{"id":1403,"date":"2017-10-12T14:30:18","date_gmt":"2017-10-12T14:30:18","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&p=1403"},"modified":"2017-10-12T14:30:18","modified_gmt":"2017-10-12T14:30:18","slug":"how-delocalized-electrons-affect-pka-values","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/how-delocalized-electrons-affect-pka-values\/","title":{"raw":"How Delocalized Electrons Affect pKa Values","rendered":"How Delocalized Electrons Affect pKa Values"},"content":{"raw":"
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\u00a0Resonance effects on acidity<\/a><\/strong><\/h4>\r\n<\/div>\r\n
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\r\n\r\nResonance effects involving aromatic structures can have a dramatic influence on acidity and basicity.\u00a0 Notice, for example, the difference in acidity between phenol and cyclohexanol.\r\n\r\n\"\"\r\n\r\nLooking at the conjugate base of phenol, we see that the negative charge can be delocalized by resonance to three different carbons on the aromatic ring.\r\n\r\n\"\"\r\n\r\nAlthough these are all minor resonance contributors (negative charge is placed on a\u00a0 carbon rather than the more electronegative oxygen), they nonetheless have a significant effect on the acidity of the phenolic proton.\u00a0 Essentially, the benzene ring is acting as an electron-withdrawing group by resonance.\r\n\r\nAs we begin to study in detail the mechanisms of biological organic reactions, we\u2019ll see that the phenol side chain of the amino acid tyrosine (see table 5 at the back of the book), with its relatively acidic pKa<\/sub>of 9-10, often acts as a catalytic proton donor\/acceptor in enzyme active sites.\r\n
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Example<\/h3>\r\nDraw the conjugate base of 2-napthol (the major resonance contributor), and on your drawing indicate with arrows all of the atoms to which the negative charge can be delocalized by resonance.\r\n\r\n\"\"\r\n\r\nSolution<\/a>\r\n\r\n<\/div>\r\n\r\n\r\n<\/div>\r\n<\/div>\r\nThe base-stabilizing effect of an aromatic ring can be accentuated by the presence of an additional electron-withdrawing substituent, such as a carbonyl.\u00a0 For the conjugate base of the phenol derivative below, an additional resonance contributor can be drawn in which the negative formal charge is placed on the carbonyl oxygen.\r\n\r\n\"\"\r\n\r\nNow the negative charge on the conjugate base can be spread out over two<\/em> oxygens (in addition to three aromatic carbons). The phenol acid therefore has a pKa<\/sub>similar to that of a carboxylic acid, where the negative charge on the conjugate base is also delocalized to two oxygen atoms.\u00a0 The ketone group is acting as an electron withdrawing group - it is 'pulling' electron density towards itself,\u00a0 through both inductive and resonance effects.\r\n
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Example<\/h3>\r\nThe position of the electron-withdrawing substituent relative to the phenol hydroxyl is very important in terms of its effect on acidity. Which of the two substituted phenols below is more acidic?\u00a0 Use resonance drawings to explain your answer.\r\n\r\n\"\"\r\n\r\nSolution<\/a>\r\n

Example<\/h3>\r\nRank the four compounds below from most acidic to least.\r\n\r\n\"\"\r\n\r\nSolution<\/a>\r\n

example<\/h3>\r\nNitro groups are very powerful electron-withdrawing groups.\u00a0 The phenol derivative picric acid (2,4,6 -trinitrophenol) has a pKa<\/sub> of 0.25, lower than that of trifluoroacetic acid. Use a resonance argument to explain why picric acid has such a low pKa<\/sub>.\r\n\r\nSolution<\/a>\r\n\r\n<\/div>\r\n\r\n\r\n<\/div>\r\nConsider the acidity of\u00a0 4-methoxyphenol, compared to phenol:\r\n\r\n\"\"\r\n\r\nNotice that the methoxy group increases the pKa<\/sub> of the phenol group - it makes it less<\/em> acidic. Why is this? At first inspection, you might assume that the methoxy substituent, with its electronegative oxygen, would be an electron-withdrawing group by induction.\u00a0 That is correct, but only to a point.\u00a0 The oxygen atom does indeed exert an electron-withdrawing inductive effect, but the lone pairs on the oxygen cause the exact opposite effect \u2013 the methoxy group is an electron-donating group by resonance<\/strong><\/em>.\u00a0 A resonance contributor can be drawn in which a formal negative charge is placed on the carbon adjacent to the negatively-charged phenolate oxygen.\r\n\r\n\"\"\r\n\r\nBecause of like-charge repulsion, this destabilizes<\/em> the negative charge on the phenolate oxygen, making it more basic. It may help to visualize the methoxy group \u2018pushing\u2019 electrons towards the lone pair electrons of the phenolate oxygen, causing them to be less 'comfortable' and more reactive. The example above is a somewhat confusing but quite common situation in organic chemistry - a functional group, in this case a methoxy group, is exerting both an inductive effect and a resonance effect, but in opposite directions<\/em> (the inductive effect is electron-withdrawing, the resonance effect is electron-donating).\u00a0 As a general rule a resonance effect is more powerful than an inductive effect - so overall, the methoxy group is acting as an electron donating group. A good rule of thumb to remember:\r\n
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When resonance and induction compete, resonance usually wins!<\/strong><\/p>\r\n\r\n<\/div>\r\n

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Example<\/h3>\r\n
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\r\n\r\nRank the three compounds below from lowest pKa to highest, and explain your reasoning.\u00a0 Hint<\/em> - think about both resonance and inductive effects!\r\n\r\n\"\"\r\n\r\nSolution<\/a>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n\r\n<\/div>\r\n
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Contributors<\/h3>\r\n