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
Nomenclature of Alkenes
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
Nomenclature of Alkenes
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
Nomenclature of Alkenes
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
Nomenclature of Alkenes
(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
Preparation of Alkenes
Examples:
Preparation of Alkenes
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.
Preparation of Alkenes
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
Preparation of Alkenes
The experimental conditions of dehydration depend on the structures of alcohols
e.g.
Preparation of Alkenes
• 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
Reactions of Alkenes
Examples:
Reactions of Alkenes
Propene reacts with HBr to give 2-bromopropane
(major product) and 1-bromopropane (minor product)
Reactions of Alkenes
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.
Reactions of Alkenes
Example:
2-bromopropane is the major product because the more stable
secondary carbocation is formed in the first step
Reactions of Alkenes
Alkenes react rapidly with Br2 in 1,1,1-trichloroethane
at room temperature and in the absence of light
Addition of Bromine
Reactions of Alkenes
e.g.
The behaviour of alkenes towards Br2 in CH3CCl3 is a useful
test for the presence of carbon-carbon multiple bonds
Reactions of Alkenes
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
Reactions of Alkenes
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.
Reactions of Alkenes
When bromic(I) acid reacts with alkenes, bromohydrin
is formed
Alkenes react with cold and concentrated H2SO4 to
form alkyl hydrogensulphates
Addition of SulphuricAcid
Reactions of Alkenes
e.g.
The large bulky –OSO3H group makes the alkyl hydrogensulphate
very unstable. Two possible further reactions take place:
1. Regeneration of alkenes
Reactions 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
Reactions of Alkenes
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
Reactions of Alkenes
(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
Reactions of Alkenes
Monomer: ethene
Depending on the conditions, two kinds of poly(ethene) are
formed
Poly(ethene)
Reactions of Alkenes