alkenes.1introduction.2nomenclature of alkenes.3physical properties of alkenes.4preparation of...

Alkenes.1.1 IntroductionIntroduction

.2.2 Nomenclature of AlkenesNomenclature of Alkenes

.3.3 Physical Properties of AlkenesPhysical Properties of Alkenes

.4.4 Preparation of AlkenesPreparation of Alkenes

.5.5 Reactions of AlkenesReactions of Alkenes

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

Their general formula is Their general formula is CCnnHH2n2n - for - for non-non-cycliccyclic alkenes alkenes

Their general formula is Their general formula is CnHCnH22n-2n-2 - for - for cyclic cyclic alkenesalkenes

Structure Of AlkenesStructure Of Alkenes

2

Angle of SP2 bond =120o

SP2Hibridization:

Introduction

Functional group of alkenes:

The C = C double bond in ethene

Each carbon is trigonal and planar. Bonding: sp2 hybridization for 3 s-bonds to the three atoms bonded to each carbon pz orbital for π-bond Typical C=C bond distance (i.e., 1.34 Å) shorter than C-C bond distance (i.e., 1.54 Å) slightly shorter C-H distance than alkanes

Nomenclature of Alkenes

1. Determine the stem name by selecting the longest possible straight chain containing the C = C double bond and use the ending ‘-ene’

2. Number the parent chain so as to include both carbon atoms of the double bond, and begin numbering with the end of the chain nearer the C = C double bond

3. Designate the position of the C = C double bond by using the number of the first atom of the double bond

Nomenclature of AlkenesNomenclature of Alkenes

4. Designate the positions of the substituent's by using the numbers obtained by application of rule 2


Examples:

1 2 3 4 5 C CC CC H2 H3

C H2 C H3

H2 H2

2 -ethyl -1-pentene

CH2CH CHCH2 CH2

CH3

C CH CH2 CH2

1,4-pentadiene

2- methyl-1,3-butadiene

5. Indicated number of double bonds by prefixes (ene, diene, triene, tetraene, etc.)

1 2 3 4 5

1 2 3 4

Examples


6. If two identical groups are present on the same side of the C = C double bond, the compound is designated as cis; if they are on opposite sides, the compound is designated as trans.

e.g.

ExampleExampleGive the IUPAC names for the following alkenes:

(a)

(b)

Answer


Solution:

(a) trans-3,4-dichlorohept-3-ene

(b) cis-3,4-dimethyloct-3-ene

Check PointCheck Point

Draw the structural formula for each of the following alkenes:

(a) cis-hex-3-ene

(b) trans-2,3-dihydroxybut-2-ene

(c) cis-1,2-dichloroethene Answer


(a)

(b)

(c)

Physical Properties of Alkenes

13

Alkenes are non polar compounds.Insoluble in water.Soluble in non polar organic solvents.They are less dense than water.Range of physical states: ≤ 4 C's are gases5 - 17 C's are liquids ≥ 18 C's are solidsThe alkenes has a boiling point which is a small number

of degrees lower than the corresponding alkanes.

Physical Properties of Alkenes

NameFormulaBoiling point (°C)

Melting point (°C)

Density at 20 °C (g cm-3)

EtheneCH2 = CH2-104-169—

PropeneCH3CH = CH2-47.7-1850.514

But-1-eneCH3CH2CH = CH2-6.3-1850.595

Pent-1-eneCH3(CH2)2CH = CH230-1650.641

Hex-1-eneCH3(CH2)2CH = CH262.9-1400.673

cis-But-2-eneCH3CH = CHCH3 (cis)4-1390.621

trans-But-2-eneCH3CH = CHCH3 (trans)

1-1060.604

2-Methylbut-1-ene

CH3CH3C(CH3) = CH231-1380.650

Preparation of Alkenes

• Dehydrohalogenation is the elimination of a hydrogen halide molecule from a haloalkane in presence of KOH in alcohol

Elimination ReactionsElimination Reactions

Dehydrohalogenation


Examples:


Dehydrohalogenation of 2° and 3° haloalkanes can take place in more than one way and a mixture of alkenes is formed

alc. KOHCH3CH2CHClCH3 CH3CH = CHCH3 + CH3CH2CH = CH2

heat

2-chlorobutane But-2-ene But-1-ene

(80%) (20%)

Note: the more highly substituted alkene is formed as major product

Br+ KOH

CH3

H

CH3

EtOH

∆ +1-methyl cyclohexeneMajo

rMinor

CH3

3 -methyl cyclohexene

Saytzeff s Rule: In every instance in which more than one alkene can be formed, the major product is the alkene with the most alkyl substituents attached on the double bonded carbon.


Dehydration is the removal of a water molecule from a reactant molecule, in the presence of Mineral acids (H2SO4, H3PO4)

Dehydration of Alcohols

The presence of H2SO4 to prevent the reversible reaction


The experimental conditions of dehydration depend on the structures of alcohols

e.g.


• hydrogenation of alkynes using Lindlar’s catalyst produces alkenes

• prevent further hydrogenation of the alkenes formed to alkanes

Addition ReactionsAddition Reactions

Hydrogenation

Reactions of Alkenes

Alkenes are more reactive than alkanes

Reason: presence of the C = C double bond

alkenes undergo addition reactions and the reactions are exothermic

Energetically favourable!!

π-bonds weaker than σ-bonds

Addition of hydrogen bromide to C = C double bond yields a bromoalkane

Electrophilic Addition ReactionsElectrophilic Addition Reactions

Addition of Hydrogen Bromide


Examples:


Propene reacts with HBr to give 2-bromopropane

(major product) and 1-bromopropane (minor product)


The formation of two possible products can be explained

by the reaction mechanism.

Markownikoff’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the carbon-carbon double bond that already has the greater number of hydrogen atoms.


Example:

2-bromopropane is the major product because the more stable

secondary carbocation is formed in the first step


Alkenes react rapidly with Br2 in 1,1,1-trichloroethane

at room temperature and in the absence of light

Addition of Bromine


e.g.

The behaviour of alkenes towards Br2 in CH3CCl3 is a useful

test for the presence of carbon-carbon multiple bonds


Add Br2 in CH3CCl3 to excess alkene

The reddish brown colour of Br2 is decolourized

In an aqueous solution of Br2, the following equilibrium exists

Addition of Bromine Water


Br2 + H2OHBr + HOBr

Bromic(I) acid

The bromine atom bears a partial positive charge while

the oxygen atom bears a partial negative charge

∵ oxygen is more electronegative than bromine

e.g.


When bromic(I) acid reacts with alkenes, bromohydrin

is formed

Alkenes react with cold and concentrated H2SO4 to

form alkyl hydrogensulphates

Addition of SulphuricAcid


e.g.

The large bulky –OSO3H group makes the alkyl hydrogensulphate

very unstable. Two possible further reactions take place:

1. Regeneration of alkenes


2. Production of alcohols

In the presence of metal catalysts (e.g. Pt, Pd or Ni), H2 is

added to each atom of C = C double bond to form an alkane

Catalytic HydrogenationCatalytic Hydrogenation


e.g.

Addition of HAddition of H22O: O: HydrationHydration

35

Only Only one productone product is possible from the addition of H is possible from the addition of H22O O in presence of in presence of acids as catalysts acids as catalysts to to symmetrical symmetrical alkenesalkenes such as ethene and cyclohexene. such as ethene and cyclohexene.

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

CH3 CH3

OH

H

+ H2O

H

Unsymmetrical akenes

Symmetrical akenes

A

AA

A+ H2O

AA

H OH

AA

H3CCH3

OH

H

+ H2O

H

H

36

Addition of HCN

A

A

A

A

+H CN

A

A

A

A

CH3 CH3 + CH3 CH3

CN

H

CH3

+CH3

H

CN

HCNH

+

H+

HCN

HCNH

+

Check Point 30-3 Check Point 30-3

(b) What is the major product of each of the following reactions?

(i)

(ii)

Answer

Reaction of Alkenes

(b) (i)

(ii)

Check PointCheck Point

(c) Give the reaction products for the following reactions:

Ni(i) CH3CH = CH2 + H2

conc. H2SO4(ii) CH3CH = CHCH3

(iii) CH3CH = CHCH3 + Br2 Answer

Reaction of Alkenes

(c) (i) CH3CH2CH3

(ii)

(iii)

Ozonolysis is a widely used method for locating the double bond of an alkene

OzonolysisOzonolysis


(unstable)

The unstable ozonide is reduced directly by treatment with Zn and H2O

Polymers: Compounds that consist of very large molecules made up of many repeating units

Monomer: Each repeating unit

Polymerization:The reaction by which monomers are joined together

Addition polymerization: alkene monomers are joined together without the elimination of small molecules

Addition polymer: The polymer produced by addition polymerization

PolymerizationPolymerization


Monomer: ethene

Depending on the conditions, two kinds of poly(ethene) are

formed

Poly(ethene)


alkenes.1introduction.2nomenclature of alkenes.3physical properties of alkenes.4preparation of...

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cc bond distance

bond typical c

c double bond3

following alkenes

solidsthe alkenes

mixture of alkenes

shorter ch distance

number of double bonds