Chapter 3 Alkenes

3.1 Lesson Outcomes3.2 Structure & Bonding in Alkenes3.4 Interesting of Alkenes IUPAC3.5 Nomenclature of Alkenes3.5 Physical Properties of Alkenes3.6 Preparation of Alkenes3.7 Chemical reaction of Alkenes

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ALKENES3

• At the end of this topic, you should be able to:- Identify alkenes Explain structure and physical properties

of alkene Name and draw alkene by using IUPAC

nomenclature Write the equation for the preparation of

alkene. Write the equation for the reactions of

alkene. Describe the sources and uses of alkene

LEARNING OUTCOMES

STRUCTURE & BONDING OF

ALKENES

Alkenes are also called olefins.

Alkenes contain a carbon—carbon double bond.

General formula: CnH2n (for one double bond)

STRUCTURE & BONDING

The double bond consists of a bond and a bond. The bond is stronger

than the bond.

Each carbon is sp2 hybridized and trigonal planar, with bond angles of

approximately 120°.

STRUCTURE & BONDING

INTERESTING OF ALKENES

Ethylene : Prepared from petroleum by a process called cracking The most widely produced organic chemicals Starting material not only for polymer polyethylene, a widely used plastic, but also many other useful organic compounds..

INETERESTING OF ALKENES

INETERESTING OF ALKENES







Organic compounds containing carbon-carbon double bonds have been isolated from natural sources.

β-carotene(orange pigment in carrot)

Zingiberene(oil of ginger)

α-farnesene(found in the waxy

coating on apple skin

CH3

HCH2C

CH3

(R)-limonene(from oranges)

CH3

H CH3C

CH2

(S)-limonene(from lemons)


The Chemistry of VisionIn the rod and cone cells in the retina of the eye, retinal in rhodopsin is found “at rest” in the cis form. When it absorbs a photon (hν) of light, one of the π- bonds is broken, causing the molecule to rotate and lock into the trans form, which has a completely different shape. This starts a long chain of chemical processes which eventually results in a visual image in the brain. The trans-retinal molecule is then twisted back into the cis form by another enzyme. When you look directly at a very bright light, the “afterimage” that you see in front of your eyes is the result of a large amount of cis-retinal having been transformed into trans-retinal all at once; the enzymes take a little bit of time to go through and “reset” all of these molecules.


The Chemistry of Vision


IUPAC NOMENCLATURE OF ALKENES

Step 1: Identify and name the longest continuous chain of C atoms which contains

the double bond(s) (#C + -ene).

NOMENCLATURE OF ALKENES


Step 2: Locate the double bond by the lower numbered carbon atom joined by the

double bond (e.g., 1-butene).

If the double bond has the same position starting from either end, use the positions of the substituents to determine beginning of

the chain.

Step 3: Write out the full name, numbering the substituents according to their position in the chain and list them in alphabetical order.


Indicate the double bond by the number of the first alkene carbon.



Compounds with two double bonds are named as dienes by changing the “-ane” ending of the

parent alkane to the suffix “–adiene”.

Compounds with three double bonds are named as trienes


In naming cycloalkenes, the double bond is located between C1 and C2. The ring is numbered

to give the first substituent the lower number.


In naming cycloalkenes: If there is a substituent on one of the double bond carbons, it gets number

1.


Compounds that contain both a double bond and a hydroxyl group are named as alkenols &

the chain (or ring) is numbered to give the OH group the lower number.


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trans-3-hexene

C CCH2CH3

H

H

H3CH2C

cis-3-heptene

C CCH2CH3

H

H3CH2CH2C

H

cis-3-methyl-3-heptene

C CCH2CH3

H

H3CH2CH2C

H3C

NOMENCLATURE OF ALKENES: CIS-TRANS

MOLECULE

trans-1,2-dimethylcyclohex-1-ene

cis-1,2-dimethylcyclohex-1-ene

PHYSICAL PROPERTIES OF

ALKENES

Alkenes exhibit only weak van der Waals interactions.

Low melting points and boiling points.

Melting & boiling points increase as the number of carbons increases because of increased surface area.

Soluble in organic solvents and insoluble in water.

Ethene, propene & butene - gases at room temperature.Alkenes are nonpolar compounds.

PHYSICAL PROPERTIES OF ALKENES

PREPARATION/SYNTHESIS OF ALKENES

X Y

Dehydration of alcohols:X = H; Y = OH

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

C CC C + X Y

SYNTHESIS OF ALKENES: ELIMINATION

REACTIONS

Heating an alcohol with excess concentrated H2SO4 at 170°C, or with excess concentrated phosphoric acid,

H3PO4 at 225°C

1. DEHYDRATION OF ALCOHOLS

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

H OH

C CC C + H2O

Examples

OHconc. H3SO4 + H2O

CH3CH2CH2OHconc. H3SO4 CH3CH=CH2

+ H2O

1. DEHYDRATION OF ALCOHOLS

Haloalkanes undergo elimination reaction when refluxed with purely alcoholic

solutions of potassium hydroxide, KOH to produce an alkene.

2. DEHYDROHALOGENATION OF ALKYL

HALIDES

H X

C CC C + HX

Reagent: NaOCH3 in methanol, or KOH in ethanol

Examples

CH3CH2CH2Br + KOH/ethanol reflux CH3CH=CH2 + KBr + H2O

CH3CCH3 + KOH/ethanol reflux CH3CH=CH2 + KCl + H2OCH3

Cl

CH3

2. DEHYDROHALOGENATION OF ALKYL

HALIDES

REACTION OF ALKENES

The characteristic reaction of alkenes is addition: the bond is broken and two new bonds are formed.

Alkenes react with Electrophiles

REACTION OF ALKENES

Alkenes undergo ELECTROPHILIC ADDITION reactions –

the C=C (C double bonds) are broken to form C-C bond (single bonds).

REACTION OF ALKENES

Addition reactions• Hydrogenation• Halogenation• Hydration• Hydrohalogenation• Halohydrin formation

Oxidation reactions• Ozonolysis• Hydroxylation with KMnO4 (room temp) • Oxidation cleavage of alkenes with

acidic KMnO4

ADDITION REACTIONS

ADDITION OF HYDROGEN (HYDROGENATION)

The addition of hydrogen to an alkene can be carried out by

passing hydrogen & the alkene, both in the gaseous state, over a metal catalyst.

General reaction

Example

C C + H HNi or Pt C C

H H

C CH

H

H

H+ H H

Ni or Pt C CH

H

H

H

H

H

ethene ethane

Hydrogenation of double bonds – used to convert unsaturated edible oil as

to solid fats like margarine.

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ADDITION OF HYDROGEN (HYDROGENATION)

Example

1

CH2=CH2 Ni / H2 CH3-CH3

CH3-C C-CH2CH3

CH3CH3

Ni / H2

CH3

2H2Ni

CH3

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

H3C

CH3

H

CH3

H

ADDITION OF HYDROGEN (HYDROGENATION

Margarine Manufacturing Process

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ADDITION OF HALOGEN (HALOGENATION)

Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal

dihalide.

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ADDITION OF HALOGEN (HALOGENATION)

Decolorisation of bromine solution – test-tube reaction to detect a double bond.

When an alkene is bubbled through a solution of hexane or tetrachlomethane (CCl4), the

orange color of bromine is rapidly decolorised.

CH2=CH2 + Br2/CCl4 CH2CH2

Br Br

room temperature

orange

colorless

2

ADDITION OF HALOGEN (HALOGENATION)2

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

HYDROHALOGENATION: ELECTROPHILIC ADDITION OF HX

Two bonds are broken in this reaction: 1. The weak bond of the alkene and the HX bond2. Two new bonds are formed: one to H and one to

X.Because the electrophilic H end of HX is attracted to the electron-rich double bond, these reactions are

called electrophilic additions.

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HYDROHALOGENATION

Addition of hydrogen halide to unsymmetrical alkenes Markonikov’s

RuleWith an unsymmetrical alkanes, HX can add to the double bond to give to positional isomers.

Markovnikov’s rule : The addition of HX to an unsymmetrical alkene, the H atom

adds to the carbon atom of the double bond with the greatest

number of carbon atoms

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HYDROHALOGENATION


Rule

3

HYDROHALOGENATION


RuleAn unsymmetrical carbon-carbon double bond is one whose 2 carbon atoms hold unequal number of hydrogen atoms.

C Cpropene

H3C

H H

H

carbon atom with the greatest number of hydrogen atom

unsymmetrical double bond

Example

2-bromobutane

CH3CH2CH=CH2 + HBr CH3CH2CHCH3

Br

CH3

+ HClCH3Cl

1-chloro-1-methylcyclohexane

3

HYDROHALOGENATION3

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

HYDROHALOGENATION3

with HBr peroxide/ ROOR , the major product is ANTI-MARKOVNIKOV.

The H from HBr will be bonded to carbon double bond is bonded directly to less

hydrogens atoms.

CH3-CH2-CH=CH2

HBr

Peroxideor

ROOR

CH3-CH2-CH2-CH2Br

(major)

CH3HBr

Peroxideor

ROOR

CH3

Br(major)

HALOHYDRIN FORMATION

Treatment of an alkene with a halogen X2 and H2O forms a halohydrin by addition of the elements of X and OH to the double bond.

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When ethene is bubbled into bromine water, the reddish-brown water is rapidly decolorised and

2-bromoethanol is produced.

CH2=CH2 + Br2/H2O CH2CH2

Br OH

room temperature

orange

colorless

Unsymmetrical alkenes: the preferred product has the electrophile X+ bonded to the carbon with more H

atom, and the nucleophile (H2O) bonded to the carbon with less hydrogen

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Although the combination of Br2 and H2O effectively forms bromohydrins from alkenes,

other reagents can also be used.Bromohydrins are also formed with N-

bromosuccinimide (NBS) in aqueous DMSO [(CH3)2S=O].

In H2O, NBS decomposes to form Br2, which then goes on to form a bromohydrin by the same

reaction mechanism.

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HYDRATION: ELECTROPHILIC ADDITION OF WATER

Hydration is the addition of water to an alkene to form an alcohol.

Markovnikov’s rule holds.

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5


5

FIVE ADDITION REACTIONS OF

CYCLOHEXENE

OXIDATION REACTIONS

OZONOLYSIS OF ALKENES1

1st step - Reaction of alkene with ozone to form ozonide.

2nd step - hydrolysis of ozonide to form aldehyde and ketone.

+ O3

C CO

O O

C C

C O CO+H2O, Zn

R R’

R”RH

H

R’

R”

ozonide

aldehyde ketone

CLEAVAGE OF DOUBLE BOND

OZONOLYSIS OF ALKENES1

examples

H

O2

acetaldehyde

O3, Zn

H2O, H+

O3, Zn

H2O, H+H

O+

acetaldehyde

O

propan-2-one

O3, Zn

H2O, H+

O

O


REACTION WITH HOT CONC. KMnO4/H+2

H2C C CH3

H

conc. KMnO4/ H+

CH H

O

+ CH

O

CH3

HCOOH

O

C CH3HO

O

CO2 & H2O

(methanal) aldehyde

carboxylic acidIf 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.


OXIDATION WITH COLD DILUTE KMnO4/H+

An alkene will react with dilute KMnO4 in acidic/alkaline solution to give a diol.

CH2=CH2 + [O] + H2O C C

H

OH

H

OH

H

H

Ethane-1,2-diol

dilute MnO4-/H+

room temperature

When ethene is added to dilute KMnO4 in alkaline conditions at room temperature, the purple color of

the KMnO4 is decolorized, producing ethane-1,2,-diol and a brown precipitate of manganese (VII)

oxide.

purple

3

REACTION WITH COLD DILUTE KMnO4/H+3

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

TRY THIS!!!

Q1: Base on the reaction below, draw the structure of the organic

products formed.

Q2:

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.

The end

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Chapter 3 Alkenes