This question deals with the reaction between benzene and 1-chloro-2-methylpropa

Question

This question deals with the reaction between benzene and 1-chloro-2-methylpropane in the presence of AlCl3. Knowing that the arenium ion intermediate has positive charge on the ortho and para carbons, draw each resonance structure - one which shows the positive charge on the ortho carbon and one which shows the positive charge on the para carbon. You will be drawing the 2 resonance structures in the same JME editor. Note, there are actually 3 resonance structures needed to show the true nature of the arenium ion, but the 2 ortho resonance structures have the same characteristics so you are only asked to draw it once.

Explanation / Answer

Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating Friedel-Crafts reactions. Contents [show] [edit]Illustrative reactions The most widely practiced example of this reaction is the ethylation of benzene. Approximately 24,700,000 tons were produced in 1999.[1] In this process, solid acids are used as catalyst to generate the incipient carbocation. All other electrophilic reactions of benzene are conducted on much smaller scale, they are valuable routes to key intermediates. The nitration of benzene is achieved via the action of the nitronium ion as the electrophile. Aromatic sulfonation of benzene with fuming sulfuric acid gives benzenesulfonic acid. Aromatic halogenation of benzene with bromine, chlorine or iodine gives the corresponding aryl halogen compounds catalyzed by the corresponding iron trihalide. The Friedel-Crafts reaction can be performed either as an acylation or as an alkylation. Typically aluminium trichloride is used, but almost any strong Lewis acid can be successful. For the acylation reaction a stoichiometric amount of aluminum trichloride must be used instead of simply a catalytic amount. [edit]Effect of substituent groups See also: Arene substitution patterns Both the regioselectivity and the speed of an electrophilic aromatic substitution are affected by the substituents already attached to the benzene ring. In terms of regioselectivity, some groups promote substitution at the ortho or para positions, while other groups increase substitution at the meta position. These groups are called either ortho-para directing or meta directing. In addition, some groups will increase the rate of reaction (activating) while others will decrease the rate (deactivating). While the patterns of regioselectivity can be explained with resonance structures, the influence on kinetics can be explained by both resonance structures and the inductive effect. Substituents can generally be divided into two classes regarding electrophilic substitution: activating and deactivating towards the aromatic ring. Activating substituents or activating groups stabilize the cationic intermediate formed during the substitution by donating electrons into the ring system, by either inductive effect or resonance effects. Examples of activated aromatic rings are toluene, aniline and phenol. The extra electron density delivered into the ring by the substituent is not equally divided over the entire ring, but is concentrated on atoms 2, 4 and 6 (the ortho and para positions). These positions are thus the most reactive towards an electron-poor electrophile. The highest electron density is located on both ortho and para positions, though this increased reactivity might be offset by steric hindrance between substituent and electrophile. The final result of the elecrophilic aromatic substitution might thus be hard to predict, and it is usually only established by doing the reaction and determining the ratio of ortho versus para substitution. On the other hand, deactivating substituents destabilize the intermediate cation and thus decrease the reaction rate. They do so by withdrawing electron density from the aromatic ring, though the positions most affected are again the ortho and para ones. This means that the most reactive positions (or, least unreactive) are the meta ones (atoms 3 and 5). Examples of deactivated aromatic rings are nitrobenzene, benzaldehyde and trifluoromethylbenzene. The deactivation of the aromatic system also means that generally harsher conditions are required to drive the reaction to completion. An example of this is the nitration of toluene during the production of trinitrotoluene (TNT). While the first nitration, on the activated toluene ring, can be done at room temperature and with dilute acid, the second one, on the deactivated nitrotoluene ring, already needs prolonged heating and more concentrated acid, and the third one, on very strongly deactivated dinitrotoluene, has to be done in boiling concentrated sulfuric acid. Functional groups thus usually tend to favor one or two of these positions above the others; that is, they direct the electrophile to specific positions. A functional group that tends to direct attacking electrophiles to the meta position, for example, is said to be meta-directing. [edit]Ortho/para directors Groups with unshared pairs of electrons, such as the amino group of aniline, are strongly activating and ortho/para-directing. Such activating groups donate those unshared electrons to the pi system. When the electrophile attacks the ortho and para positions of aniline, the nitrogen atom can donate electron density to the pi system (forming an iminium ion), giving four resonance structures (as opposed to three in the basic reaction). This substantially enhances the stability of the cationic intermediate. Compare this with the case when the electrophile attacks the meta position. In that case, the nitrogen atom cannot donate electron density to the pi system, giving only three resonance contributors. For this reason, the meta-substituted product is produced in much smaller proportion to the ortho and para products.

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