'A' Level Chemistry Problem Analysis: Adding Aqueous Bromine To 4-aminophenol |
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If aqueous bromine (also known as bromine water) is added to 4-aminophenol, what will be the organic product(s)? Thought process: The phenolic hydroxy group and the amino group are located para to each other. Both groups withdraw electrons by induction but donate electrons by resonance (and resonance effects outweigh induction effects here), and are hence (because they donate electrons by resonance) ortho-para directing. NH2 is a (only slightly) stronger activating substituent compared to OH, because while they both donate a lone pair by resonance, O is more electronegative than N, and hence OH withdraws by induction (only slightly) more than NH2. So based on electronics, you might expect the incoming electrophile to be directed ortho to NH2 rather than OH (para positions are out, for obvious reasons). However, NH2 presents a (only slightly) greater steric hinderance compared to OH, due to the additional hydrogen on NH2. So based on sterics, you might expect the incoming electrophile to be directed ortho to OH rather than NH2. In practice, and without any further info given by the question on the yield ratios, you may state that a mixture of isomeric products are obtained. Directing effects largely cancel out in this particular case (for reasons discussed above). At lower temperatures and/or with limiting bromine, you would obtain a mixture of mono or di bromination, rather than tri or tetra bromination, due to steric hinderance and/or limiting bromine. At much higher temperatures and with excess bromine, in theory you would be able to achieve tetrabromination. That is to say, in theory you can obtain an end-product of 4-amino-2,3,5,6-tetrabromophenol a.k.a. 4-hydroxy-2,3,5,6-tetrabromoaniline. There is certainly steric hinderance, but both phenolic and amino groups strongly activate the benzene ring towards electrophilic aromatic substitution (even though the bromine substituents added on are slightly deactiving in nature as they withdraw electrons by induction a little stronger than they donate electrons by resonance); and bulky molecules (such as hexabromobenzene, methyltetrabromophenol, etc) are indeed known to exist, and are regularly synthesized by chemical industries. Having said all this, remember that ultimately, theory is supposed to HELP you, not limit you. Be open to all possibilities, and use your theoretical knowledge to help you interpret understand experimental findings and observations of the universe around you. Note:As an 'A' level examination candidate, if given such a question (which is unlikely in the actual 'A' level exam; more likely the directing effects of substituents will be clear cut, and/or experimental isomeric provided by the question), then your duty is to predict and explain (just about everything discussed above, ideally) based on your own understanding of organic chemistry principles, of electronics, directing effects, sterics and temperature (ie. activation energies), what the possible products are, and why. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums. |
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