l17a - CH 203 O R G A N I C C H E M I S T R Y I Addition to...

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Addition to carbon–carbon double bonds I • Reaction syllabus © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Addition to carbon–carbon double bonds I: Reaction syllabus The carbon–carbon double bond typically undergoes addition, that is, it is degraded to a single bond and new atoms add to the carbon atoms that made up that degraded double bond. The thermodynamic feasibility of such reactions is assessed by comparing the strengths E of the bonds formed against the strengths of the bonds broken: C C + A B C C A B Bonds broken Bonds formed C–C ! C–A " A–B " C–B " H º = E (C–C ! ) + E (A–B " ) – E (C–A " ) – E (C–B " ) Let’s use the data in the table below to calculate the H º values of some of the reactions we are to study. Bond E [kJ/mol] Bond E [kJ/mol] Bond E [kJ/mol] Bond E [kJ/mol] C–C ! 264 H–H " 436 C–C " 348 F–F " 153 H–C " 414 C–O " 348 Cl–Cl " 243 H–O " 465 C–F " 486 Br–Br " 193 H–F " 565 C–Cl " 339 I–I " 151 H–Cl " 431 C–Br " 285 H–Br " 366 C–I " 214 H–I " 297 Problem Calculate the H º of hydrogenation (addition of hydrogen): C C + H H C C H H Answer H º = E (C–C ! ) + E (H–H " ) – 2 E (C–H " ) = –128 kJ
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Addition to carbon–carbon double bonds I • Reaction syllabus © Bruno I. Rubio 2 Problem Calculate the H º of all of the four hydrohalogenations (addition of hydrogen halide HX, where X = F, Cl, Br, I): C C + H X C C H X Answer H º = E (C–C ! ) + E (H–X " ) – E (C–H " ) – E (C–X) X F Cl Br I H º [kJ/mol] –71 –58 –69 –67 Problem Calculate the H º of all of the four halogenations (addition of ha- loogen X 2 , where X = F, Cl, Br, I) C C + X X C C X X Answer H º = E (C–C ! ) + E (X–X " ) – 2 E (C–X) X F Cl Br I H º [kJ/mol] –555 –171 –113 –13 Problem Calculate the H º of hydration (addition of water): C C + H OH C C H OH Answer H º = E (C–C ! ) + E (H–OH " ) – E (C–H) – E (C–O) = –43 kJ The results of the preceding problems suggest that additions to the carbon– carbon double bond tend to be quite favorable. This lecture focuses on only one addition reaction, namely, the addition of a hydrogen halide (HF, HCl, HBr, HI) to the carbon–carbon double bond because the reaction illustrates many of the concepts, chemical species and theoreti- cal tools that we encounter in our study of organic reactions. Among these are: (1) Reaction mechanism -- A description at the molecular level of the events that take place when reactants are transformed into products.
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Addition to carbon–carbon double bonds I • Reaction syllabus © Bruno I. Rubio 3 (2) Nucleophiles and electrophiles -- Nucleophiles are electron-rich species; electrophiles are electron-poor species. Nucleophiles attack electrophiles: this makes sense because a species that has a lot of electrons (a nucleo- phile) has an excess of negative charge and is attracted to a species that has a deficit of electrons (an electrophile).
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