6.10 Acid-Catalyzed Hydration of Alkenes 19

Acid-Catalyzed Hydration of Alkenes OH C H + H—OH reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O 2

Follows Markovnikov's Rule OH C CH2CH3 CH3 H3C CH3 H C 50% H2SO4 50% H2O (90%) 3

Follows Markovnikov's Rule OH CH3 50% H2SO4 50% H2O CH2 (80%) 4

involves a carbocation intermediate Mechanism involves a carbocation intermediate is the reverse of acid-catalyzed dehydration of alcohols to alkenes OH C CH3 H3C C CH2 H+ + H2O 5

Step (1) Protonation of double bond Mechanism Step (1) Protonation of double bond H3C C CH2 O + H : + slow H3C O H : + C CH3 + H3C H 6

Step (2) Capture of carbocation by water Mechanism Step (2) Capture of carbocation by water + H3C C CH3 H O : fast + H O : C CH3 6

Step (3) Deprotonation of oxonium ion Mechanism Step (3) Deprotonation of oxonium ion H O : C CH3 O H : + + fast H O : C CH3 .. O + H : + 6

Acid-catalyzed hydration Relative Rates Acid-catalyzed hydration ethylene CH2=CH2 1.0 propene CH3CH=CH2 1.6 x 106 2-methylpropene (CH3)2C=CH2 2.5 x 1011 The more stable the carbocation, the faster it is formed, and the faster the reaction rate. 9

Principle of Microscopic Reversibility OH C CH3 H3C C CH2 H+ + H2O In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order. 11

6.11 Hydroboration-Oxidation of Alkenes 19

Suppose you wanted to prepare 1-decanol from 1-decene? Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? OH 13

Suppose you wanted to prepare 1-decanol from 1-decene? Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule. OH 13

Synthesis Two-step reaction sequence called hydroboration- oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule. 1. hydroboration 2. oxidation OH 13

Hydroboration Step C H BH2 C + H—BH2 Hydroboration can be viewed as the addition of borane (BH3) to the double bond. But BH3 is not the reagent actually used. 14

Hydroboration reagents: Hydroboration Step C H BH2 C + H—BH2 Hydroboration reagents: H Diborane (B2H6) normally used in an ether- like solvent called "diglyme" H2B BH2 H 14

Hydroboration reagents: Hydroboration Step C H BH2 C + H—BH2 Hydroboration reagents: + O BH3 – •• Borane-tetrahydrofuran complex (H3B-THF) 14

Oxidation Step H2O2, HO– H BH2 C H OH C Organoborane formed in the hydroboration step is oxidized with hydrogen peroxide. 17

Example 1. B2H6, diglyme 2. H2O2, HO– OH (93%) 13

Example H C CH3 OH H3C CH3 1. H3B-THF 2. H2O2, HO– C H3C H (98%) 13

Features of Hydroboration-Oxidation hydration of alkenes regioselectivity opposite to Markovnikov's rule no rearrangement stereospecific syn addition 18

Example OH 1. B2H6, diglyme 2. H2O2, HO– (82%) 13

6.12 Stereochemistry of Hydroboration-Oxidation 19

Features of Hydroboration-Oxidation hydration of alkenes regioselectivity opposite to Markovnikov's rule no rearrangement stereospecific syn addition 18

only product is trans-2-methylcyclopentanol (86%) yield syn-Addition H and OH become attached to same face of double bond H CH3 HO CH3 H 1. B2H6 2. H2O2, NaOH only product is trans-2-methylcyclopentanol (86%) yield 20

6.13 Mechanism of Hydroboration-Oxidation 19

1-Methylcyclopentene + BH3 syn addition of H and B to double bond B adds to less substituted carbon 22

Organoborane intermediate 23

OH replaces B on same side Add hydrogen peroxide OH replaces B on same side 23

trans-2-Methylcyclopentanol 24