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Home , Addition reaction, Alkene, Carbocation, Catalysis, Electrophile, Electrophilic addition

Chapter 6: Reactions of : Addition Reactions

6.1: of Alkenes – addition of H-H (H2) to the π-bond of alkenes to afford an . The reaction must be catalyzed by such as Pd, Pt, Rh, and Ni.

H H H H Pd/C ΔH° = -136 KJ/mol + C C hydrogenation C C H H H EtOH H H H H H

C-C π-bond H-H C-H = 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 . • The catalyst assists in breaking the π-bond of the and the H-H 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

• -carbon π-bond of alkenes and can be reduced to the corresponding saturated C-C bond. Other π-bond bond such as C=O (carbonyl) and C≡N are not easily reduced by catalytic hydrogenation. The C=C bonds of aryl rings are not easily reduced.

O O

H2, PtO2

O

H2, Pd/C C5H11 OH CH3(CH2)16CO2H

Linoleic (unsaturated ) Steric Acid (saturated fatty acid) O O H , Pd/C OCH3 2 OCH3 ethanol

H , Pd/C C 2 C N N ethanol 128 6.2: Heats of Hydrogenation -an be used to measure relative stability of isomeric alkenes

H H H CH3 trans is ~3 KJ/mol H3C CH3 H3C H more stable than the cis-2- trans-2-butene cis isomer

ΔH° : -2710 KJ/mol -2707 KJ/mol

H H H CH3 H2, Pd H2, Pd CH3CH2CH2CH3 H3C CH3 H3C H

cis-2-butene trans-2-butene

ΔH°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): Heats of Hydrogenation of Some Alkenes

Alkene !H° (KJ/mol)

H2C=CH2 136

H H monosubstituted 125 - 126 H3C H

H H 117 - 119

H3C CH3

H CH3 disubstituted 114 - 115 H3C H

H3C H 116 - 117

H3C H

H3C H trisubstituted 112 H3C CH3

H3C CH3 tetrasubstituted 110 H3C CH3 130 6.3: of Alkene Hydrogenation Mechanism: H H H2C CH2 H H H2C CH2

H2C CH2

H2

H H H H H H C C C H H H H H C H

The addition of H2 across the π-bond is syn, i.e., from the same face of the

H CH3 CH CH H 3 H2, Pd/C 3

EtOH CH CH CH3 3 3 H H

syn addition Not observed of H2 131

6.4: of Halides to Alkenes

C-C σ-bond: ΔH°= 368 KJ/mol C-C π-bond: ΔH°= 243 KJ/mol

π-bond of an alkene can act as a !!

Electrophilic H H Br H C C + H-Br C C H H H H H H nucleophile

Bonds broken Bonds formed

C=C π-bond 243 KJ/mol H3C-H2C–H -410 KJ/mol H–Br 366 KJ/mol H3C-H2C–Br -283 KJ/mol calc. ΔH° = -84 KJ/mol 132 expt. ΔH°= -84 KJ/mol of HX correlates with acidity: slowest HF << HCl < HBr < HI fastest 6.5: Regioselectivity of Hydrogen Halide Addition: H Markovnikov's Rule Br H H Br C H H-Br R C R C C H + R C C H H H H H H none of this R Br H H Br C H H-Br R C R C C H + R C C H H R H R H none of this R Br H H Br C H H-Br R C R C C R + R C C R R R H R H none of this R Br H H Br C H H-Br H C R C C R + R C C R' R' H H H H Both products observed For the electrophilic addition of HX across a C=C bond, the H (of HX) will add to the carbon of the double bond with the most H’s (the least substitutent carbon) and the X will add to the carbon of the double bond that has the most groups. 133

Mechanism of electrophilic addition of HX to alkenes

6.6: Mechanistic Basis for Markovnikov's Rule: Markovnikov’s rule can be explained by comparing the stability of the intermediate 134 For the electrophilic addition of HX to an unsymmetrically substituted alkene: • The more highly substituted intermediate is formed. • More highly substituted carbocations are more stable than less substituted carbocations. () • The more highly substituted carbocation is formed faster than the less substituted carbocation. Once formed, the more highly substituted carbocation goes on to the final more rapidly as well.

135

6.7: Carbocation Rearrangements in 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 and methyl shifts. H Cl H H H

H3C C H H-Cl H C C H 3 + H C C H C C C C 3 C C H3C H C H H H 3 H H H C H 3 Cl H ~ 50% ~ 50% expected product

H Cl H H3C H H3C C H H-Cl H C C H 3 + H C C H C C C C 3 C C H C 3 H3C H CH3 H CH H H C H 3 3 Cl H

Note that the shifting or group moves with its pair.

A MORE STABLE CARBOCATION IS FORMED. 136 6.8: Free- Addition of HBr to Alkenes H Br H H Br C H H-Br + Polar mechanism H3CH2C C H3CH2C C C H H3CH2C C C H H H H H H (Markovnikov addition) none of this H Br H H Br C H H-Br + Radical mechanism H3CH2C C H3CH2C C C H H3CH2C C C H H H H H H (Anti-Markovnikov addition) (RO-OR) none of this

H H-Br Br H H Br C H R C C H + R C C H R C ROOR H H H H The regiochemistry of H (peroxides) none of this HBr addition is reversed R H-Br Br H H Br C H + in the presence of R C R C C H R C C H ROOR H R H R H peroxides. none of this R Br H H Br C H H-Br Peroxides are radical R C R C C R + R C C R R ROOR R H R H initiators - change in none of this mechanism R Br H H Br C H H-Br H C R C C R + R C C R' R' ROOR H H H H 137 Both products observed

The regiochemistry of free radical addition of H-Br to alkenes reflects the stability of the radical intermediate. H H R • R C R C • R C • H R R Primary (1°) < Secondary (2°) < Tertiary (3°)

138 6.9: Addition of to Alkenes (please read) 6.10: Acid-Catalyzed Hydration of Alkenes - addition of (H-OH) across the π-bond of an alkene to give an ; opposite of dehydration

H3C H3C H2SO4, H2O C CH2 C OH H3C H C 3 H3C

This addition reaction follows Markovnikov’s rule The more highly substituted alcohol is the product and is derived from The most stable carbocation intermediate.

Reactions works best for the preparation of 3°

139

Mechanism is the reverse of the acid-catalyzed dehydration of alcohols: Principle of Microscopic Reversibility

140 6.11: of Addition-Elimination Equlibria H3C H3C H2SO4 + H O C OH C CH2 2 H3C H3C H3C

Bonds broken Bonds formed

C=C π-bond 243 KJ/mol H3C-H2C–H -410 KJ/mol H–OH 497 KJ/mol (H3C)3C–OH -380 KJ/mol calc. ΔH° = -50 KJ/mol ΔG° = -5.4 KJ/mol ΔH° = -52.7 KJ/mol ΔS° = -0.16 KJ/mol

How is the position of the equilibrium controlled? Le Chatelier’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. Oxymercuration: a general (2-step) method for the Markovnokov hydration of alkenes H H OH H OH 2) NaBH C H 1) Hg(OAc)2, H2O C Hg(OAc) 4 C H C H C 4 9 C4H9 C C4H9 C H H H H H O NaBH4 reduces the C-Hg Ac= acetate = C bond to a C-H bond H3C O

142 6.12: -Oxidation of Alkenes - Anti-Markovnikov addition of H-OH; syn addition of H-OH

1) B2H6, THF H CH3 CH3 2) H2O2, NaOH, H2O

H HO 6.13: Stereochemistry of Hydroboration-Oxidation 6.14: Mechanism of Hydroboration-Oxidation -

Step 1: syn addition of the H2B–H bond to the same face of the π-bond in an anti-Markovnikov sense; step 2: oxidation of the

B–C bond by basic H2O2 to a C–OH bond, with retention of stereochemistry

143

6.15: Addition of to Alkenes

X2 = Cl2 and Br2

X2 X X C C C C ( dihalide)

alkene 1,2-dihalide 6.16: Stereochemistry of Addition - 1,2-dibromide has the anti stereochemistry

Br Br

+ Br2 + Br Br not observed

Br CH3 CH3 Br2

Br H

144 6.17: Mechanism of Halogen Addition to Alkenes: Halonium - Bromonium intermediate explains the

stereochemistry of Br2 addition

145

6.18: Conversion of Alkenes to Vicinal "X-OH" X OH C C C C

alkene X X2, H2O + HX OH

anti stereochemistry Mechanism involves a intermediate

146 For unsymmterical alkenes, halohydrin formation is Markovnikov-like in that the orientation of the addition of X-OH can be predicted by considering carbocation stability

+ CH ! 3 more δ+ charge on the + Br ! more substituted carbon !+

H2O 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

HO CH3 CH3 Br2, H2O + HBr Br H

Br adds to the double bond first (formation of bromonium ion) and is on the least substituted end of the double bond 147

Organic are sparingly soluble in water as . The reaction is often done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as he electrophilic source. O OH O Br DMSO, H2O + N Br + N H

O O Note that the aryl ring does not react!!! 6.19: Epoxidation of Alkenes - (oxirane): three- membered ring, cyclic . O H O O HO OH Reaction of an alkene with a peroxyacid: H3C O H3C OH peroxyacetic acid peroxyacetic acetic acid acid

O H OH O H3C O + O H3C O 148 Stereochemistry of the epoxidation of alkenes: syn addition of . 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. O H H H3CCO3H H H R R R R cis-alkene cis-epoxide

H CCO H O H R 3 3 H R R H R H trans-alkene trans-epoxide 6.20: of Alkenes - oxidative cleavage of an alkene to carbonyl compounds ( and ). The π- and σ-bonds of the alkene are broken and replaced with C=O double bonds. C=C of aryl rings, C≡N and C=O do not react with , 149 C≡C react very slowly with ozone

electrical + Ozone (O3): 3 O2 discharge 2 O3 O _ O O mechanism

O3, CH2Cl2 O R1 R3 R R -78 °C O O O O Zn 1 3 R1 R 3 -or- O + O R1 R3 R R2 R4 2 O R4 (H3C)2S R2 R4 R2 R4 molozonide + ZnO or (H3C)SO

1) O3 2) Zn O + O

1) O3 2) Zn H H + O C H O

1) O3 2) Zn O

H O

150 6.21: Introduction to Organic 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

OH ??

H2SO4 H2, Pd/C

OH ??

151

CH3 CH3 Br ??

H

Br

6.22: Reactions of Alkenes with Alkenes: (please read)

152