• Unsaturated chemical compound containing at least one carbon-carbon double bond, where rotation about the C=C is very difficult.

• To show the presence of the double bond, the –ane suffix from the alkane name is changed to –ene.

• Also called olefins( fat dissolving)• sp2 atomic orbitals• Trigonal planar, 120o degree

Geometric Isomerism

• Cis-trans isomerism-isomers that have same order

of atom attachment but a different arrangement of their atoms in space.

General Formula

CnH2n

• where n is the number of carbon atoms in the molecule

Physical Properties• Physical state -The first lower

member like ethene, propene and butene are colorless gases.

• Density - lighter than water. • Solubility - insoluble in water and

soluble in nonpolar organic solvents.• more reactive than alkanes due to

their double carbon-carbon bond.

• Boiling point -The boiling points of alkenes gradually increase with an increase in the molecular mass.

• The cis isomer ( example cis-2-butene, b.p.= 3.7°C) is higher in bpt than its trans isomers (example, trans-2-butene, b.p.= 1°C)

• Melting point The melting points of alkenes increase with an increase in the molecular mass.

Natural Sources

• Isolated from petroleum.• Plant material like plant

hormone, like Ethylene – a natural ripening agent and Terpenes – found in essential oil.

Some Common Alkene Polymers and their Uses

•Ethylene H2C=CH2 Polyethene, Polythene Packaging, cable insulation, films and sheets

•Tetrafluoroethene F2C=CF2 Polytetrafluoroethene, PTFE, Teflon Coatings, gaskets.

•Chloroethene (vinyl chloride) H2C=CHCl Polyvinyl chloride, PVC, Tedlar Insulation, films, pipes

•Styrene H2C=CHC6H5 Polystyrene, Styron Foam for packaging etc.

•Vinyl acetate H2C=CHOCOCH3 poly(vinyl acetate), PVA

•Paints, adhesives.

Preparations of Alkenes

1. Dehydrohalogenation of Alkyl halides2. Dehydration of Alcohol3. Dehalogenation of Vicinal Halides4. Reduction of Alkynes

Preparations of alkenes1. Dehydrohalogenation of alkyl halides

C

C

XH

+ KOHalcohol C C

+ KX + H2O

Ease of dehydrohalogenation of alkyl halides

3° > 2° > 1°

Example:

CH3CH2CH2CH2Cl KOHCH3 CH2 HC CH2

n-butyl chloride 1-butene

KOH is an OH- donor use to abstract H

+

CH3CH2CHClCH3KOH

CH3 HC CHCH3 + CH3CH2HC CH2sec - butyl chloride 2 - butene (80%) 1 - butene (20%)

Dehydrohalogenation of Alkyl Halides

• Is an Elimination reaction. The term "elimination" describes the fact that a small molecule is lost during the process.

Different mechanisms are possible:– Loss of the LG to form a carbocation, removal of

H+ and formation of C=C bond – Simultaneous H+ removal, C=C bond formation

and loss of the LG – Removal of H+ to form a carbanion, loss of the LG

and formation of C=C bond.

2. Dehydration of alcohol

C C

H OH

acidC C

+ H2O

alkene

Ease of dehydration of alcohols

3° > 2° > 1°

ex.

CH C

H

H

H

H

OH

H2SO4 CH

H

C H

H

+ H2Oethylene

ethyl alcohol

acid serves as H+ donor

CH3CH2CH2CH2OHH2SO4 CH3CH2HC CH2 CH3 HC CHCH3

n-butyl alcohol 1-butene 2-butene (chief product)

CH3CH2 HC CH3

OH

H2SO4

Al2O2 in

heated tube

CH3 HC CHCH3 + CH3CH2HC CH2

sec-butyl alcohol 2-butene 1-butene (chief product)

Dehydration of Alcohols

• It is the elimination of water molecule from alcohol to convert into alkene.

• Lost of H and OH from adjacent carbons

• An acid catalyst alkene.

Mechanism of Alcohol Dehydration

Step 1 : Alcohol unites with a hydrogen ion to form the protonated alcohol

Step 2 : Alcohol associates into water and carbonium ion.

Step 3 : The carbonium ion then loses a hydrogen ion to form alkene.

3. Dehalogenation of vicinal dihalides (same side)

C

C

X X

+ Zn C

C

+ ZnX2

dihalides

Example:

CH3 HC CH CH3

Br Br

ZnCH3 HC CHCH3 + ZnX2

2,3- Dibromobutane 2- butene

4. Reduction of alkynes

R C C R

LindlarCatalyst

Pd, NiBr

Na or Li

C C

HH

R R

C C

R

H

H

R

syn (cis)

anti (trans) NH3

Reduction of Alkynes• Reducing Alkynes to form trans or cis

Alkenes.Using Na/ NH3

Step 1:Sodium transfer an electron to the alkyne giving a radical anion.

Step 2: The radical anion removes a proton from the ammonia in an acid/base reaction

Step 3: A second atom of sodium transfers

another electron to the alkyne giving an anion.

Step 4 :the anion removes proton from the

ammonia in an acid/base reaction.

Reactions of Alkene

• Halogenation• Hydration• Hydrogenation• Addition of hydrogen Halides• Addition of sulfuric acid• Addition of Carbenes• Addition of Free Radical

• Allylic Hydrogenation• Dimerazation• Alkylation• Polymerization• Hydroxylation• Halohydrins formation• Ozonolysis

• Hydroboration-oxidation

• Epoxidation

C

C

+ X2 C

C

X X

X2 = CL2, Br2

I2 - unreactive with alkane

Reactions of Alkenes

1. Addition of Halogens (X2)

Example.

HCH3C CH2

Br2CH3CHBrCH2Br

propene(propylene) 1,2 dibromopropane (propylene bromide)

CCl4

Halogenations – Addition of Halogens

• When an alkene is treated at room temperature with a solution of bromine or chlorine in carbon tetrachloride or some other inert solvent, the halogens adds rapidly to the double bond of the alkene to give the corresponding vicinal dihalide ( two halogens attached adjacent carbons

HCH3C CH2 CH3CH2CH3

H2, Ni

propene(propylene) propane

C C

+ H2

Pt, Pd or NiC

C

H H

2. Addition of Hydrogen (catalytic hydrogenation)

Ex.

Hydrogenation of Alkenes

• The relationship between reactants and products in addition reactions can be illustrated by the hydrogenation of alkenes yield alkanes.Hydrogenation is the addition of H2 to a multiple bond.

C

C

+ HX C

C

H X

HX = HCL, HBr, HI

3. Addition of hydrogen halides

HCH3C CH2HBr

no peroxides

peroxides

H3C C

Br

H

CH3

2 - bromopropane(Isopropyl bromide)

Markovnikov addition.

CH3CH2CH2Br1 - bromopropane

(n - propyl bromide)

anti - Markovnikov addition.

HCH3C CH2HI CH3CHICH3

propene 2 - iodopropane ( isopropyl halides)

Ex.

Ex.

actual product+ HI H3C CHI CH3H3C CH CH2

HCH3C CH2 + HI H3C CH CH2

H I

HI CH3CHICl

1-Chloro

-1

-iodoethane

H3CHC=C

CH3

CH3 + HI CCH2CH3

CH3

CH3

I

CH3CH=CHCH3 + HI CH3CHICH2CH3

CH2=CHCl

vinyl chloride

Addition of hydrogen halides

– In the addition of an acid to the C=C of an alkene, the hydrogen of the acid attaches itself to the carbon that already holds the greater number of hydrogens

– The reactivity of alkene, with halogen acids is in the order;.

HI > HBr > HCl

C C

+ H2SO4 C

C

H OSO3Halkyl hydrogen sulfates

4. Addition of sulfuric acid

Ex.

HCH3C CH2

propene

80% H2SO4H2O, heat

CHCH3

OSO3H

CH3

CHCH3

OH

CH3

Isopropyl alcohol

CH2=CH298%H2SO4

CH3CH2OSO3HH2O, heat

CH3CH2OH + H2SO4

CCH3

CH3

CH2

63%H2SO4

5. Addition of water. HYDRATION

C

C

+ HOH C

C

H OH

H

+

H3C C

CH3

CH2H2O, H

+

CH3 C

CH3

CH3

OHIsobutylene

tert - butyl alcohol

CH4

Ex.

HCH3C CH2H2O, H

+

propene Isopropyl alcohol (2 - propanol)

CHCH3CH3

OH

Addition of Water - Hydration

– When heated with water in the presence of an acid catalyst, alkenes yield alcohol ROH.

– The process is called hydration of alkenes because it involves the addition of water across the double bond.

– The addition of the HOH across the double bonded carbon that bears the greater number of hydrogen atoms and the hydroxyl groups goes to the other double-bonded carbon

C

C

+ X2 + H2O C

C

H OH

+ HX X2 = Cl2, Br2

HCH3C CH2

Cl2, H2OH3C CH CH2

HO Clpropylene(propene)

propylene chlorohydrin

follow Markovnikov rule

6. Halohydrin formation

Ex.

C

C

HH3C

CH3 H

Cl2, H2O Cl

CH3

H

H OH

CH3cis - 2 - butene

threo (stereospecific)

3 - chloro - 2 - butanol

C

C

CH3H

CH3 H

trans 2 - butene

Cl2, H2O H

CH3

Cl

H OH

CH3

erythro

3 - chloro - 2 - butanol

Sterospecific:

7. Dimerization (di = two, mer = part, product contains exactly twice the # of C & H atom as the original).

H3C C

CH3

CH2+ H3C C

CH3

CH2H2SO4

H3C C

CH3

CHCH3

C

CH3

CH3+ H3C C

CH3

CH2CH3

C

CH3

CH2isobutylene isobutylene

2,4,4 - trimethyl - 2 - pentene 2,4,4 - trimethyl - 1 - pentene

Mechanism:

Addition of the tert-butyl cation to iso butylene; the orientation of addition is duch to yield the more stable tertiary cation. Step(2) brings about the union of two : isobutylene units, which is of course necessary for the product.

H3C C

CH3

CH2+ H:Br H3C C

CH3

CH3+

+ : Braddition of a hydrogen ionto isobutylene to formthe carbon

H3C C

CH3

CH3+

+ H3C C

CH3

CH2 H3C C

CH3

CH2

CH3

C

CH3

CH3+H3C C

CH3

CH

CH3

C

CH3

CH3

8. Alkylation

C

C

+ R Hacid

C C

RHex.

H3C C CH2

CH3

isobutylene+ H3C C H

CH3

CH3

H2SO4 H3C C CH2 C CH3

CH3 CH3

CH3Hisobutane 2, 2, 4 - trimethyl pentane

mechanism:

H3C C CH2

CH3

+ H3C C

CH3

CH3

+ H3C C CH2 C CH3

CH3 CH3

CH3

+

H3C C CH2 C CH3

CH3

CH3

CH3

++ H C CH3

CH3

CH3

H3C C CH2 C CH3

CH3 CH3

CH3H

+

C CH3

CH3

CH3

+

Addition of a hydrogen ion to form carbocation

Addition of a tert-butyl carbocation to isobutylene

Carbocation abstracts a hydrogen atom with its pair of electrons from a molecule of alkane. This abstraction of hydride ion yields an alkane of 8 carbons and a new carbocation to continue the chain.

H3C C CH2

CH3

+ H:B H3C C CH3

CH3

+ :B+

A carbocation may:

a.) combine with a negative ion or other basic molecule

b.) rearrange to a more stable cabocation

c.) eliminate a hydrogen ion to form an alkene

d.) add to an alkene to form a larger carbocation

e.) abstract a hydride ion from an alkane

9. Oxymercuration - demercuration

•Oxymercuration – involves addition to the C=C of OH and HgOAc (mercuric ion)

•Demercuration – the HgOAc is replaced by H

C C + H2O+ Hg(OAc)2 C C

AcOHgHO

NaBH4C C

HHO

oxymercuration demercuration

mercuric acetate

organomercurial cmpd. alcoholMarkovnikov orientation

Ex. Undergo the process of oxymercuration, involves addition to the carbon – carbon double bond of –OH+ -HgOAc

CH2CH3C CH2

CH3

Hg(OAc)2, H2O NaBH4 CH2CH3C CH3

OH

CH3

2 - methyl - 1 - butene

tert - pentyl alcohol

CH3 CH3

OH

Hg(OAc)2, H2O NaBH4

1 - methylcyclopentene 1 - methylcyclopentanol

H3C C

CH3

CH3

CH CH2 H3C C

CH3

CH3

HC CH3

OH

Hg(OAc)2, H2O NaBH4

3, 3 - dymethyl - 1 - butene 3, 3 - dymethyl - 2 - butanol

H3C(H2C)3HC CH2Hg(OAc)2, H2O NaBH4

1 - hexene2 - hexanol

CH(CH2)3CH3

OH

CH3

Follows Markovnikov addition

10. Hydroboration – oxidation

With the reagent Diborane, alkenes undergo hydroboration to yield alkylboranes, which on oxidation give alcohols.

(BH3)2 CH3CH2BH2 (CH3CH2)2BH (CH3CH2)3BH2C=CH2 H2C=CH2 H2C=CH2

diborane triethylboron

(CH3CH2)3B + 3H2O2 3CH3CH2OH + B(OH)3

OH-

ethanol boric acid

C C + (BH3)2 C C

H B

C C

H OH

H2O2

OH-

Diborane

Anti - markovnikov orientation

Mechanism:

Hydroboration involves the addition of the double bond of BH3 w H becoming attach to one doubly bonded carbon and boron to the other. The alkylborane can then undergo oxidation in which the boron is replaced by –OH. Thus, the 2 – stage reaction process of hydroboration oxidation permits the effect. The addition to the carbon – carbon double bond of elements of H-OH.

C C + H - B C C

B

C C

OHhydroboration oxidation

H B = H - BH2, H - BHR, H - BR2

CH3CH=CH2(BH3)2 H2O2, OH

-CH3CH2CH2OH

n-propyl alcoholpropylene

CH3CH2CH=CH2

(BH3)2 H2O2, OH-

CH3CH2CH2CH2OH

n-propyl alcohol1

-butene

C=CH2CH3

CH3(BH3)2 H2O2, OH

-

CHCH2OH

CH3

CH3

Isobutyl alcoholIsobutylene

11. Addition of free radicals

C C + Y Z C C

Y Z

peroxides

or light

n - C6H13CH CH2 + BrCCl3 n - C6H13CH

Br

CH2 CCl3peroxide

1 - octane bromotrichloromethane

3 - bromo - 1,1,1 - trichlorononanestability of radical: 3º > 2º > 1º CH3

RCH CH2+ CCl4 RCH CH2 CCl3

Cl

peroxides

Mechanism:

peroxides rad.

Rad. + Cl:CCl3 Rad:Cl+ .CCl3

.CCl3+ RCH=CH2 RCH - CH2 - CCl3

RC.H - CH2 - CCl3 + Cl:CCl3 RCH - CH2 - CCl3

Cl

+ .CCl3

Electrophilic addition : Markonikov orientation

HCH3C CH2

HBr

CH3 CH CH3+

2º cation

CH3 CH CH3

Brisopropyl bromide

CH3 CH2 CH2+

1º cation

propylene

HCH3C CH2

propylene

Br.

CH3 CH CH2Br

2º free radical

CH3 CH2 CH2Br

CH3 CH2 CH2

Br1º free radical

HBr.

.

Free – radical Addition : Anti – Markovnikov orientation

12. Polymerization of Alkenes

Polymerization – the joining together of many small molecules to form very large molecules.Monomers – the simple compounds form which polymers are made.

Ex.

nCH2 CH2

O2, heat, presH2C CH2 CH2 CH2 CH2 CH2

or

H2C CH2

polythylene(plastic material of packaging film)

•5 processes of polymerization

1. Free - radical polymerization2. Cationic polymerization3. Anionic polymerization4. Condensation polymerization5. Coordination polymerization

Free – radical polymerization

nCH2 CH

Cl

peroxideH2C CH CH2 CH CH2 CH CH2

Cl Cl Cl

or

H2C CH2

Cl n

poly (vinyl chloride)

Polyvinyl chloride - use to make phonograph, records, plastic pipes, when plasticized with high boiling esters – raincoats, shower curtains and coatings for metal and upholstery fabrics.Peroxide – initiator, required in small amount in polymerization-Free radical initiator

peroxide rad. decomposition of peroxides to form free radical

Mechanism:

Free radical adds to molecule of alkenes which for another free radical

rad H2C CH

G

radCH2 CH

G

. chain initiating step

This radical adds to another molecule of alkene to generate another free radical. This radical adds to another molecule of alkene to generate a still larger radical

radCH2 CH

G

.+ H2C CH

G

radCH2 CH

G

CH2 CH

G

. chain propagating step

13. Addition of Carbenes. Cycloadditioncarbenes – derivative of methylene

H2C N N+ -

CH2 N2UV lightphotolysis

diazomethans methylene

H2C C O CH2 COUV light +

ketene methylene

+

CH2: or H :C. ...

H

H : C....H

Methylene exist into 2 different forms

singlet methylene – unshared electrons are paired, less stable & generated first in photolysis : stereospecific addition

triplet methylene – unshared electrons are not paired, free radical (diradical) : nonstereospecific addition

HCH3C CHCH3 + CH2N2light

CH2

HCH3C CHCH32 - butene diazomethane

1, 2 - dimethycyclopropane

+ N2

Cycloaddition: addition of the carbon – carbon double bond

Photolysis of diazomethane into in liquid

And in liquid

Sterospecific: (addition of methylene can occur with 2 different kinds of stereochemistry.)

cis - 2 -butene CH2N2 cis 1, 2 - dimethylcyclopropane+

trans 2 -butene CH2N2 trans 1,2 -dimethylcyclopropane+

CH2: + C C

C C

CH2

C C

CH2Singlet methyleneStereospecificElectrophilic additionElectron deficient and can find electrons at the C-C double bondingle

CH2N2Non – stereospecific: cis/trans2 – butene + both cis and trans 1,2 - dimethylcyclopropane

Triplet methyleneNon - StereospecificFree radical additionff. by addition

CH2 + C C

C C

CH2

C C

CH2

....

C

H + CH2 C

CH2 H

HCH2C CHCH3 + CHCl3t - BuO

-K

+

HCH3C CHCH3

C

Cl Cl

+ t - BuOH + KCl

2 - butene chloroform

3, 3 - dichloro - 1, 2 - dimethylcyclopropane

Methylene undergoes intersection

Addition of substituted carbenes: 1,1 - elimination

t - BuO-.. + H..CCl3

..CCl3-

..CCl3-

..CCl2 Cl -+ t - BuO

..H

dichlorocarbene

+

HCH3C CH3CH+ ..CCl2 HCH3C CHCH3

C

Cl Cl

Reaction involves a divalent carbon compound, a derivative of methylene: dichlorocarbene: CU2.Generated in 2 steps, initiated by attack on chloroform by the strong base tert-butoxide ion and then adds to the alkene.

Mechanism:

Because of the presence of halogen atom, the singlet form is the more stable form of dichlorocarbene and is the one adding to the double bond.

Addition of Carbenes

• Carbenes are intermediates of the general formula CH2:. The derivatives of methylene (CH2) are the carbenes.

• Methylene is formed by the photolysis of either diazomethane, CH2N2 or ketene, CH2=C=O.

C

C

+ KMnO4 or HCO2OH C

C

OH OH

HCH3C CH2propylene

cold, dil. KMnO4 or HCO2OHH3C CH CH2

OHOH+ 2MnO2

1,2 - propanediol (propylene glycol)

14. Hydroxylation. Glycol formation

Example.

Oxidizing agents that bring about hydroxylation

a. cold alkaline potassium permanganate, KMnO4

b. peroxy acids, such as peroxyformic acid HCO2OH

KMnO4

OHOH

HCO2OH

OH

OH +OH

OH

syn-hydroxylation

anti-hydroxylation

cis-1,2

-cyclopentanediol

trans-1,2

-cyclopentanediol

CH

C

C

+ X2heat

CX

C

C

X2 = Cl2, Br2

low conc.

HCH3C CH2propylene

Cl2, 600ºCl - CH2CH CH2

allyl chloride

+ NBS

Br

cychlohexeneN - bromosuccinimide

3 - bromocyclohexene

15. Halogenation. Allylic substitution ( same mechanism with substitution in alkenes)

Ex.

CH2=CH-CH3

alkene-like site of addition alkane

-like site of substitution

CH2=CH-CH3

heterolytic attack, addition Free-radical attack, substitution

Can we direct the attack to just one of these sites? Yes, by our choice of rxn. Conditions.

Conditions:1.alkenes undergo substitution by halogen at high temp. or under the influence of UV light, generally in gas phase.2.it can also undergo addition of halogen at low temp. in the absence of light and generally in liquid state(phase).

CH CH2CH3

propylene

Cl2

CH3 CH CH2

Cl Cl

Cl CH2 CH CH2

low T

CCl4 soln

500 - 600ºgas phase

ClH

1,2 - dichloropropane

3 - chloro -1 - propane

heterolytic add'n

free radical subs

N – bromosuccinimide a reagent used for the specific purpose of brominating alkenes at the allylic position provides a constant low conc. of bromine.

NBS

NC

C

CH2

CH2

O

O

BrHBr +

NC

C

CH2

CH2

O

O

HBr2 +

succinimide

C

C

C

H

H

H

H

vinylic hydrogen: hard to abstract

allylic hydrogen: easy to abstract.

Ease of abstraction of hydrogen atoms:

Ease of formation of free radicals: allyl > 3º > 2º > 1º > CH3. > vinyl

Vinylic hydrogen- hydrogens attached to C=C

Alylic hydrogen – hydrogens attached to a carbon atom next to a double bond

Allylic >3º > 2º > 1º > CH4 > Vinylic

Example:

CH3(CH2)3CH2CH2CH=CH2NBS

16. Ozonolysis (Cleavege rxn)

C

C

+ O3 OO

O

O

O

H2O, ZnC O

+ O C

ozone

ozonide

aldehyde ketone

Cleavage – a rxn in which the double bond is completely broken and the alkene molecules converted into 2 smaller molecule.

Reducing agent (Zn) – prevent formation of hydrogen peroxidewill not react with aldehyde and ketone (aldehyde are often

converted to acid, RCOOH for ease of isolation.)

CH3CH2CH=CH2O3 H2O, Zn CH3CH2CHO + CH2=O

C=CH2

CH3

CH3O3 H2O, Zn

C=O

CH3

CH3 + CH

H

O

CH2CH3C

H

O + O C

H

CH2CH3H2O / Zn O3 CH2CH3C

H

C CH2CH3

H

aldehydes 3 - hexene

CH2CH3C

H

O + O C

CH3

CH3H2O / Zn O3 CH2CH3C

H

C CH3

CH3

aldehydes 2 - methyl - 2 - penteneketone

Ozonolysis – is a typical means of degradation

CH3CH2CH2CHO + CH3CHOH2O, Zn O3

CH3CH2CH2CH=CHCH3

17. Cleavage with periodate ( cleavage with a diol)

C

C

C

C

OH OH

KMnO4

cold, dil.

NaIO4acids, ketones. CO2

RCOOH are generally obtained instead of aldehydes, RCHO a terminal ==CH2 group is oxidized to CO2.

CH3CH2CH2COOH + CO2KMnO4NaIO4

HCH2CH2CH3C CH2

carboxylic acid carbon dioxide

Example.

CH3COOH + O C

CH3

CH3 CH C

CH3

CH3CH3KMnO4

NaIO4carboxylic acid ketone 2 - methyl - 2 - butene

Cleavage of cycloalkenes

CH

CH

CH2

CH2

H2C

H2C

O3 H2O, Zn

KMnO4

NaIO4

CHO

CHO

CH2

CH2

H2C

H2C

COOH

COOH

CH2

CH2

H2C

H2C

di - aldehyde

di - acid

18. Epoxidation of Alkenes

C=C +CHC=C +CH33COOH C C + COOH C C + CH3COHCH3COH O

O O

Alkene Peroxyacetic Acid

Epoxide Carboxylic acid

H2C=CH(CH2)9CH3 + CH3COOHCH2-CH(CH2)9CH3 + CH3COH

O O

1-Dodecane Peroxyacetic acid 1,2-epoxydodecane

Acetic acid

Example