20.1 Org Rxn Mech
Transcript of 20.1 Org Rxn Mech
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Organic Reactions
Orig.: Larry Schefer Retired, Lincoln HighSchool
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Reaction Pathways and
mechanisms Most organic reactions proceed by a defined
sequence or set of steps. The detailedpathwaywhich an organic reaction follows is called a
mechanism. Knowing a reaction mechanism is very valuable
information. It allows the chemist to predict whatproducts will be formed when a chemical reactionoccurs.
The organic chemist can use this information tomodify compounds and to synthesize newcompounds with certain desired characteristics.
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Substitution Reactions In a substitutionreaction, one atom or
group of atoms, takes the place of
another in a molecule Eamples
!"#!"2$r% &!'!"#!"2!'% &$r
(!"#)#!!l% 'aO"
(!"#)# !O"% 'a!l!l2 % !"* !"#!l % "!l
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+hree $asic Steps in a
ree Radical -echanism Chain initiation
+he chain is initiated (started) by ./ lightbreaking a chlorine molecule into free radicals0
!l2
2!l.
Chain propagation reactions+hese are the reactions which keep the chaingoing0
!"*
% !l. !"#
. % "!l
!"#. % !l2 !"#!l % !l
. Chain termination reactions
+hese are reactions which remoe free radicalsfrom the system without replacing them by
new ones0. . .
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ree Radical -echanism3
+he Initiation Step +he ultraiolet light is a source of energy that
causes the chlorine molecule to break apartinto 2 chlorine atoms, each of which has an
unpaired electron +he energies in ./ are eactly right to break
the bonds in chlorine molecules to producechlorine atoms0
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"omolytic ission ree radicals are formed if a bond
splits eenly 3 each atom getting
one of the two electrons0 +he namegien to this is homolytic fssion.
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ree Radical Propagation +he productive collisionhappens if a chlorine radical
hits a methane molecule0 +he chlorine radical remoes a hydrogen atom from the
methane0 +hat hydrogen atom only needs to bring one
electron with it to form a new bond to the chlorine, andso one electron is left behind on the carbon atom0 6 newfree radical is formed 3 this time a methyl radical, !"#0
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ree Radical Propagation II If a methyl radical collides with a chlorine
molecule the following occurs8
!"#.
% !l2
!"#!l % !l.
+he methyl radical takes one of the chlorineatoms to form chloromethane
In the process generates another chlorine free
radical0 +his new chlorine radical can now go throughthe whole se9uence again, It will produce yetanother chlorine radical 3 and so on and so on0
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+ermination Steps+he free radical propagation does
not go on for eer0 If two free radicals collide the
reaction is terminated0
2!l. !l2
!"#. % !l 0 !"#!l
!"#. % !"#
0 !"#!"#
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Eercise
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Solution Initiation !l2 2!l
0 Propagation
+ermination 2!l0 !l2
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'ucleophilic Substitution A nucleophileis a molecule or ion that has ahigh electron density.
It is attracted to atoms in molecules with a
lower electron density. It may replace another group in an organic
molecule.
The molecule to which the nucleophile is
attracted is called the substrate The group that the nucleophile replaces is
called the leaving group
These reactions are known as nucleophilic
substitutions. 12
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'ucleophilic Substitution ne covalent bond is broken as a new
covalent bond is formed
The general form for the reaction is !u"# $ %#& %#!u $ &"#
'ucleophile Substrate Product ?eaing group
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'ucleophilic Substitution !u"# $ %#& %#!u $ &"
The bond to the leaving group is broken
The leaving group takes both electrons thatformed the bond with it
The nucleophile provides the electrons to form
the new bond
'ucleophile Substrate Product ?eaing group
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'ucleophilic Substitution$imolecular or S
'2
6 reaction is bimolecularwhenthe rate depends on both the
concentration of the substrateand the nucleophile0
S'2 mechanisms occur most
readily with methyl compoundsand primaryhaloalkanes
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S'2 -echanism
+he general form for an S'2 mechanism is shown aboe0
'u83 @ nucleophile
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'ucleophilic Substitution.nimolecular or S
'1
6 unimolecular reaction occurs whenthe rate of reaction depends on the
concentration of the substrate but notthe nucleophile0 6 unimolecular reaction is a two step
process since the subtrate and the
nucleophile cannot both appear in therate determining step S'1 mechanisms occur most readily
with tertiaryhaloalkanes and some
secondary haloalkanes0 2>
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S'1 -echanism
+he Arst step is the formation of the carbocation0 It is theslow step0 +he rate of the reaction depends only on theconcentration of the substrate0
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S'
1 and S'
2 Reactions
SN SN!
Rate "#$R%& "#$R%&$Nuc:'&
(arbocationintermediate)
*es No
Stereochemistr
y
mi+ nversion o-
conguration
Rearrangement
/H, / (H0
possibleNo
rearrangements
2#
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Electrophilic 6ddition
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6ddition -echanisms Electrophilic additionoccurs in reactions
inoling carbon3carbon double bonds 3
the alkenes0 6n electrophileis a molecule or ion that
is attracted to electron3rich regions inother molecules or ions0
$ecause it is attracted to a negatieregion, an electrophile carries either apositie charge or a partial positie
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-arkonikoBCs RuleActually there are two possible carbocations that could beformed. In may cases this would result in two possible products.However only one form is preferred
The hydrogen ion will tend to migrate to the side with the
greater number of hydrogen atoms. This preference is known
as Markovnikoffs Rule.
D$irds of a feather ock togetherFG
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Electrophilic 6dditions 6n addition reaction is a reaction in which two
molecules Hoin together to make a largermolecule0 +here is only one product0 6ll the
atoms in the original molecules are found inthe single product molecule0
6n electrophilic addition reaction is an additionreaction which happens because what wethink of as the important molecule isattacked by an electrophile0 +he importantmolecule has a region of high electron densitywhich is attacked by something carrying somedegree of positie charge0
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Eercise 1rite a mechanism -or the electrophilic addition
o- H2r to 'butene.
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Solution 1rite a mechanism -or the electrophilic addition
o- H2r to 'butene.
Solution
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Elimination Reactions
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Elimination Reactions 6n elimination reactionis a type
of organic reaction in which two
substituents are remoed from amolecule in either a one or two3step mechanism
In most organic eliminationreactions the unsaturation leel ofthe molecule increases0
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E1.nimolecular
Elimination Occurs in two steps Reaction rate depends primarily on
the concentration of the substrate
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E1 .nimolecular
elimination Occurs in two steps8 irst there is the
formation of the intermediate and then theformation of the !@!0
Occurs in tertiary and secondaryhaloalkanes0
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Option L8 urther Organic
Electrophilic Substitution
+he displacement reactions of the alkyl halidesdo not usually work for aromatic (aryl) halidesunless a halogen is part of a side chain0
6 halogen atom held to a double bondedcarbon atom is usually rather unreactie,?ikewise a halogen atom attached to aben=ene ring is ery stable and unlikely to
react0 -ost aromatic substitution reactions proceed
by a mechanism known as electrophilic
substitution*#
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Electrophilic Substitution 6n eample of an electrophilic substitution is
the reaction of chlorine with a ben=ene ring0 +he oerall reaction is
+he mechanism for this reaction inoles # steps
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Electrophilic Substitution
3# Steps +he initial step is the formation of the electrophile06catalyst may be re9uired0
e!l# % !l2 e!l*3 % !l%
+he second step is the attachment of the electrophileto the ben=ene ring forming the carbocation0
+he Anal step is the loss of hydrogen to form the product0
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Ring Substitution !ase 2
+he presence of the presence of a carboylgroupon the ring causes the chlorine to attach atthe third position0 It is called a meta
director
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Ring Substitution !ertain groups to the ben=ene ring cause new groups
to attach at carbons 2 and *0 +hey are calledorthoMpara directors0 Other groups cause the newgroup to attach at carbons # and +hey are known as
metadirectors
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Ring 6ctiation
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