rganic hemistry, 5 th Edition L. G. Wade, Jr. hapter 8 Reactions of Alkenes Jo Blackburn Richland ollege, allas, TX allas ounty ommunity ollege istrict 2003, Prentice all Reactivity of = Electrons in pi bond are loosely held. Electrophiles are attracted to the pi electrons. arbocation intermediate forms. Nucleophile adds to the carbocation. Net result is addition to the double bond. hapter 8 2 Electrophilic Addition Types of Additions Step 1: Pi electrons attack the electrophile. E E Step 2: Nucleophile attacks the carbocation. E _ Nuc: hapter 8 3 E Nuc hapter 8 4 Addition of X (1) Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated. 3 Addition of X (2) 3 3 3 3 3 _ 3 3 3 X 3 3 3 3 3 3 _ 3 3 3 _ 3 3 3 hapter 8 5 hapter 8 6 1
Regiospecificity Markovnikov s Rule: The proton of an acid adds to the carbon in the double bond that already has the most s. Rich get richer. More general Markovnikov s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate. l,, and I add to alkenes to form Markovnikov products. hapter 8 7 Free-Radical Addition of In the presence of peroxides, adds to an alkene to form the anti- Markovnikov product. nly has the right bond energy. l bond is too strong. I bond tends to break heterolytically to form ions. hapter 8 8 Free Radical Initiation Propagation Steps Peroxide - bond breaks easily to form free radicals. heat R R R R ydrogen is abstracted from. R R Electrophile hapter 8 9 omine adds to the double bond. ydrogen is abstracted from. hapter 8 10 Electrophile Anti-Markovnikov?? ydration of Alkenes 3 3 3 X 3 3 3 3 Tertiary radical is more stable, so that intermediate forms faster. 3 3 hapter 8 11 2 alkene Reverse of dehydration of alcohol Use very dilute solutions of 2 S 4 or 3 P 4 to drive equilibrium toward hydration. alcohol hapter 8 12 2
Mechanism for ydration 2 2 2 3 hapter 8 13 rientation for ydration Markovnikov product is formed. 3 3 3 3 3 3 2 3 2 3 3 3 3 3 2 hapter 8 14 Indirect ydration xymercuration-emercuration Markovnikov product formed Anti addition of - No rearrangements ydroboration Anti-Markovnikov product formed Syn addition of - xymercuration (1) Reagent is mercury(ii) acetate which dissociates slightly to form g(ac). g(ac) is the electrophile that attacks the pi bond. 3 g 3 3 _ g 3 hapter 8 15 hapter 8 16 xymercuration (2) The intermediate is a cyclic mercurinium ion, a three-membered ring with a positive charge. Ac g g(ac) hapter 8 17 xymercuration (3) Water approaches the mercurinium ion from the side opposite the ring (anti addition). Water adds to the more substituted carbon to form the Markovnikov product. 2 Ac g 2 Ac g Ac g hapter 8 18 3
emercuration Sodium borohydride, a reducing agent, replaces the mercury with hydrogen. Ac g _ 4 NaB 4 4 4 NaB() 4 _ 4 g 4 Ac hapter 8 19 Predict the Product Predict the product when the given alkene reacts with aqueous mercuric acetate, followed by reduction with sodium borohydride. 3 (1) g(ac) 2, 2 (2) NaB 4 anti addition 3 hapter 8 20 Alkoxymercuration - emercuration If the nucleophile is an alcohol, R, instead of water,, the product is an ether. (1) g(ac) 2, 3 3 g(ac) 4 3 hapter 8 21 ydroboration Borane, B 3, adds a hydrogen to the most substituted carbon in the double bond. The alkylborane is then oxidized to the alcohol which is the anti-mark product. (1) B 3 B 2 (2) 2 2, hapter 8 22 Borane Reagent Borane exists as a dimer, B 2 6, in equilibrium with its monomer. Borane is a toxic, flammable, explosive gas. Safe when complexed with tetrahydrofuran. Mechanism The electron-deficient borane adds to the least-substituted carbon. The other carbon acquires a positive charge. adds to adjacent on same side (syn). 2 B 2 6 2 B - TF TF. B 3 hapter 8 23 hapter 8 24 4
Actually, Trialkyl xidation to Alcohol 3 3 3 3 3 B 3 B 3 3 3 3 Borane prefers least-substituted carbon due to steric hindrance as well as charge distribution. hapter 8 25 xidation of the alkyl borane with basic hydrogen peroxide produces the alcohol. rientation is anti-markovnikov. 3 3 B 2 2, Na 2 3 3 hapter 8 26 Predict the Product Predict the product when the given alkene reacts with borane in TF, followed by oxidation with basic hydrogen peroxide. ydrogenation Alkene 2 Alkane atalyst required, usually Pt, Pd, or Ni. Finely divided metal, heterogeneous Syn addition 3 (1) B 3, TF (2) 2 2, - 3 syn addition hapter 8 27 hapter 8 28 Addition of arbenes Insertion of - 2 group into a double bond produces a cyclopropane ring. Three methods: iazomethane Simmons-Smith: methylene iodide and Zn(u) Alpha elimination, haloform hapter 8 29 iazomethane N N 2 N N 2 diazomethane N N 2 heat or uv light N 2 carbene Extremely toxic and explosive. hapter 8 30 5
Simmons-Smith Best method for preparing cyclopropanes. 2 I 2 Zn(u) I 2 ZnI a carbenoid 2 I 2 Zn, ul Alpha Elimination aloform reacts with base. and X taken from same carbon l 3 K K - l 3 2 l l l l - l l l 3 l K, 2 l hapter 8 31 hapter 8 32 Stereospecificity is-trans isomerism maintained around carbons that were in the double bond. 3 3 3 Na, 2 3 3 Addition of alogens l 2, 2, and sometimes I 2 add to a double bond to form a vicinal dibromide. Anti addition, so reaction is stereospecific. 2 hapter 8 33 hapter 8 34 Mechanism for alogenation Pi electrons attack the bromine molecule. A bromide ion splits off. Intermediate is a cyclic bromonium ion. Mechanism (2) alide ion approaches from side opposite the three-membered ring. hapter 8 35 hapter 8 36 6
Examples of Stereospecificity Test for Unsaturation Add 2 in l 4 (dark, red-brown color) to an alkene in the presence of light. The color quickly disappears as the bromine adds to the double bond. ecolorizing bromine is the chemical test for the presence of a double bond. hapter 8 37 hapter 8 38 Formation of alohydrin Regiospecificity If a halogen is added in the presence of water, a halohydrin is formed. Water is the nucleophile, instead of halide. Product is Markovnikov and anti. 2 3 2 hapter 8 39 The most highly substituted carbon has the most positive charge, so nucleophile attacks there. hapter 8 40 Predict the Product Epoxidation Predict the product when the given alkene reacts with chlorine in water. 3 l 2, 2 3 l Alkene reacts with a peroxyacid to form an epoxide (also called oxirane). Usual reagent is peroxybenzoic acid. R R hapter 8 41 hapter 8 42 7
Mechanism ne-step concerted reaction. Several bonds break and form simultaneously. R R Epoxide Stereochemistry Since there is no opportunity for rotation around the double-bonded carbons, cis or trans stereochemistry is maintained. 3 3 Ph 3 3 hapter 8 43 hapter 8 44 pening the Epoxide Ring Acid catalyzed. Water attacks the protonated epoxide. Trans diol is formed. 3 2 2 ne-step Reaction To synthesize the glycol without isolating the epoxide, use aqueous peroxyacetic acid or peroxyformic acid. The reaction is stereospecific. 3 hapter 8 45 hapter 8 46 Syn ydroxylation of Alkenes Alkene is converted to a cis-1,2-diol, Two reagents: smium tetroxide (expensive!), followed by hydrogen peroxide or old, dilute aqueous potassium permanganate, followed by hydrolysis with base hapter 8 47 Mechanism with s 4 oncerted syn addition of two oxygens to form a cyclic ester. s s hapter 8 48 s 4 2 2 8
Stereospecificity If a chiral carbon is formed, only one stereoisomer will be produced (or a pair of enantiomers). 2 3 2 3 cis -3-hexene (1) s 4 (2) 2 2 2 3 2 3 meso -3,4-hexanediol xidative leavage Both the pi and sigma bonds break. = becomes =. Two methods: Warm or concentrated or acidic KMn 4. zonolysis Used to determine the position of a double bond in an unknown. hapter 8 49 hapter 8 50 leavage with Mn4 - Example Permanganate is a strong oxidizing agent. Glycol initially formed is further oxidized. isubstituted carbons become ketones. Monosubstituted carbons become carboxylic acids. Terminal = 2 becomes 2. hapter 8 51 3 KMn 4 3 3 (warm, conc.) 3 3 hapter 8 52 3 3 3 3 3 zonolysis zonolysis Example Reaction with ozone forms an ozonide. zonides are not isolated, but are treated with a mild reducing agent like Zn or dimethyl sulfide. Milder oxidation than permanganate. Products formed are ketones or aldehydes. hapter 8 53 3 3 3 3 3 3 3 zonide ( 3 ) 2 S 3 3 3 S 3 3 MS hapter 8 54 9
Polymerization ationic Polymerization An alkene (monomer) can add to another molecule like itself to form a chain (polymer). Three methods: ationic, a carbocation intermediate Free radical Anionic, a carbanion intermediate (rare) hapter 8 55 Electrophile, like or BF 3, adds to the least substituted carbon of an alkene, forming the most stable carbocation. 3 3 3 3 3 hapter 8 56 Radical Polymerization In the presence of a free radical initiator, like peroxide, free radical polymerization occurs. Ph R R Ph Ph Ph Ph R hapter 8 57 - Anionic Polymerization For an alkene to gain electrons, strong electron-withdrawing groups such as nitro, cyano, or carbonyl must be attached to the carbons in the double bond. 3 N 3 N 3 N 3 3 N N hapter 8 58 End of hapter 8 hapter 8 59 10