PS_3_Answer_Key

PS_3_Answer_Key - Massachusetts Institute of Technology...

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This material may not be reproduced or posted on an internet website without written permission of Professors Danheiser and Jamison. Massachusetts Institute of Technology Organic Chemistry 5.12 Problem Set #3 Answer Key H. K. Kwon, Yu-Pu Wang & Phil Hamzik March 9, 2011 1. Identify sites of significant nucleophilic and electrophilic character in each of the following molecules. Remember that some molecules can exhibit both nucleophilic and electrophilic character and may also possess multiple sites that have the same character. ANSWER: In general, identifying lone pairs and π -bonds as possible nucleophilic sites and consideration of all significant resonance contributors to determine partial charge distribution are quick ways to determine a molecule's reactivity profile. Finally, one should see what product would result if a proposed nucleophilic or electrophilic site were to react with a generic electrophile (E + ) or nucleophile (Nu - ). (a) strongly electrophilic site very weakly nucleophilic sites Cl Cl O Cl Cl O From the resonance contributors shown, we can see there will be significant partial positive charge on the sp 2 carbon, and partial negative charge on the oxygen. The partial positive charge is increased by the inductive effect exerted by the chlorine atom. Thus, the sp 2 carbon would be a very electrophilic site. The oxygen, while somewhat nucleophilic, is only weakly nucleophilic due to the inductive effect of the carbon- chlorine bond, the electronegativity of oxygen, and the fact that the lone pairs are in sp 2 orbitals, which lowers the energy of the electrons relative to lone pairs in sp 3 orbitals. (b) Just like alkenes, the π -system in benzene has moderate nucleophilicity due to the polarizability of electrons in p-orbitals. The product of reaction of benzene with a generic electrophile also enjoys resonance stabilization of the generated positive charge within the ring. E + E E E
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This material may not be reproduced or posted on an internet website without written permission of Professors Danheiser and Jamison. (c) OLi O Nucleophilic sites O Li Li First we should keep in mind that an oxygen-lithium bond is very polar, so the oxygen atom has significant anionic character. Considering the resonance structures of the anion, it is clear that the oxygen and the terminal sp 2 carbon are nucleophilic sites, since both possess partial negative charge and anionic character. The carbonyl carbon is not very electrophilic because the molecule has a net negative charge. Lithium ion is not electrophilic because lithium has very low electronegativity and accommodates positive charge very well. (d) Br Electrophilic sites Nucleophilic ! -bond Br " + " + "# Weakly nucleophilic The C-C π bond, like other alkenes, possesses nucleophilic character, though it is a weaker nucleophile than most C-C double bonds due to the inductive effect of the electronegative bromine atom. The sp 3 carbon is electrophilic due to the polarized nature of the carbon-bromine bond and the relative weakness
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This note was uploaded on 02/14/2012 for the course CHEMISTRY 5.12 taught by Professor Saraho'connor during the Fall '09 term at MIT.

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PS_3_Answer_Key - Massachusetts Institute of Technology...

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