Aromatic compounds exhibit predictable reactivity trends in electrophilic substitution
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Peer-reviewed organic chemistry literature reports that aromatic compounds undergo electrophilic substitution with predictable reactivity trends and regioselectivity governed by resonance and inductive substituent effects.
The development of stable and tunable polycyclic aromatic compounds (PACs) is crucial for the advancement of organic optoelectronics. Conventional PACs, such as acenes, often suffer from poor stability due to photooxidation and oligomerization, which are linked to their frontier molecular orbital energy levels. To address these limitations, we designed and synthesized a new class of π-expanded indoloindolizines by merging indole and indolizine moieties into a single polycyclic framework. We developed a scalable synthetic protocol to produce a wide range of π-expanded derivatives. The structural, electronic, and optical properties of these compounds were extensively characterized. We achieved precise modulation of the electronic structure by controlling the aromaticity of specific rings. Benzannulation at targeted positions allowed fine-tuning of the HOMO–LUMO gap, leading to distinct shifts in the optoelectronic properties. Single-crystal X-ray diffraction confirmed their molecular structures, while theoretical calculations provided insights into the observed experimental trends. These indoloindolizines exhibit vivid colors and fluorescence across the visible spectrum and enhanced stability against photooxidation. Reactivity studies demonstrated high regioselectivity in electrophilic substitutions, highlighting the indole-like behavior of these compounds and opening avenues for further functionalization. To showcase the practical utility of our design rules, we fabricated organic field-effect transistors (OFETs) using the newly developed indoloindolizines, which revealed remarkable performance with ambipolar charge transport properties. Overall, our work establishes indoloindolizines as a promising platform for the development of stable, tunable organic materials for optoelectronic applications. Through rational molecular design, we have provided a new pathway for molecular innovation in organic electronics.
This chapter assesses electrophilic aromatic substitution. Formation of the enol tautomer is catalysed by acid or by base, and because the ketone and enol are in equilibrium, enolization in the presence of D2O can lead to replacement of the protons in the α positions of ketones by deuterium atoms. Because the enolization and deuteration process can be repeated, eventually all of the α-protons are replaced by deuterium. The way this ketone is deuterated provides evidence that its enol form exists, even though the keto/enol equilibrium greatly favours the ketone form at equilibrium. The chapter discusses similar reactions of a compound that exists entirely in its enol form. That very stable enol is phenol and its stability is a consequence of the aromaticity of its benzene ring. The chapter then looks at alkyl benzenes, halogens, and the Friedel–Crafts chemistry.
This competition between inductive electron withdrawal and conjugative electron donation was discussed earlier in the context of substituent effects on electrophilic aromatic substitution. Here, it was noted that amino groups were strongly electron donating (resonance effect >> inductive effect), alkoxy groups were slightly less activating, acyloxy groups still less activating (resonance effect > inductive effect) and chlorine was deactivating (inductive effect > resonance effect). In the illustration on the right, R and Z represent the remainder of a benzene ring. This analysis also predicts the influence these substituent groups have on the reactivity of carboxylic acid derivatives toward nucleophiles (Z = O in the illustration). Inductive electron withdrawal by Y increases the electrophilic character of the carbonyl carbon, and increases its reactivity toward nucleophiles. Thus, acyl chlorides (Y = Cl) are the most reactive of the derivatives. Resonance electron donation by Y decreases the electrophilic character of the carbonyl carbon.
By clicking the "Toggle Effect" button the electron shift in both effects will be displayed sequentially. This competition between inductive electron withdrawal and conjugative electron donation was discussed earlier in the context of substituent effects on electrophilic aromatic substitution. Here, it was noted that amino groups were strongly electron donating (resonance effect >> inductive effect), alkoxy groups were slightly less activating, acyloxy groups still less activating (resonance effect > inductive effect) and chlorine was deactivating (inductive effect > resonance effect). In the illustration on the right, R and Z represent the remainder of a benzene ring. This analysis also predicts the influence these substituent groups have on the reactivity of carboxylic acid derivatives toward nucleophiles (Z = O in the illustration). Inductive electron withdrawal by Y increases the electrophilic character of the carbonyl carbon, and increases its reactivity toward nucleophiles. Thus, acyl chlorides (Y = Cl) are the most reactive of the derivatives. Resonance electron donation by Y decreases the electrophilic character of the carbonyl carbon.
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.
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.
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