Chapter 5
Alkenes Structure,
Nomenclature, and
an introduction to
Reactivity •
Thermodynamics
and Kinetics
Paula Yurkanis Bruice
University of California,
Santa Barbara
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Saturated and
Unsaturated Hydrocarbons
Saturated hydrocarbons have no double bonds.
Unsaturated hydrocarbons have one or more double bonds.
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Introduction
Alkenes contain a C=C double bond
H
H
H
H
C=C double bond consists of
sp2-sp2  bond
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p-p  bond
Introduction
compared to alkanes, bond lengths decrease in alkenes
133 pm
H
116.6o
H
154 pm
108 pm
H
H

121.7
H
C C
C C
H
H
H
109 pm

109.6
H
H
compared to alkanes, bond angles increase in alkenes
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Alkenes
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Introduction
• Typical representatives are
– Ethene, plant growth hormone, affects seed
germination, flower maturation, and fruit ripening.
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H
H
H
H
Introduction
• Typical representatives are
– citronellol, in rose and geranium oils
4
5
3
2
6 1
OH
7
8
7
8
6
5
4
3
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1
2
Geranium “Mavis Simpson”
Introduction
• Typical representatives are
– limonene, in lemon and orange oils
1
2
3
1
4
6
2
5
5
3
Citrus limon
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6
4
Introduction
• Typical representatives are
– -phellandrene, in oil of eucalyptus
1
2
3
1
6
2
6
5
3
5
4
4
Eucalyptus globulus
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Molecular Formulas
• Alkane: CnH2n+2
• Alkene: CnH2n
– or CnH2n+2- 2P
– P = number of double bonds
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
+ 2H
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Molecular Formulas
• Alkane: CnH2n+2
• Ring: CnH2n
– or CnH2n+2- 2R
– R = number of rings
H
H
H
H
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H
H
H
H
H
H
H
H
H
H
H
H
+ 2H
Molecular Formulas
• Alkene:
CnH2n+2- 2P-2R
P = number of double bonds
R = number of rings.
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Nomenclature of Alkenes
• The functional group is the center of reactivity
in a molecule.
• The IUPAC system uses a suffix to denote
certain functional groups.
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Nomenclature of Alkenes
• 1-1. Find the longest carbon chain.
• 1-2. Enumerate the carbons such that the
functional group, here the double bond, gets the
lowest possible number.
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Stereoisomers of an Alkene are named
using a cis or trans Prefix
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Nomenclature of Alkenes
• 2. Substituents are cited before the parent longest
chain, along with a number indicating its position
at the chain.
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Nomenclature of Alkenes
• 3. If a chain has more than one double bond, we
first identify the chain by its alkane name,
replacing the “ne” ending with the appropritate
suffix: diene, triene, etc.
• 4. If a chain has more than one substitutent,
substituents are cited in alphabetical order.
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Nomenclature of Dienes
two double bonds = diene
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Nomenclature of Alkenes
• 5. If the same number for alkene is obtained in
both directions, the correct name is the name that
contains the lowest substituent number.
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Nomenclature of Alkenes
• 6. A number is not needed to denote the position of
the double bond in a cyclic alkene because the double
bond is always placed between carbons 1 and 2.
• 7. Numbers are needed if the ring has more than one
double bond.
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Nomenclature of Alkenes
• Remember that the name of a substituent is
stated before the name of the parent
hydrocarbon, and the functional group suffix
is stated after that.
[substitutent] [parent hydrocarbon] [fucntional group
suffix]
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Nomenclature of Alkenes
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Nomenclature of Cyclic Alkenes
A number is not needed to denote the position of the
functional group; it is always between C1 and C2.
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Vinylic and Allylic Carbons
vinylic carbon: the sp2 carbon of an alkene
allylic carbon: a carbon adjacent to a vinylic carbon
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Reactions of Organic Compounds
Organic compounds can be divided into families.
All members of a family react in the same way. The family a
compound belongs to depends on its functional group.
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Each family can be put in one
of four Groups
The families in a group react in similar ways.
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How Alkenes React ; Curved Arrows
The functional group is the center of reactivity of a
molecule.
In essence, organic chemistry is about the interaction
between electron-rich atoms or molecules and
electron-deficient atoms or molecules. It is these
forces of attraction that make chemical reactions
happen.
A very simple rule:
Electron-rich atoms or molecules are attracted to
electron-deficient atoms or molecules!
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Electrophiles vs Nucleophiles
Electrophile: electron-deficient atom or molecule that
can accept a pair of electrons.
H
CH3CH2
BF3
Nucleophile: electron-rich atom or molecule that has a
pair of electrons to share.
HO
Cl
CH3NH2
H2O
A very simple rule restated:
A nucleophile reacts with an electrophile!
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Electrophiles vs Nucleophiles
Nucleophiles:
Organic molecules with double bonds
(alkenes, alkynes) are also nucleophilic.
Examples:
CH3
H3C C
H3C
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CH
Electrophiles
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Nucleophiles
A nucleophile has
a negative charge
a lone pair
a double bond
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A Nucleophile reacts
with an Electrophile
Your first organic reaction is a two-step reaction.
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Curved Arrows
first step of the reaction
Curved arrows are used to show the mechanism of a reaction.
The mechanism of a reaction is the step-by-step description of
the process by which reactants are converted into products.
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Curved Arrows
second step of the reaction
The curved arrow shows where the electrons start from
and where they end up.
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How to draw Curved Arrows
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How to draw Curved Arrows
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How to draw Curved Arrows
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How to draw Curved Arrows
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A Reaction Coordinate Diagram
A reaction
coordinate diagram
shows the energy
changes that take
place in each step
of a reaction.
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Thermodynamics and Kinetics
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The Equilibrium Constant
The equilibrium constant gives the concentration of reactants
and products at equilibrium.
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Exergonic and Endergonic Reactions
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Gibbs Free-Energy Change (∆G°)
ΔG° = free energy of the products –
free energy of the reactants
ΔH° = heat required to break bones –
heat released from forming bonds
ΔS° = freedom of motion of the products –
freedom of motion of the reactants
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A Reduction Reaction
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Catalytic Hydrogenation
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Using ∆H° Values to determine
the Relative Stabilities of Alkenes
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Using ∆H° Values to determine
the Relative Stabilities of Alkenes
The relative energies (stabilities) of three alkenes that can be catalytically
hydrogenated to 2-methylbutane. The most stable alkene has the smallest heat of
hydrogenation. (Notice that when a reaction coordinate diagram shows ∆H° values,
the y-axis is potential energy; when it shows ∆G° values, the y-axis is free energy
[Figure 5.2].)
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Relative Stabilities of Alkenes
Alkyl substituents that are bonded to the sp2 carbons
of an alkene have a stabilizing effect on the alkene.
The more alkyl substituents bonded to the sp2
carbons of an alkene, the greater is its stability.
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Stability
The stability of alkenes depends
upon number of substituents
R
H
R
H
<
H
H
R
H
<
H
R
R
R
R
R
<
R
R
The more substituents, the more stable
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Relative Stabilities of
Cis and Trans Alkenes
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Relative Stabilities of
Dialkyl-Substituted Alkenes
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Stability
Steric repulsion (Steric strain) is responsible for
energy differences among the disubstituted alkenes
H3C
H
H
H
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H
H
>
>
H3C
H3C
H3C
CH3
H
CH3
Kinetics: How fast is the
product formed?
∆G‡ = free energy of the transition state - free energy of the reactants
The greater the energy barrier, the slower the reaction.
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Rate of a Chemical Reaction
Increasing the concentration increases the rate.
Increasing the temperature increases the rate.
The rate can also be increased by a catalyst.
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An Electrophilic Addition Reaction
Transition states have partially formed bonds.
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Reaction Coordinate Diagram for each step
of the addition of HBr to 2-Butene
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Reaction Coordinate Diagram for
the addition of HBr to 2-Butene
The rate-limiting step of the reaction is the step that has its
transition state at the highest point of the reaction coordinate
diagram.
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Transition State
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Transition state
The chemical species that exists
at the transition state, with old
bonds in the process of breaking
and new bonds in the process of
forming:
TS 1

bond breaking
TS 2
H3C
CH2

H2C C
H
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CH3

Br
bond
forming
bond
forming
H
O 
H
H
A Catalyst
A catalyst provides a pathway for a reaction with a lower
energy barrier.
A catalyst does not change the energy of the starting point
(the reactants) or the energy of the end point (the products).
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Enzymes
Most biological reactions require a catalyst.
Most biological catalysts are proteins called enzymes.
The reactant of a biological reaction is called a substrate.
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The Active Site of an Enzyme
An enzyme binds its substrate at its active site.
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Enzyme Side Chains that
bind the Substrate
Some enzyme side chains bind the substrate.
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Enzyme Side Chains that
catalyze the Reaction
Some enzyme side chains are acids, bases, and
nucleophiles that catalyze the reaction.
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