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Chapter 7- Alkenes: Structure and Reactivity
Ashley Piekarski, Ph.D.
Alkene
• What is an alkene func<onal group? • hydrocarbon with carbon-carbon double bond • occurs in many natural materials (flavors,
fragrances, vitamins)
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Why am I learning this, Dr. P?
• C-‐C double bonds are present in most organic and biological molecules
• Alkene stereochemistry
• Focus on a general alkene reac<on: Electrophilic addi4on
Use of alkenes
• Ethylene and propylene are the most important organic chemicals produced
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Degree of Unsaturation
• Are alkenes saturated or unsaturated? • Formula for saturated acyclic compound is
CnH2n+2
• Degree of unsatura<on: number of mul<ple bonds or rings • Each ring or multiple bond replaces 2H’s
Learning check
• How many degrees of unsaturated are in C6H10? What are some possible structure for this molecular formula?
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With other elements
• Organohalogens (X: F, Cl, Br, I) • Halogen replaces a hydrogen
• For example, C4H6Br2 and C4H8 have one degree of unsatura<on
• Organoxygen compounds (C, H, O) • These don’t affect the total count of H’s
Organonitrogen compounds
• Nitrogen has three bonds • So if it connects where H was, it adds a
connection point • Subtract one H for equivalent degree of
unsaturation in hydrocarbon
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Summary
• Count pairs of H’s below CnH2n+2
• Add number of halogens to number of H’s • Ignore oxygens • Subtract N’s-‐ they have two connec<ons
Learning check
• Calculate the degree of unsatura<on for C10H12N2O3
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Nomenclature
• Name the parent hydrocarbon • Number carbons in chain so the double-‐bond carbons have the lowest possible numbers
• Rings have a “cyclo” prefix
Common names
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Learning check
(E)-4-methylhept-2-ene(E)-hex-2-ene
4-ethylcyclopent-1-ene
Cl
(1S,3S)-1-chloro-3-ethylcyclopentane
(1R,3S)-1,3-diethylcyclopentane
Cl
(Z)-4-chloro-3-methyloct-3-ene (Z)-4-ethyl-2,3-dimethylhept-3-ene
+ HBr
Cl
• Name the following alkenes:
(E)-4-methylhept-2-ene(E)-hex-2-ene
4-ethylcyclopent-1-ene
Cl
(1S,3S)-1-chloro-3-ethylcyclopentane
(1R,3S)-1,3-diethylcyclopentane
Cl
(Z)-4-chloro-3-methyloct-3-ene (Z)-4-ethyl-2,3-dimethylhept-3-ene
+ HBr
Cl
Stereochemistry
• Make a model of 1,2-‐dichloroethene. • Can you rotate the double-‐bond, like an alkane?
• This creates two possible isomers for alkenes!
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Cis- and Trans- Isomers
• The presence of a carbon-‐carbon double bond can create two possible structures • cis isomer- 2 similar groups on the same side
of the double bond • trans isomer- similar groups on opposite sides
• Each carbon must have two different groups for these isomers to occur
• Now, draw the cis-‐ and trans-‐ isomers for 1,2-‐dichloroethene
Cis- and Trans- Isomers
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Sequence Rules: The E, Z Designation
• Neither compound is clearly “cis” or “trans” • Substituents on C1 are different than those on
C2 • We need to define “similarity” in a precise way
to distinguish the two stereoisomers • Cis, trans nomenclature only works for disubs<tuted double bonds
E, Z Stereochemistry
• Compare where higher priority groups are with respect to bond and designate as prefix • E –entgegen, opposite sides • Z –zusammen, together on the same side
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Ranking Priority Rules!
• Rule 1 • Rank atoms that are connected at comparison
point • Higher atomic number gets higher priority
Ranking Priority Rules!
• Rule 2 • If atomic numbers are the same, compare at
next connection point at same distance • Compare until something has higher atomic
number • Do not combine- always compare
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Ranking Priority Rules!
• Rule 3 • Substituent is drawn with connections shown
and no double or triple bonds • Added atoms are valued with 0 ligands
themselves
Learning check
• Name the following alkenes using E, Z configura<on:
(E)-4-methylhept-2-ene(E)-hex-2-ene
4-ethylcyclopent-1-ene
Cl
(1S,3S)-1-chloro-3-ethylcyclopentane
(1R,3S)-1,3-diethylcyclopentane
Cl
(Z)-4-chloro-3-methyloct-3-ene (Z)-4-ethyl-2,3-dimethylhept-3-ene
+ HBr
Cl
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Stability of Alkenes
• Cis alkenes are less stable than trans alkenes • Compare heat given off on hydrogena<on • Less stable isomer is higher in energy
Comparing Stabilities of Alkenes
• Evaluate heat given off when C=C is converted to C-‐C bond
• More stable alkene gives off less heat
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Hyperconjugation
• Hyperconjuga3on is the stabilizing interac<on between filled pi orbital and a neighboring filled C-‐H sigma bond on a subs<tuent. The more subs<tuents there are, the greater the stabiliza<on of the alkene
Electrophilic Addition of Alkenes- Mechanism
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Electrophilic Addition Energy Diagram
• Two step process • First transi<on state is high energy point
Electrophilic Addition Examples
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Markovnikov’s Rule
• Addi<on is regiospecific • Halide will attack the carbon that is more
substituted
Markovnikov’s Rule
• Why? • What is the intermediate of an electrophilic
addition reaction?
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Learning check
• Predict the product for the following reac<on:
(E)-4-methylhept-2-ene(E)-hex-2-ene
4-ethylcyclopent-1-ene
Cl
(1S,3S)-1-chloro-3-ethylcyclopentane
(1R,3S)-1,3-diethylcyclopentane
Cl
(Z)-4-chloro-3-methyloct-3-ene (Z)-4-ethyl-2,3-dimethylhept-3-ene
+ HBr
Cl
Learning check
• Retrosynthesis: predict the reactants to make the following product
(E)-4-methylhept-2-ene(E)-hex-2-ene
4-ethylcyclopent-1-ene
Cl
(1S,3S)-1-chloro-3-ethylcyclopentane
(1R,3S)-1,3-diethylcyclopentane
Cl
(Z)-4-chloro-3-methyloct-3-ene (Z)-4-ethyl-2,3-dimethylhept-3-ene
+ HBr
Cl
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Carbocation Structure and Stability
• Carboca<ons 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
Carbocation Structure and Stability
• The stability of the carboca<on (measured by energy needed to form it from R-‐X) is increased by the presence of alkyl subs<tuents
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Inductive stabilization
The Hammond Postulate
• If carboca<on intermediate is more stable than another, why is the reac<on through the more stable one faster? • The relative stability of the intermediate is related to
an equilibrium constant (ΔGº) • The relative stability of the transition state (which
describes the size of the rate constant) is the activation energy (ΔG‡)
• The transition state is transient and cannot be examined
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Transition State Structures
• A transi3on state is the highest energy species in a reac<on step
• By defini<on, its structure is not stable enough to exist for one vibra<on
• But the structure controls the rate of reac<on
• So we need to be able to guess about its proper<es in an informed way
• We classify them in general ways and look for trends in reac<vity – the conclusions are in the Hammond Postulate
Hammond Postulate
• A transi<on state should be similar to an intermediate that is close in energy
• Sequen4al states on a reac4on path that are close in energy are likely to be close in structure -‐ G. S. Hammond
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Competing Reactions
• Normal Expecta<on: Faster reac<on gives more stable intermediate
• Intermediate resembles transi<on state
Rearrangements of Carbocations
• Carboca<ons undergo structural rearrangements following set pajerns
• 1,2-‐H and 1,2-‐alkyl shiks occur • Goes to give more stable carboca<on • Can go through less stable ions as intermediates
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