6) chapter 8 alkenes and alkynes ii

CCB1013 Organic ChemistryBy Dr. Oh Pei Ching

1

Chapter 8:

Alkenes and Alkynes II –

Addition Reactions

2

Alkenes and Alkynes II

CCB1013 Organic Chemistry

� By the end of this lecture, you should be able to:

• Use curved arrow notation in addition mechanism

• Understand additions to alkenes and alkynes

(hydrogen halides, sulfuric acid, water and halogens)

• Identify proton donors and acceptors in proton

transfer steps

• Understand stereochemistry, regiochemistry, and

conditions for use of a variety of reagents that add to

alkenes and alkynes

3

Learning Outcomes


� Example:

4

Introduction: Addition Reactions to Alkenes


Electrophilic Nucleophilic

5



� How to Understand Additions to Alkenes

� An addition reaction results in the conversion of one π

bond and one σ bond into two σ bonds. Generally,

addition reactions are exothermic.

� How to Understand Additions to Alkenes

� Since π bonds are formed from the overlapping of p

orbitals, π electron clouds are above and below the

plane of the double bond. The electrons are loosely held

and are a source of electron density (nucleophilic).

6



C C π electron

clouds

7



� Electrophilic Addition

� C=C and C≡C π bonds are particularly susceptible to

electrophilic reagents (electrophiles).

� Common electrophiles: H+, X+ (X = Cl, Br, I), Hg2+, etc.

� In addition reactions, the alkene changes from a nucleophile

in the first step to an electrophile in the second.

Recall:

Electrophiles can accept e‒

Nucleophile can donate e‒

8

Electrophilic Addition of Hydrogen Halides to

Alkenes: Mechanism and Markovnikov’s Rule


� Hydrogen halides add to the double bond of alkenes:

� Markovnikov’s Rule:

� For symmetrical substrates, no problem for regiochemistry.

C C

H

H

H

HE Nu

E

CC H

H

H

H

E

C CH

H

H

H

E

C CH

H

H

H

Nu E

CC H

H

H

H

Nu

Nu

same

as

same

as

9





� But for unsymmetrical substrates, two regioisomers are

possible.

E NuC C

H

H3C

H

H E

CC H

H

CH3

H

E

C CCH3

H

H

H

E

C CCH3

H

H

H

Nu E

CC H

H

CH3

H

Nu

Nu Nu

or

different

from

10





� Addition of HX to an alkene proceeds so that the hydrogen

atom adds to the carbon that already has the most hydrogen

atoms.

� Example:

� Addition of HBr to propene occurs to give 2-bromopropane as

the major product.

11




� Mechanism for addition of hydrogen halide to alkene

Rate-

determining

step

12




� Free-energy diagram for addition of hydrogen halide to alkene

� The reaction has a highly endergonic first step (rate

determining) and a highly exergonic second step.

13




� Theoretical Explanation of Markovnikov’s Rule:

� The product with the more stable carbocation intermediate

predominates (because it is formed faster).

� The most stable carbocation is formed fastest because it has a

lower ∆G‡.

� The transition state for the rate determining step (first step) resembles a

carbocation and is stabilized by factors which stabilize carbocations.

14




� Theoretical Explanation of Markovnikov’s Rule:

� The product with the more stable carbocation intermediate

predominates (because it is formed faster).

� The most stable carbocation is formed fastest because it has a

lower ∆G‡.

� The transition state for the rate determining step (first step) resembles a

carbocation and is stabilized by factors which stabilize carbocations.

∆G‡(2o) < ∆G‡(1o)

→ 2o carbocation forms faster

15




� Mechanism for addition of HBr to 2-methylpropene

This

reaction

takes place

This

reaction

does not

occur to any

appreciable

extent

16




� Modern Statement of Markovnikov’s Rule:

� In the ionic addition of an unsymmetrical reagent to a double

bond, the positive portion of the adding reagent attaches itself

to a carbon atom of the double bond so as to yield the more

stable carbocation as an intermediate.

� Regioselective Reactions:

� A reaction that can potentially yield two or more

constitutional isomers actually produces only one or a

predominance of one isomer.

� Markovnikov additions of hydrogen halides to alkenes are

regioselective.

17




� Exception to Markovnikov’s Rule:

� Anti-Markovnikov addition occurs when alkenes are treated

with HBr in the presence of peroxides (ROOR).

� Addition occurs via a radical mechanism.

� This anti-Markovnikov addition does not take place with HI,

HCl, and HF, even when peroxides are present.

Anti-

Markovnikov

Product

18

Stereochemistry of the Ionic Addition to an Alkene


� Addition of HX to 1-hexene yields a chiral molecule.

� A racemic mixture is produced because the intermediate

carbocation is achiral.

C C

H

H

Bu

HH X

C CH2 HH

Bu

X

X

achiral

trigonal planar

carbocation

C CH3H

X

Bu

(S)-2-Halohexane(50%)

attack from top

rac

em

ate

C CH3

H

X

Bu

(R)-2-Halohexane(50%)

(R)-2-Halohexane

(50%)

1-Hexene donates a

pair of electrons to

the proton of HX to

form an achiral

carbocation.attack from bottom

19

Addition of Sulfuric Acid to Alkenes


� Addition of cold concentrated sulfuric acid to alkenes leads to

alkyl hydrogen sulfates which are soluble in the acid.

� The addition follows Markovnikov’s rule.

20

Addition of Sulfuric Acid to Alkenes


� Alcohols from Alkyl Hydrogen Sulfates:

� Alkyl hydrogen sulfates can be hydrolyzed by heating with

water.

� The net result is Markovnikov addition of water to an alkene.

21

Addition of Water to Alkenes:

Acid-Catalyzed Hydration


� The reaction of alkenes with dilute aqueous acid (e.g. dilute

H2SO2 and H3PO4) leads to Markovnikov addition of water.

22




� Mechanism:

Rate-

determining

step

23




� The hydration of alkenes and the dehydration of alcohols are

simply reverse reactions of one other.

H The reaction is governed by the position of all the equilibria.

H Hydration is favored by addition of a small amount of acid and a large amount

of water.

H Dehydration is favored by concentrated acid with very little water present

(removal of water produced also helps favor dehydration).

� Rearrangements:

� One complication associated with alkene hydration is the

occurrence of rearrangements, as carbocation formed initially

may rearrange to a more stable one.

How to avoid

rearrangement?

Oxymercuration

-demercuration

24

Alcohols from Alkenes through Oxymercuration-

Demercuration: Markovnikov Addition


� Oxymercuration-demercuration procedure gives high yields of

alcohols and avoids rearrangements.

Alkene mercuric acetate (hydroxyalkyl)mercury

Water and mercuric acetate add to the double bond

sodium alcohol

borohydride

Sodium borohydride reduces the acetoxymercury group and replaces it with hydrogen

OAc

25




� Oxymercuration–demercuration is also highly regioselective and

follows Markovnikov’s rule.

� Specific Examples:

No

rearrange-

ment

26




� Mechanism of Oxymercuration

� The mechanism involves formation of a bridged mercurinium ion.

27




� Mechanism of Oxymercuration

� The mechanism involves formation of a bridged mercurinium ion.

28

Alcohols from Alkenes through Hydroboration-

Oxidation: Anti-Markovnikov Syn Addition


� Hydroboration-Oxidation:

� The reaction leads to syn and anti-Markovnikov addition of water to

alkenes.

� Hydroboration: Synthesis of Alkylboranes

� The elements of hydrogen and boron are added across the double bond.

� In practice, a borane complex with the solvent tetrahydrofuran (THF) is often used.

29




� Hydroboration: Mechanism

� When terminal alkene (e.g. propene) is treated with solution

containing BH3:THF, boron hydride adds successively to three

molecules of alkene.

� Boron becomes attached to the least substituted carbon of the

double bond.

� The bulky boron group can approach the least sterically hindered carbon more

easily.

� This orientation also allows a �+ charge in the transition state to reside at the most

substituted carbon.

� This orientation leads to anti-Markovnikov product.

30




� Hydroboration: Mechanism

� Boron becomes attached to the least substituted carbon of the double

bond.

� The boron and hydride add with syn stereochemistry.

Boron attachment

31




� Oxidation and Hydrolysis of Alkylboranes

� Oxidation and hydrolysis to the alcohol takes place with retention of

stereochemistry at the carbon bonded to boron.

32




� Oxidation of trialkylboranes: Mechanism

33




� The hydroboration-oxidation of 1-methylcyclopentene gives the anti-

Markovnikov product with syn addition of the elements of water.

� Summary of Alkene Hydration Methods:

� Acid-catalyzed hydrolysis: Markovnikov addition

� Oxymercuration-demercuration: Markovnikov addition

� Hydroboration-oxidation: anti-Markovnikov and syn addition

34

Addition of Bromine and Chlorine to Alkenes


� Addition produces vicinal dihalides.

� The addition of halogens is an anti addition to the double bond.

35



� Bromine serve as a test for the presence of carbon-carbon multiple

bonds.

� If bromine is added to an alkene (or alkyne), the red color of the bromine

reagent disappears when an alkene (or alkyne) is present in excess.

� Alkanes do not react appreciably with bromine or chlorine at room

temperature in the dark.

36



� Mechanism:

� A bromonium ion intermediate results instead of the carbocation seen in

other addition reactions.

37



� Stereochemistry:

� The net result is anti addition because of SN2 attack on the bromonium

ion intermediate.

� When cyclopentene reacts the product is a racemic mixture of trans-1,2-

dibromocyclopentane enantiomers.

38



� Stereospecific Reactions:

� A reaction is stereospecific if a particular stereoisomeric form of the

starting material reacts in such a way that it gives a specific

stereoisomeric form of the product.

� Example: cis- and trans-2-butene give stereoisomeric products when

halogenated.

� Halogenation of double bonds is stereospecific.

Reaction 1

Reaction 2

39



� Halohydrin Formation:

� If halogenation is carried out in aqueous solvent (not nucleophilic

solvent such as CCl4), the water molecule can act as a nucleophile to open

the halonium ion.

� The product is a halohydrin.

40



� Halohydrin Formation Mechanism:

41



� Halohydrin Formation:

� In unsymmetrical alkenes, the bromonium ion will have some of its δ +

charge density on the most substituted of the two carbons.

� The most substituted carbon can best accommodate δ+ charge.

� The water nucleophile will tend to react at the carbon with the most δ+

charge.

42

Addition of Bromine and Chlorine to Alkynes


� Addition of halogen to alkynes can occur once or twice depending on

how many equivalents of the halogen are added.

� Addition of one equivalent usually proceeds to give the trans dihalide

through anti addition.

43

Addition of Hydrogen Halides to Alkynes


� Addition of hydrogen halides occurs once or twice depending on how

many molar equivalent of hydrogen halide are added.

� Both additions are Markovnikov and give gem-halides.

44

Addition of Hydrogen Halides to Alkynes


� HBr can be generated by reaction of acetyl bromide and alumina.

� Anti-Markovnikov addition of HBr occurs in the presence of peroxide.

6) chapter 8 alkenes and alkynes ii

Documents

addition mechanism

addition reactions2alkenes

addition of hx

addition of hbr

addition reaction results

double bond of alkenes

ccb1013 organic chemistryby

alkyneshydrogen halides