ORGANIC CHEMISTRY 1 CHM 207

CHAPTER 3: ALKENESNOR AKMALAZURA JANI

ALKENES Also called olefins Contain at least one carbon-carbon double bond (C=C) General formula, CnH2n (n=2,3,) Classified as unsaturated hydrocarbons (compound with double or triple carbon-carbon bonds that enable them to add hydrogen atoms. sp2-hybridized For example: C2H4 - ethylene CH2 CH2

Naming Alkenes

IUPAC RULESRULE 1. Select the longest continuous carbon chain that contains a double bond.

This chain contains 6 carbon atoms

RULE 2. Name this compound as you would an alkane, but change ane to ene for an alkene.

This chain contains 8 carbon atoms

Name the longest This isthe parent compound chain. continuous octene. Select it as the parent compound.

RULE 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon to indicate the position of the double bond. Place this number in front of the alkene name.

IUPAC RULESThis end of the chain is closest to the double bond. Begin numbering here.

IUPAC RULESThe name of the parent compound is 1-octene. 4 3 2 1

5 6

78

RULE 4. Branched chains and other groups are treated as in naming alkanes. Name the substituent group, and designate its position on the parent chain with a number.

IUPAC RULESThe is an ethyl is attached to carbon 4. Thisethyl groupgroup.4 3 2 1

5 6

4-ethyl-1-octene

78

A compound with more than one double bond. - Two double bond: diene - Three double bond: triene - Four double bond: tetraene * Numbers are used to specify the locations of the double bonds.CH2 C H1 2

C CH2 H

3

4

7

6

5

4

3

CH3 C C H H

C H

C H

2

C H

1

CH2

IUPAC names: 1,3-butadiene new IUPAC names: buta-1,3-diene1 8 7 6 5 2 3 4

1,3,5-heptatriene hepta-1,3,5-triene

IUPAC names: 1,3, 5, 7-cyclooctatetraene new IUPAC names: cycloocta-1,3,5,7-tetraene

ALKENES AS SUBSTITUENTS Alkenes names as substituents are called alkenyl groups. Can be named systematically as ethenyl, propenyl, etc. or by common names such as vinyl, ally, methylene and phenyl groups.

CH2methylene group (methylidene group)

-CH=CH2vinyl group (ethenyl group)

-CH2-CH=CH2allyl group (2-propenyl group)

CH=CH2 CH2 CHCHCH2CH CH23-methylenecyclohexene 3-vinyl-1,5-hexadiene 3-vinylhexa-1,5-diene

CYCLOALKENES Contains C=C in the ring

cyclopropene

cyclobutene

cyclopentene

cyclohexene

Nomenclature of cycloalkenes: - Similar to that alkenes - Carbons atoms in the double bond are designated C1 and C26 5 4 3 1 2

CH3

5 4 3 1 2

1-methylcyclohexene

1,5-dimethylcyclopentene

NOMENCLATURE OF cis-trans ISOMERSH3C H CH2CH3 H cis-2-pentene H3C H H C C CH2CH3 trans-2-pentene

C

C

cis two particular atoms (or groups of atoms) are adjacent to each other trans the two atoms (or groups of atoms) are across from each other

PHYSICAL PROPERTIES OF ALKENESBoiling points and densities:

- Most physical properties of alkenes are similar to those alkanes.- Example: the boiling points of 1-butene, cis-2-butene, trans-2-butene and n-butane are close to 0oC. - Densities of alkenes: around 0.6 or 0.7 g/cm3.

- Boiling points of alkenes increase smoothly with molecular weight.- Increased branching leads to greater volatility and lower boiling points.

Polarity: - relatively nonpolar. - insoluble in water but soluble in non-polar solvents such as hexane, gasoline, halogenated solvents and ethers. - slightly more polar than alkanes because: i) electrons in the pi bond is more polarizable (contributing to instantaneous dipole moments). ii) the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment).

Alkyl groups are electron donating toward double bond, helping to stabilize it. This donating slightly polarizes the vinylic bond, with small partial positive charge on the alkyl group and a small negative charge on the double bond carbon atom. For example, propene has a small dipole moment of 0.35 D.Vinylic bonds

H3C C H C

H H

H3C C H C

CH3 H

H3C C H C

H CH3

propene, = 0.35 D

Vector sum = Vector sum = 0 propene, = 0.33 D propene, = 0 cis-2-butene, bp 4oC trans-2-butene, bp 1oC

In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans-disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans-disubstituted, the dipole moment is zero.

H C H

H3C C H C

CH3 H

H3C C H C

H CH3

C

Vector sum = Vector sum = 0 propene, = 0.33 D propene, = 0 cis-2-butene, bp 4oC trans-2-butene, bp 1oC

Cis- and trans-2-butene have similar van der Waals attractions, but only cis isomer has dipole-dipole attractions.

Because of its increased intermolecular attractions, cis-2-butene must be heated to a slightly higher temperature (4oC versus 1oC) before it begins to boil.

H C H

H3C C H C

CH3 H

H3C C H C

H CH3

C

Vector sum = Vector sum = 0 propene, = 0.33 D propene, = 0 cis-2-butene, bp 4oC trans-2-butene, bp 1oC

PREPARATION OF ALKENES Alkenes can be prepared in the following ways: i) Dehydration of alcohols R-CH2-CH2-OHconc. H2SO4

R-CH=CH2 + H2O

ii) Dehydrohalogenation of haloalkanesR-CH2-CH2-XNaOH/ethanol reflux

R-CH=CH2 + HX

NaOH can be replaced by KOH

Saytzeff rule: - A reaction that produces an alkene would favour the formation of an alkene that has the greatest number of substituents attached to the C=C group.Dehydration of alcoholsH+ H+

CH3CH2-CH-CH3 OH2-butanol

CH3CH2-CH=CH2 + H2O1-butene

CH3CH=CH-CH3 + H2O2-butene major product

Dehydrohalogenation of haloalkanesCH3CH-CH-CH2 H Br H2-bromobutane

KOH

alcohol reflux

CH3CH=CH-CH32-butene (major product)

CH3CH2CH=CH21-butene

REACTIVITY OF ALKENESMore reactive than alkanes because:

i)

A carbon-carbon double bond consists of a and a bond. It is easy to break the bond while the bond remains intact. The electrons in the double bond act as a source of electrons (Lewis base). Alkenes are reactive towards electrophiles which are attracted to the negative charge of the electrons. bond will broken, each carbon atom becomes an active site which can form a new covalent bond with another atom. One bond is converted into 2 bonds.

ii)

iii)

REACTIONS OF ALKENESCatalytic hydrogenation: - hydrogenation: addition of hydrogen to a double bond and triple bond to yield saturated product. - alkenes will combine with hydrogen in the present to catalyst to form alkanes.

C C

HH

Pt or Pd 25-90 Co

C C HH

- Plantinum (Pt) and palladium (Pd) Catalysts - Pt and Pd: temperature 25-90oC - Nickel can also used as a catalyst, but a higher temperature of 140oC 200oC is needed.

EXAMPLES:H2C CH2 ethylene H2 Pt low pressure H3C CH3 ethane

CH3CH2CH2CH2CH CH2 hexene

H2

Pt low pressure

CH3CH2CH2CH2CH2CH3 hexane

Addition of halogens: i) In inert solvent: - alkenes react with halogens at room temperature and in dark. - the halogens is usually dissolved in an inert solvent such as dichloromethane (CH2Cl2) and tetrachloromethane (CCl4). - Iodine will not react with alkenes because it is less reactive than chlorine and bromine. - Fluorine is very reactive. The reaction will produced explosion.inert solvent

C C

X X

C C X X

X X = halogen such as Br2 or Cl2Inert solvent = CCl4 or CH2Cl2

EXAMPLES:H H H C C Hetheneinert solvent (CCl4)

Br Br

H H H C C H Br Br1,2-dibromoethane

* the red-brown colour of the bromine solution will fade and the solution becomes colourless.

Br2 cyclohexene CH3CH=CH2 propene

CCl4

Br Br 1,2-dibromocyclohexane

Cl2

CCl4

Cl Cl CH3CH CH2 1,2-dichloropropane

Addition of halogens: ii) In water / aqueous medium: - chlorine dissolves in water to form HCl and chloric (l) acid (HOCl). Cl2 (aq) + H2O(l) HCl(aq) + HOCl (aq)- same as bromine Br2 (aq) + H2O(l)

HBr(aq) + HOBr(aq)

* Reaction of alkenes with halogens in water (eg. chlorine water and bromine water) produced halohydrins (an alcohol with a halogen on the adjacent carbon atom).

EXAMPLES:CH3CH=CH2 + propene Br2H2O

CH3 CH CH2 CH3 CH CH2 Br Br OH Br 1-bromo-2-propanol 1,2-dibromopropane (major product) (minor product)

* Br atom attached to the carbon atom of the double bond which has the greater number of hydrogen atoms. CH3CH2CH=CH2 1-buteneCl2, H2O

CH3 CH2 CH CH2 OH Cl 1-chloro-2-butanol

Addition of hydrogen halides: - Addition reaction with electrophilic reagents. - Alkenes react with hydrogen halides (in gaseous state or in aqueous solution) to form addition products. - The hydrogen and halogen atoms add across the double bond to form haloalkanes (alkyl halides). - General equation:

C Calkene

HX

H X C Chaloalkane

- Reactivity of hydrogen halides : HF < HCl < HBr < HI

* Reaction with HCl needs a catalyst such as AlCl3

H2C CH2EXAMPLES:

HCl

AlCl3

CH3CH2Cl

H-I cyclopentene

Iiodocyclopentane

CH3CH=CHCH3 + H-Br2-butene

Br CH3CH2CHCH32-bromobutane

MARKOVNIKOVS RULE There are 2 possible products when hydrogen halides react with an unsymmetrical alkene. It is because hydrogen halide molecule can add to the C=C bond in two different ways.H H CH3 C C H H-I H H CH3 C C H H I1-iodopropane

H H CH3 C C H H-I

H H CH3 C C H I H2-iodopropane (major product)

Markovnikovs rules:- the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms.

Mechanism of electrophilic addition reactions: - C=C : electron rich part of the alkene molecule - Electrophiles: electron-seeking Step 1: Formation of carbocation. Attack of the pi bond on the electrophile to form carbocation.

C C

+

-

E

Y

C C Ecarbocation

Y-

Step 2: Rapid reaction with a negative ion. The negative ion (Y-) acts as nucleophile and attacks the positively charged carbon atom to give product of the addition reaction.

C C E

Y-

C C E Y

ADDITION OF HYDROGEN HALIDES TO UNSYMMETRICAL ALKENES AND MARKOVNIKOVS RULECH3CHCH23

CH3CH=CH2 Propene

2

1

HCl

H Cl1-chloropropane

CH3CHCH2 Cl H2-chloropropane (major product) according to Markovnikov's rules

MECHANISM: Step 1: Formation of carbocation H H CH3 C C H H Cl H H H H H H ClH C C C H or H C C C H H H H Hless stable carbocation (1 carbocation)o

more stable carbocation (2o carbocation)

- 2o carbocation is more stable than 1o carbocation. - 2o carbocation tends to persist longer, making it more likely to combine with Cl- ion to form 2-chloromethane (basis of Markovnikov's rule).

Step 2: Rapid reaction with a negative ion H H H H C C C H H H ClH H H H C C C H H Cl H2-chloromethane (major product)

Addition reaction with concentrated sulfuric acid: hydration of alkenes- the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold (0-15oC). - involves the addition of H atom and HSO4 group across the carbon-carbon double bond. - follows Markovnikovs rule.

H H H C C H

H H H OSO3H(H2SO4)

H C C H H OSO3Hethyl hydrogensulphate (CH3CH2HSO4)

When the reaction mixture is added to water and warmed, ethyl hydrogensulphate is readily hydrolysed to ethanol CH3CH2OSO3H + H-OH(H2O)

CH3CH2OH + H2SO4

*ethene reacts with concentrated H2SO4 to form ethanol* or *alkene reacts with concentrated H2SO4 to form alcohol*

Addition reaction with acidified water (H3O+): hydration of alkenes Hydration: The addition of H atoms and OH groups from water molecules to a multiple bond. Reverse of the dehydration reaction. Direct hydration of ethene: - passing a mixture of ethene and steam over phosphoric (v) acid (H3PO4) absorbed on silica pellets at 300oC and a pressure of 60 atmospheres. - H3PO4 is a catalyst.

CH2=CH2 (g)ethene

H2O(g)

H3PO4

300 C, 60 atm

o

CH3CH2OH (g)ethanol

C Calkene

H2O

H+

H OH C Calcohol

Markovnikovs rule is apply to the addition of a water molecule across the double bond of an unsymmetrical alkene. For examples:

CH3 CH3 C CH2 H OHH 25oC+

CH3 CH3 C CH2 OH Htert-butyl alcohol

2-methylpropene

CH3CH=CH2 + H2OpropeneH+ = catalyst

H+

CH3CHCH3 OH2-propanol

MECHANISM OF ACID CATALYSED HYDRATION OF ALKENES Step 1: Protonation to form carbocation H H CH3 C C H H H H H+ H C C C H H Hmore stable carbocation (2o carbocation)

Step 2: Addition of H2O to form a protonated alcohol H H H H C C C H H H O H H CH3CHCH3 O H H

Step 3: Loss of a proton (deprotonated) to form alcohol CH3CHCH3 O H H CH3CHCH3 OH H+H+ = catalyst

ANTI-MARKOVNIKOVS RULE: FREE RADICAL ADDITION OF HYDROGEN BROMIDE When HBr is added to an alkene in the absence of peroxides it obey Markovnikovs rule. When HBr (not HCl or HI) reacts with unsymmetrical alkene in the presence of peroxides (compounds containing the OO group) or oxygen, HBr adds in the opposite direction to that predicted by Markovnikovs rule. The product between propene and HBr under these conditions is 1-bromopropane and not 2-bromopropane.

CH3CH=CH2

HBr

peroxide

CH3CH2CH2Br1-bromopropane (major product) anti-Markovnikov's orientation

Anti-Markovnikovs addition: - peroxide-catalysed addition of HBr occurs through a free radical addition rather than a polar electrophilic addition.

- also observed for the reaction between HBr and many different alkenes. - not observed with HF, HCl or HI.

Combustion of alkenes: The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water. For example, ethene burns as follows : C2H4 + 3O2 2CO2 + 2H2O

OXIDATION Oxidation: reactions that form carbonoxygen bonds. Oxidation reaction of alkenes: i) epoxidation ii)hydroxylation iii)Ozonolysis

EPOXIDATION OF ALKENES Epoxide / oxirane: a three-membered cyclic ether.O C Calkene

R C

O O H

O C Cepoxide (oxirane)

O R C OHacid

peroxyacid

Examples of epoxidizing reagent:O CH3 C O O H peroxyacetic acid O C O OH peroxybenzoic acid (PhCO3H) O Cl O H O

m-chloroperoxybenzoic acid (MCPBA)

Examples:MCPBA CH2CI2, 25 C cyclohexene 1,2-epoxycyclohexaneo

O

MCPBA CH2CI2, 25 C cycloheptene 1,2-epoxycycloheptaneo

O

HYDROXYLATION OF ALKENES Hydroxylation: - Converting an alkene to a glycol requires adding a hydroxyl group to each end of the double bond. Hydroxylation reagents: i) Osmium tetroxide (OsO4) ii)Potassium permanganate (KMnO4)

C C

OsO4

H2O2

C C OH OHglycol

(or KMnO4, -OH)

CH2 CH2ethene

KMnO4 (aq), OHcold

CH2 CH2 OH OH1,2-ethanediol

MnO2

CH3 CH CH2propene

KMnO4 (aq), OHcold

CH3 CH CH2 OH OH1,2-propanediol

MnO2

* Also known as Baeyers test

OZONOLYSIS OF ALKENES Ozonolysis:- The reaction of alkenes with ozone (O3) to form an ozonide, followed by hydrolysis of the ozonide to produce aldehydes and /or ketone. - Widely used to determine the position of the carbon-carbon double bond.

- Ozonolysis is milder and both ketone and aldehydes can be recovered without further oxidation.R C C R H R' O3 R O R'(CH3)2S

R C O Rketone

R' O C Haldehyde

C C R O O H or H2O, Zn/H+ozonide

EXAMPLES:i) O3 3-nonene ii) (CH3)2S

H O CH3O O O H H

O

CH3O H

OCH3 H

i) O3 ii) (CH3)2S

OCH3 O O H

REACTIONS OF ALKENES WITH HOT, ACIDIFIED KMnO4R C C R H R'KMnO4/H+

R R' R C C H OH OH

R R C O O C

R' OHacid

R R C O O C

R' Haldehyde

ketone

ketone

Example:KMnO4/H 4-methyl-4-octene+

C

O

HO O

C

2-pentanone

butanoic acid

POLYMERIZATION OF ALKENES Polymer: A large molecule composed of many smaller repeating units (the monomers) bonded together. Alkenes serves as monomers for some of the most common polymers such as polyethylene (polyethene), polypropylene, polystyrene, poly(vinyl chloride) and etc. Undergo addition polymerization /chain-growth polymer: - a polymer that results from the rapid addition of one molecule at a time to a growing polymer chain, usually with a reactive intermediate (cation, radical or anion) at the growing end of the chain.

repeating unit

H C C H

CI H

H C C Hvinyl chloride

CI H

H C C H

CI H

H C H

CI H C H C H

Cl H C H C H

Cl C H

n poly(vinyl chloride)

UNSATURATION TESTS FOR ALKENES1) Reactions of alkenes with KMnO4

- KMnO4 is a strong oxidising agent. - alkenes undergo oxidation reactions with KMnO4 solution under two conditions: a) Mild oxidation conditions using cold, dilute, alkaline KMnO4 (Baeyers test). b) Vigorous oxidation conditions using hot, acidified KMnO4.

a) Reaction of alkenes with cold, dilute, alkaline KMnO4 (Baeyers test) - the purple colour of KMnO4 solution disappears and a cloudy brown colour appears caused by the precipitation of manganese (IV) oxide, MnO2. - test for carbon-carbon double or triple bonds.

- a diol is formed (containing two hydroxyl groups on adjacent carbon atoms).

C C

KMnO4 (aq), OHcold

C

C

MnO2

OH OHa diol

b) Bromine- A solution of bromine in inert solvent (CH2CI2 or CCI4) and dilute bromine water are yellow in colour. - The solution is decolorised when added to alkenes or organic compounds containing C=C bonds.

C C

Br2

CH2CI2

C

C

Br Br Br2(aq) H2O

C C

C

C

C

C

OH Br

Br Br

DETERMINATION OF THE POSITION OF THE DOUBLE BONDa) Ozonolysis of alkenes: - For example, ozonolysis of an alkene produces methanal and propanone.H H C Omethanal

CH3 O C CH3

propanone

remove the oxygen atoms from the carbonyl compounds and joining the carbon atoms with a double bond.

H H C

CH3 C CH3 H

H C

CH3 C CH3

2-methylpropene

b) Reaction of alkenes with hot, acidified KMnO4 - by using hot, acidified KMnO4, the diol obtained is oxidised further. - cleavage of carbon-carbon bonds occurs and the final products are ketones, carboxylic acids or CO2.CH3 CH3 C CH22-methylpropene

KMnO4/H+

CH3 CH3 C Opropanone (ketone)

CO2

+

H2O

Example:An alkene with the molecular formula C6H12 is oxidised with hot KMnO4 solution. The carboxylic acids, butanoic acid (CH3CH2CH2COOH) and ethanoic acid (CH3COOH), are produced. Identify the structural formula of the alkene.i) cleavage of the double bond gives a mixture of carboxylic acids H H R C C R' KMnO4/H+

OH R C O

OH O C R'

ii) location of the double bond is done by taking away the oxygen atoms from the carboxylic acids and then joining the carbon atoms by the double bond. RCOOH and R'COOH CH3CH2CH2COOH butanoic acid and RCH=CHR' CH3COOH ethanoic acid CH3CH2CH2CH=CHCH3 2-hexene

USES OF ALKENES Ethylene and propylene are the largest-volume industrial organic chemicals. Used to synthesis a wide variety of useful compounds.H H C C H Hn polyethylene polymerize

O CH3 C Hacetaldehyde oxidize

Ooxidize

CH3 C OHacetic acid

O H2C CH2H+ H2O ethylene oxide

O2 Ag catalyst

H H C Cethylene

H H

Cl2

CH2 CH2 CI CIethylene dichloride NaOH

H2O catalyst

CH2 CH2 OH OHethylene glycol

CH3 CH2 OHethanol

H H C

CI C H

vinyl chloride

POLYETHENE (PE) The most popular plastic. Uses: i) Grocery bags ii)Shampoo bottles iii)Children's toy iv)Bullet proof vests v)Film wrapping vi)Kitchenware

POLYVINYL CHLORIDE (PVC)H H H C C CIvinyl chloride polymerize

H C H

CI H C C H H

CI H C C H n H

CI C H

USES OF PVC: Clothing - PVC fabric has a sheen to it and is waterproof. - coats, shoes, jackets, aprons and bags. As the insulation on electric wires. Producing pipes for various municipal and industrial applications. For examples, for drinking water distribution and wastewater mains. As a composite for the production of accessories or housings for portable electronics. uPVC or Rigid PVC is used in the building industry as a low-maintenance material. Ceiling tiles.

poly(vinyl chloride) PVC, "vinyl"

USES OF ETHANOL Motor fuel and fuel additive. As a fuel to power Direct-ethanol fuel cells (DEFC) in order to produce electricity. As fuel in bipropellant rocket vehicles. In alcoholic beverages. An important industrial ingredient and use as a base chemical for other organic compounds include ethyl halides, ethyl esters, diethyl ether, acetic acid, ethyl amines and to a lesser extent butadiene. Antiseptic use. An antidote. Ethanol is easily miscible in water and is a good solvent. Ethanol is less polar than water and is used in perfumes, paints and tinctures. Ethanol is also used in design and sketch art markers. Ethanol is also found in certain kinds of deodorants.