carbon number 1. 

When the difference between substituents is in configuration, then in general Z> E and R> S. However, the formal definition is that an olefinic ligand in which the substituent of higher sequence priority is on the same side of the alkene double bond as the chiral centre takes priority. This definition does not correspond with either E,Z- or cis/trans- (change to the rules in 1982), The subscript n (for new ) is used in cases of doubt. 

The E/Z nomenclature was introduced in Chapter 7, and now that you have read Chapter 16 we can be more precise with our definition. For disubstituted alkenes, Ecorrespondsto trans and Zcorresponds to cis. To assign Eor Zto tri- or tetrasubstituted alkenes, the groups at either end of the alkene are given an order of priority according to the same rules as those outlined for R and S in Chapter 16. Ifthe two higher priority groups are cis, the alkene is Z if they are trans the alkene is E. Of course, molecules don t know these rules, and sometimes (as in the second example here) the Ealkene is less stable than the Z. 

In order to describe the absolute configuration of chiral allenes, the Cahn-Ingold-Prelog (CIP) rules are first used to ascertain the priorities at the each terminal carbon, as is done for alkenes at this point the similarity with alkenes ends. The method will be demonstrated with the trisubstituted allene 14a ... 

A carbonyl group in ABC=0 is prochiral provided A differs from B the same holds for the faces of both carbons in an alkene R R2C=CR3R4, and for both faces of one carbon in RlR2C=CH3. The upper face of a carbonyl carbon in ABC=0 is given the symbol Re if, after using the CIP sequence rules, the priorities, say, O > A > B decrease clockwise when looking at the face from above . In such a case the opposite face of the carbonyl carbon is Si. 

For di-, tri- and tetra-substituted alkenes (with two, three and four substituents, respectively, on the double bond), alkenes can be named using the E,Z nomenclature. The groups on the double bond are assigned priorities based on a series of sequence rules. 

As noted above, synthetically important prochiral centers are the carbonyl of a ketone or aldehyde and the double bond of an alkene. These functional groups do not contain a pro-R or pro-S group but it is clear that delivery of a fourth point ligand from one face or the other will lead to an (R) or (5) chiral center, as in conversion of 108 to 109 and/or 110. If the carbonyl is oriented as in ketone lllA, priorities can be assigned to the three atoms connected to the prochiral atom, based on the CIP rules. For lllA, the a b c priority is... 

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