Transcript Chapter 1 Chemical Bonding and Chemical Structure

Chapter 4
Introduction to Alkenes.
Structure and Reactivity
Alkenes
• Hydrocarbons containing one or more carboncarbon double bonds
• Sometimes called olefins
• Classified as unsaturated hydrocarbons
• Ethylene is the simplest alkene
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Physical Properties
• Very similar to corresponding alkanes:
– Flammable
– Non-polar
– Insoluble in Water
– Low melting and boiling points
– Low density compared to water
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Bond Length
• Carbon-carbon multiple bonds are shorter
than their carbon-carbon single bond
counterparts
133.9 pm
153.6 pm
Dipole moments and polarization
Structure and Bonding in Alkenes
• Carbons involved in the carbon-carbon double
bond exhibit trigonal planar geometry
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Carbon Hybridization in Alkenes
• sp2-hybridization  Trigonal planar geometry
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• sp2 = 33% s character, 66% p character
• More s character → electrons are held closer
to the nucleus
Carbon Hybridization in Alkanes
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Carbon-carbon single bond in propene = 150 pm
Carbon-carbon single bond in propane = 154 pm
Due to sp2 vs. sp3 hybridization
Bonds with more s character are shorter
Hybrid Orbital Picture of Ethylene
4.1 Structure and Bonding in Alkenes
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• s bond: head-to-head overlap of orbitals
• Cylindrically symmetric
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 bond: side-to-side overlap of p orbitals
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 Orbital System in Ethylene
4.1 Structure and Bonding in Alkenes
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EPM of Ethylene
• The filled  molecular orbital is the  bond
• Most of the important reactions of alkenes
involve the electrons of the  bond
4.1 Structure and Bonding in Alkenes
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Rules for Naming Alkenes
1) Find the longest chain that includes the double bond and
name this as the parent chain by changing the ending from
“-ane” to “-ene”
• Hexane  Hexene
• If there are two possibilities, choose the principle chain
to be the one with the greatest number of double bonds
2) Number the chain so that the double bond has the lowest
number possible and place that number before the parent
chain
• The double bond takes precidence over the substituents
when numbering the chain
3) Identify the substituents with number and name
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Problems
• Name the following:
4) If there are more than one double bonds in
the parent chain, identify each double
bond’s position by number and indicate
how many are present using Greek prefixes
preceded by the letter “a.”
– Example: -adiene, -atriene, etc.
Problems
• Name the following:
Substituents Containing Alkenes
• Some common groups with alkenes
• Nonsystematic traditional names
4.2 Nomenclature of Alkenes
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Problems
• Name the following compounds:
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Problems
• Name the following compounds:
CH3CH=CHCH3
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• Stereoisomers: Molecules that have the same
molecular formula, same atom connectivity, but a
different spatial arrangement
– Cis-trans isomers:
• Recall: Constitutional/Structural Isomers: same
molecular formula, different atom connectivity
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Restricted Rotation of Alkenes
4.1 Structure and Bonding in Alkenes
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Properties of 2-butene
• Cis-2-Butene
– M.P. = -139°C
– B.P. = 3.7°C
– Density = 0.6213 g/mL
• Trans-2-Butene
– M.P. = - 105°C
– B.P. = 1°C
– Density = 0.6041 g/mL
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Requirements for Cis-Trans Isomerism
• Only when both carbons are bonded to two
different groups are cis-trans isomers possible
• Compounds that have one of their carbons
bonded to two identical groups can’t exist as
cis-trans isomers
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Problems
1) Which of the following compounds can exist
as pairs of cis-trans isomers? Draw each cistrans pair.
a)
b)
c)
d)
CH3CH=CH2
CH3CH2CH=CHCH3
(CH3)2C=CHCH3
ClCH=CHCl
2) There are two isomers for 3-methyl-2pentene. Draw both of them. Which one is
cis and which one is trans?
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E,Z Nomenclature System
• cis vs trans is not always clear
• Cahn-Ingold-Prelog system: Assign priorities
• Z (zusammen): “together”
– On Zee Zame Zide
• E (entgegen): “across”
– E = Enemies = On Opposite Sides
4.2 Nomenclature of Alkenes
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Priority Assignment Rules
1) Examine each of the double bond carbons
separately. Identify the two atoms directly attached
to the C and rank them according to atomic #.
– Higher atomic # = higher priority
– Higher isotopic mass = higher priority
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2) If the first atoms connected to the double
bond carbon are the same, continue moving
outward until the first point of difference
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3) Multiple-bonded atoms are equivalent to the
same number of single bonded atoms
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Problems
• Using the Cahn-Ingold-Prelog rules, identify
whether the following molecules are E or Z:
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Problems
• Give the IUPAC names for each of the
following molecules, including the E, Z
designation.
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What’s the Structure of C6H10?
• Saturated is C6H14
– Therefore 4 H's are not
present
• So, what does it look like?
– Double bond(s)?
– Triple bond(s)?
– Ring(s)?
– Ring and double bond?
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Degree of Unsaturation
• Gives info on number of rings and/or  bonds
• Maximum # of H’s in a hydrocarbon: CnH2n+2
• Each ring and/or  bond reduces number of
hydrogens by 2
• aka: unsaturation number
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• For C6H10 the unsaturation # is 2
– Two double bond
– Two rings
– One double bond and a ring
– One triple bond
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Calculating Degree of Unsaturation for Compounds
Containing Elements Other than Carbon and Hydrogen
• Halogens are counted as 1 H:
4.3 Unsaturation Number
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• Nitrogen increases H count by 1:
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• Ignore Oxygens
– O’s form two bonds, don’t affect the formula of an
equivalent hydrocarbon
– No change in number of H atoms if insert an O
into a hydrocarbon compound
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Problems
• Calculate the degree of unsaturation for the
following molecules
1)
2)
3)
4)
5)
6)
C12H20
C4H6
C6H5N
C6H5NO2
C8H9Cl3
Draw as many structures as you can for #2
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Alkene Stability
• Who’s more stable?
Product Ratio
• Interconversion between cis and trans isomers can
be made to happen using a strong acid catalyst
• More stable product is favored
Heats of combustion
ΔH°combustion = -2685.5 kJ/mol
ΔH°combustion = -2682.2 kJ/mol
• The cis configuration is more strained = higher energy
Heats of Hydrogenation
• More stable alkene gives off
less heat
Effects of Branching on Alkene Stability
Relative Stabilities of Alkene Isomers
• From heats of hydrogenation (DH°hydrog):
• More alkyl substituents on double bond → more
stable alkene
• In general, the number of alkyl groups is more
important than the identity
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Hyperconjugation
• A stabilizing interaction between a vacant p orbital or antibonding π orbital on one atom with the electrons in a
neighboring σ bond (usually C-H or C-C)
– In alkenes, occurs between the unfilled anti-bonding C=C π
bond orbital and the electrons of a C-H σ bond of a
neighboring substituent
• More substituents present = more opportunities for
hyperconjugation = more stable alkene
Bond Strength
• sp2-sp3 bond is stronger than sp3-sp3
• More highly substituted alkenes have higher ratio of sp2-sp3 to
sp3-sp3
Addition Reactions of Alkenes
• Characteristic reaction of alkenes
• The  bond of carbon-carbon double bond is:
– Broken
– Nucleophilic
• Three main types of alkene addition rxns:
– Rxn with hydrogen halides (ex: HCl)
– Catalytic hydrogenation
– Hydration
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Alkene Reactions with Hydrogen Halides
• Called “Electrophilic Addition of Alkenes”
• HF, HCl, HBr, HI add to alkenes → alkyl halides
• HF is rarely used due to its extreme toxicity
– Lung and cornea damage
– Eventual cardiac arrest and death
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Electrophilic Addition Energy Path
• Two step process
• First transition state is
high energy point
Problems
• Draw the mechanism for the following reactions
Writing Organic Reactions
Markovnikov’s Rule
• In the addition of HX to an alkene:
– the H attaches to the carbon with fewer alkyl substituents
– the carbocation forms on the more substituted carbon of
the C=C
– the X attaches to the more substituted carbon
• When one of the possible isomer products predominates, the
reaction is said to be regiospecific or regioselective
Problems
1) Complete the following reaction showing the
complete mechanism
2) What alkene would you start with to prepare
the following alkyl halide?
Regioselectivity of Addition
• Unsymmetrical alkenes can give regioisomers
• If both carbons of the C=C have similar
substitution, then reaction is not regiospecific
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Carbocation Structure and Stability
• The more highly substituted carbocation is more
stable
– Formation of more substituted carbocation
intermediate is favored
Why?
1) Inductive Effect
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Electrons from groups neighboring the carbocation
can shift toward the positive charge
Larger groups can “donate” electrons more easily
than H can
2)
Hyperconjugation
• Carbocations are planar and the tricoordinate carbon is
surrounded by only 6 electrons in sp2 orbitals
• The fourth orbital on carbon is a vacant p-orbital
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Alkyl groups can share electrons with the empty
neighboring p orbital
Problems
• Predict the products of the following reactions:
Problems
• Which alkenes would you start with to
prepare the following alkyl halides?
Carbocation Rearrangements
• Sometimes, we get an unexpected product
• In a Carbocation Rearrangement, a group (alkyl, aryl,
or hydride) moves to a different position so that a
more stable carbocation can form
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Alkyl Shift
Hydride Shift
Hydride shifts in biological molecules
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Problem
• On treatment with HBr, vinylcyclohexane
undergoes addition and rearrangement to
yield the following product. Propose a
mechanism to account for this result
Catalysis
• Catalyst: A substance that increases the
reaction rate without being consumed
– Lowers the Ea
• Heterogeneous
• Homogeneous
4.9 Catalysis
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Catalytic Hydrogenation of Alkenes
• Catalyst: Pt or Pd as
– In powdered form on carbon
• Heterogeneous catalyst
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• Example:
• Aromatic  bonds are less reactive
4.9 Catalysis
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Problems
1) What product would you obtain from the
catalytic hydrogenation of
(CH3)2C=CHCH2CH3?
2) What product would you obtain from the
reaction of 3,3-Dimethylcyclopentene with
Palladium on Carbon in the presence of H2?
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Hydration of Alkenes
Alkene
• Acid-catalyzed addition of H2O across an
double bond
• Homogeneous catalyst
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Problems
• Draw out the mechanism and products for the
following reactions:
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Problems
• What alkenes might the following alcohols
have been prepared from?
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Enzymes Catalysis
• Nature produces catalysts called enzymes
• Most biological mechanisms would be too
slow to be useful without enzymes
4.9 Catalysis
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Principle of Microscopic Reversibility
• Generally, the reverse reaction follows the
exact reverse of forward mechanism
• Dehydration is the reverse of hydration
4.9 Catalysis
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Reaction Rates
• Commonly, two or more reactions are in
competition
4.8 Reaction Rates
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The Transition State
• Forward and reverse reaction have same
transition state (‡)
4.8 Reaction Rates
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The Energy Barrier
• Molecules must possess enough energy to get
over the transition state
• Maxwell-Boltzmann distribution
4.8 Reaction Rates
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Multistep Reactions
• Intermediates are formed in many reactions
• Rate-determining step: Controls reaction rate
4.8 Reaction Rates
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Hammond’s Postulate
• The structure and energy of the transition
state can be approximated by the structure
and energy of the intermediate
4.8 Reaction Rates
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