Synthesise 3-chloro-4-methylbenzene sulfonic acid from toluene
Note that the orientations in each category change depending on whether the groups have similar or opposite individual directing effects. The products from substitution reactions of compounds having a reinforcing orientation of substituents are easier to predict than those having antagonistic substituents. For example, the six equations shown below are all examples of reinforcing or cooperative directing effects operating in the expected manner.
Symmetry, as in the first two cases, makes it easy to predict the site at which substitution is likely to occur. Note that if two different sites are favored, substitution will usually occur at the one that is least hindered by ortho groups. The first three examples have two similar directing groups in a meta-relationship to each other. In examples 4 through 6, oppositely directing groups have an ortho or para-relationship.
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The major products of electrophilic substitution, as shown, are the sum of the individual group effects. The strongly activating hydroxyl —OH and amino —NH 2 substituents favor dihalogenation in examples 5 and six. Substitution reactions of compounds having an antagonistic orientation of substituents require a more careful analysis. If the substituents are identical, as in example 1 below, the symmetry of the molecule will again simplify the decision. Case 3 reflects a combination of steric hindrance and the superior innate stabilizing ability of methyl groups relative to other alkyl substituents.
Oxidation of Alkyl Side-Chains The benzylic hydrogens of alkyl substituents on a benzene ring are activated toward free radical attack, as noted earlier.
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Furthermore, S N 1, S N 2 and E1 reactions of benzylic halides , show enhanced reactivity, due to the adjacent aromatic ring. The possibility that these observations reflect a general benzylic activation is supported by the susceptability of alkyl side-chains to oxidative degradation, as shown in the following examples the oxidized side chain is colored.
Such oxidations are normally effected by hot acidic pemanganate solutions, but for large scale industrial operations catalysed air-oxidations are preferred. Interstingly, if the benzylic position is completely substituted this oxidative degradation does not occur second equation, the substituted benzylic carbon is colored blue. These equations are not balanced. Two other examples of this reaction are given below, and illustrate its usefulness in preparing substituted benzoic acids.
Reduction of Nitro Groups and Aryl Ketones Electrophilic nitration and Friedel-Crafts acylation reactions introduce deactivating, meta-directing substituents on an aromatic ring. The attached atoms are in a high oxidation state, and their reduction converts these electron withdrawing functions into electron donating amino and alkyl groups. Examples of these reductions are shown here, equation 6 demonstrating the simultaneous reduction of both functions.
2-Amino-4-chloro-5-methylbenzenesulfonic acid Specification
Note that the butylbenzene product in equation 4 cannot be generated by direct Friedel-Crafts alkylation due to carbocation rearrangement. The zinc used in ketone reductions, such as 5, is usually activated by alloying with mercury a process known as amalgamation. Several alternative methods for reducing nitro groups to amines are known. These include zinc or tin in dilute mineral acid, and sodium sulfide in ammonium hydroxide solution. The procedures described above are sufficient for most cases.
This provides a powerful tool for the conversion of chloro, bromo or iodo substituents into a variety of other groups. Many reactions of these aryl lithium and Grignard reagents will be discussed in later sections, and the following equations provide typical examples of carboxylation, protonation and Gilman coupling. Metal halogen exchange reactions take place at low temperature, and may be used to introduce iodine at designated locations.
An example of this method will be displayed below by clicking on the diagram. In this example care must be taken to maintain a low temperature, because elimination to an aryne intermediate takes place on warming. Hydrolysis of Sulfonic Acids The potential reversibility of the aromatic sulfonation reaction was noted earlier.
The following equation illustrates how this characteristic of the sulfonic acids may be used to prepare the 3-bromo derivative of ortho-xylene. Direct bromination would give the 4-bromo derivative. Direct nitration of phenol hydroxybenzene by dilute nitric acid gives modest yields of nitrated phenols and considerable oxidative decomposition to tarry materials; aniline aminobenzene is largely destroyed. Bromination of both phenol and aniline is difficult to control, with di- and tri-bromo products forming readily.
Because of their high nucleophilic reactivity, aniline and phenol undergo substitution reactions with iodine, a halogen that is normally unreactive with benzene derivatives.
The mixed halogen iodine chloride ICl provides a more electrophilic iodine moiety, and is effective in iodinating aromatic rings having less powerful activating substituents. By acetylating the heteroatom substituent on phenol and aniline, its activating influence can be substantially attenuated. For example, acetylation of aniline gives acetanilide first step in the following equation , which undergoes nitration at low temperature, yielding the para-nitro product in high yield.
The modifying acetyl group can then be removed by acid-catalyzed hydrolysis last step , to yield para-nitroaniline.
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The following diagram illustrates how the acetyl group acts to attenuate the overall electron donating character of oxygen and nitrogen. The non-bonding valence electron pairs that are responsible for the high reactivity of these compounds blue arrows are diverted to the adjacent carbonyl group green arrows. It should now be apparent that an extensive "toolchest" of reactions are available to us for the synthesis of substituted benzenes. Six proposed syntheses are listed in the following diagram in rough order of increasing complexity.
You should try to conceive a plausible reaction sequence for each. Once you have done so, you may check suggested answers by clicking on the question mark for each. Compounds in which two or more benzene rings are fused together were described in an earlier chapter , and they present interesting insights into aromaticity and reactivity. The smallest such hydrocarbon is naphthalene. Naphthalene is stabilized by resonance. Three canonical resonance contributors may be drawn, and are displayed in the following diagram. The two structures on the left have one discrete benzene ring each, but may also be viewed as pi-electron annulenes having a bridging single bond.
The structure on the right has two benzene rings which share a common double bond. As expected from an average of the three resonance contributors, the carbon-carbon bonds in naphthalene show variation in length, suggesting some localization of the double bonds. The C1—C2 bond is 1. This contrasts with the structure of benzene, in which all the C—C bonds have a common length, 1. Naphthalene is more reactive than benzene, both in substitution and addition reactions, and these reactions tend to proceed in a manner that maintains one intact benzene ring. The following diagram shows three oxidation and reduction reactions that illustrate this feature.
Electrophilic substitution reactions take place more rapidly at C1, although the C2 product is more stable and predominates at equilibrium. Examples of these reactions will be displayed by clicking on the diagram. The kinetically favored C1 orientation reflects a preference for generating a cationic intermediate that maintains one intact benzene ring. By clicking on the diagram a second time , the two naphthenonium intermediates created by attack at C1 and C2 will be displayed.
The structure and chemistry of more highly fused benzene ring compounds, such as anthracene and phenanthrene show many of the same characteristics described above.
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The chief products are phenol and diphenyl ether see below. This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an S N 1 or S N 2 pathway. The presence of electron-withdrawing groups such as nitro ortho and para to the chlorine substancially enhance the rate of substitution, as shown in the set of equations presented on the left below. To explain this, a third mechanism for nucleophilic substitution has been proposed.
This two-step mechanism is characterized by initial addition of the nucleophile hydroxide ion or water to the aromatic ring, followed by loss of a halide anion from the negatively charged intermediate. This is illustrated by clicking the "Show Mechanism" button next to the diagram. The sites over which the negative charge is delocalized are colored blue, and the ability of nitro, and other electron withdrawing, groups to stabilize adjacent negative charge accounts for their rate enhancing influence at the ortho and para locations.
2-CHLOROAMINOTOLUENESULFONIC ACID (2B ACID)
Three additional examples of aryl halide nucleophilic substitution are presented on the right. Only the 2- and 4-chloropyridine isomers undergo rapid substitution, the 3-chloro isomer is relatively unreactive. Nitrogen nucleophiles will also react, as evidenced by the use of Sanger's reagent for the derivatization of amino acids. The resulting N-2,4-dinitrophenyl derivatives are bright yellow crystalline compounds that facilitated analysis of peptides and proteins, a subject for which Frederick Sanger received one of his two Nobel Prizes in chemistry.
Such addition-elimination processes generally occur at sp 2 or sp hybridized carbon atoms, in contrast to S N 1 and S N 2 reactions. When applied to aromatic halides, as in the present discussion, this mechanism is called S N Ar. Some distinguishing features of the three common nucleophilic substitution mechanisms are summarized in the following table.
Elimination There is good evidence that the synthesis of phenol from chlorobenzene does not proceed by the addition-elimination mechanism S N Ar described above. However, ortho-chloroanisole gave exclusively meta-methoxyaniline under the same conditions.