6.10 Acid-Catalyzed Hydration of Alkenes. Acid-Catalyzed Hydration of Alkenes

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1 Acid-atalyzed ydration of Alkenes 6.10 Acid-atalyzed ydration of Alkenes O O reaction is acid catalyzed; typical hydration medium is 50% 2 SO 4-50% 2 O Follows Markovnikov's Rule Follows Markovnikov's Rule % 2 SO 4 50% 2 O 3 3 O (90%) % 2 SO 4 50% 2 O O (80%) involves a carbocation intermediate is the reverse of acid-catalyzed dehydration of alcohols to alkenes 2 2 O Mechanism 3 3 O 3 Step (1) Protonation of double bond Mechanism 2 O 3 slow O

2 Step (2) apture of carbocation by water Mechanism 3 O fast 3 O 3 Step (3) Deprotonation of oxonium ion 3 3 O O O 3 Mechanism fast O Relative Rates Principle of Microscopic Reversibility ethylene propene 2-methylpropene Acid-catalyzed hydration 2 = = x 10 6 ( 3 ) 2 = x The more stable the carbocation, the faster it is formed, and the faster the reaction rate O In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order. 3 3 O 3 ydration-dehydration Equilibrium 2OO 6.11 Thermodynamics of Addition-Elimination Equlibria ow do we control the position of the equilibrium and maximize the product?

3 Le hatelier s s Principle Le hatelier s s Principle A system at equilibrium adjusts so to minimize any stress applied to it. For the hydration-dehydration equilibria, the key stress is water. Adding water pushes the equilibrium toward more product (alcohol). Removing water pushes the equilibrium toward more reactant (alkene). At constant temperature and pressure a reaction proceeds in a direction which is spontaneous or decreases free energy (G). The sign of G is always positive, but ΔG G can be positive or negative. ΔG G = G products G reactants Spontaneous when ΔG G < 0 Le hatelier s s Principle For a reversible reaction aa bb c dd The relationship between ΔG G and ΔG o is R = J/(mol. K) and T is the temperature in K Le hatelier s s Principle At equilibrium ΔG G = 0 and the following becomes true Substituting K eq into the previous equation gives ΔG o = - RT lnk eq Reactions for ΔG o positive are endergonic and for ΔG o negative are exergonic. Synthesis 6.12 ydroboration-oxidation of Alkenes Suppose you wanted to prepare 1-decanol from 1-decene? O Needed a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule.

4 Synthesis Two-step reaction sequence called hydroboration- oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule. 1. hydroboration 2. oxidation O ydroboration Step B 2 B 2 ydroboration can be viewed as the addition of borane (B 3 ) to the double bond. But B 3 is not the reagent actually used. ydroboration Step ydroboration Step B 2 B 2 B 2 B 2 2 B ydroboration reagents B 2 Diborane (B 2 6 ) normally used in an ether-like solvent called "diglyme" ydroboration reagents O B3 Borane-tetrahydrofuran complex ( 3 B-TF) Oxidation Step Example 2 O 2, O B 2 O Organoborane formed in the hydroboration step is oxidized with hydrogen peroxide. (93%) 1. B 2 6, diglyme 2. 2 O 2, O O

5 Example Features of ydroboration-oxidation B-TF 2. 2 O 2, O 3 3 O (98%) 3 hydration of alkenes regioselectivity opposite to Markovnikov's rule no rearrangement stereospecific syn addition Example 1. B 2 6, diglyme 2. 2 O 2, O O 6.13 Stereochemistry of ydroboration-oxidation (82%) Features of ydroboration-oxidation hydration of alkenes regioselectivity opposite to Markovnikov's rule no rearrangement stereospecific syn addition and O become attached to same face of double bond 3 syn Addition 1. B O 2, NaO O 3 only product is trans-2-methylcyclopentanol (86%) yield

6 1-Methylcyclopentene B Mechanism of ydroboration-oxidation syn addition of and B to double bond B adds to less substituted carbon Organoborane Intermediate Add ydrogen Peroxide O replaces B on same side trans-2-methylcyclopentanol 6.15 Addition of alogens to Alkenes

7 General features Example X 2 X X 3 ( ( 3 ) 2 2 l ( ( 3 ) 2 electrophilic addition to double bond forms a vicinal dihalide (100%) Scope limited to l 2 and 2 F 2 addition proceeds with explosive violence I 2 addition is endothermic vicinal diiodides dissociate to an alkene and I Stereochemistry of alogen Addition anti addition Example Example 2 l 2 l l trans-1,2- -1,2-Dibromocyclopentane 80% yield; only product trans-1,2- -1,2-Dichlorocyclooctane 73% yield; only product

8 Mechanism is electrophilic addition 6.17 Mechanism of alogen Addition to Alkenes alonium Ions 2 is not polar, but it is polarizable two steps (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide ethylene Relative Rates of omination 2 = Mechanism? 2 2 propene 3 = 2 61? 2-methylpropene 2,3-dimethyl dimethyl-2-butene ( 3 ) 2 = ( 3 ) 2 =( 3 ) 2 920,000 More highly substituted double bonds react faster. Alkyl groups on the double bond make it more electron rich. No obvious explanation for anti addition provided by this mechanism. 2 Mechanism Formation of omonium Ion yclic bromonium ion Mutual polarization of electron distributions of 2 and alkene

9 Electrons flow from alkene toward 2 Formation of omonium Ion δ π electrons of alkene displace from Formation of omonium Ion δ δ Stereochemistry attack of from side opposite bond of bromonium ion gives anti addition Example 2 trans-1,2- -1,2-Dibromocyclopentane 80% yield; only product yclopentene 2 omonium ion

10 omide ion attacks the bromonium ion from side opposite carbon-bromine bond trans-stereochemistry in vicinal dibromide X 2 X X 6.18 onversion of Alkenes to Vicinal alohydrins alkenes react with X 2 to form vicinal dihalides X 2 X X 2 Examples O 2 2 O (70%) alkenes react with X 2 to form vicinal dihalides alkenes react with X 2 in water to give vicinal halohydrins l 2 2 O O X 2 2 O X O l X anti addition only product

11 O bromonium ion is intermediate water is nucleophile that attacks bromonium ion Mechanism O 2 Examples 2 2 l 2 2 O 2 O 2 2 O l (70%) O anti addition only product yclopentene l 2 hloronium ion trans-stereochemistry in oxonium ion Water attacks chloronium ion from side opposite carbon-chlorine bond

12 Regioselectivity O 3 3 O 2 (77%) trans-2-hlorocyclopentanol Markovnikov's rule applied to halohydrin formation the halogen adds to the carbon having the greater number of hydrogens. O δ Explanation δ O 3 3 δ δ 2 δ δ transition state for attack of water on bromonium ion has carbocation character; more stable transition state (left) has positive charge on more highly substituted carbon