Chapter 7: Alkenes: Reactions and Synthesis

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1 hapter 7: Alkenes: Reactions and Synthesis alcohol alkane halohydrin 1,2-diol 1,2-dihalide carbonyl halide halide Addition Y Y Elimination Electrophilic Addition Dehydrohalogenation: loss of from an alkyl halide to form an K Et () K 2 1

2 ydration: addition of water (-) across a double bond to give an alcohol 2 Dehydration: Loss of water (-) from an alcohol to give an 2 Addition of alogens ( 2 ) to Alkenes: 1,2-dihalides 2 1,2-dihalide 2 not observed 1,2-dibromide has the anti stereochemistry omonium ion intermediate controls the stereochemistry 2

3 alohydrin Formation "-" halohydrin 2, 2 anti stereochemistry rganic molecules are sparingly soluble in water as solvent. The reaction is often done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as the electrophilic bromine source. N DMS, 2 N Note that the aryl ring does not react!!! For unsymmterical s, halohydrin formation is Markovnikov-like in that the orientation of the addition of - can be predicted by considering carbocation stability d d 3 d more d charge on the more substituted carbon 2 adds in the second step and adds to the carbon that has the most d charge and ends up on the more substituted end of the double bond 3 2, 2 3 adds to the double bond first (formation of bromonium ion) and is on the least substituted end of the double bond 3

4 ydration of Alkenes: addition of water (-) across the p-bond of an to give an alcohol. 1. Acid catalyzed hydration- Markovnikov addition of - Not a good method for hydration of an 2. xymercuration- Markovnikov addition - 3 1) g(ac) 2, 2 2) NaB ydroboration- Anti-Markovnikov addition of -, Syn addition of - 3 1) B 2 6, TF 2) 2 2, Na, 2 3 B B B 2 6 (diborane) tetrahydrofuran (TF) _ 2 3 B borane-tf complex 4

5 Reaction of Alkenes with arbenes to give yclopropanes arbene: highly reactive, 6-electron species. (sp 2 -hybridized) Generation and Reaction of arbenes: l 3 K l 2 : 2 Kl dichlorocarbene l 3, K l l Simmons-Smith Reaction (cyclopropanation) 2 I 2 Zn(u) I- 2 -Zn-I = 2 : methylene carbene 2 I 2, Zn(u) 5

6 The cyclopropanation reaction of an with a carbene takes place in a single step. There is N intermediate. As such, the geometry of the is preserved in the product. Groups that are trans on the will end up trans on the cyclopropane product. Groups that are cis on the will end up cis on the cyclopropane product. R R cis- 2 I 2, Zn(u) R R cis-cyclopropane R R trans- 2 I 2, Zn(u) R R trans-cyclopropane ydrogenation: Addition of 2 across the p-bond of an to give an alkane. This is a reduction. 2, Pt 2 The reaction uses 2 and a precious metal catalyst. The catalysts is not soluble in the reaction media, thus this process is referred to as a heterogenous catalysis. The catalyst assists in breaking the p-bond of the and the - s-bond. The reaction takes places on the surface of the catalyst. Thus, the rate of the reaction is proportional to the surface area of the catalyst. To increase the surface area of the catalyst it is finely dispersed on an inert support such as charcoal (carbon, ) arbon-carbon p-bond of s and alkynes can be reduced to the corresponding saturated - bond. ther p-bond bond such as = (carbonyl) and N are not easily reduced by catalytic hydrogenation. The = bonds of aryl rings are not easily reduced. 6

7 atalysts: Pt 2 (Adam s catalyst) or Pd/ mechanism: The addition of 2 across the p-bond is syn 2, Pt , Pd/ 3 ( 2 ) 16 2 Linoleic Acid (unsaturated fatty acid) Steric Acid (saturated fatty acid) 3 2, Pd/ 3 N 2, Pd/ N 3 2, Pt syn addition of 2 3 Not observed 7

8 xidation of Alkenes to 1,2-Diols and arbonyl ydroxylation: formal addition of - across the p-bond of an to give a 1,2-diol. This is an overall oxidation. 1) s 4 2) NaS 3 syn addition s osmate ester intermediate - not usually isolate - NaS 3 breaks down the osmate ester to the product zonolysis: oxidative cleavage of an to carbonyl compounds. The p- and s-bonds of the are broken and replaced with = doubled bonds. = of aryl rings, N and = do not react with ozone, react very slowly with ozone zone ( 3 ): electrical discharge 3 _ R 1 R 3 R 2 R 4 3, 2 l 2-78 R 1 R 3 R 1 R 3 R 2 R 4 R2 R 4 Zn R 1 R 2 R 3 R 4 molozonide ozonide Zn 8

9 1) 3 2) Zn 1) 3 2) Zn = 2 1) 3 2) Zn xidative leavage of 1,2-Diols to arbonyl ompounds R 1 R2 R 4 R 3 NaI 4 TF, 2 I R 1 R 3 R2 R 4 R 1 R 3 R 2 R 4 Zn periodate intermediate NaI 4 9