Alkenes, Alkynes

&

EA

Reaction

Department of

Biochemistry Faculty of Pharmacy

University of Santo Tomas

Alkenes CnH2n

Alkynes

Alkenes

Cycloalkenes

Alkynes

Alkenes

Physical Properties

Alkenes & alkynes are

nonpolar

Physical properties are

similar to those of alkanes

with same C skeletons

Those that are liquid at RT

have densities <1.0g/mL

Physical Property: Boiling point

Alkenes have diffuse e- cloud; alkanes have stronger

interaction; & alkyne’s e- cloud is tighter than alkenes

STRUCTURE OF ALKENES

ISOMERISM

Alkenes: cis & trans isomerism

R

H H

R H

R H

R

All substituents are

on one side

of bond

All substituents are

on different sides

of bond

cis trans

Alkenes: cis & trans isomerism

Alkenes: cis & trans isomerism

Alkenes: cis & trans isomerism

Cis/trans Isomerism in Cycloalkenes

For cycloalkenes with 7 or fewer C

atoms, the cis configuration is preferred

Cis/trans Isomerism in Di-, Tri-, &

Polyenes

Total # of cis-trans isomers possible = 2n

n = # of double bond

2,4-heptadiene

Double bond

C2 – C3 C4 – C5

trans trans

trans cis

cis trans

cis cis

What is the correct systematic name for the following?

Br

1. 1-bromo-2-hexene2. 6-bromo-4-hexene3. cis-1-bromo-2-hexene4. trans-1-bromo-2-hexene5. trans-6-bromo-4-hexene

Alkenes: E/Z System

For more than two substituents the cis/trans,the E/Z system is used.

Alkenes: E/Z System

H3C

I H

F

(E)-3-fluoro-2-iodo-2-propene

H

H3C CH2

CH2

OH

CH3

path goes to O,

not H

path goes to C,

not H

comparison

stops here

C

H

O

H

C

H

C

H

Alkenes: E/Z System

H3C

H CH

CH2

CH2

CH3

CH CH2 C C

C H

C

HH

CH2 CH3 C C

H H

H

HH

Which is the correct structure for E-3-bromo-2-chloro-2-pentene?

C C

CH3

Cl

CH2CH3

Br

C C

CH3

Cl

Br

CH2CH3

C C

CH3

Br

Cl

CH2CH3

C C

CH3

Br

CH2CH3

Cl

1. 2.

3. 4.

Alkenes: Stability

The stability of alkenes depends upon

number of substituents

Stability of Disubstituted Alkenes

H3C

H3C H

H H3C

H H

CH3 H

H3C H

CH3

> >

REACTION OF ALKENES

ELECTROPHILIC ADDITION & OXIDATION

Reactive Sites of Alkenes & Alkynes

CH3

H3C

H3C C CH

Alkenes: Electrophilic Addition

Electrophilic addition to the double bond is a

two-step process

R

R

R

R

+ Y+ + Z-

R

R

Y

Z

RR

electrophile

nucleophile

Alkenes: Electrophilic Addition

Alkenes: Electrophilic Addition

Alkenes: AE - Hydrogenation

CH3

+ H2

Pt/CCH

3H

H

With the help of metal catalysts, H2 adds to

the double bond.

(Catalytic Reduction)

Alkenes: AE - Hydrogenation

Alkenes: AE - Hydrogenation

Alkenes: AE - Halogenation

Alkenes: AE - Halogenation

Alkenes: AE - Hydrohalogenation

Electrophilic reagents we can use are hydrogen halides:

HF, HCl, HBr, HI

Alkenes: AE - Hydrohalogenation

Alkenes: AE - Hydration

H3C

CH2

CH

CH2

+ H2O

HCl, trace

OH

H3C

CH2

CH

CH2

H

1-butene 2-butanol

Alkenes: AE - Hydration

AE – Markovnikov’s Rule

We obtain both products; however, one is the

major product.

HII

H I

H

1-methylcycohexene 1-iodo-1-methyl-cyclohexane

2-iodo-1-methyl-cyclohexane

Major product Minor product

+

We have a regioselective reaction.

AE – Markovnikov’s Rule

In the addition of HX or H2O to an

alkene, H adds to the C of the double

bond having the greater number of

hydrogens.

AE – Markovnikov’s Rule

AE – Markovnikov’s Rule

Carbocation Stability

Carbocation Stability

AE – Markovnikov’s Rule

AE – Markovnikov’s Rule

HBr

Br

H

+H

Br1-bromo-1-methyl-cyclopentane

1-bromomethyl-cyclopentane

A B

AE – Markovnikov’s Rule

Alkenes: AE –Addition of Alcohols

Alcohols react the same way as water

C CH2

H3C

H3C

+ CH3CH2OHH+ H3C

CH3

CH3

O

H2C CH32-methylpropene ethanol

tert-butyl-ethyl ether

Note: the reaction works only in the

presence of acid.

Alkenes: AE –Addition of Alcohols

Alkenes: OxidationCold, alkaline KMnO4

acidic KMnO4

Alkenes: Oxidation

Ozonolysis

Dihydroxylation with OsO4

Alkenes: Oxidation

Hydroboration oxidation

Polymerization of Alkenes

• Polymers are large molecules that

contain a large number of repeating

units of a small molecule (monomer).

• Biopolymers are typically synthesized

by organisms.

• Synthetic Polymers are made in organic

laboratories.

Polymerization of AlkenesAlkenes can be used as a monomeric unit to

form chain-growth polymers

styrene

n

polystyrene

Polymerization of Alkenes

Polymerization of Alkenes

For the polymerization, electrophilic addition

reactions can be used.

BF3F3B

In the initiation step an electrophile (BF3)

adds to the alkene to form a carbocation

intermediate.

Polymerization of Alkenes

F3B

+F

3B

Subsequent propagation steps grow the

polymer until all monomers are consumed.

Polymerization of Alkenes

Termination can also be accomplished by

adding a nucleophile.

F3B

n

Nu

F3B

Nu

n

Polymerization of AlkenesAn alternative way to initiate polymerization is

starting with a radical initiator.

RO ORD

(hn) 2 RO

radicals

RO

RO

R R

R

The radical adds onto the alkene to form a new

radical. Propagation is accomplished via subsequent

radical additions.

PREPARATION OF ALKENES

-ELIMINATION REACTION

Preparation of Alkenes

-Elimination Reactions

Dehydrogenation of alkanes

Dehydration of alcohols

Dehydrohalogenation of alkyl halides

Dehydrogenation of Alkanes

Dehydration of Alcohols

Zaitsev’s Rule

Dehydrohalogenation of RX

CONJUGATED DIENES

ADDITION & DIELS-ALDER REACTIONS

Reaction of Conjugated Dienes

undergo two-step electrophilic addition

reactios just as do simple alkenes

1,2 & 1,4-addition

Reaction of Conjugated Dienes

CH2 CH CH CH3 CH2 CH CH CH3

Less Substituted Double Bond

(Secondary carbocation)

More Substituted Double Bond

(Primary carbocation)

+

Diels-Alder Reaction: Cycloaddition

REACTION OF ALKYNES

ADDITION, OXYDATION, & ACIDITY OF TERMINAL ALKYNES

Reactivity of Alkynes

Hydrohalogenation

Hydrohalogenation

Halogenation

Hydration

Hydration of Alkyne & Tautomerism

Draw the ENOL tautomers of the following ketones:

Catalytic Reduction

Oxidation

Acidity of Terminal Alkynes

HC CH + NH2

HC C + H3N

Acidity of Terminal Alkynes

-FIN-