Alkenes Nomenclature of Alkenes Naming of Cycloalkenes

Geometric Isomers Properties of Alkenes Synthesis of Alkenes

Principal Reactions of Alkenes

Unsaturated Hydrocarbon - Alkenes

Chapter 2.2

Introduction

Classification of Hydrocarbon

Hydrocarbon

Aliphatic Aromatic

Alkanes Alkenes Alkynes

CycloalkanesCycloalkenes

Saturated Unsaturated

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AliphaticAliphaticAliphaticAliphatic

AlkenesAlkenesAlkenesAlkenes

• Alkenes are hydrocarbons that contain a carbon-carbon double bond.

CC CC

HH HH

HH HH

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AliphaticAliphaticAliphaticAliphatic

CycloAlkenesCycloAlkenesCycloAlkenesCycloAlkenes

• Alkenes whose carbon atoms are joined in rings.

Alkenes

• Unsaturated hydrocarbon

• Contain carbon – carbon double bonds

• Alkenes

• General formula

•Aliphatic hydrocarbons

CYCLOALKENES

•Alkenes whose carbon atoms are joined in rings

•General formula Cn H2n-2 where n =3,4,5…..

Eg:

Cyclopentene C5H8

Cyclobutene C4H6

CH3 CH3

1,3- dimethylcyclopentene

Naming Alkenes

We must account for double bond:

• Longest chain must contain the double bond

• Number of carbons so that the double bond has the lowest number

• Indicate the position of the double bond

• Change ending to - ene

• use the same rules for the side chains and halides

• The ending of the alkenes with more than one double bond should be change from -ene to:

diene – if there are two double bonds triene – if there are three double bonds

CH2

CH3

CH2

1

CH2

4

1,3-butadiene

CH2

1

CH2

CH3

CH4

CH5

CH6

CH3

7

1,3,5-heptatriene

1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene.

2) Number through the double bond in thedirection that gives the lower number to the first-appearing substituent.

6-Ethyl-1-methylcyclohexene6-Ethyl-1-methylcyclohexeneCHCH33

CHCH22CHCH33

11

2233

44

55 66

Naming cycloalkenes

• Number substituted cycloalkenes in the way that gives the carbon atoms of the double bond the 1 and 2 positions.

• That also gives the substituent groups the lower numbers at the first point of difference.

5

1

4

2

3

CH32

3

1

4

6

5

CH3CH3

1-methylcyclopentene 3,5-dimethylcyclohexene

• Alkenes nomenclature must specify whether a given molecule is cis or trans if it is a geometric isomer

• Eg:

CH3

C = C

CH3

HH

cis-2-butene

CH3CH2

C = C

H

CH2CH3H

trans-3-hexene

CH3CH2

C = C

CH2CH3

CH2CH2CH3H

trans-4- ethyl –3-heptene

CH3CH2

C = C

CH2CH2CH3

CH3H

cis-4-methyl-3-heptene

Geometric Isomer

Trigonal planar

2-butene

However, Cis – trans stereoisomerism (geometry isomer) in alkenes is not possible when :

• When one of the doubly bonded carbons bears two identical substituents

Using the prefix E and Z

Although the prefixes cis and trans can be used to distinguish disastereomers when two alkyl groups are bonded to the C C , they cannot be used when there are three or four alkyl groups bonded to the C C.

A completely unambiguous system for specifying double bond stereochemistry has been adopted by the IUPAC based on an atomic number criterion for ranking substituents on the doubly bonded carbons.

• When atoms of higher atomic number are on the same side of double bond, we say that the double bond has the Z configuration.

• When atoms of higher atomic number are on opposite site of double bond, we say that the double bond has the E configuration.

C C

H

Br

F

CIHigherHigher

Lower Lower

Z configuration

C C

H

F

Br

CI Lower

Lower

Higher

Higher

E configuration

* Refer to page 189 in our text book

Physical Properties

Physical Properties

Similar to alkanes

Solubility

Low density, boiling point and melting point

Properties vary based on chain size

Interesting physical properties

•Alkenes with several double bonds will have a color associated with them

• Soluble in non polar solvent• Not soluble in water

Colored Alkenes

Physical Properties of Alkenes

Synthesis of alkenes

XX YY

•dehydrogenation of alkanes:X = Y = H

•dehydration of alcohols:X = H; Y = OH

•dehydrohalogenation of alkyl halides:X = H; Y = Br, Cl

CC CCCC CC ++ XX YY

Synthesis of alkenes: elimination reactions

• Elimination of molecule H2

• limited to industrial synthesis of ethylene, propene

750°C+ H2

Synthesis of alkenes

Dehydrogenation of alkanes

H H C CC C + H2

Example

ethene (ethylene)ethane

H HC C

HHHH

HH

C C

H

H

H

H

HH22SOSO44

160°C160°C++ H2O

Dehydration of alcohol

Loss of H and OH from adjacent carbons. Acid catalyst is necessary.

HH OHOH CC CCCC CC ++ HH22OO

Example

Synthesis of alkenes

ethanol

H OHC C

HHHH

HH

ethene (ethylene)

C C

H

H

H

H

10 % 90 %

H2SO4

80°C++

• A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.

Regioselectivity in alcohol dehydration

2-methyl-2-butanol 2-methyl-1-butene 2-methyl-2-butene

Synthesis of alkenes

C

C

C C

OH

CH

H

H

H

H

H

H

H

H

H

H C

C

C CCH

HH

H

H

H

H

H

H

H C

C

C CCH

H

H

H

H

HH

H

H

H

• When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the carbon having thefewest hydrogens.

RR OHOH

CHCH33

CC CC

HH

RR CHCH22RR

Zaitsev Rule

2H on this 2H on this carbon carbon

RR

RR

CHCH22RR

CHCH33

CC CC

only 1H on this only 1H on this carbon carbon

Synthesis of alkenes

Dehydrohalogenation of alkyl halides Loss of H and halogen (X) from an alkyl halideLoss of H and halogen (X) from an alkyl halide In the presence of strong base in solvent likewise In the presence of strong base in solvent likewise NaOCHNaOCH33 in methanol, or KOH in ethanol in methanol, or KOH in ethanol

NaOCHNaOCH22CHCH33

ethanol, 55°Cethanol, 55°C

HH XX CC CCCC CC ++ HXHX

Example

++ HClHClHydrogenchloride

(Sodium ethoxide)

Synthesis of alkenes

Ethyl chlorides

H ClC C

HHHH

HH

ethene (ethylene)

C C

H

H

H

H

Reactions of alkenes

Reaction of Alkenes

• Primarily reactions involve the double bond

• The key reaction of double bond is addition reaction (Breaking the bond and adding something to each carbon)

+ A - B

A B

• The major alkene reactions include additions of hydrogen (H2),halogen ( CI2 or Br2), water (HOH) or hydrogen halides (HBr or HCI)

Reaction of Alkenes

Hydrogenation – Addition of H2

• Addition of a molecule of H2

• Results in the formation of an alkane

• Usually requires heat, pressure and a catalyst like Pt, Pd or Ni

Reaction of Alkenes

Hydration: Addition of H2O

• The addition of water to an alkene

• produces an alcohol

• One carbon get an H, the other an OH

The reaction requires a small amount of acid to be present to work

Reaction of Alkenes

Reaction of Alkenes

Reaction of Alkenes

Reaction of Alkenes

Hydrohalogenation

• Addition of HX to an alkene

• HX – HF, HCI, HBr, HI

It follows Markonikov’s rule where the H ends up on the carbon with the most hydrogen to start with

Reaction of Alkenes

Reactions of alkenes

Halogenation: Addition of X2

● The addition of halogen to an alkene

● produces a haloalkane or alkyl halide

C C X2R R RCCR

XX

Simple laboratory test for unsaturation.

Ozonolysis of alkenes •1st step - reaction of alkene with ozone to form

ozonide.

•2nd step - hydrolysis of ozonide to form aldehyde

and ketone.

+ O+ O33

CC CCOO

OO OO

CC CC

CC OO CCOO++HH22O, ZnO, Zn

R R’

R”RH

H

R’

R”

ozonide

aldehyde ketone