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• Also called an olefin but alkene is be>er • Hydrocarbon that contains a C-‐C double bond • Includes many naturally occurring materials
– Flavors, fragrances, vitamins
Alkene -‐ Hydrocarbon With Carbon-‐Carbon Double Bond
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Industrial Prepara1on and Use of Alkenes
Ethylene and propylene are important organic chemicals produced
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Calcula1ng Degree of Unsatura1on
• Relates molecular formula to possible structures
• Degree of unsatura0on: number of mul1ple bonds or rings • Formula for a saturated acyclic compound is CnH2n+2
• Alkene has fewer hydrogens than an alkane with the same number of carbons —CnH2n because of double bond
• Each ring or mul1ple bond replaces 2 H's
5
Vinylic and Allylic Carbons
vinylic carbon: the sp2 carbon of an alkene
allylic carbon: a carbon adjacent to a vinylic carbon 6
Naming Alkenes • Name the parent hydrocarbon
• Number the carbons in chain so that double bond carbons have lowest possible numbers
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Naming Alkenes Write the full name-‐ Number subs1tuents according to: 1) Posi1on in chain, 2) Alphabe1cally
Rings have “cyclo” prefix
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Cis-‐Trans Isomerism in Alkenes
• Carbon atoms in a double bond are sp2-‐hybridized
– Three equivalent orbitals at 120º separa1on in plane – Fourth orbital is atomic p orbital
• Combina1on of electrons in two sp2 orbitals of two atoms forms σ bond between them
• Addi1ve interac1on of p orbitals creates a π bonding orbital – Subtrac1ve interac1on creates a π an1-‐bonding orbital
• Occupied π orbital prevents rota1on about σ-‐bond • Rota1on prevented by π bond -‐ high barrier, about 268 kJ/mole in ethylene
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• Rota1on of π bond is prohibi1ve • This prevents rota1on about a carbon-‐carbon double bond
(unlike a carbon-‐carbon single bond). • Creates possible alterna1ve structures
Cis-‐Trans Isomerism in Alkenes
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• The presence of a carbon-‐carbon double bond can create two possible structures – cis isomer -‐ two similar groups on 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
Cis-‐Trans Isomerism in Alkenes
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Cis-‐Trans Isomerism in Alkenes
Cis-‐Trans Isomeriza1on requires that end groups differ in pairs
Bo>om pair cannot be superposed without breaking C=C
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Alkene Stereochemistry and the E,Z Designa1on
• Cis-‐Trans naming system discussed thus far only works with disubs+tuted alkenes
• Tri, Tetra-‐ subs1tuted double bonds, and conjugated systems require more general method, referred to as the E,Z system.
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Alkene Stereochemistry and the E,Z Designa1on (Con1nued): E,Z Stereochemical Nomenclature
• Priority rules of Cahn, Ingold, and Prelog
• 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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Alkene Stereochemistry and the E,Z Designa1on Cahn-‐Ingold-‐Prelog Rules
RULE 1
• Must rank atoms that are connected at comparison point
• Higher atomic number gets higher priority
– Br > Cl > S > P > O > N > C > H
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RULE 2 • If atomic numbers are the same, compare at next connec1on point
at same distance • Compare un1l something has higher atomic number • Do not combine – always compare
Alkene Stereochemistry and the E,Z Designa1on Cahn-‐Ingold-‐Prelog Rules
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RULE 3 • Mul1ple-‐bonded atoms are equivalent to the same number of
single-‐bonded atoms • Subs1tuent is drawn with connec1ons shown and no double or
triple bonds • Added atoms are valued with 0 ligands themselves
Alkene Stereochemistry and the E,Z Designa1on Cahn-‐Ingold-‐Prelog Rules
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Stability of Alkenes • Cis alkenes are less stable than trans alkenes • Compare heat given off on hydrogena1on: ΔHo
• Less stable isomer is higher in energy – And gives off more heat – tetrasubs1tuted > trisubs1tuted > disubs1tuted > monosusb1tuted
– hyperconjuga:on stabilizes
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Rela1ve Stabili1es of Alkenes • Evaluate heat given off when C=C is converted to C-‐C • More stable alkene gives off less heat
– trans-‐Butene generates 5 kJ less heat than cis-‐butene
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Stability of Alkenes-‐ Hyperconjuga1on
• Electrons in neighboring filled σ orbital stabilize vacant an1bonding π orbital – net posi1ve interac1on
• Alkyl groups are be>er than H
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Electrophiles
An electrophile has a posi1ve charge, a par1al posi1ve charge, or an incomplete octet.
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Curved Arrows
Curved arrows are used to show the mechanism of a reac1on.
The mechanism of a reac1on is the step-‐by-‐step descrip1on of the process by which reactants are converted into products.
first step of the reac1on
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second step of the reac1on
The curved arrow shows where the electrons start from and where they end up.
Curved Arrows
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Reac1on Coordinate Diagram for the Addi1on of HBr to 2-‐Butene
The rate-‐limi1ng step of the reac1on is the step that has its transi1on state at the highest point of the reac1on coordinate diagram. 29
Electrophilic Addi1on of Alkenes • The reac1on is successful with HCl and with HI as well as
HBr
• HI is generated from KI and phosphoric acid
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Orienta1on of Electrophilic Addi1ons: Markovnikov’s Rule
• In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other
• If one orienta1on predominates, the reac1on is regioselec0ve • Markovnikov observed in the 19th century that in the addi1on of
HX to alkene, the H a>aches to the carbon with more H’s and X a>aches to the other end (to the one with more alkyl subs1tuents) – This is Markovnikov’s rule
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• Addi1on of HCl to 2-‐methylpropene • Regiospecific – one product forms where two are possible • If both ends have similar subs1tu1on, then not regiospecific
Example of Markovnikov’s Rule
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Markovnikov’s Rule • More highly subs1tuted carboca1on forms as intermediate
rather than less highly subs1tuted one
• Ter1ary ca1ons and associated transi1on states are more stable than primary ca1ons
33
Carboca1on Structure and Stability
• Carboca1ons 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 • the stability of the carboca1on (measured by energy needed to
form it from R-‐X) is increased by the presence of alkyl subs1tuents
35
Carboca1on Structure and Stability
A plot of ΔH dissocia1on shows that more highly subs1tued alkyl halides dissociate more easily than less highly subs1tuted ones
37
The Hammond Postulate
• If a carboca1on intermediate is more stable than another, why is the reac1on through the more stable one faster? – the rela1ve stability of the intermediate is related to an equilibrium constant (ΔGº)
– the rela1ve stability of the transi1on state (which describes the size of the rate constant) is the ac1va1on energy (ΔG‡)
– the transi1on state is transient and cannot be examined – What does the Hammond Postulate state? “the structure of a transi1on state resembles the structure of the nearest stable species. Transi1on states for endergonic steps structurally resemble products, and transi1on states for exergonic steps structurally resemble reactants”
38
The Hammond Postulate (Con1nued): Transi1on State Structures
• A transi0on state is the highest energy species in a reac1on step
• By defini1on, its structure is not stable enough to exist for one vibra1on
• But the structure controls the rate of reac1on • So we need to be able to guess about its proper1es in an
informed way • We classify them in general ways and look for trends in
reac1vity – the conclusions are in the Hammond Postulate
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Examina1on of the Hammond Postulate
• A transi1on state should be similar to an intermediate that is close in energy
• Sequen1al states on a reac1on path that are close in energy are likely to be close in structure -‐ G. S. Hammond
40
Compe1ng Reac1ons and the Hammond Postulate
• Normal Expecta1on: Faster reac1on gives more stable intermediate
• Intermediate resembles transi1on state
41
Evidence for the Mechanism of Electrophilic Addi1on: Carboca1on Rearrangments
• Carboca1ons undergo structural rearrangements following set pa>erns
• 1,2-‐H and 1,2-‐alkyl shits occur
• Goes to give moststable carboca1on
• Can go through less stable ions as intermediates
42