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3/30/2011 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis A carbene, R2C:, is a neutral molecule containing a divalent carbon with only six electrons in its valence shell • One simple method for generating dichlorocarbene is by treatment of CHCl3 with KOH • Carbenes behave as electrophiles, adding to alkenes to yield cyclopropanes CHP 8 Problems 8.1-22, 29-54, 58, 61-63. Addition of Carbenes to Alkenes: Cyclopropane Synthesis Mechanism of the formation of dichlorocarbene Addition of Carbenes to Alkenes: Cyclopropane Synthesis • Dichlorocarbene carbon atom is sp2-hybridized with a vacant p orbital extending above and below the plane of the three atoms with an unshared pair of electrons occupying the third sp2 lobe 1 3/30/2011 Addition of Carbenes to Alkenes: Cyclopropane Synthesis • Reaction of dichlorocarbene with an alkene results in a dichlorocyclopropane • Addition is stereospecific, meaning that only a single stereoisomer is formed as product 8.10 Radical Addition to Alkenes: Alkene Polymers Radicals add to alkene double bonds • Radicals remove one electron from double bond • One electron left behind yielding a new radical Polymer • A large molecule built up by repetitive bonding together of many smaller molecules called monomers • Cellulose (glucose polymer) Radical Addition to Alkenes: Alkene Polymers • Proteins (amino acid polymers) • Nucleic acid (nucleotide polymer) 2 3/30/2011 Radical Addition to Alkenes: Alkene Polymers Simplest polymerization • Result when an alkene is treated with a small amount of a radical as an initiator Radical Addition to Alkenes: Alkene Polymers Initiation 1. 2. 3. Small amount of benzoyl peroxide catalyst is heated breaking weak O-O bonds and yielding radicals Benzoyloxy radical adds to C=C bond of ethylene forming a carbon radical a) One electron from C=C bond pairs up with electron of benzoyloxy radical to form C-O bond b) Other electron remains on carbon (a carbon-centered radical) Radical Addition to Alkenes: Alkene Polymers Propagation • Polymerization occurs when the carbon radical adds to another ethylene molecule to yield another radical Termination • Chain process ends by a reaction that consumes a radical • Combination of two growing chains 2-R–CH2CH2 → R–CH2CH2CH2CH2–R 3 3/30/2011 Radical Addition to Alkenes: Alkene Polymers Vinyl monomers • Substituted ethylene Undergo polymerization to yield polymer with substituted groups regularly spaced in alternating carbon atom long chain • • Polypropylene • Styrene Radical Addition to Alkenes: Alkene Polymers Polymerization of unsymmetrically substituted vinyl monomers Propylene or Styrene • Radical addition steps can take place at either end of the double bond to yield: • • • • A primary radical intermediate (RCH2.) A secondary radical (R2CH.) Similar to electrophilic addition reaction More highly substituted, secondary radical is formed Worked Example 8.4 Predicting the Structure of a Polymer Show the structure of poly(vinyl chloride), a polymer made from H2C=CHCl, by drawing several repeating units 4 3/30/2011 8.11 Biological Additions of Radicals to Alkenes Radical vs. Electrophilic Addition Reactions Electrophilic addition • • Reaction occurs once Intermediate is then quenched and reaction stops. Biological Additions of Radicals to Alkenes Radical vs. Electrophilic Addition Reactions Radical addition • • • Difficult to control Limited use in the laboratory Reaction intermediate is not quenched so reaction continues Biological Additions of Radicals to Alkenes Biological Reactions • Only one substrate molecule at a time is present in the active site of the enzyme where the reaction occurs (necessary reactant groups nearby) • More controlled • More common than laboratory radical reactions 5 3/30/2011 Biological Additions of Radicals to Alkenes Step 1 – formation of a carbon radical at C13 Step 2 – C13 radical reacts with O2 at C11 through resonance form Biological Additions of Radicals to Alkenes Step 3 – Oxygen radical reacts with C8-C9 double bond forming carbon radical at C8 Step 4 – C8 radical adds to C12-C13 double bond forming carbon radical at C13 Biological Additions of Radicals to Alkenes Step 5 – Resonance form of C13 carbon radical adds at C15 to a second O2 molecule Step 6 – Reduction of O-O bond gives prostaglandin H 2 6