Electrophilic Addition
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1 .
2 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. 2
3 Electrophilic Addition Step 1: Pi electrons attack the electrophile. E + E + + Step 2: Nucleophile attacks the carbocation. E _ + + Nuc: E Nuc 3
4 Types of Additions 4
5 Addition of X (1) Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated Br _ Br X
6 Addition of X (2) Br Br _ Br _ Br 6
7 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, Br, and I add to alkenes to form Markovnikov products. 7
8 Free-Radical Addition of Br In the presence of peroxides, Br adds to an alkene to form the anti-markovnikov product. nly Br has the right bond energy. l bond is too strong. I bond tends to break heterolytically to form ions. 8
9 Free Radical Initiation Peroxide - bond breaks easily to form free radicals. heat R R R + R ydrogen is abstracted from Br. R + Br R + Br Electrophile 9
10 Propagation Steps Bromine adds to the double bond. Br + Br ydrogen is abstracted from Br. Br Br + Br + Br Electrophile 10
11 Anti-Markovnikov?? Br X Br Br Tertiary radical is more stable, so that intermediate forms faster. 11
12 ydration of Alkenes alkene alcohol Reverse of dehydration of alcohol Use very dilute solutions of 2 S 4 or 3 P 4 to drive equilibrium toward hydration. 12
13 Mechanism for ydration
14 rientation for ydration Markovnikov product is formed
15 Indirect ydration xymercuration-demercuration Markovnikov product formed Anti addition of - No rearrangements ydroboration Anti-Markovnikov product formed Syn addition of - 15
16 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 16
17 xymercuration (2) The intermediate is a cyclic mercurinium ion, a three-membered ring with a positive charge. Ac g + + g(ac) 17
18 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. Ac g + Ac g Ac g
19 Demercuration Sodium borohydride, a reducing agent, replaces the mercury with hydrogen. Ac g _ 4 + NaB NaB() 4 _ + 4 g + 4 Ac 19
20 Predict the Product Predict the product when the given alkene reacts with aqueous mercuric acetate, followed by reduction with sodium borohydride. D 3 (1) g(ac) 2, 2 (2) NaB 4 3 D anti addition 20
21 Alkoxymercuration - Demercuration If the nucleophile is an alcohol, R, instead of water,, the product is an ether. (1) g(ac) 2, 3 g(ac) (2) NaB
22 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 (2) 2 2, - B 2 22
23 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. 2 + B B - TF TF. B 3 23
24 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). 24
25 Actually, Trialkyl B 3 B Borane prefers least-substituted carbon due to steric hindrance as well as charge distribution. 25
26 xidation to Alcohol xidation of the alkyl borane with basic hydrogen peroxide produces the alcohol. rientation is anti-markovnikov. 3 3 B 2 2, Na
27 Predict the Product Predict the product when the given alkene reacts with borane in TF, followed by oxidation with basic hydrogen peroxide. 3 (1) B 3, TF D (2) 2 2, - syn addition 3 D 27
28 ydrogenation Alkene + 2 Alkane atalyst required, usually Pt, Pd, or Ni. Finely divided metal, heterogeneous Syn addition 28
29 Addition of arbenes Insertion of - 2 group into a double bond produces a cyclopropane ring. Three methods: Diazomethane Simmons-Smith: methylene iodide and Zn(u) Alpha elimination, haloform 29
30 Diazomethane N N 2 N N 2 diazomethane N N 2 heat or uv light N 2 + carbene Extremely toxic and explosive. 30
31 Simmons-Smith Best method for preparing cyclopropanes. 2 I 2 + Zn(u) I 2 ZnI a carbenoid 2 I 2 Zn, ul 31
32 Alpha Elimination aloform reacts with base. and X taken from same carbon l 3 + K K + - l l l l l + l - l l 3 l K, 2 l 32
33 Stereospecificity is-trans isomerism maintained around carbons that were in the double bond. 3 3 Br 3 Na, 2 3 Br 3 Br 33
34 Addition of alogens l 2, Br 2, and sometimes I 2 add to a double bond to form a vicinal dibromide. Anti addition, so reaction is stereospecific. Br + Br 2 Br 34
35 Mechanism for alogenation Pi electrons attack the bromine molecule. A bromide ion splits off. Intermediate is a cyclic bromonium ion. Br + Br Br + Br 35
36 Mechanism (2) alide ion approaches from side opposite the threemembered ring. Br Br Br Br 36
37 Test for Unsaturation Add Br 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. Decolorizing bromine is the chemical test for the presence of a double bond. 37
38 Formation of alohydrin 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. Br Br Br
39 Epoxidation Alkene reacts with a peroxyacid to form an epoxide (also called oxirane). Usual reagent is peroxybenzoic acid. + R + R 39
40 Mechanism ne-step concerted reaction. Several bonds break and form simultaneously. R + R 40
41 ne-step Reaction To synthesize the glycol without isolating the epoxide, use aqueous peroxyacetic acid or peroxyformic acid. The reaction is stereospecific. 3 41
42 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 42
43 Mechanism with s 4 oncerted syn addition of two oxygens to form a cyclic ester. s s s 4 43
44 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. 44
45 leavage with Mn4 - Permanganate is a strong oxidizing agent. Glycol initially formed is further oxidized. Disubstituted carbons become ketones. Monosubstituted carbons become carboxylic acids. Terminal = 2 becomes 2. 45
46 Example 3 KMn (warm, conc.)
47 zonolysis 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. 47
48 zonolysis Example zonide ( 3 ) 2 S S 3 DMS 3 48
49 TANKS Dr. Alka Tangri Associate Professor Department f hemistry Brahmanand ollege, Kanpur. hapter 8 49
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