14-3 Nucleophilic Substitution of Allylic Halides: Allylic halides can also undergo Sx2 reactions. nemllo he p orbital radicalformed,xture of CHCH-OH CH-CHCH.C+CH-CHCH C T3 OICH-CHCHO CH.CHCHC+CHCHCH Cr I 14-5 Two Neighboring Double Bonds:Conjugated arbon N Proronp vatives h iheeo double bonds are amed CH:-CH-CH-CH CH,-C-CH -aa4mG- 好 人 llylic lithium and Grignard reagents can function as nucleophile CH:-c-CH: 个，个 3 3 Nucleophilic Substitution of Allylic Halides: SN1 and SN2 14-3 Allylic halides undergo SN1 reactions. Different allylic halides may give identical products if they dissociate into the same allylic cation. If an unsymmetrical allylic radical is formed, a mixture of products may be obtained. Allylic halides can also undergo SN2 reactions. SN2 reactions of allylic halides with good nucleophiles are faster than those of the corresponding saturated haloalkanes due to transition-state stabilization by overlap of the double bond and the p orbital. Allylic Organometallic Reagents: Useful Three-Carbon Nucleophiles 14-4 Alkyl lithium reagents can be made from propene derivatives by proton abstraction by an alkyl lithium. The Grignard derivative can also be prepared: Allylic lithium and Grignard reagents can function as nucleophiles. Two Neighboring Double Bonds: Conjugated Dienes 14-5 Hydrocarbons with two double bonds are named dienes. Dienes can be either non-conjugated (separated by at least one saturated carbon atom) or conjugated (immediately adjacent to one another). In a conjugated system the single atomic p orbital on each outer sp2 hybridized carbon atom overlaps one of the two perpendicular p orbitals on the inner sp hybridized carbon atom. Naming of conjugated and non-conjugated dienes is straightforward. The longest chain incorporating both double bonds is found and numbered to indicate the locations of substituent groups. Cis-trans, or E,Z prefixes indicate the configuration about the double bonds