H EtOH. C-C π-bond H-H C-H = 243 KJ/mol = 435 KJ/mol = 2 x -410 KJ/mol = -142 KJ/mol

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1 hapter 6: eactions of Alkenes: Addition eactions 6.1: ydrogenation of Alkenes addition of - ( 2 ) to the π-bond of alkenes to afford an alkane. The reaction must be catalyzed by metals such as Pd, Pt, h, and Ni. Pd/ Et Δ hydrogenation = -136 KJ/mol - π-bond - - = 243 KJ/mol = 435 KJ/mol = 2 x -410 KJ/mol = -142 KJ/mol 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 π-bond of the alkene and the - σ-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. 127 arbon-carbon π-bond of alkenes and alkynes can be reduced to the corresponding saturated - bond. ther π-bond bond such as = (carbonyl) and N are not easily reduced by catalytic hydrogenation. The = bonds of aryl rings are not easily reduced. 2, Pt 2 ethanol , Pd/ 3 ( 2 ) 16 2 Linoleic Acid (unsaturated fatty acid) Steric Acid (saturated fatty acid) 3 2, Pd/ ethanol 3 N 2, Pd/ N ethanol 128

6.2: eats of ydrogenation -an be used to measure relative stability of isomeric alkenes 3 3 cis-2-butene Δ combustion : -2710 KJ/mol 3 3 trans-2-butene -2707 KJ/mol trans isomer is ~3 KJ/mol more

2 6.2: eats of ydrogenation -an be used to measure relative stability of isomeric alkenes 3 3 cis-2-butene Δ combustion : KJ/mol 3 3 trans-2-butene KJ/mol trans isomer is ~3 KJ/mol more stable than the cis isomer 3 3 2, Pd , Pd 3 3 cis-2-butene trans-2-butene Δ hydrogenation : -119 KJ/mol -115 KJ/mol trans isomer is ~4 KJ/mol more stable than the cis isomer The greater release of heat, the less stable the reactant. 129 Table 6.1 (pg 228): eats of ydrogenation of Some Alkenes Alkene 2 = 2! (KJ/mol) 136 monosubstituted disubstituted trisubstituted tetrasubstituted

6.3: Stereochemistry of Alkene ydrogenation Mechanism: 2 2 2 2 2 2 2 The addition of 2 across the π-bond is syn, i.e., from the same face of the double bond 3 2, Pd/ 3 3 Et 3 3 3 syn addition of 2 Not observed 131 6.

3 6.3: Stereochemistry of Alkene ydrogenation Mechanism: The addition of 2 across the π-bond is syn, i.e., from the same face of the double bond 3 2, Pd/ 3 3 Et syn addition of 2 Not observed : Electrophilic Addition of ydrogen alides to Alkenes Electrophilic addition reaction nucleophile - σ-bond: Δ = 368 KJ/mol - π-bond: Δ = 243 KJ/mol π-bond of an alkene can act as a nucleophile!! - electrophile Bonds broken Bonds formed = π-bond 243 KJ/mol KJ/mol 366 KJ/mol KJ/mol calc. Δ = -84 KJ/mol expt. Δ = -84 KJ/mol 132

4 eactivity of X correlates with acidity: slowest F << l < < I fastest 6.5: egioselectivity of ydrogen alide Addition: Markovnikov's ule ' For the electrophilic addition of X across a = bond, the (of X) will add to the carbon of the double bond with the most s (the least substitutent carbon) and the X will add to the carbon of the double bond that has the most alkyl groups ' Both products observed 133 Mechanism of electrophilic addition of X to alkenes 6.6: Mechanistic Basis for Markovnikov's ule: Markovnikov s rule can be explained by comparing the stability of the intermediate carbocations 134

For the electrophilic addition of X to an unsymmetrically substituted alkene: The more highly substituted carbocation intermediate is formed.

5 For the electrophilic addition of X to an unsymmetrically substituted alkene: The more highly substituted carbocation intermediate is formed. More highly substituted carbocations are more stable than less substituted carbocations. (hyperconjugation) The more highly substituted carbocation is formed faster than the less substituted carbocation. nce formed, the more highly substituted carbocation goes on to the final product more rapidly as well : arbocation earrangements in ydrogen alide Addition to Alkenes - In reactions involving carbocation intermediates, the carbocation may sometimes rearrange if a more stable carbocation can be formed by the rearrangement. These involve hydride and methyl shifts l -l 3 3 l 3 3 ~ 50% ~ 50% expected product 3 3 l 3 Note that the shifting atom or group moves with its electron pair. A ME STABLE ABATIN IS FMED. 3 3 l 3 l 136

6 6.8: Free-radical Addition of to Alkenes peroxides (-) Polar mechanism (Markovnikov addition) adical mechanism (Anti-Markovnikov addition) ' - (peroxides) ' Both products observed The regiochemistry of addition is reversed in the presence of peroxides. Peroxides are radical initiators - change in mechanism 137 The regiochemistry of free radical addition of - to alkenes reflects the stability of the radical intermediate. Primary (1 ) < Secondary (2 ) < Tertiary (3 ) 138

7 6.9: Addition of Sulfuric Acid to Alkenes (please read) 6.10: Acid-atalyzed ydration of Alkenes - addition of water (-) across the π-bond of an alkene to give an alcohol; opposite of dehydration S 4, This addition reaction follows Markovnikov s rule The more highly substituted alcohol is the product and is derived from The most stable carbocation intermediate. eactions works best for the preparation of 3 alcohols 139 Mechanism is the reverse of the acid-catalyzed dehydration of alcohols: Principle of Microscopic eversibility 140

8 6.11: Thermodynamics of Addition-Elimination Equlibria S Bonds broken Bonds formed = π-bond 243 KJ/mol KJ/mol 497 KJ/mol ( 3 ) KJ/mol calc. Δ = -50 KJ/mol ΔG = -5.4 KJ/mol Δ = KJ/mol ΔS = KJ/mol ow is the position of the equilibrium controlled? Le hatelier s Principle - an equilibrium will adjusts to any stress The hydration-dehydration equilibria is pushed toward hydration (alcohol) by adding water and toward alkene (dehydration) by removing water 141 The acid catalyzed hydration is not a good or general method for the hydration of an alkene. xymercuration: a general (2-step) method for the Markovnokov hydration of alkenes 4 9 Ac= acetate = 3 1) g(ac) 2, g(ac) 2) NaB NaB 4 reduces the -g bond to a - bond 142

9 6.12: ydroboration-xidation of Alkenes - Anti-Markovnikov addition of -; syn addition of - 3 1) B 2 6, TF 2) 2 2, Na, : Stereochemistry of ydroboration-xidation 6.14: Mechanism of ydroboration-xidation - Step 1: syn addition of the 2 B bond to the same face of the π-bond in an anti-markovnikov sense; step 2: oxidation of the B bond by basic 2 2 to a bond, with retention of stereochemistry : Addition of alogens to Alkenes X 2 = l 2 and 2 X 2 X X (vicinal dihalide) alkene 1,2-dihalide 6.16: Stereochemistry of alogen Addition - 1,2-dibromide has the anti stereochemistry 2 not observed

10 6.17: Mechanism of alogen Addition to Alkenes: alonium Ions - omonium ion intermediate explains the stereochemistry of 2 addition : onversion of Alkenes to Vicinal alohydrins "X-" X alkene X 2, 2 halohydrin X anti stereochemistry X Mechanism involves a halonium ion intermediate 146

11 For unsymmterical alkenes, halohydrin formation is Markovnikov-like in that the orientation of the addition of X- can be predicted by considering carbocation stability!! 3! more δ charge on the more substituted carbon 2 adds in the second step and adds to the carbon that has the most δ 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 147 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 he electrophilic bromine source. N DMS, 2 Note that the aryl ring does not react!!! N 6.19: Epoxidation of Alkenes - Epoxide (oxirane): threemembered ring, cyclic ethers. eaction of an alkene with a peroxyacid: peroxyacetic acid 3 peroxyacetic acid 3 acetic acid peroxide

12 Stereochemistry of the epoxidation of alkenes: syn addition of oxygen. The geometry of the alkene is preserved in the product Groups that are trans on the alkene will end up trans on the epoxide product. Groups that are cis on the alkene will end up cis on the epoxide product. cis-alkene 3 3 cis-epoxide trans-alkene : zonolysis of Alkenes - oxidative cleavage of an alkene to carbonyl compounds (aldehydes and ketones). The π- and σ-bonds of the alkene are broken and replaced with = double bonds. = of aryl rings, N and = do not react with ozone, react very slowly with ozone trans-epoxide 149 zone ( 3 ): mechanism electrical discharge 3 _ , 2 l Zn -or- ( 3 ) 2 S molozonide ozonide Zn or ( 3 )S 1) 3 2) Zn 1) 3 2) Zn 1) 3 2) Zn 150

13 6.21: Introduction to rganic hemical Synthesis Synthesis: making larger, more complex molecules out of less complex ones using known and reliable reactions. devise a synthetic plan by working the problem backward from the target molecule?? 2 S 4 2, Pd/?? ?? 6.22: eactions of Alkenes with Alkenes: Polymerization (please read) 152

Double Bonds. Hydration Rxns. Hydrogenation Rxns. Halogenation. Formation of epoxides. Syn addition of 2 OH. Ozonolysis

Double Bonds. Hydration Rxns. Hydrogenation Rxns. Halogenation. Formation of epoxides. Syn addition of 2 OH. Ozonolysis Double Bonds What do we do with double bonds? We do addition reactions. In an addition reaction, something is added to both carbons involved in a double bond (or not involved in the double bond, in the