1559T_ch12_220-24611/02/0521:55Pa9e22 EQA 224.Chapter 12 REACTIONS OF ALKENES 12-9. Diazomethane ,Carbe sand Cyclopropane Synthesis which have the h-energy spec cannot be synt In thand e a co ang an octet,ca stored but have【 section,look at the a vs in the mecha nism for this process.The electron pair in the bond of the alkene these two bond-forming processes happen together,making this an example of a concerted reaction.one in which multiple happen sim from alkenes these are called ca benoids they act like carbens without carbenes Like carbenes they contan a spcia supplying ar 12-10,12-11,and 12-12.Oxidation of Alkenes These sections present several reactions that attach oxygen to both the double-bonded carbons of alkenes Petch rea bonding changes occur in one step. R808 that adds to give oxacyclopropanes. abo The ing compound 安 cleavage of carbon-carbon b ompoun R 0 R4 12-13.Radical Additions to Alkenes the the radicalatom Unlike additions of positively chard ctophiles how hcaocation but ather radical.Like the procesese studied in Chapter 3.these l chain mechanisms 224 • Chapter 12 REACTIONS OF ALKENES 12-9. Diazomethane, Carbenes, and Cyclopropane Synthesis This section introduces an unusual electrophile, the carbene, which has a specific use in synthesis. Carbenes, which have the general structure R2C:, have a neutral but electron-deficient carbon capable of electrophilic ad￾ditions. Lacking an octet, carbenes are high-energy species that cannot be synthesized and stored but have to be prepared in the presence of whatever substance they are intended to react with. In the presence of alkenes, carbenes react rapidly to form bonds with both alkene carbons, giving a cyclopropane as a result. In the text section, look at the arrows in the mechanism for this process. The electron pair in the
bond of the alkene moves to make one bond between one of the alkene carbons and the electrophilic carbene carbon, and the elec￾tron pair on the carbene carbon itself moves to make the second bond with the other alkene carbon. Typically these two bond-forming processes happen together, making this an example of a concerted reaction, one in which multiple bonding changes happen simultaneously. Some alternative species are encountered that also make cyclopropanes from alkenes; these are called car￾benoids because they act like carbenes without actually being carbenes. Like carbenes they contain a special carbon atom capable of reacting as an electrophile with the
bond of the alkene to form one carbon–carbon bond and also supplying an electron pair to form a second carbon–carbon bond. 12-10, 12-11, and 12-12. Oxidation of Alkenes These sections present several reactions that attach oxygen to both the double-bonded carbons of alkenes. Each reaction is an example of a concerted process, in which several bonding changes occur in one step. Peroxycarboxylic acids such as MCPBA contain an electrophilic oxygen O B ROCOOOO  OH that adds to give oxacyclopropanes. Osmium tetroxide and ozone both participate in a process where three pairs of electrons move in a circle to simultaneously form new COO bonds at both the double-bonded carbons. The products of these concerted cycloadditions are ring compounds: Notice, for synthetic purposes, that ozonolysis is the first process you’ve seen that can lead to complete cleavage of carbon–carbon bonds. Ozonolysis followed by reduction breaks double bonds, giving two carbonyl compounds: 12-13. Radical Additions to Alkenes Although typical radicals are neutral, they are electron deficient in the sense that they are one electron short of having a full octet. Reaction between the
electrons of an alkene double bond can take place, completing the octet around the radical atom. Unlike additions of positively charged electrophiles, however, the result of this addition is not a carbocation but another radical. Like the processes we studied in Chapter 3, these reactions follow radical chain mechanisms. C C R4 R1 R2 R3 1. O3, CH2Cl2 2. Zn, CH3COH O C O  O R1 R2 C R4 R3 O O Os O O O C C O O C C 1559T_ch12_220-246 11/02/05 21:55 Page 224