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Chapter 4
ALKENES
STRUCTURES, NOMENCLATURE, ANDAN INTRODUCTION TO
REACTIVITY
THERMODYNAMICS AND KINETICS
ORGANIC CHEMISTRY, 2ND EDITION
PAULA YURKANIS BRUICE
RAED M. AL-ZOUBI, ASSISTANT PROFESSOR OF CHEMISTRY
DEPARTMENT OF CHEMISTRY - N4L0
JORDAN UNIVERSITY OF SCIENCE AND TECHNOLOGY
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Properties of Alkenes
• Alkenes are hydrocarbons containing at least one double bond (Pi-bond)
Alkene = alkane – 2H• Alkenes are compounds with general formula (CnH2n)
Every one Pi-bond = 2H• Degree of unsaturation = no. of Pi-bond
or (2nX2+2)- (no. of H)2
1 double bond is equal to 1 degree of saturation1 ring is equal to 1 degree of saturation1 triple bond is equal to 2 degree of saturation
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Q: An unknown hydrocarbon have 9 carbons, 2 rings, 1 double bond and 1 triple bond.
What is the molecular formula?
C9H20 is the molecular formula for alkane based on no. of carbon.
Degree of saturation = 55 X 2 = 10 (no. of missing H)
So, molecular formula = C5H10
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Plane of symmetry in Alkenes
• Alkenes are flat hydrocarbons. All Sp2-Sp2 bonds and Sp3-Sp2 bonds are in the same
plane!!!
• So, 6 carbon atoms are in the same plane.
Q: How many carbons in the same plane in Methylcyclohexene?
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Nomenclature of Alkenes
1. Longest continuous chain containing the functional group:
2. Number the chain so that the functional group gets the lowest number
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3. When there are both a functional group and a substituent, the functional group gets the lowest number:
4. Cite the substituents in alphabetical order:
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5. Name with the lowest functional group number and then the lowest substituent numbers:
6. For a compound with two double bonds, the suffix is “diene”:
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1. In a cyclic alkene, the alkene functional group is always given the numbers 1 and 2.
2. The alkene substituent is always given the number 1
Nomenclature of Cycloalkenes (CnH2n-2)
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Q: What is the IUPAC name of these cycloalkenes?
1
2
6
1
2
3
1
2
5
1,6-dimethylcyclohexene
3-isopropyl-1-methylcyclohexene
1,5-dimethylcyclohexene
Answer:
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Nomenclature of Cycloalkenes with two or more double bonds
Cycloalkadiene (CnH2n-4)Cycloalkadiene (CnH2n-4)
1. In a cyclic diene, the diene functional groups are always given the least numbers and with numbers 1 and 2 on the first double bond.
1
2
3
1
2
3
2
1
3
4
4
4
6
1,3-cyclohexadiene
3-isopropyl-1-methyl-1,3-cyclohexadiene
2,6-dimethylcyclo-1,3-hexadiene
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Special Nomenclatures
Q: How many 2o allylic carbons are in methylcyclohexene?
Two secondary allyic carbons
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Alkene Isomers (Cis-Trans VS E-Z relationships)
One condition:Each Sp2 carbon should have at
least two different groups.
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E and Z Nomenclature:
Z, Zusammen E, Entgegen(Means together) (meansopposite)
E,Z is from German:
Is this Cis or Trans??
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Naming by the E,Z System
Rule 1: Consider the atomic number of the atoms bonded directly to a specific sp2 carbon:53 35 17 16 8 7 6 1I > Br > Cl > S > O > N > C > H
Higher atomic number has higher priority
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Rule 2: If there is a tie, consider the atoms attached to the tied atoms:
Rule 3: Multiple bonds are treated as attachment of multiple single bonds using “divide-duplicate.”
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Using divide-duplicate:
Rule 4: Rank the priorities by mass number in isotopes:
• Divide -bonds.• Duplicate atoms attached to each -bond.
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How alkenes react?Electrophilic addition reaction
Addition of HBr to an Alkene
• 1st Step: Addition of a proton to the alkene. The proton is the electrophile.
• 2nd Step: Bromide traps the carbocation. Bromide is the nucleophile.
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A Reaction Coordinate Diagram Describes the Reaction Pathway
Transition states have partially formed bondsIntermediates have fully formed bonds
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Transition State Versus Intermediate
Transition states have partially formed bonds
Intermediates have fully formed bonds
intermediate
intermediate
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Electrophilic Addition of HBr to 2-Butene
The rate-limiting step controls the overall rates of the reaction
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Exothermic Reaction-DGo
Endothermic Reaction
+DGo
Exothermic VS Endothermic steps
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The free energy between the transition state and the reactants.
Slow Vs Fast steps
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Q: Given the following reaction coordinate diagram for the reaction of A to D, answer the questions below?
How many steps?How many intermediates?How many transition states?Which intermediate is the most stable?Which TS is the most stable?Which step is the most exothermic?
Which step is the most endothermic?which step is the rate determining step?Which step is the slowest step?Which step is the fastest step?Is the overall reaction exothermic or endothermic?
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Q: Name the alkene below according to the IUPAC system.
A) 2-bromo-5-heptane
B) 6-bromo-2-heptene
C) 2-bromo-5-heptene
D) 5-bromo-2-heptene
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Q: How many alkenes 3-ethylpentane can produce?
A) 2
B) 3
C) 4
D)5
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Q: Which alkene can have cis/trans stereoisomers?
A) B)
C) D)
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Q: Assign the configuration of the alkenes below as Z or E, respectively.
A) E,E
B) E,Z
C) Z,Z
D) Z,E
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Q: Which one of the following groups has the highest rank when precedence is assigned according to the Cahn-Ingold-Prelog system?
A) -CH2Cl
B) -CH=O
C) -C(CH3)3
D) CH2NH2
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Q: How many alkenes, including stereoisomers, contain a tert-butyl group and have the molecular formula C7H14?
A) 3
B) 4
C) 5
D) 6
