Acid-Catalyzed Hydration of Alkenes

H—OHC C + OHC CH

Acid-Catalyzed Hydration of Alkenes

reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O

Follows Markovnikov's Rule

Hydrohalogenation & hydration reactions are at equilibrium.

Hydration – Thermodynamics

How could Le Châtelier’s principle be used to shift the equilibrium to the right or left?

SEE: CONCEPTUAL CHECKPOINT 9.11.

Hydration – Thermodynamics

Similar to hydrohalogenation, the stereochemistry of hydration reactions is controlled by the geometry of the carbocation.

Draw the complete mechanism for the reaction above to show WHY a racemic mixture is formed.

SEE: SKILLBUILDER 9.3.

Hydration – Thermodynamics

(90%)

50% H2SO4

50% H2O

H3C

H3C CH3

H

C C

OH

C CH2CH3CH3

CH3

Markovnikov's Rule

50% H2SO4

50% H2O

Follows Markovnikov's Rule

(80%)

OH

CH3

CH2

+ H2OH+

involves a carbocation intermediate

is the reverse of acid-catalyzed dehydrationof alcohols to alkenes

H3C

H3C

C CH2

OH

C CH3CH3

CH3

Mechanism

Addition of Water to Alkene(alcohols)

Addition of Water to Alkenes(alcohols)

Question

Which alkene will undergo acid-catalyzed hydrolysis at the fastest rate?

A) B)

C) D)

In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order.

+ H2OH+

H3C

H3C

C CH2

OH

C CH3CH3

CH3

Principle of Microscopic Reversibility

H2O addition: Markovnikov's Rule

Question

The product isolated from the acid-catalyzed hydration of (Z)-3-methyl-2-pentene is:

A) 2-ethyl-2-butanol

B) 2-ethyl-1-butanol

C) 3-methyl-2-pentanol

D) 3-methyl-3-pentanol

Question

The product isolated from the acid-catalyzed hydration of (E)-3-methyl-2-pentene is:

A) chiral

B) achiral

Because rearrangements often produce a mixture of products, the synthetic utility of Markovnikov hydration reactions is somewhat limited.

OXYMERCURATION-DEMERCURATION is an alternative process that can yield Markovnikov products avoiding rearrangements

Oxymercuration-Demercuration

OXYMERCURATION begins with mercuric acetate.

How would you classify the mercuric cation? As a nucleophile or an electrophile?

As a Lewis acid or Lewis base?

Oxymercuration-Demercuration

Similar to how we saw the alkene attack a proton previously, it can also attack the mercuric cation.

Resonance stabilizes the mercurinium ion. Can you draw a reasonable resonance hybrid?

Oxymercuration-Demercuration

The mercurinium ion is also a good electrophile, and it can easily be attacked by a nucleophile, even a weak nucleophile such as water.

NaBH4 is generally used to replace the –HgOAc group with a –H group via a free radical mechanism.

Oxymercuration-Demercuration

Addition of Alcohol to Alkenes(ethers)

Regiochemistry withstanding, in order to understand thestereochemistry of the product, you must consider two things:

(1) Stereochemistry of the starting alkene (cis or trans;

Z or E)

(2) Stereochemistry of the addition (syn or anti)

Stereochemistry of Addition to Alkenes

C C + E—Y C CE Y

Optically inactive reactants produce optically inactive products.(Racemic mixtures are optically inactive)

The correlary is that an optically active starting material MAY produce an optically active product depending on the mechanism.

Stereochemistry of Addition to Alkenes

C C + E—Y C CE Y

Question

The product isolated from the acid-catalyzed hydration of (E)- or (Z)-3-methyl-2-pentene is:

A) optically active

B) an optically inactive racemic mixture

C) an optically inactive enantiomer

Hydroboration-Oxidation of Alkenes

To achieve anti-Markovnikov hydration, hydroboration-oxidation is often used.

Note that the process occurs in two steps.

Hydroboration-Oxidation

Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule.

Suppose you wanted to prepare 1-decanol from 1-decene?

Synthesis

OH

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

1. hydroboration

2. oxidation

Synthesis

OH

+ H—BH2C C H BH2C C

Hydroboration can be viewed as the addition ofborane (BH3) to the double bond. But BH3 is

not the reagent actually used.

Hydroboration Step

+ H—BH2C C H BH2C C

Hydroboration reagents:

H2B

H

H

BH2

Diborane (B2H6)

normally used in an ether-like solventcalled "diglyme"

Hydroboration Step

+ H—BH2C C H BH2C C

Hydroboration reagents:

Borane-tetrahydrofurancomplex (H3B-THF)

Hydroboration Step

+O

BH3–

••

H2O2, HO–

H BH2C C H OHC C

Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.

Oxidation Step

1. B2H6, diglyme

2. H2O2, HO–

(93%)

Example

OH

(98%)

H3C

H3C

CH3

H

C C1. H3B-THF

2. H2O2, HO–

H

C CCH3

CH3

CH3

H OH

Example

hydration of alkenes

regioselectivity opposite to Markovnikov's rule

no rearrangement

stereospecific syn addition

Features of Hydroboration-Oxidation

1. B2H6, diglyme

2. H2O2, HO–

(82%)

Example

OH

Stereochemistry of Hydroboration-Oxidation

hydration of alkenes

regioselectivity opposite to Markovnikov's rule

no rearrangement

stereospecific syn addition

Features of Hydroboration-Oxidation

H and OH become attached to same face of double bond

1. B2H6

2. H2O2, NaOH

only product is trans-2-methylcyclopentanol(86%) yield

syn Addition

CH3

H H

CH3

H

HO

Question

Hydroboration-oxidation of which one of the following yields a primary alcohol as the major product?

A) B)

C) D)

H3C

H3C

CH3

H

C C1. H3B-THF

2. H2O2, HO–

H

C CCH3

CH3

CH3

H OH

Question

A) The product is achiralB) The product is optically activeC) The product is a racemic mixtureD) The product is a single enantiomer

Conversion of Alkenes to Vicinal Halohydrins

Addition of Halogens in the Presence of Water

(halohydrins)

+ X2 X XC CC C

alkenes react with X2 to form vicinal dihalides

+ X2 X XC CC C

+ H2O OH

+ H—X

+ X2 X C CC C

alkenes react with X2 to form vicinal dihalides

alkenes react with X2 in water to give vicinal

halohydrins

Cl2

anti addition: only product

H2O

H2C CH2 BrCH2CH2OH+ Br2

H2O

(70%)

Examples

H

H

OH

Cl

H

H

Cl2

anti addition: only product

H2O

H2C CH2 BrCH2CH2OH+ Br2

H2O

(70%)

Examples

H

H

OH

Cl

H

H

Perspective formula

Fischer projection

(77%)

H3C

C CH2

H3C

CH3

OH

C CH2BrCH3

Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens.

Br2

H2O

Regioselectivity

CH2

H3C

CH3C

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

H3C CH2

H3C

Br: :

H

OH ..

Br: :

H

O H..

C

Explanation

Question

A. C.

B. D.

Br2, H2O

Br Br

OH

Br

OH

Br

OH

OH

What is the product (in addition to its enantiomer) of the following reaction?

Conversion of Alkenes to Vicinal Diols

SYN dihydroxylation adds across the C=C double bond in ONE step.

Syn Dihydroxylation

MnO41- is similar to OsO4 but more reactive.

SYN dihydroxylation occurs with KMnO4 only under mild conditions (cold temperatures).

Diols are commonly further oxidized by MnO41-, and in addition

MnO41- is reactive toward many other functional groups as

well. It is very useful in qualitative analysis due to a pronounced color change.

SEE: CONCEPTUAL CHECKPOINT 9.33.

Syn Dihydroxylation