Conjugated systems allylic carbocation

SOLUTION TO 11a First we need to determine which of the terminal sp carbons of the conjugated system is going to be the C-1 carbon. The proton will be more apt to add to the indicated sp carbon because the carbocation that is formed shares its positive charge with a tertiary allylic and a secondary allylic carbon. If the proton were to add to the sp carbon at the other end of the conjugated system, the carbocation that would be formed would be less stable because its positive charge would be shared by a primary allylic and a secondary allylic carbon. Therefore, 3-chloro-3-methylcyclohexene is the 1,2-addition product and 3-chloro-l-methylcyclohexene is the 1,4-addition product. 3-Chloro-3-methylcyclohexene is the kinetic product because of the chloride ion s proximity to C-2, and 3-chloro-l-methylcyclohexene is the thermodynamic product because its more highly substituted double bond makes it more stable. 

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

The allyl carbocation is another example of a conjugated system. The three carbon atoms of the. allyl carbocation—the positively charged carbon atom and the two that form the double bond— are sp hybridized with a p orbital. The p orbitals for the double bond carbons each contain an electron, whereas the p orbital for the carbocation is empty. 

Dienes are conjugated systems of two pi bonds. Above, the simplest diene, 13-butadiene, adds the electrophile to the end to produce a resonance stabilized allylic carbocation. As with alkenes, the more substituted the diene is, the more reactive. 

At its essence, a conjugated system involves at least one atom with ap orbital adjacent to at least one tt bond. The adjacent atom with the p orbital can be part of another ir bond, as in 1,3-butadiene, or a radical, cationic, or anionic reaction intermediate. If an example derives specifically from a propenyl group, the common name for this group is allyl. In general when we are considering a radical, cation, or anion that is adjacent to one or more TT bonds in a molecule other than propene, the adjacent position is called allylic. Below we show the formula for butadiene, resonance hybrids for the allyl radical and an allylic carbocation, and molecular orbital representations for each one. 

As described in Sections 10.1-10.4, allylic carbocations, radicals, and anions are conjugated TT-electron systems involved as intermediates in chemical reactions. The remaining sections of this chapter focus on stable molecules, especially hydrocarbons called conjugated dienes, which contain two C=C units joined by a single bond asinC=C—C=C. It begins by comparing their structure and stability to isolated dienes, in which the two... 

Benzylic carbocations, radicals, and anions resemble their allylic counterparts in being conjugated systems stabilized by electron delocalization. This delocalization is describable in resonance, valence bond, and molecular orbital terms. 

The chemistries of the benzyiic and allylic positions are very similar. Intermediate carbocations, free radicals and carbanions formed at these positions are stabilized by delocalization with the adjacent ir system, the benzene ring in the case of the benzyiic position. Another aspect of arene chemistry is the enhanced stability of unsaturated arenes having double bonds conjugated with the benzene ring. This property is akin to the stability of conjugated di- and polyenes. 

In the presence of a Lewis acid (such as Et2AlCl), allylsilanes react with electrophiles in a regiospecific manner. The intermediate (3-carbocation is stabilized by (a-Tc)-conjugation with the C-Si bond. The most important feature of this reaction is that the electrophile reacts with the terminus (y-carbon) of the allyl system, and the n-system is relocated adjacent to its original position. Even substituted allylic silanes can be acylated at the more hindered site. Because of this predictability and their high nucleophilicity, allylsilanes are valuable in many synthetic transformations. 

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