Reactions of Alkenes Typical type: addition CC + YZ CC Y Z substrate reagent product Mechanism: Usually not one simple step. Weaker π-bond broken, also Y - Z bond. Energy provided by formation of C - Y and C - Z bonds. π-bond: source of electrons. Reagent: typically electrophile or free radical. Therefore, electrophilic or free radical addition. Addition of Acids & Markovnikov's Rule CC+ HX CC acid H X alkene alkyl halide + HX = HCl, HBr, HI (or KI + H3PO4), HOH (with H3O ) H H H H H H CC + HCl H CCCH3 H CCCH3 H CH3 H Cl Cl H major product minor product Markovnikov rule: Polar (ionic) addition of an acid to a double bond --- the hydrogen preferentially attaches itself to the carbon which already has the greater number of hydrogens directly attached. This preference is strong. HCl, HBr, HI (or KI + H3PO4) and H2O follow this rule. However, in the presence of peroxides, R - O - O - R', HBr (only) will add the opposite way: anti- Markovnikov These are highly regioselective or regiospecific reactions. 2 (1) CC + HZ CC + Z H + HZ = HCl, HBr, HI, H3O (2) CC + Z CC H HZ Step 1 is rate limiting. eg CH3 CH3 H3CCHC 2 + HCl H3CCHC 2 + Cl 3o H CH3 3o carbocation is formed faster than 1o: H3CCHC 2 H CH3 CH3 H3CCHC 2 + little or no H3CCHC 2 Cl H HCl 3 eg H H H3CCHC 2 + HOH2 H3CCHC 2 + H2O 2o H H 2o carbocation is formed faster than 1o: H3CCHC 2 H H H H3CCHC 2 + little H3CCHC 2 H2O H HOH2 H H H3CCHC 2 + little H3CCHC 2 + H HO H HOH 4 (Fun) fact #1 about carbocations: The order of stability of carbocations is: 3o (tertiary) > 2o (secondary) > 1o (primary) > methyl most stable least stable Reason: Charge and electrons are most delocalized in 3o and most localized in methyl. [It can be shown by quantum mechanics that spreading electrons over a larger volume of space --- delocalization --- leads to lower energy.] The electrons are more delocalized in the more highly substituted carbocations because of an inductive effect: alkyl groups are larger and more polarizable than hydrogen atoms, and a hyperconjugation effect. Thus, orientation of addition (regioselectivity) seems to depend on the formation of the more stable carbocation; in other words the more stable carbocation is formed (much) faster. Why? 5 It is also usually true that the relative reactivity of two different alkenes will depend on which forms the more stable carbocation. (Not so fun) fact #2 about carbocations: They rearrange, eg CH3 CH3 H3CCCH CH2 + HCl H3CCCH CH2 + Cl 2o H H H formed faster than: CH3 H3CCCH CH2 H H 1o CH3 CH3 rearrangement H3CCCH CH2 H3CCCH CH2 H H 3o, H H more stable than 2o Cl CH3 Cl H3CCCH CH2 CH3 H Cl H H3CCCH CH2 Cl H H 6 Electrophilic Addition with Electrophiles Other Than H+ Halogenation --- CCl4 CC + X2 CC X X X2 = Br2 or Cl2 eg CCl4 + Br Br Br CH3 but no Br Br CH3 CH3 CH3 Br CH3 CH3 ANTI ADDITION ONLY This may be contrasted with the addition of HX to an alkene where syn and anti addition are observed --- eg + HBr HCH3 + HBr CH3 CH3 CH3 Br CH3 CH3 ANTI AND SYN ADDITION 7 Mechanism — Br δ − bromine: polarized by π bond Br δ + Br CC a bromonium ion CC Br Br C C CC Br Br A proton, unlike bromine, is not large enough to form a 3- membered ring intermediate; it forms an open carbocation. 8 Alcohols via Hydroboration - Oxidation --- Anti-Markovnikov, syn addition of H and OH to a double bond to form an alcohol. 3 + "BH3" CC C C H B H C C H a trialkyl borane "BH3"= B2H6 or OBH3 NaBH BF B H - 4 + 3 2 6 H2O2, OH 3 CC H OH An example --- H & OH same side: syn addition 1. "BH3" OH H + B(OH)3 2. H2O2, KOH H CH3 H CH3 H attaches here: anti-Markovnikov 9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis --- A carbene is a neutral molecule containing a divalent carbon with 6 electrons in its valence shell --- R2C:, where R = hydrogen, alkyl, halogen. The simplest carbene is methylene aka carbene: hν H2CNN CH2 + N2 diazomethane Methylene is highly reactive; it can add to double bonds to make a cyclopropane ring, but it can also insert itself into C-H bonds, resulting in low yields for the addition. Addition --- CH2 CH2 + Insertion --- CH + CH2 CCH2 H 10 Addition of Dichlorocarbene to an Alkene to Give a Dichlorocyclopropane --- K O-H + HCCl3 K CCl3 + H2O K CCl3 K Cl + CCl2 dichlorocarbene H C CH 3 3 H3CCHCCl2 3 CC + CCl2 CC H H HH H C H 3 H3CHCCl2 CC + CCl2 CC H CH3 HCH3 stereospecific syn addition This reaction is stereospecific and syn. 11 Simmons-Smith Reaction: Alkene to Cyclopropane --- H C CH 3 3 H3CCHCH2 3 CC + CH2I2 + Zn(Cu) CC+ ZnI2 H H HH H3C H H3CHCH2 CC + CH2I2 + Zn(Cu) CC+ ZnI2 H CH3 HCH3 stereospecific syn addition Mechanism: CH2I2 + Zn(Cu) ICH2ZnI ICH2 ZnI + CC CC CC CH2 CH2 I Zn I a carbenoid reaction; + ZnI2 :CH2 is carbene This reaction gives a cyclopropane ring cleanly; σ-bond insertion is not a problem here. 12 Ozonolysis: Structure Determination by Degradation --- O O O3 O O CC C C CC OO a molozonide an ozonide Zn, + CO+ O C aldehydes & ketones H2O, H These carbons were joined by a double bond. 13.