Keynotes in Organic Chemistry. Oxford; Malden, MA: Blackwell Science, 2003 - Taylor, R. Electrophilic Aromatic Substitution. Chichester, West Sussex, England; New York: J. Wiley, 1990 Problems - Label the hybridization on all the carbons in a) reacting benzene ring, b) intermediate (including resonance forms), and c) product (monosubstituted benzene ring) - Is the energy of activation higher in the first step or second step of the mechanism? Explain your reasoning. - If you wanted to halogenate benzene, what sort of reagent and catalyst (if needed) would you use? - Which hydrogren is used in order to regain aromaticity after the electrophile has added to the ring? - Critical Thinking Question: Mentioned above was the fact that electrophilic aromatic substitution can and does happen when there are substituents already present on the ring. Already present substituents will determine where something adds onto the ring in relation to itself (ortho, meta, or para position). What sort of factors do you think influence where addition will occur?

This chapter evaluates the orientation of electrophilic substitution reactions. When an arene C 6 H 5 X is substituted by an electrophile R + , there are three principal sites for bonding: C-2 ( ortho , o ), C-3 ( meta , m ), and C-4 ( para , p ). Halogen atoms and most of the commonly encountered substituent groups, apart from alkyl units, are more electronegative than the sp 2 hybridized carbon atoms which constitute the benzene ring. As a result, a dipole is created and charge is withdrawn from the ring. The chapter then looks at resonance effects within sigma intermediaries. While resonance normally has the dominant role in determining the site adopted by the entering electrophile (the electromeric effect), the rate of the reaction is also influenced by the electron withdrawing power of the original substituent (the inductive effect). The chapter also considers ipso substitution, kinetic and thermodynamic control, and Birch reduction.

In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group (EDG) or electron releasing group (ERG, Z in structural formulas) is an atom or functional group that donates some of its electron density into a conjugated In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group (EDG) or electron releasing group (ERG, Z in structural formulas) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance (mesomerism) or inductive effects (or induction)—called +M or +I effects, respectively—thus making the π system more nucleophilic. As a result of these electronic effects, an aromatic ring to which such a group is attached is more likely to participate in electrophilic substitution reaction. EDGs are therefore often known as activating groups, though steric effects can interfere with the reaction. An electron withdrawing group (EWG) will have the opposite effect on the nucleophilicity of the ring. The EWG removes electron density from a π system, making it less reactive in this type of reaction, and therefore called deactivating groups. EDGs and EWGs also determine the positions (relative to themselves) on the aromatic ring where substitution reactions are most likely to take place. Electron donating groups are generally ortho/para directors for electrophilic aromatic substitutions, while electron withdrawing groups (except the halogens) are generally meta directors. The selectivities observed with EDGs and EWGs were first described in 1892 and have been known as the Crum Brown–Gibson rule. Electr… In general, the resonance effect of elements in the third period and beyond is relatively weak. This is mainly because of the relatively poor orbital overlap of the substituent's 3p (or higher) orbital with the 2p orbital of the carbon. Due to a stronger resonance effect and inductive effect than the heavier halogens, fluorine is anomalous. The partial rate factor of electrophilic aromatic substitution on fluorobenzene is often larger than one at the para position, making it an activating group. Conversely, it is moderately deactivated at the ortho and meta positions, due to the proximity of these positions to the electronegative fluoro substituent.

This chapter considers benzene as one of the most fascinating organic molecules. Six carbon atoms in benzene are linked in a planar hexagon and, as each carbon atom is bonded to only one hydrogen atom, benzene is an unsaturated hydrocarbon. The chapter determines what aromatic, antiaromatic, and nonaromatic compounds are and give examples of each. It identifies reagents and reaction mechanisms to explain how benzene undergoes halogenation, nitration, sulfonation, Friedel–Crafts alkylation, and Friedel–Crafts acylation. It also talks about how the electronic and steric effects of substituents on benzene rings influence the rates and regioselectivities of electrophilic substitution reactions and how substituents on benzene rings can be converted into other substituents by redox reactions or by forming diazonium ions.

considered. Relative reactivity The first is the relative reactivity of the compound compared with benzene itself. Experiments have shown that substituents on a benzene ring can influence reactivity in a profound manner. For example, a hydroxy or methoxy substituent increases the rate of electrophilic substitution about ten thousand fold, as illustrated by the case of anisole in the virtual demonstration (above). In contrast, a nitro substituent decreases the ring's reactivity by roughly a million. This activation or deactivation of the benzene ring toward electrophilic substitution may be correlated with the electron donating or electron withdrawing influence of the substituents, as measured by molecular dipole moments. In the following diagram we see that electron donating substituents (blue dipoles) activate the benzene ring toward electrophilic attack, and electron withdrawing substituents (red dipoles) deactivate the ring (make it less reactive to electrophilic attack).