l17b

l17b - 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 II • 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 II: Reaction syllabus In this lecture we continue and conclude our discussion of addition to the carbon–carbon double bond. Halogenation The elements of X 2 (X = halogen) add across the carbon–carbon double bond; the product is a vicinal dihalide: C C + X X C C X X a vicinal dihalide Although halogenation is not particularly useful, it has been extensively studied mostly to satisfy curiosity about the reaction’s unusual cyclic in- termediate. The first and rate-determining step of the reaction involves the attack of X 2 by the alkene to form a cyclic halonium cation intermediate: cyclic halonium cation C C + X X C C + X X slow Why should the electron-rich carbon–carbon double bond be attracted to a molecule like X 2 , which consists of two atoms of equal electronegativity and consequently has no electron-poor atoms? When X 2 is far from the ! bond of the alkene, the electrons in X 2 are symmetrically distributed and neither X atom is surrounded by more electron density than the other. As the alkene ap- proaches X 2 , the electrons surrounding X 2 attempt to flee the electrons of the ! bond of the alkene; this happens in order to minimize unfavorable elec- tron–electron repulsion. Eventually, one of the X atoms in X 2 finds itself surrounded by less electron density than the other; in other words, one end of the X–X bond becomes electron-poor whereas the other becomes electron- rich. This distortion of positive and negative charge caused by the approach-
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Addition to carbon–carbon double bonds II • Reaction syllabus © Bruno I. Rubio 2 ing alkene is called an induced dipole. The X 2 molecule finally undergoes at- tack by the alkene and the halogen–halogen bond breaks: C C X X C C X X C C X X ! + !" Note that the intermediate is not a carbocation: it is a cyclic halonium cation in which the halogen is bonded simultaneously to the two carbons that made up the carbon–carbon double bond of the reactant alkene. A cyclic halo- nium cation forms in preference to a carbocation because all atoms in the cy- clic halonium cation have octets whereas a carbocation has a carbon atom that has been deprived of its octet and is thus less stable: carbocationic carbon missing its octet: usually less favorable C C + X X C C + X X all atoms have an octet: usually more favorable C C + X X The cyclic halonium cation is an electrophile. In the second step of the halogenation mechanism, X (a nucleophile) attacks one of the carbons in the three-membered ring and the cyclic halonium cation opens to afford the vici- nal dihalide product:
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Addition to carbon–carbon double bonds II • Reaction syllabus © Bruno I. Rubio 3 C C + X X C C X X fast This step is fast because opening the cyclic halonium cation relieves the strain of the three-membered ring.
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l17b - 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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