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  Reactions of Substituent Groups Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions of Substituent Groups

A. Oxidation of Alkyl Side-Chains

As noted earlier, the benzylic hydrogens of alkyl substituents on a benzene ring are activated in the direction of free radical attack. Additionally, SN1, SN2 and E1 reactions of benzylic halides, display improved reactivity, because of the adjacent aromatic ring. The probability that these observations reflect a usual benzylic activation that is supported by the susceptibility of alkyl side-chains to oxidative degradation, as displayed in the following illustrations (the oxidized side chain is colored). Such types of oxidations are generally influenced by hot acidic pemanganate solutions, but for large scale industrial operations catalyzed air-oxidations are preferred. Interestingly, if benzylic position is entirely substituted this oxidative degradation does not take place (2nd equation, the substituted benzylic carbon is colored blue).

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These equations are not balanced. The permanganate oxidant is reduced, generally to Mn(IV) or Mn(II). Two other illustrations of this reaction are given below in the diagram, and depict its usefulness in preparing substituted benzoic acids.

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B. Reduction of Nitro Groups and Aryl Ketones

Friedel-Crafts acylation and Electrophilic nitration reactions introduce deactivating, the meta-directing substituents on an aromatic ring. The attached atoms are in the high oxidation state and their reduction converts these electron withdrawing functions into electron donating alkyl and amino groups. Reduction is simply get either by catalytic hydrogenation (H2 + catalyst), or with reducing metals in acid. Instances of these reductions are displayed here, equation 6 illustrating the concurrent reduction of both functions. Note: the butylbenzene product in the equation 4 cannot be generated by direct Friedel-Crafts alkylation because of carbocation rearrangement. The zinc employed in ketone reductions, like equation 5, is generally activated by alloying with mercury (a process termed as amalgamation).

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Various alternative techniques for reducing nitro groups to amines are known. These comprise tin or zinc in dilute mineral acid and sodium sulfide in ammonium hydroxide solution. The procedures that are explained above are enough for most cases.

C. Conversion of Halogens to Organometallic Reagents

The reaction of aryl and alkyl halides with reactive metals (generally Li & Mg) to give nucleophilic reagents has been noted. This gives a powerful tool for the conversion of bromo, chloro or iodo substituents into a range of other groups. Several reactions of these aryl Grignard and lithium reagents will be discussed after, and the following equations provide typical illustrations of carboxylation, protonation and Gilman coupling. The Metal halogen exchange reactions occur at low temperature, and may be employed to introduce iodine at designated locations. In this instance care must be taken to maintain a low temperature because elimination to an aryne intermediate occurs on warming.

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D. Hydrolysis of Sulfonic Acids

The potential reversibility of the aromatic sulfonation reaction was noticed before. The following equation demonstrates how these features of the sulfonic acids may be employed to prepare the 3-bromo derivative of ortho-xylene. The Direct bromination would give the 4-bromo derivative.

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E. Modifying the Influence of Strong Activating Groups

The strongest activating and ortho/para-directing substituents are the amino (-NH2) and hydroxyl (-OH) groups. Direct nitration of phenol with dilute nitric acid offers modest yields of nitrated phenols and significant oxidative decomposition to tarry materials; aniline (aminobenzene) is largely destroyed. The Bromination of both phenol and aniline is hard to control, with di- and tri-bromo products forming readily. Due to their high nucleophilic reactivity, phenol and aniline go through substitution reactions with iodine, a halogen that is generally unreactive with benzene derivatives. The mixed halogen iodine chloride (ICl) gives a more electrophilic iodine moiety, and is useful in iodinating aromatic rings having less powerful activating substituents.

C6H5-NH2  +  I2  +  NaHCO3    p-I-C6H4-NH2  +  NaI  +  CO2  +  H2O

From acetylating the heteroatom substituent on phenol and aniline, its activating effect can be substantially attenuated. For an instance, acetylation of aniline gives acetanilide (first step in the following equation), which goes through nitration at low temperature, yielding the para-nitro result in high yield. The modifying acetyl group can then be apart by acid-catalyzed hydrolysis (last step), to yield para-nitroaniline. Even though the activating effect of the amino group has been reduced by this method, the acetyl derivative remains an ortho/para-directing and activating substituent.

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The diagram demonstrates how the acetyl group acts to attenuate the overall electron donating character of nitrogen and oxygen. The non-bonding valence electron pairs which are responsible for the high reactivity of these compounds (blue arrows in the diagram) are diverted to the adjacent carbonyl group (green arrows in the diagram). Though, the overall effect of the modified substituent is still activating and ortho/para-directing.

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