6. alkenes: structure and reactivity - portale docentidocenti.unicam.it/tmp/3490.pdfalkenes...
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4. Alkenes: Structure and
Reactivity
Based on McMurry’s Organic Chemistry, 6th edition
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Alkenes
Hydrocarbons With Carbon-Carbon Double Bond
Also called olefins but alkenes is better
Because of their double bond, alkenes have fewer hydrogens than related alkanes and are therefore referred to as unsaturated(general formula CnH2n)
Includes many naturally occurring materials
Flavors, fragrances, vitamins
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Important industrial products
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Naming Alkenes
Are named according to a series of rules similar to those used for alkanes, with the suffix –ene in place of -ane
Find longest continuous carbon chain for root
Number carbons in chain so that double bond carbons have lowest possible numbers
If more than one double bond is present, indicate the position of each and use one of the suffixes –diene, -triene, and so on
Rings have “cyclo” prefix
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Naming Alkenes
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Many Alkenes Are Known by Common Names
Ethylene = ethene
Propylene = propene
Isobutylene = 2-methylpropene
Isoprene = 2-methyl-1,3-
butadiene
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Electronic Structure of Alkenes
Carbon atoms in a double bond are sp2-hybridized
Three equivalent orbitals at 120º separation in plane
Fourth orbital is atomic p orbital
Combination of electrons in two sp2 orbitals of two atoms forms bond between them
Additive interaction of p orbitals creates a orbital
Occupied orbital prevents rotation about -bond
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Cis-Trans Isomerism in Alkenes
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 two similar groups on opposite sides
Each carbon must have two different groups for these isomers to occur
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Sequence Rules: The E,Z Nomenclature
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 disubstituted double bonds
a system for comparison of Priority of Substituents has been developed
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E,Z Stereochemical Nomenclature
Priority rules of Cahn, Ingold, and
Prelog
Rank atoms that are connected at
comparison point
Higher atomic number gets higher
priority
Br > Cl > O > N > C > H
Compare where higher priority group
is with respect to bond and designate
as prefix
E -entgegen, opposite sides
Z - zusammen, together on the
same side10
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E,Z Stereochemical Nomenclature
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Extended Comparison
If atomic numbers are the same, compare at next connection point at same distance
Compare until something has higher atomic number
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Dealing With Multiple Bonds
Substituent is drawn with connections shown and no double or triple bonds
Added atoms are valued with 0 ligands themselves
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Multiple bonds
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Comparing Stabilities 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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Alkene Stability: electronic and steric effects
Less stable isomer is higher in energy
And gives off more heat
Cis alkenes are less stable than trans alkenes: minor steric
strain
Alkyl substituents (electron donors) increase the electron density
on the double bond and are better separated by a double bond
(120° vs 109°):
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Kinds of Organic Reactions
In general, we look at what occurs and try to learn how it happens
There are four broad types of organic reactions (describe the
changes)
Addition reactions – two molecules combine
Elimination reactions – one molecule splits into two
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Kinds of Organic Reactions
Substitution reactions – parts from two molecules exchange
Rearrangement reactions – a molecule undergoes changes in
the way its atoms are connected
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How Organic Reactions Occur: Mechanisms
In a clock the hands move but the mechanism behind the face is
what causes the movement
In an organic reaction, we see the transformation that has occurred.
The mechanism describes the steps behind the changes that we can
observe
Reactions occur in defined steps that lead from reactant to product
A step involves either the formation or breaking of a covalent bond
Steps can occur in individually or in combination with other steps
When several steps occur at the same time they are said to be
concerted18
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Types of Steps in Reaction Mechanisms
Formation of a covalent bond
Homogenic or heterogenic
Breaking of a covalent bond
Homolytic or heterolytic
Oxidation of a functional group
Reduction of a functional group
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Homogenic Formation of a Bond
One electron comes from each fragment
No electronic charges are involved
Not common in organic chemistry
Heterogenic Formation of a Bond
One fragment supplies two electrons
One fragment supplies no electrons
Combination can involve electronic charges
Common in organic chemistry
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Homolytic Breaking of Covalent Bonds
Each product gets one electron from the bond
Not common in organic chemistry
Heterolytic Breaking of Covalent Bonds
Both electrons from the bond that is broken become associated
with one resulting fragment
A common pattern in reaction mechanisms
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Indicating Steps in Mechanisms
Curved arrows indicate breaking and
forming of bonds
Arrowheads with a “half” head (“fish-
hook”) indicate homolytic and homogenic
steps (called ‘radical processes’)
Arrowheads with a complete head
indicate heterolytic and heterogenic steps
(called ‘polar processes’)
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Radical Reactions and How They Occur
Note: Polar reactions are more common
A “free radical” is an “R” group on its own:
CH3 is a “free radical” or simply “radical”
Has a single unpaired electron, shown as: CH3.
Its valence shell is one electron short of being complete
Radicals react to complete electron octet of valence shell
A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule
A radical can add to an alkene to give a new radical, causing an addition reaction
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Steps in Radical Substitution
Three types of steps
Initiation – homolytic formation of two reactive species with
unpaired electrons
Example – formation of Cl atoms form Cl2 and light
Propagation – reaction with molecule to generate radical
Example - reaction of chlorine atom with methane to give
HCl and CH3.
Termination – combination of two radicals to form a stable
product: CH3. + CH3
. CH3CH3
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Polar Reactions and How They Occur
Molecules can contain local unsymmetrical electron distributions
due to differences in electronegativities
This causes a partial negative charge on an atom and a
compensating partial positive charge on an adjacent atom
The more electronegative atom has the greater electron density
Carbon bonded to a more electronegative element has a partial
positive charge (+)
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Polarity of common functional groups
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Generalized Polar Reactions
Polar reactions occur between regions of high electron density and regions of low electron density
An electrophile, an electron-poor species, reacts with a nucleophile, an electron-rich species
An electrophile is a Lewis acid
A nucleophile is a Lewis base
The electron movement is represented with a curved arrow from nucleophile to electrophile
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Alkenes Reactivity
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H+
C
H
CH3
CH3
H3C
H3CC
H
CH3
CH3
H3CH3C
Br
H-Br
H+ + Br -:.... ..
Br -:....
..
The bond of an alkene (the substrate) is electron-rich, allowing it to
function as a nucleophile
H-Br is electron deficient at the H since Br is much more electronegative,
making HBr an electrophile
Let for a moment imagine H-Br dissociated in H+ and Br-
H+ adds to the part of C-C double bond, giving a carbocation
Br- adds to the positive carbon
The results is the addition of H-Br
The reaction is named “electrophilic addition” (electrophilic because the
electrophile first react with the substrate)
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Alkenes Reactivity: Electrophilic Addition
General reaction mechanism: electrophilic addition
The alkene bond attacks the electrophile (such as HBr)
Produces carbocation and bromide ion
Carbocation is an electrophile, reacting with nucleophiles (bromide ion)
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Writing Organic Reactions: Not necessarily balanced
Reactants can be before or on arrow
Solvent, temperature, details, on arrow
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Electrophilic Addition mechanism
The reaction is successful with HCl and with HI as well as HBr30
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Reaction Diagram for Addition of HBr to Ethylene
Two separate steps, each
with a own transition state
Energy minimum between
the steps belongs to the
carbocation reaction
intermediate.
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Orientation of Electrophilic Addition: 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 orientation predominates, the reaction is regiospecific
Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents)
This is Markovnikov’s rule
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+ Br_
+
+CH3 C
CH3
CH CH3
H
CH3 C
CH3
CH CH3
H
H Br
CH3 C
CH3
CH CH3
X
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Orientation of Electrophilic Addition: Markovnikov’s Rule
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Energy of Carbocations and Markovnikov’s Rule
More stable carbocation forms faster
Tertiary carbocations and associated transition states are more
stable than primary carbocations
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Normal Expectation: Faster reaction gives more stable intermediate
Intermediate resembles transition state
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Regiospecificity in Addition Reactions
If addition involves a
carbocation intermediate
and there are two
possible ways to add
the route producing the
more alkyl substituted
cationic center is lower
in energy
alkyl groups stabilize
carbocation
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Carbocation Structure and Stability
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
The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents, which tend to donate electrons to the positively charged carbon atom
Therefore stability of carbocations: 3º > 2º > 1º > +CH3
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Hyperconjugation
Electrons in neighboring filled orbital stabilize vacant antibonding orbital – net positive interaction
Alkyl groups are better than H
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End of chapter 4