Transcript Lecture 11

Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
Heterolytic nucleophilic
substitution:SN1 and SN2:
halogenoalkanes with
nucleophiles
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Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
When carbon is bonded
This can result in
to a more
complete
electronegative atom
either heterolytic
breakage of the
bond.
This atom applies an
inductive effect on the
electrons in the bond,
pulling the electrons
towards it.
or
This can result in
charge
separation along
the bond.
If the bond breaks
heterolytically, two
ions, a halide and a
carbocation, are
made.
Charge separation
creates an electron
deficient carbon
atom which is a
reactive site with a
center of positive
charge.
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Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
These two possibilities lead to two different reaction mechanisms:
SUBSTITUTION BY A NUCLEOPHILE; FIRST ORDER
KINETICS: SN1
Stage 1
The carbon-halogen
bond breaks
heterolytically.
Stage 2
The carbocation is attacked by a nucleophile.
This step is very fast, so the rate determining
step is stage 1. The rate of stage 1 depends
only on the concentration of halogen
compound, so the kinetics are first order.
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Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
These two possibilities lead to two different reaction mechanisms:
SUBSTITUTION BY A NUCLEOPHILE; FIRST ORDER
KINETICS: SN1
Stage 3
In the product, the halogen atom has been substituted by the
nucleophile so the result is a substitution reaction. Because the
carbocation is planar, it can be attacked equally well form either side.
If the product has an asymmetric carbon atom, equal amounts of each
optical isomer will be made. The product, although it contains
optically active particles, contains equal amounts of each kind and so
will not affect plane polarized light. It is called a racemic mixture.4
Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
SUBSTITUTION BY A NUCLEOPHILE; SECOND ORDER
KINETICS: SN2
Stage 1
Initial attack is by nucleophile on the electron deficient carbon
atom.
This initial step is the rate determining step and involves a
collision between two particles, the halogen compound and the
nucleophile, so the kinetics are second order.
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Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
SUBSTITUTION BY A NUCLEOPHILE; SECOND ORDER
KINETICS: SN2
Stage 2
The intermediate has a high energy, which means that the overall
rate of reaction is slow.
The carbon being attacked is saturated and has a full outer shell
because carbon cannot expand its outer shell. As the electron pair
from the nucleophile moves in, the pair forming the sigma bond
to the halogen moves away. This bond breaks heterolytically.
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Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
SUBSTITUTION BY A NUCLEOPHILE; SECOND ORDER
KINETICS: SN2
Stage 3
In the product the halogen has been substituted by the nucleophile so
the reaction is a substitution.
The halide ion is a leaving group. The strength of the bond to the
leaving group and the stability of the leaving group will affect the
amount of product make.
If the product has an asymmetric carbon atom, only one optical
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isomer will be made.
Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles
Words
Words and Expressions
heterolytic nucleophilic substitution
halogenoalkane
inductive effect
carbocation
substitute; substitution
intermediate
asymmetric
optical isomer
racemic
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Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds with
hydrogen cyanide
Heterolytic nucleophilic addition and
addition/elimination: carbonyl
compounds with hydrogen cyanide
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Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds with
hydrogen cyanide
Hydrogen cyanide adds across the double bond in a
carbonyl compound, if there is a trace of the catalyst
potassium cyanide present.
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Heterolytic nucleophilic addition and addition/elimination:carnonyl
compounds with hydrogen cyanide
Stage 1
• Initial attack is by the lone pair of a nucleophile on an electron
deficient carbon atom. The hydrogen cyanide is not a good
nucleophile, because it does not have a prominent lone pair. But the
cyanide ion does.
• The positive reactive site on the carbon atom is due to the electron
withdrawing or inductive effect of the more electronegative oxygen
atom bonded to it.
• The carbon atom is unsaturated. As the lone pair from the nucleophile
moves in, the p pair between the carbon and the oxygen moves onto
the oxygen at the same time. This movement of p electrons is called
a
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mesomeric(中介的) shift and is the heterolytic breaking of the p bond.
Heterolytic nucleophilic addition and addition/elimination:carnonyl
compounds with hydrogen cyanide
Stage 2
The intermediate made has a four bonded carbon atom. This is the usual
bonding for carbon and so the energy of the intermediate is low. A low
activation energy means a fast reaction.
If there is a good leaving group joined
to the carbon, the lone pair on the
oxygen may move back between the
oxygen and carbon making the p bond
again as the leaving group moves away.
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Heterolytic nucleophilic addition and addition/elimination:carnonyl
compounds with hydrogen cyanide
Stage 3
The negatively charged oxygen atom made when the p pair moves onto it
(becoming a lone pair) is a good base.
This base deprotonates the protonated form of the nucleophile, here the
hydrogen cyanide, replacing the cyanide ion which did the first attack.
So the cyanide ion is a catalyst.
Notice that the result of the whole
reaction is the addition of a substance
across the double bond. The hydrogen
always ends up joined to oxygen.
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Heterolytic nucleophilic addition and addition/elimination:carnonyl
compounds with hydrogen cyanide
Because the carbonyl group is planar, the nucleophile has an equal
chance of attacking from above or below the plane of the molecule.
This means that there are two possible products made in equal
quantities.
The two products can be optical isomers. As they are made in equal
quantities, the product mixture is a racemate.
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Umpolung & Benzoin Condensation
• Umpolung (polarity inversion) is the chemical modification that
reverses the polarity of a functional group. This modification
allows secondary reactions of this functional group that would
otherwise not be possible.
• The concept was introduced by
D. Seebach & E.J. Corey.
• Benzoin condensation is a
classical example of umpolung,
in which the cyanide ion is an
umpolung reagent.
Umpolung: 极性翻转
Benzoin：安息香
Mechanism of
Benzoin condensation
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Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds with
hydrogen cyanide
Words
Words and Expressions
addition/elimination
add across a double bond
cyanide
withdraw
mesomeric shift
racemate
Umpolung
Benzoin condensation
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Heterolytic electrophilic substitution: the nitration of benzene
Heterolytic electrophilic substitution:
the nitration of benzene
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Heterolytic electrophilic substitution: the nitration of benzene
Stage 1
Initial attack is by the p electrons of the aromatic system on the
added electrophile.
Only very strong electrophiles can react without the help of a
catalyst. The usual catalyst is aluminum chloride and is called a
Friedel-Crafts catalyst.
Here the electron shifts happen one after another. Firstly, a p pair
from the ring moves out. This movement of p electrons means
that the delocalization stability of the ring is lost, so the process
has a high activation energy.
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Heterolytic electrophilic substitution: the nitration of benzene
Stage 2
The deprotonation of the intermediate by some basic particle
in the reaction mixture.
Once the hydrogen has been removed, the delocalization stability
can be restored.
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Heterolytic electrophilic substitution: the nitration of benzene
Stage 3
Production of a stable substitution product
The nitro group has replaced or substituted for a hydrogen on the
benzene ring. Aromatic stability is restored.
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Heterolytic electrophilic addition: alkenes with halogens and
hydrogen halides
Heterolytic electrophilic addition:
alkenes with halogens and hydrogen
halides
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Heterolytic electrophilic addition: alkenes with halogens and
hydrogen halides
Stage 1
Inititial attack is by p electrons of an unsaturated system on an
electrophile.
Sometimes the electrophile is induced or caused by the p system.
Notice here that, unlike the mechanism above, the electron shifts
happen at the same time. As the p pair moves towards the
electrophile a s pair moves onto the far end of the bond in the
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electrophile. This bond is breaking heterolytically.
Heterolytic electrophilic addition: alkenes with halogens and
hydrogen halides
Stage 2
Nucleophilic attack on the intermediate. The intermediate is
attacked from the other side by the nucleophile that was made in the
heterolysis.
p electrons are less strongly held, so the activation energy of the first
step is low and the reaction goes fast in the cold.
If the reaction is done in water, then the second attack might be by a
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water molecule, which has nucleophilic properties.
Heterolytic electrophilic addition: alkenes with halogens and
hydrogen halides
Stage 3
Production of trans-addition product
The result of the whole reaction is addition across the double bond.
Because the second attack is from the other side, the addition is
always trans so that one part is joined on one side of the chain and
the other on the other side of the chain.
If the substance being added is HX, then the H goes onto the carbon
with the most hydrogen atoms already bonded to it (Markovnikov’s
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Rule).
Heterolytic electrophilic addition: the nitration of benzene
alkenes with halogens and hydrogen halides
Words
Words and Expressions
nitration
nitro group
restore
trans-addition
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Homolytic substitution: a free radical photochemical chain reaction
Homolytic substitution: a free radical
photochemical chain reaction
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Homolytic substitution: a free radical photochemical chain reaction
Stage 1
Initiation: the production of free radicals.
A chlorine molecule absorbs ultraviolet radiation. This energy
makes the bond in the molecule break homolytically producing
two free radicals.
Because this reaction is started by light, it is a photochemical
reaction.
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Homolytic substitution: a free radical photochemical chain reaction
Stage 2
Propagation(生长): the reaction between a free radical and a
molecule to make another free radical.
There are many possible steps.
Because each step makes another reactive free radical, the
reaction is a chain reaction.
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Homolytic substitution: a free radical photochemical chain reaction
Stage 3
Termination: the reaction between two free radicals making an
unreactive molecule.
Because any two free radicals can meet, this type of reaction
always makes a mixture of products.
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Hemolytic addition: A free radical polymerization reaction
Homolytic addition:
A free radical polymerization reaction
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Hemolytic addition: A free radical polymerization reaction
Stage 1
Initiation: the production of free radicals.
At initiator, usually an organic peroxide, is heated. This energy
makes the oxygen-oxygen bond in the molecule break
homolytically producing two free radicals.
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Hemolytic addition: A free radical
polymerization reaction
Stage 2
• Propagation:
the
reaction
between a growing chain, which
is a free radical, and a monomer
molecule to make a longer
growing chain radical.
• There are many possible steps.
• Because each step makes another
reactive free radical, the reaction
is a chain reaction.
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Hemolytic addition: A free radical polymerization reaction
Stage 3
Termination: two growing chains meet leading to the
disappearance of the radical active sites.
Because any two free radicals can meet, this type of reaction
always makes a mixture of chains of different lengths.
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Hemolytic addition: A free radical polymerization reaction
Words
Words and Expressions
chain reaction
initiation; propagation; termination
photochemical reaction
monomer
polymer; polymerization
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