Alkenes

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AliphaticAliphaticAliphaticAliphatic

AlkenesAlkenesAlkenesAlkenes

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

CC CC

HH HH

HH HH

Alkenes

• Unsaturated hydrocarbon

• Contain carbon – carbon double bonds

• Alkenes

• General formula

•Aliphatic hydrocarbons

•Two simplest alkenes; ethene (ethylene) and propene (propylene) are major raw materials for the chemical industry.

Naming the alkenes (IUPAC Nomenclature)• Longest carbon must contain the double bond. •The C=C functional and position of substituent must be at lower number. •However, position of C=C is of higher priority (LOWER NUMBER) than substituent.

CH3-CH2-CH=CH-CH2-CH2-CH3

CH3-C=C-C-CH3

H H CH3

H Br-CH2-CH2-CH-CH-CH2-CH3

CH=CH2

CH33-heptene

1 2 3 4

1 2 3 4 5

4-methyl-2-pentene

12

345

5-bromo-3-sec-butyl-1-pentene

•For alkenes which show cis-trans isomerism, prefix cis or trans is written before the IUPAC name.

C C

H

H3C

H

CH3

C C

HH3C

H CH3

cis-2-butene trans-2-butene

CH3-CH=CH-CH=CH2

CH3-CH-CH2-CH=CH-CH2-CH=CH2

CH3

1,3-pentadiene

7-methyl-1,4-octadiene

8 1234567

12345

• 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

Let’s name the structural formulae

CH3CH2

C = C

H

CH2CH3H

trans-3-hexene

CH3CH2

C = C

CH2CH3

CH2CH2CH3H

cis-4- ethyl –3-heptene

Physical PropertiesSimilar to alkanes

Solubility

Low density, boiling point and melting point

Properties vary based on chain size. Alkenes have week Van der Waals forces between molecules; for example the lower carbon alkenes exist as gases at room conditions. Interesting physical properties

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

• Soluble in non polar solvent• Not soluble in water

Table below shows some physical properties of alkenes and cycloalkenes

Name Structural

Formula

Melting Point (°C)

Boiling Point

(°C)

Ethene CH2=CH2 -169 -104

Propene CH2=CH-CH3 -185 -48

1-butene CH2=CH-CH2-CH3 -185 -6.3

Cyclopentene -135 44

Stability of Alkenes• There are 3 factors that influence the

stability of alkenes:– Degree of substitution : more highly

alkylated alkenes are more stables, so tetra> tri > di > mono-substituted

– Stereochemistry : trans > cis due to reduced steric interactions when R groups are on opposite sides of the double bond.

– Conjugated alkenes are more stable than isolated alkenes.

Synthesis of alkenes

XX YY

a) Dehydration of alcohols:X = H; Y = OH

b) Dehydrohalogenation of alkyl halides:X = H; Y = Br, Cl

c) Dehalogenation of dihalides

d) Hydrogenation @ Reduction of alky

CC CCCC CC ++ XX YY

Synthesis of alkenes: elimination reactions

HH22SOSO44

160°C160°C++ H2O

A) Dehydration of alcohol

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

HH OHOH CC CCCC CC ++ HH22OO

Example

ethanol

H OHC C

HHHH

HH

ethene (ethylene)

C C

H

H

H

H

More example on dehydration of alcohols:-

C

H

H

H

C

H

OH

H CH

H

C

H

H H2Oconc. H2SO4

1700Cethene

i)

CH

H

H

C C

OH

H H

H

Hconc. H2SO4

1700C

CH3-CH=CH2 H2O

ii)

OH

conc. H2SO4

1700CH2O

iii)

iv) CH3-CH-CH2-CH3

OH

conc. H2SO4

1700CCH2=CH-CH2-CH3 CH3-CH=CH-CH3

(minor) (major)

In dehydration reaction, i) excess H2SO4 is heated at 170ºC or

ii) excess H3PO4 heated at 225ºC or

iii) alcohol vapor is passed over alumina at 350ºC.

Saytzeff’s Rule; in elimination reactions, the major reaction product is the alkene with the more highly substituted (more stable).

B) 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)

Ethyl chlorides

H ClC C

HHHH

HH

ethene (ethylene)

C C

H

H

H

H

CH

H

H

C

H

Br

C

H

H

HKCH3CH2O

CH3CH2OHCH3CH=CH2 HBr

Cl

HCl

CH3-CH2-CH-CH3

Br

KOH, CH3CH2OH CH3-CH2-CH=CH2 CH3-CH=CH-CH3 HBr(minor) (major)

i)

ii)

iii)

More examples on dehydrohalogenation of alkyl halides

C) Dehalogenation of dihalides

CC CC

Br Br

2 NaCl2 NaCl

acetoneacetoneCC CC ClCl22++

2 NaBr2 NaBr

++

@@

ZnZn

CHCH33COOHCOOH ZnBrZnBr22

D) Hydrogenation @ Reduction of alkynes

• Depending on the condition & catalyst that will be used.

Catalyst Product

H2 / Pt CH3CΞCCH3 → CH3CH=CHCH3

H2 / Ni2B @ P - 2 CH3CH2CΞCCH2CH3 → Cis - alkene

H2, Pd / CaCO3 & Lindlar’s catalyst

Li @ Na in NH3 CH3(CH2)2CΞC(CH2)2CH3 → Trans - alkene

NH2C2H5 / low temp.

CC CC

HH

CH3CH2CH3CH2

CC CC

H

H

(CH2)2CH3

CH3(CH2)2

Reaction of alkenes • Addition of symmetric reagents

a) Hydrogenation (H2)b) Halogenation (X2)

• Addition of unsymmetric reagentsc) Hydration (H2O)d) Hydrohalogenation (HX)

• Oxidation reactionsa) Ozonolysisb) Hydroxylation with KMnO4 (room temp) c) Oxidation cleavage of alkenes with acidic

KMnO4

• 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)

• Alkenes are more reactive than alkane due to the presence of π bond. The bond has high electron density and susceptible to be attacked by electrophiles ( electron deficient species and low electron density).

• Alkenes undergo ELECTROPHILIC ADDITION reactions which means the C=C (C double bonds) are broken to form C-C bond (single bonds).

a) 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

Examples on Hydrogenation•Can be carried out by using hydrogen gas in the presence of catalysts such as Ni/ Pd/ Pt.

CH2=CH2 Ni / H2 CH3-CH3

CH3-C C-CH2CH3

CH3CH3

Ni / H2 CH3-C CH CH2CH3

CH3CH3

CH3

2H2Ni

CH3

(2 mole C=C requires 2 mole of H2 gas)

b) 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.

Examples on Halogenation•Using Cl2 or Br2 in inert solvents such as tetrachloromethane, CCl4.

CH3-CH2-CH=CH2 CH3-CH2-CH-CH2Br

Cl2 / CClH2C CH2

Cl Cl

Br2 / CCl4

Br

CH2 CH3

2Br2 / CCl4Br

Br

Br

Br

CH2CH3

CH2=CH2

•This reaction can be used to detect the presence of C=C (alkene). The observation is the reddish brown colour of bromine solution decolourises to colourless.•Reaction mechanism (Electrophilic Addition)

C C

H

H

H

H C C

H

H

H

H

Br

Br

Br Br

slow

C C

H

H

H

H

Br

Brfast

C

H

H C

Br

H

H

Br

•The Br2 molecule polarised by C=C undergo heterolytic fission to form electrophile, (Brδ+) and nucleophile.

•The bromide ion, Br- (nucleophile) attacks the charge on C as it is, electron deficient.

c) Hydration: Addition of H2O

• The addition of water to an alkene

• produces an alcohol

• One carbon get an H, the other an OH

• Reagents used are concentrated sulphuric acid followed by addition of water and high temperature or dilute warm aqueous acids such as H2SO4.

d) 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

• HX in the gaseous state or dissolved in organic solvent. (i.e: CCl4)

Examples on Hydrohalogenation•Using HX (X=Cl/ Br/ I) to attack the (double bond (C=C) whereby the HX molecule is the electrophile.

CH3-CH2-CH-CH3

HCl CH3-CH2Cl(one product)

CH3-CH=CH-CH3 HBr CH3-CH-CH2-CH3

Br(one product)

CH3-CH2-CH=CH2 HCl

Cl

CH3-CH2-CH2-CH2-Cl

(major)

(minor)

CH2=CH2

CH3-CH2-CH=CH2

HBr

Peroxideor

ROOR

CH3-CH2-CH2-CH2Br

(major)

However, with HBr peroxide/ ROOR (not HCl or HI), the major product is ANTI-MARKOVNIKOV. The H from HBr will be bonded to carbon double bond is bonded directly to less hydrogens atoms.

CH3HBr

Peroxideor

ROOR

CH3

Br(major)

a) 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

Oxidation reactions

Examples on Ozonolysis

C C

H

H

H

H

O3 , Zn

H2O / H+C

H H

O

C

H

H

O

C C

H CH3

CH3H3C O3 , Zn

H2O / H+C HH3C

O

C CH3H3C

O

H3CO3 , Zn

H2O / H+ H2C

C-CH2-C

CCH3

HO

H

H

O

b) Hydroxylation With Potassium Manganate (VII) (Baeyer test)

C C

H

H

H

H

KMnO4 / H+

cold C C

H

H

OH

H

H

OH

CH3

CH3 KMnO4 / H+

cold

CH3

CH3

OH

OH

Reaction with potassium manganate (VII) solution either acidic or alkaline at room temperature will result addition of 2-OH groups at the carbon double bonds.

This test can be used as a chemical test to detect the presence of C=C (alkene) functional group. The purple color of potassium manganate (VII) solution will turn colourless.

c) Reaction with hot, conc acidified KMnO4

H2C C CH3

H

conc. KMnO4/ H+

CH H

O

+ CH

O

CH3

HCOOH

O

C CH3HO

O

CO2 & H2O

(methanal) aldehyde

carboxylic acid

If the product is methanal, it will oxidised to HCOOH and then form carbon dioxide and water. If the product is other aldehyde containing more than one carbon atom, it will oxidised to carboxylic acid. However, if ketones is formed, as ketone DOES NOT undergo oxidation, it will remain as ketone.

Base on the reaction below, draw the structure of the organic products formed.

• An alkene X, containing 8 carbon atoms reacts with hot acidified potassium manganate (VII) solution to form propanoic acid and pentanoic acid. Draw the two forms of which exist as cis-trans isomers.

Uses of alkenes• The most important use of alkene are:

– Manufacture of plastics

Ethene is used to produce plastics such as polyethene, polychloroethene and polyester (textile).

– Manufacture of ethane-1,2-diol

Ethane-1,2-diol is used as antifreeze in car radiators and for making detergents.

– Manufacture of ethanol

Ethanol is used as a solvent for vanishes, cosmetic and toilet preparations and also in manufacture of ethanal.