Review and Preview
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Alkenes and alkynes are hydrocarbons
Newman projections illustrate conformations
Cycloalkanes use the cis/trans nomenclature.
Physical properties of alkanes and cycloalkanes
are similar to each other.
• Alkene and alkyne stability relate to their structure
and degree of substitution.
• Pi bonds in alkenes and alkynes gives them types
of reactivity that alkanes lack
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As You Study…
As you study, here are some concepts and skills of particular
importance:
• How do alkanes, alkenes, and alkynes differ from one another?
• What orbitals are involved in forming bonds in each class of
compound?
• What configurational isomers are possible for alkenes and how
do we provide names that distinguish these isomers from one
another?
• What aspects must be considered when thinking about ring
compounds that incorporate double or triple bonds?
• What effect do substituents have on the relative stabilities of
alkenes and alkynes, and how are relative stabilities
measured?
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Alkanes and Alkenes
• Alkanes have the formula CnH2n+2.
• Cycloalkanes with one ring have a different
formula: CnH2n.
• A second family of hydrocarbons also has the
formula CnH2n. These are known as alkenes.
• Alkynes are a third family of hydrocarbons having
the formula CnH2n–2.
• Alkenes and alkynes are unsaturated; they contain
fewer hydrogen atoms than alkanes.
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Ethene (the Simplest Alkene)
• Ethene has two sp2hybridized C atoms
• Each C has one
unhybridized p orbital
as well
• The sp2 orbitals are
in a trigonal plane,
120°apart.
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2pz Orbitals and the Pi (p) Bond
• One p orbital on each C of ethene form a bond
known as a pi bond.
• Overlapping sp2 orbitals form a sigma (s) bond.
• The p bond is formed at a 90°angle from the s
bond.
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2pz Orbitals and the Pi (p) Bond
• The figure shows p orbitals (blue) overlapping to
form a pi bond and the sp2 orbitals (green)
overlapping to form a sigma bond. A line bond
structure of ethene is shown on the right.
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Derivatives and Isomers of
Alkenes
• Derivatives of ethylene yield new compounds.
• Replacing one H atom with some “X” group gives
the vinyl compound.
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Derivatives and Isomers of
Alkenes (cont.)
• Consider a simple alkene in which a methyl
group and an X substituent are at either side of
the double bond. There are two possible
isomers of
such a
compound:
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Derivatives and Isomers of
Alkenes (cont.)
• A cis- or Z- label is used for disubstituted alkenes
like these in which the non-H substituents are
pointing towards the same side of the C=C
• A trans- or E- label is used for disubstituted alkenes
like these in which the non-H substituents are
pointing towards opposite sides of the C=C
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Derivatives and Isomers of
Alkenes (cont.)
• A label is added prior to the name of the alkene to
tell which isomer it is:
• Demonstrating when and how these labels are
used is a key purpose in this chapter.
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Cis-Trans and E-Z Isomerism
• Similar groups on same side of double bond,
alkene is cis
• Similar groups on opposite sides of double bond,
alkene is trans
• E-Z nomenclature used when cis-trans no longer
applicable.
• If high priority groups are on the same side, the
name is Z (for zusammen).
• If high priority groups are on opposite sides, the
name is E (for entgegen).
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IUPAC Nomenclature
• Parent is longest chain containing the double bond
-ane changes to -ene (or -diene, -triene)
• Chains are numbered to give the double bond the
smallest possible number.
• In a ring, the double bond is assumed to be
between carbon 1 and carbon 2.
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Nomenclature Examples
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E-Z Nomenclature
• Uses the Cahn-Ingold-Prelog rules to assign
priorities to groups attached to each carbon in the
double bond
– Step 1-Use the priority system to distinguish atoms on
the basis of atomic number.
The atom of higher atomic number has the higher priority.
– Step 2-For isotopes, atomic mass is used to break the
tie in atomic number.
– Step 3-Nonisotopic ties are broken by looking at the
groups attached to the tied atoms.
– Step 4-Multiple bonds attached to alkenes are treated
as multiplied bonds.
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E-Z Nomenclature (cont.)
• Rule 1:
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E-Z Nomenclature (cont.)
• Rule 2:
• Rule 3:
Section 3.5
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E-Z Nomenclature (cont.)
• Rule 4: A special treatment is needed for
substituents that have multiple bonds. We treat
them as multiplied
single bonds:
• When prioritizing:
As a substituent, the unit on the left is treated as
the unit on the right
Section 3.5
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Alkene Stability: Heats of
Formation (DHof)
• DHof of a compound is the enthalpy of formation
from its constituent elements in their standard
states
• Observe the pair of disubstituted alkenes; note the
cis- and trans-3 hexene.
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Cis- versus Trans- and
Degree of Substitution
• Cis-isomerism forces groups to eclipse each other
(less stable; more positive DHof value).
• The trans-isomer is the more stable compound
(more negative DHof value).
• More substituted alkenes are more stable.
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Cycloalkenes
• Rings larger than cyclopropene can have more
than one double bond.
• Cis-cycloalkenes are more stable than transcycloalkenes due to ring strain.
• Trans-cycloalkenes are severely twisted and highly
destabilized.
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Cis- and Trans- Example
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Natural Cycloalkenes
• Many known cyclic and polycyclic compounds are
found in nature.
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Bredt’s Rule
• A bridged bicyclic compound cannot have a
double bond at a bridgehead position unless one
of the rings contains at least eight carbon atoms.
• The bridgehead position is the point at which
the bridges meet.
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Bicyclic Systems
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Physical Properties of Alkenes
• Differ only slightly from alkanes
• Odor is more pungent (trivial name is olefins,
referring to their smell)
• In general, alkane mp and bp increase as
molecular weight increases.
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Physical Properties of Alkenes (cont.)
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Alkynes
• C-C triple bond is shorter than a C-C double bond,
which is shorter than a C-C single bond
• Greater s character leads to shorter bond
• Internal alkynes are more stable than terminal
alkynes
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Relative Heats of
Formation for Alkynes
• DHf for alkynes much less stable than their
constituent elements
• The more p bonds in a molecule, the more positive
the DHf.
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Triple Bonds in Cyclics
• These can occur in rings.
• Undergo severe ring strain due to 180°linear
geometry
• Overlap between 2p orbitals of a p bond is reduced,
forming the ring.
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Strain in Cyclic Alkynes
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Alkyne Physical Properties
• Similar to alkanes and alkene
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Alkyne Acidity
• Alkanes/alkenes have high pKa (40–44) values.
• Terminal alkynes differ; they are relatively strong
proton donors.
• An acetylide anion is produced by alkyne
deprotonation
• Alkynes are more acidic due to higher s
character (50% s)
• Greater s character = lower energy of electron =
more stable anion
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Degree of Unsaturation
• DoU for a hydrocarbon = total number of p bonds
and rings in the molecule
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Alkene and Alkyne Addition Rxns
• Pi bond acts as a proton acceptor and HX is the
proton donor
• H-X s bond is broken and new bonds are formed to
the carbons of the old p system
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HX Addition Mechanism
• Alkene is protonated by acid first.
• Halide addition occurs in the second step as soon
as the carbocation is generated.
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Regiochemistry of the Addition
Rxn
• Addition to either end of a symmetrical alkene
generates no preference for the emerging
carbocation made after alkene protonation.
• Regiochemistry (a chemical distinction) is
important when you have an unsymmetrical alkene.
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Regiochemistry of Addition
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Order of Carbocation Stability
• More substituted carbocation is preferred upon
alkene protonation.
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Alkene Hydration
• Alkene + acid catalyst  alcohol.
• Alkene is protonated by H+ first, then water
molecule attacks newly formed carbocation.
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Hydration Mechanism
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Study Guide for Chapter 3
Slides
3-5
6-9
10
11-14
15-16
17-18
19-23
24
25
26
27-28
29
30
31
32
33
34-36
37-38
Section(s)
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.12
3.13
3.14
3.15
3.16
3.17
3.18
3.19
Topic
Preview
Alkenes: Structure and Bonding
Derivatives and Isomers of Alkenes
Nomenclature
The Cahn–Ingold–Prelog Priority System
Relative Stability of Alkenes: Heats of Formation
Double Bonds in Rings
Physical Properties of Alkenes
Alkynes: Structure and Bonding
Relative Stability of Alkynes: Heats of Formation
Triple Bonds in Rings
Physical Properties of Alkynes
Acidity of Alkynes
Molecular Formulas and Degrees of Unsaturation
An Introduction to Addition Reactions
Mechanism of Addition of HX
Regiochemistry of the Addition Reaction
A Catalyzed Addition to Alkenes: Hydration
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Chapter 3 Checklist
Be Able To:
 Describe the bonding model for alkenes and alkynes and sketch the orbitals involved
 Name alkenes, including proper use of cis-/trans- and E-/Z- notation for geometric
isomers
 Use the Cahn-Ingold-Prelog Rules to assign priorities to substituents
 Correlate stability of alkenes with structure in terms of electronic and steric arguments
 Discuss how ring size and cis-/trans- isomerism dictate placement of multiple bonds in
cyclic molecules
 Use Bredt’s Rule to predict possible placement of multiple bonds in a polycyclic
 Compare/contrast physical properties of alkanes, alkene, and alkynes
 Discuss relative stabilities of alkenes on the basis of cis/trans isomerism and degree
of substitution
 Rationalize the acidity of alkynes compared to that of alkanes or alkenes
 Predict products of addition reactions to alkenes (Addit. Of HX and Hydration)
 Provide and discuss the mechanism of addition reactions and give rationale for
observed regiochemistry
 Do all problems at the end of Chapter 3 in your text; pay particular focus to any
assigned homework:
By
, you should be a master of Chapter 3 fundamentals.
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