Aromatic Compounds Help
Benzene, (C6 H6) is the prototype of aromatic compounds, which are unsaturated compounds showing a low degree of reactivity. Benzene exists as a resonance hybrid of two structures shown below. As a result, all of the C–C bonds and all the hydrogens in benzene are equivalent.
There are three disubstituted benzenes—the 1,2-, 1,3-, and 1,4-position isomers—designated as ortho, meta, and para, respectively.
Each carbon atom in benzene is sp2 hybridized. The σ bonds comprise the skeleton of the molecule. Each carbon also has a p orbital at right angles to the plane of the ring, forming a cyclic, conjugated π system containing 6 electrons. This π system is parallel to and above and below the plane of the ring.
The six π electrons in the π system are associated with all six C's. They are therefore more delocalized, accounting for the great stability of benzene and other aromatic rings.
Aromaticity and Hückel's Rule
Hückel's rule states that if the number of π electrons in a planar, cyclic, conjugated structure is equal to 4n + 2, where n equals zero or a whole number, the species is aromatic. This rule applies to heterocycles (rings containing a non-carbon atom such as nitrogen or sulfur) as well as to carbocycles (like benzene).
Polycyclic Aromatic Compounds. The prototype of this class of compounds is naphthalene, C10H8, although there are many others. Although the Hückel 4n + 2 rule is rigorously derived for monocyclic systems, it can also be applied to other compounds.
Planar cyclic conjugated species with 4n π electrons are called antiaromatic and are quite unstable. 1,3-Cyclobutadiene (n = 1), for which two equivalent contributing resonance structures can be writtten, is an extremely unstable antiaromatic molecule.
Some benzene derivatives are usually referred to by their common, nonsystematic names, such as toluene (C6H5CH3), xylene (C6H4(CH3)2), phenol (C6H5OH), and aniline (C6H5NH2). Derived names combine the name of the substituent as a prefix with the word benzene. Examples include nitrobenzene (C6H5NO2) and ethylbenzene (C6H5CH2CH3). Some common aromatic groups that are substituents (Ar–) are C6H5– (phenyl), C6H5–C6H4– (biphenyl), and p–CH3C6H4(p-tolyl). Another common group is C6H5CH2–, known as benzyl.
The unusual stability of the benzene ring dominates the chemical reactions of benzene and naphthalene. Both compounds resist addition reactions which lead to destruction of the aromatic ring. Rather, they undergo substitution reactions, in which a group or atom replaces an H from the ring, thereby preserving the stable aromatic ring. Atoms or groups other than H may also be replaced.
Reduction. Benzene is resistant to catalytic hydrogenation (high temperatures and high pressures of H2 are needed) and to reduction with Na in alcohol. Reduction with lithium in liquid ammonia (known as the Birch reduction) produces 1,4-cyclohexadiene.
Oxidation. Benzene is very stable to oxidation except under very vigorous conditions. In fact, when an alkylbenzene is oxidized, the alkyl group is oxidized to a COOH group, while the benzene ring remains intact. For this reaction to proceed, there must be at least one H atom on the C attached to the ring.
Electrophilic Aromatic Substitution. Aromatic rings undergo substitution reactions (replacement of a hydrogen with another group) with strong electrophiles. The mechanism for these reactions involve attack of the electrophile on the π electrons of the ring, followed by loss of a proton to reestablish aromaticity. These reactions typically require Lewis acid catalysts to help generate the electrophiles. The first step in this reaction is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation loses a cation (most often H+) to give the substitution product, rather than adding a nucleophile to give the addition product.
A wide range of different electrophiles can be used, as demonstrated below.
The introduction of acyl groups is accomplished by treating benzene with an acid chloride and AlCl3. This reaction is known as the Friedel-Crafts Acylation.
The 5 ring H's of monosubstituted benzenes are not equally reactive. The ring substituents determine the orientation of E (meta, or a mixture of ortho and para), and the reactivity of the ring toward substitution. Electron-donating groups (–OCH3, –NR2, alkyl) make the ring more reactive, and direct electrophilic attack to the ortho and para positions. These groups are known as ortho, para directors. Electron withdrawing groups (–NO2, acyl) are meta directors and deactivate the ring toward electrophilic attack. Halogens are ortho, para directors, but weak deactivators.
Order of Introducing Groups
In the preparation of highly substituted aromatic compounds, it is essential to introduce the substituents in the proper order. For example, if benzene is first treated with CH3Cl in the presence of AlCl3, then with HNO3/H2SO4 the para product will be formed. Reversing the order of these steps yields the meta product.
Reactivity of the Benzylic Positions
Benzylic carbons are adjacent to an aromatic ring. The chemistry of benzylic and of allylic positions are very similar. Intermediate carbocations, free radicals, and carbanions formed at these positions are stabilized by delocalization with the adjacent π system, the benzene ring in the case of the benzylic position. Benzylic halides can be prepared in good yield through free-radical halogenation, as shown below.
Benzylic halides are highly reactive, even reacting with nucleophiles as weak as water. Alkyl halides do not undergo nucleophilic substitutions with such weak nucleophiles.
Practice problems for these concepts can be found at: Aromatic Compounds Practice Problems
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