ORGANIC CHEMISTRY 171

Section 201

2

Alkenes,Chapter 3

3

Unsaturated hydrocarbons can be

1-open-chain (linear and branched alkenes)

2- cyclic (cycloalkenes)

Alkenes and Cycloalkenes

4

Unsaturated HydrocarbonsUnsaturated Hydrocarbons

Hydrocarbons that contain at least one C=C ( alkenes) are called unsaturated hydrocarbons

CH3

CH2C

CH2CH3

CH2

5

Alkenes are acyclic unsaturated hydrocarbons that contain at least one C=C

C2H4

6

• Generic formula: Start with CnH2n+2 and minus two for each C=C

• one C=C e.g., C2H4, C3H6, etc.

C2H4

CnH2n

Nomenclature of Alkenes

8

Common Names

• Usually used for small molecules.

• Examples:

CH2 CH2

ethylene

CH2 CH CH3

propylene

CH2 C CH3

CH3

isobutylene

=>

IUPAC Nomenclature of Alkenes

• 1. Find the longest continuous chain containing the double bond.

• 2. Name the corresponding alkane and change the “ane” ending to “ene” for alkenes.

• 3. Number the chain so as to give the double bond the lowest number. Place a numerical prefix in front of the parent name to indicate the position of the first carbon in the double bond.

• Number and name alkyl groups as with alkanes.

10

Nomenclature

• alkenes: parent chain contains C=CC=C gets lowest numbersposition of C=C indicated by lower of the two numbers

CH3 CH2 CH CH2

Br

1-butene

4-methyl-1-butene

3-bromocyclohexene

11

Nomenclature

• C=C and OH: alkenol

higher priority group (OH) gets last suffixand lowest number

OH

OH

OH

2-propen-1-ol

5-methyl-4-hexen-2-ol

2-cyclohexenol

12

Nomenclature

• as side groups:

H2C CH

H2C CH CH2

H2C

C

H3C

CH2

ethenyl (vinyl)

2-propenyl (allyl)

1-methylethenyl (isopropenyl)

methylene

CH2

vinyl chloride

allyl alcohol

isopropenyl bromide

Cl

OH

Br

Examples

1-vinylcyclohexene methylenecyclopentane

Name the following compound:

C

H2C

CH

H2C CH3

CH3

H3C

The longest continuous chain containing the double bond is 5 carbons long and is indicated in blue, below:

C

H2C

CH

H2C CH3

CH3

H3C

The parent compound is derived from pentane. The parent alkene is pentene.

14

NomenclatureE-Z notation

H

C

H3C

C

Cl

CH2CH3

H

C

H3C

C

CH2CH3

Cl

(E)-3-chloro-2-pentene (Z)-3-chloro-2-pentene

1. Determine the higher priority group on each end of the alkene.2. If the higher priority groups are:

on opposite sides: E (entgegen = opposite)on the same side: Z (zusammen = together)

CH3 > HCl > CH2CH3

15

Name These Alkenes

CH2 CH CH2 CH3

CH3 C

CH3

CH CH3

CH3

CHCH2CH3H3C

1-butene

2-methyl-2-butene

3-methylcyclopentene

2-sec-butyl-1,3-cyclohexadiene

3-n-propyl-1-heptene =>

16

ALKENE ALKENE STRUCTURE AND STRUCTURE AND BONDINGBONDING

17

ALKENE STRUCTURE AND BONDINGALKENE STRUCTURE AND BONDING

sp2

sp2

C C

R

R

R

R

SHAPE IS TRIGONAL PLANAR

18

Orbital Description

• Sigma bonds around C are sp2 hybridized.• Angles are approximately 120 degrees.• No nonbonding electrons.• Molecule is planar around the double bond.• Pi bond is formed by the sideways overlap of

parallel p orbitals perpendicular to the plane of the molecule. =>

19

THE BOND ANGLE OF AN ALKENETHE BOND ANGLE OF AN ALKENE

120o

120o

120o

20

Bond Lengths and Angles

• Hybrid orbitals have more s character.

• Pi overlap brings carbon atoms closer.

• Bond angle with pi orbitals increases.– Angle C=C-H is 121.7 – Angle H-C-H is 116. 6 =>

21

C CH H

H H

2p

(sp2C + 1sH)

(sp2C + sp2

C)

overlapp orbitals C C

H H

H H

bond

no free rotation

HC

HC

H

H

trigonal planarsp2

22

Isomerism in alkenesIsomerism in alkenes

Cis and Trans Isomers

• Some alkenes can have the same connection of atoms, but have a different arrangement in three dimensional space.

• This is due to the lack of free rotation about the double bond.

• The different arrangements are geometric isomers.

• One of the isomers is cis- the other is trans.

24

CISCIS // TRANSTRANS ISOMERS ISOMERS

ciscis transtrans

substituents onthe same side ofmain chain

substituents onopposite sides ofmain chain

C C

H

C

C

H

C C

C

H

C

H

25

C C

R

H

R

H

C C

H

R

R

H

RR

R

R

COMPARE COMPARE cis / transcis / trans ISOMERS IN RING COMPOUNDS ISOMERS IN RING COMPOUNDS

cis trans

In alkenes and rings cis / trans isomers are called stereoisomers or geometric isomers.

2-butene

C

CH3

C

H

cis-2-butene

H3C

H

C

CH3

C

H3C

trans-2-butene

H

H

mp = -139oC

CH3 groups same side CH3 groups opposite sides

mp = -106oC

Insert figure 19.11

Geometric Isomers of 2-butene

28

Physical Properties

• Low boiling points, increasing with mass.• Branched alkenes have lower boiling points.• Less dense than water.• Slightly polar

– Pi bond is polarizable, so instantaneous dipole-dipole interactions occur.

– Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. =>

29

Polarity Examples

= 0.33 D = 0

=>

cis-2-butene, bp 4°C

C CH

H3C

H

CH3

trans-2-butene, bp 1°C

C CH

H

H3C

CH3

Preparation

I. Preparation

• 1. Dehydration

C C

OH

C C + H2O

excess conc. H2SO4 at 170oC

or Al2O3/SiO2 at 350oC

This is intermolecular dehydration.

What kinds of dehydration?

H

H

H

H

H

OH

O H

H

H

H

H

H

H

H

H

H + OH2

conc.H2SO4

140 Co

Another reaction occur!

I. Preparation - Dehydration

Saytzeff Rule:Hydrogen is preferably removed from the carbon with least no. of hydrogen since the alkene formed is more highly branched and is energetically more stable.

C C C C

H HOH

C C C C C C C C+

(major) (minor)

How do you which one is major product?

33

Dehydration Mechanism

C

H

C

O H

S

O

O

O HOH C

H

C

O H

H

HSO4 _

C

H

C

O H

H

C

H

CH2O:

C C H3O+ =>

I. Preparation

• 2. Dehydrohalogenation

Example:

alcoholic KOH

EtO- (ethoxide ion) in EtOH (ethanol)

H H

H

X

H

H

strong base

alcoholic reflux+ HX

KX + OH2

H

H

H

H

EtOH EtO + H+

weakconjugate acid

strongconjugate base

Press

35

Hofmann Product

• Bulky bases abstract the least hindered H+

• Least substituted alkene is major product.

CH3 C

H

H

C

CH3

Br

CH2

H

CH3CH2O

CH3CH2OH

_

C CCH3

CH3H

H3CC C

H

HH3C

CH3CH2

71% 29%

72%28%

C CH

HH3C

CH3CH2

C CCH3

CH3H

H3C_

CH3CH2OHCH3 C

H

H

C

CH3

Br

CH2

H

(CH3)3CO =>

I. Preparation

3.Dehalogenation

C C

X X

+ Zn dust C C + ZnX2

alcoholic reflux

(vicinal dihalide)

(c.f. gem-dihalide )

X

X

I. Preparation - dehalogenation (application)

H OH

H

H

H

H

HH

H OH

O

H

HHH[O]

X2

H OH

H

H

H

H

HH

X X

KMnO4 / H+

H OH

O

H

HHH

X X

Zn dustalcoholic reflux

I. Preparation

• 4. Hydrogenation

– This makes use of a catalyst which activity has been decreased by sulphur containing compound. E.g. Pd (palladium) in BaSO4

C C + H2 C C

H HPd/BaSO4

Reactions of Alkenes

Chapter 8 40

Reactivity of C=C

• Electrons in pi bond are loosely held.

• Electrophiles are attracted to the pi electrons.

• Carbocation intermediate forms.

• Nucleophile adds to the carbocation.

• Net result is addition to the double bond. =>

Markownikoff’s rule– The more electronegative atom (or group of

atoms) attached to carbon having least no. of H.In general, the greater the no. of alkyl grops present, or the larger is the alkyl group, the more stable is the carbonium ion.

– Stability of carbonium ion:– 3ry C+ > 2ry C+ > 1ry C+ > CH3

+

(It undergoes addition reaction.)

• Electrophilic Addition Reactions

1-With HX

C C + HX C C

H X

(Mechanism of Addition Reactions)

CH2CH3CH + H X

CH3CH2CH2X CH3CHCH3

X

CH3CH2CH2

+X

(I)

CH3CHCH3

+X

(II)

• Electrophilic Addition Reactions (cont’d)

2-With conc. sulphuric acidH OSO3H+ -

C C + H .H SO 4 C C

H O SO 3 H

C C

H O H

H 2Obo il

alkyl hydrogen sulphate

Hence, this is used in preparation of alcohol.

3-Addition of halogen to alkene (Halogenation)

C C

Br

Br

C C

Br

C C

Br

+

+

Br

C C

Br

Br

Proof for the formation of brominium ion

+ Br2

CCl4 / C2H5OH Br

Br

Trans-addition(anti-addition)

The bromide ions attack carbon of the ring from the side opposite to that of the “positive” brominium ion.

• Addition Reactions (NOT electrophilic)

4-Hydrogenation

C C + H2 C C

H H

a. It is used analytically to find the number of mole of double bond or triple bond by the number of mole of hydrogen absorbed per mole of molecule.

b. It is used in converting vegetable oil.

5-Ozonolysis

C C

R

R' R''

R'''

+ O3

ice-cold

chloroformC

O

C

O O

R

R' R''

R'''ozonide

H2O /H

Zn dustC O

R'

R

CO

R''

R'''

6-Oxidationa.at room temperature (Hydroxylation)

(addition)

C C + [O] + H2O C C

OH OHfrom MnO4

-/OH-

b.at vigorous condition (bond breaking)

C C

H H

R R' + [O] C O CO

H H

R R'+

C O

OH

R

MnO4- / H+

Further oxidation

In acidic condition, the products will be oxidised to acid or ketone.

7-Addition Polymerization– This is a process by which simple molecules are joined

up to form large molecule with same empirical formula.

– condition : high temperature and pressure– with Ziegler’s catalyst

C Cn C C( )n

C C

R'

Rn C C

R

R'

( )n

• Free radical addition mechanismInitiation:

Propagation:

RO OR 2RO

RO C C

R'C C

R'

RO

C C

R'

C C

R'

RO C C C C

R'R'

RO

Termination:

C C C C

R'R'

RO CCCC

R' R'

RO

RO OR RO OR

C C C C

R'R'

RO OR C C C C

R'R'

RO OR

C C OR

R'

RO

n n

n m

n+m+2

e.g. polythenemoleclar mass: 50000g

melting point: 126oC – 135oC (having diff. Isomer)

Properties:Light, inert (strong sigma bonds) and water-resistance (do

not form H-bonding), tough and capable of moulding. It is a thermo-plastic since chains of hydrocarbons causes the chain to move apart and to come closer again on cooling.

Uses:1. Make water-proof sheeting

2. Electrical cable insulator