chapter7

chapter7 - 7. Alkenes: Reactions and Synthesis Based on...

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Unformatted text preview: 7. Alkenes: Reactions and Synthesis Based on McMurrys Organic Chemistry , 7 th edition 2 Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) 3 Why this chapter? To begin a systematic description of major functional groups Begin to focus on general principles and patterns of reactivity that tie organic chemistry 4 7.1 Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by elimination of HX from alkyl halide ( dehydrohalogenation ) Uses heat and KOH elimination of H-OH from an alcohol ( dehydration ) require strong acids (sulfuric acid, 50 C) This is how they would be prepared in a laboratory setting or for making a pharmaceutical. How is this done in industry? 5 GENERAL REACTION MECHANISM FOR ELECTROPHILIC ADDITION TO AN ALKENE (You MUST know this general mechanism!) CH 3 E + CH 3 E - complex CH 3 E bridged intermed iate CH 3 E most stab le carbocation E N CH 3 N N N N e lectroph ile (e lectron loving) nucleoph ile (nucleus loving) add ition product CH 3 E N pathway when the e lectroph ile is a proton to make the most stab le carbocation pathway when the e lectroph ile is a larger atom like Br + or Hg + Questions you must ask: 1 . What kind of reactions do a lkene participate in? (e lectroph ilic add ition reactions) 2. What is the e lectroph ile, i.e. E + = H + , Br + , Hg + , etc. 3. What is the nucleoph ile, i.e. N ;- or N : = HO- , C l- , Br- , H 2 0, ROH , RNH 2 etc., 4. How do I use arrow forma lism to show e lectron movement as bonds are broken and new bonds made? 6 7.2 Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process F 2 is too reactive and I 2 does not add Cl 2 reacts as Cl + Cl- Br 2 is similar 7 Addition of Br 2 to Cyclopentene Addition is exclusively trans ANTI addition 8 Mechanism of Bromine Addition Br + adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition + - You can assume that Br + or Br + are the electrophiles 9 Bromonium Ion Mechanism Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile ANTI Addition 10 The Reality of Bromonium Ions Bromonium ions were postulated more than 60 years ago to explain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene Olah showed that bromonium ions are stable in liquid SO 2 with SbF 5 and can be studied directly Experimental evidence for bromonium ion....
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This note was uploaded on 04/07/2008 for the course CHGN 222 taught by Professor Cowley during the Spring '08 term at Mines.

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chapter7 - 7. Alkenes: Reactions and Synthesis Based on...

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