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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

1#

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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

1*

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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

1:

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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

charge 24

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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

2:

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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

2;

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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

#1

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Elimination Reactions

#4

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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

*4

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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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