Unsaturated HydrocarbonsAlkenes

1435-14362014-2015

Learning ObjectivesChapter two discusses the following topics and the student by the end of this chapter will:

Know the structure, hybridization and bonding of alkenes

Know the common and IUPAC naming of alkenes

Know the geometry of the double bond i.e. cis/trans isomerization

Know the physical properties of alkenes

Know the different methods used for preparation of alkenes (elimination reactions ; dehydrogenation, dehydration and alkenes stability (Zaitsev’s rule) play an important role in understanding these reactions

Know the addition reactions of alkenes and the effect of Markovnikov’s rule in determining the regioselectivity of this reaction.

Alkenes

They are unsaturated hydrocarbons – made up of C and H atoms and contain one or more C=C double bond somewhere in their structures.

Their general formula is CnH2n - for non-cyclic alkenes

Their general formula is CnH2n-2 - for cyclic alkenes

Structure Of Alkenes

3

Alkenes

Trigonal planar

2s2 2p3 3 x sp2 2p

sp2 Hybridization Of Orbitals In Alkenes

The electronic configuration of a carbon atom is 1s22s22p2

Thus

promotion hybridization

2p2 2s1

Alkenes

In ethylene (ethene), each carbon atom use an sp2 orbital to form a single

C-C bond. Because of the two sp2 orbitals overlap by end- to- end the resulting bond is called σ bond. The pi (π) bond between the two carbon atoms is formed by side- by-side overlap of the two unhybridized p- orbitals (2p–2p ) for maximum overlap and hence the strongest bond, the 2p orbitals are in line and perpendicular to the molecular plane. This gives rise to the planar arrangement around C=C

bonds. Also s orbitals of hydrogen atoms overlap with the sp2 orbitals in carbon atoms to form two

C-H bonds with each carbon atom. The resulting shape of ethene molecule is planar with

bond angles of 120º and C=C bond length is 1.34 Å

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Orbital Overlap In Ethene

Alkenes

two sp2 orbitals overlap to form a sigma bond between the two carbon atoms

Orbital Overlap In Ethene

two 2p orbitals overlap to form a pi bond between the two carbon atoms

s orbitals in hydrogen atoms overlap with the sp2 orbitals in carbon atoms to form C-H bonds

the resulting shape is planar with bond angles of 120º and C=C (1.34 Å)

sp2 hybridized carbon atoms

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Alkenes

Nomencalture Of Alkenes And Cycloalkenes

1. Alkene common names:

Substituent groups containing double bonds are: H2C=CH–   Vinyl group

H2C=CH–CH2–   Allyl group

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BrCl

Common: Allyl bromide Vinyl chlorride

H2C CH2 CH3-CH CH2 C CH2

CH3

H3C

Common: IsobuteneEthylene Propylene

Alkenes

2. IUPAC Nomenclature Of Alkenes

Find the longest continuous Carbon chain containing the double bond this determines the root name then add the suffix -ene.

Number the C- chain from the end that is nearer to the double bond. Indicate the location of the double bond by using the number of the first atom of the double bond just before the suffix ene or as a prefix.

Indicate the positions of the substituents using numbers of carbon atoms to which they are bonded and write their names in alphabetical order (N.B. discard the suffixes tert-, di, tri,---when alphabetize the substituents) and if more than one substituent of the same type are present use the prefixes di- or tri or tetra or penta,--- to indicate their numbers.

CH3CH CHCH2CH2CH3

Hex-2-ene or 2-Hexene(not 4-Hexene)

1 2 43 5 6

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H2C CH CH2CH3

But-1-ene or 1-Butene (not 3-Butene)

1 2 3 4

Alkenes

CH3

CH3C CH

2-Methyl-but-2-eneor 2-Methyl-2-butene(not 3-Methyl-2-butene)

31CH3

2 4

CH3

Cl

3-Chloro-2-hexene(not 2-Chloro-1-methyl-1-pentene)

2

1

3

4

5

62

3

4

5

6

7

8

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

2,2-Dibromo-3-heptene(not 6,6-Dibromo-4-heptene)

1567 234(CH3CHCHCH2OCH3)

1-Methoxy-but-2-ene(not 4-Methoxy-but-2-ene)

OCH32

3 1

8

12

56

734

2,3,7-Trimethyl-non-3-ene(not 2-Isopropyl-6-methyl--2-octene)

9CN

4-Cyano-2-ethyl-1-pentene(not 2-Ethyl-4-cyano-1-pentene)

1

23

45

5-Methylcyclopenta-1,3-dieneAn ''a'' is added due to inclusion of di put two consonants consecutive

1

23

CH3

=

Br

Br

6-Methyl-2-octene

1

4

Alkenes

In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.

If the substituents on both sides of the = bond are at the same distance, the numbering should start from the side that gives the substituents with lower alphabet the lower number.

103-tert-Butyl-7-isopropyl-cycloheptene

(not 3-Isopropyl-7-tert-butylcycloheptene)

CH36 2

3

4

5

1-Methyl cyclopentene(not 2-Methylcyclopeneten)

CH3H3C

3,5-Dimethyl-cyclohexene(not 4,6-Dimethylcyclohexen)

(not 1,5-Dimethyl-2-cyclohexen)

1

1

Alkenes

When the longer chain cannot include the C=C, a substituent name is used.

6

2

3

5

1

1

CH CH2

Vinyl-cyclohexane

CH CH2

3-Vinyl-cyclohexene

4

Alkenes

Geometrical Isomerism In Alkenes

G. I. found in some, but not all, alkenes It occurs in alkenes having two different groups / atoms attached to each carbon

atom of the = bond

G. I. x G. I. X G. I. G. I.

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BA

DC

A=C or B=D No Cis or transe

(G. I. X)

A≠C B≠D, A =B or C=D Cis / A =D or C= B transe

G. I.

Alkenes

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It occurs due to the Restricted Rotation of C=C bonds so the groups on either end of the bond are ‘fixed’ in one position in space; to flip between the two groups a bond must be broken.

X

Geometrical isomers can not convert to each at room temperature.

Alkenes

Geometrical Isomerism In Alkenes

A) Cis / trans isomerism in alkenes Exhibited by alkenes having two H’s and two other similar groups or

atoms attached to each carbon atom of the = bond (or generally the alkene have only two types of atom or groups i.e. ABC=CAB)

Cis prefix used when hydrogen atoms on both carbon atoms are on the SAME side of C=C bond

Trans prefix used when non-hydrogen groups / atoms are on the opposite sides of C=C bond

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Types Of Geometric Isomerism

Cis-But-2-ene Trans-But-2-ene

Alkenes

CisGroups / atoms are on theSame Side of the double

bond

TransGroups / atoms are on

Opposite Sides across the double bond

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H

ClCl

H H

ClH

Cl

cis-1,2-Dichloro-ethene trans-1,2-Dichloro-ethene

Trans-Oct-4-eneH H

Cis-Oct-4-ene

=

Alkenes

16

If the groups attached to the C=C are different, we distinguish the two isomers by adding the prefix Z (from German word Zusammen) if the higher-priority groups are together in the same side or E (from German word Entgegen) if the higher-priority groups are opposite sides depending on the atomic number of the atoms attached to each end of the C=C.

Atoms with higher atomic numbers receive higher priority I> Br > Cl > F > O > N > C > H

B) Z/ E isomerism in alkenes

Alkenes

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CH3

BrI

ClBr

CH3I

Cl

CH

CH3

O

OH

CH3

I

Cl

H

CH2

Z-2-bromo-1-chloro-1-iodopropene E-2-bromo-1-chloro-1-iodopropene

Z Z

Alkenes

Exercise

Q1-Which of the following compounds can exhibit cis / trans isomerism

a) 2-Methylpropene

b) 1-Butene

c) 2-Methyl-2-pentene

d) 2-Butene

e) 3-Methyl-2-hexene

Q2- Name the following compounds according to IUPAC system

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c)b)a)

Alkenes

19

Br12

3

4

3-Bromo propene1

23

45

2-Ethyl-4-methyl pentene

CH3

Cl

CH3

1

23

4

56

4-Chloro-3,6-dimethylcyclohexene 3-Chloro-2,5-dimethylcyclohexene

CH3

Cl

CH3

6

12

3

45

C C

H

Br Br

H

C C

H

H Cl

Cl

Cis-1,2-DibromoetheneGeometrical isomerism

1,1-Dichloroethene

Trans-trans-2,4-heptadiene Trans-1,3,5-heptatriene

Cis-cis-2,5-heptadiene Home work

not geometrical isomerism

1

2

3

4

5

6

7

Alkenes

C C

H

H3C CH3

CH2CH3

1

2 3

4 5E-3-Methyl-2-pentene

Physical Properties of Alkenes

Alkenes are nonpolar compounds thus: Insoluble in water Soluble in nonpolar solvents ( hexane, benzene,…) The boiling point of alkenes increase as the number of carbons

increase.

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Alkenes

Preparation Of Aalkenes1- Dehydration of alcohols ( removal of OH group and a proton from

two adjacent carbon atoms ) using mineral acids such as H2SO4 or H3PO4

CH3CH2OHH

+/ heat

CH2 CH2 + OH2

OH

H

+ OH2

H+/ heat

cyclohexanol cyclohexene

Ethanol Ethene

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Alkenes

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Zaitsev’sRule

If there are different protons can be eliminated with the hydroxyl group or with halogen atom, in this case more than one alkene can be formed, the major product will be the alkene with the most alkyl substituents attached to the double bonded carbon.

H3CCH3

OH

H / Heat

H2CCH3

H3CCH3

+ H2O

+ H2O

1- Butene Minor

2- Butene Major

Zaitsev rule: an elimination occurs to give the most stable, more highly substituted alkene

Alkenes

2- Dehydrohalogenation of alkyl halides using a base

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Alkenes

or NaOH

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3. Dehalogenation of vicinal dihalides

For example: Dehalogenation of 1,2-Dibromobutane leads to the formation of 1-Butene. In the presence of catalyst.

Br

Zn/AcOH

Br

Alkenes

Reactions Of Alkenes

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Reactions of Alkenes

Oxidation Reactions

Addition(Electrophilic) reaction: - Hydrogenation - Halogenation - Hydrohalogenation - Halohydrin formation -Hydration

KMnO4

Ozonolysis

Alkenes

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Alkenes

An electrophile, an electron-poor species,(from the Greek words meaning electron loving). It is a species (any molecule, ion or atom) that accept a pair of electrons to form a new covalent bond.

A nucleophile, an electron-rich species, ,(from the Greek words meaning nucleus loving). It is a species (any molecule, ion or atom) that donate an electron pair to form a new covalent bond.

C , H , Br , Cl , I , etc., AlCl3 , BF3 , FeCl3 , FeBr3 , etc.

C , OH , Br , Cl , I , etc., H2N, HS, etc., H2O ,

CH3OH , RNH2 , R2NH , R3N , , etc.

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Electrophilic Addition Reaction

1- Additions To The Carbon-Carbon Double Bond

1.1 Addition Of Hydrogen: Hydrogenation

Alkenes

A

A

A

A

+ H2

H H

A

A

A

A

An alkene An alkane

Pt or Ni or Pd

CH2 CH2 + H2Pt

CH3 CH3

CH3CH2

+ H2Pt CH3 CH3

CH3

CH3

+ H2Pt

CH3

CH3

Cis-1,2-Dimethyl cyclohexane

1.2.Addition of Halogens( Halogenation)

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Alkenes

A

A

A

A

+ X2

X X

A

A

A

A (X= Cl or Br)

CH3CH3

+ Cl2CCl4 CH3 CH3

Cl

Cl

+ Br2

CCl4

Br

Br

CH3

CH3

+ Br2

CCl4

CH3

Br

Br

CH3

Trans-1,2- Dibromo-1,2-Dimethyl cyclohexane

1.3. Addition of Hydrogen Halides

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However, if the double bond carbon atoms are not structurally equivalent, i.e. unsymmetrical alkenes as in molecules of 1- propene, 1-butene, 2-methyl-2-butene and 1-methylcyclohexene, the reagent may add in two different ways to give two isomeric products. This is shown for 1-propene in the following equation.

Only one product is possible from the addition of these strong acids to symmetrical alkenes such as ethene, 2-butene and cyclohexene.

A

AA

A

+ HX

AA

H X

AA

(x= Cl or Br or I)

+ HClH3C

CH3

Cl

H

+ HIH

I

Alkenes

30

Alkenes

HBrCH3CHCH3

CH3CH3CH3

CH3CH=CH2

Br

Br

CH3CHCH3

CH3CH2CH2Br

Br2o Carbocation

1o Carbocation

maijor

minor

Stability of carbocation

C

CH3H3C

CH3

CH

H3C

CH3

CH2CH2 CH3

3o 2o 1o

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Alkenes

Markovnikov’s rule stats that : In addition of unsymmetrical reagent to unsymmetrical alkenes the positive ion adds to the carbon of the alkene that bears the greater number of hydrogen atoms and the negative ion adds to the other carbon of the alkene.

However when the addition reactions to such unsymmetrical alkenes are carried out, it was found that 2-bromopropane is nearly the exclusive product. Thus it said the reaction proceeded according to Markovnikov’s rule

+ HClH3C

CH3

CH3

H3CCH3

CH3Cl

1.4. Addition of HOX halogen in aqueous solution ( -OH, X+): Halohydrin formation

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Only one product is possible from the addition of HOX acids (formed from mixture of H2O and X2) to symmetrical alkenes such as ethene and cyclohexene.

A

AA

A

+ H2O / X2

AA

OH X

AA (x= Cl or Br )

H3CCH3

Cl

OH

+ H2O / Cl2

Symmetrical akenes

Alkenes

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Alkenes

However, addition reactions to unsymmetrical alkenes will result in the formation of Markovonikov’s product preferentially.

Cl

+ H2O / Cl2 OH

Unsymmetrical akenes

CH2Br

OH

+ H2O / Br2

1.5. Addition of H2O: Hydration

34

Only one product is possible from the addition of H2O in presence of acids as catalysts to symmetrical alkenes such as ethene and cyclohexene.

However, addition reactions to unsymmetrical alkenes will result in the formation of Markovonikov’s product preferentially.

CH3 CH3

OH

H

+ H2O

H

Unsymmetrical akenes

Symmetrical akenes

A

AA

A+ H2O

AA

H OH

AA

H3CCH3

OH

H

+ H2O

H

H

Alkenes

1- Ozonolysis (Oxidative cleavage): This reaction involves rupture of the C=C to give aldehydes or ketones according to the structure of the original alkene.

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A

AA

A

+ O3

AA

O O

AA

O

Zn /H2O- H2O2

O

A

A

+ O

A

A

( A= H or R)

i) O3

ii) Zn /H2O O + O

i) O3

ii) Zn /H2O O + O

H

i) O3

ii) Zn /H2O

O

O

Alkenes

Oxidation Reaction:

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2- Oxidation with KMnO4 (Oxidative addition):

OH

OH

KMnO4 / OH

Alkenes

Cis- doil

Thank You for your kind attention !

Questions?

Comments

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Alkenes