factors affecting nuecleophilic substitution reaction
TRANSCRIPT
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Factors affecting Nuecleophilic substitution
reaction
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Steric nature of the reactant
SN2----- CH3-X, R-CH2X, allylic,
benzylic
SN1----- R3-CX, allylic, benzylic
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Nature of nucleophile
small, strong Nu- favors SN2----
OH-, CH3O-, CH3CH2O-, CN-, RS-, RCN-, Br-, I-
weak, small Nu- favors SN1----- H2O, CH3OH, CH3CH2OH, RSH, NH3, F
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Solvent used
SN2---- low moderate polarity solvent as:
acetone, NMF
SN1----- moderate to high polarity solvents as:
water, methanol, ethanol
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Leaving group
all halogens, except F , are good leaving group
water is good leaving group ( not OH)
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Question: Explain WHY? then show the mechanism
R OH + HBr R-Br + H2O
R OH + HBr
H+
no reaction
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Electrophilic Substitution Reactions
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Benzene is treated with a mixture of concentrated nitric acid
and concentrated sulphuric acid at a temperature not
exceeding 50°C. As temperature increases there is a greater
chance of getting more than one nitro group, -NO2,
substituted onto the ring.
Nitrobenzene is formed.
or:
HH22SOSO44
heatheat
THE NITRATION OF BENZENETHE NITRATION OF BENZENE
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If you are going to substitute an -NO2 group into the ring,
then the electrophile must be NO2+. This is called the
"nitronium ion" or the "nitryl cation", and is formed by
reaction between the nitric acid and sulphuric acid
The formation of the electrophile
The equation
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THE HALOGENATION OF BENZENETHE HALOGENATION OF BENZENE
Benzene reacts with chlorine or bromine in an electrophilic
substitution reaction, but only in the presence of a catalyst.
The catalyst is either aluminium or ferric chloride (or
aluminium (ferric) bromide if you are reacting benzene with
bromine) or iron.
FeCl3
FeBr3
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The formation of the electrophile
As a chlorine molecule approaches the benzene ring, the delocalised electrons in the ring repel electrons in the chlorine-chlorine bond
it is the slightly positive end of the chlorine molecule which acts as the electrophile. The presence of the aluminium chloride helps this polarisation.
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Friedel-Crafts Acylation of Benzene Friedel-Crafts Acylation of Benzene
Named after Friedel and Crafts who discovered the reaction.
Reagent : normally the acyl halide (e.g. usually RCOCl) with
aluminum trichloride, AlCl3, a Lewis acid catalyst.
The AlCl3 enhances the electrophilicity of the acyl halide by
complexing with the halide.
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FRIEDEL-CRAFTS ACYLATION OF FRIEDEL-CRAFTS ACYLATION OF BENZENE BENZENE
Electrophilic species : the acyl cation or acylium ion (i.e. RCO
+ ) formed by the "removal" of the halide by the Lewis acid
catalyst, which is stabilised by resonance as shown below.
Other sources of acylium can also be used such as acid anhydrides with AlCl3
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Addition Reactions
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16
• In an addition reaction, new groups X and Y are added to the starting material. A bond is broken and two bonds are formed.
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17
• Addition and elimination reactions are exactly opposite. A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.
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The double bond dissolves back to single bond and new bonds reach out to A and B whose bond is also dissolving
C C C C
BA
C C
A B
A-B can be :
H-H H-OH H-X OH-OH OH-X
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Draw the product of each of these examples of A-B when they add to 1-propene.
C C
H
H H
CH3
H-H H-OH H-X OH-OH OH-X
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Addition reactions Addition reactions
A reaction in which an atom or group of atoms is added
to a molecule. divided into:
1.1- Electrophlic Addition
mechanism
H2C CH2 + HX CH3CH2X
X = Cl, Br, I
a. H2C CH2 H X+ H3C CH2 + X-
b. H3C CH2 + X- H3C CH2Xfast
slow
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Electrophilic addition reactions - the general picture
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Why?Why?
addition to unsymmetrical alkenes
According to Markovnikov's RuleAccording to Markovnikov's Rule
•Which is an empirical rule based on Markovnikov's experimental
observations on the addition of hydrogen halides to alkenes.
•The rule states that :
"when an unsymmetrical alkene reacts with a hydrogen halide to give
an alkyl halide, the hydrogen adds to the carbon of the alkene that
has the greater number of hydrogen substituentshas the greater number of hydrogen substituents, and the
halogen to the carbon of the alkene with the fewer number of
hydrogen substituents"
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Addition of H-X Reactivity rank: HI > HBr > HCl > HF.
It is an electrophilic addition reaction.
It Follows Markovnikov`s rule.
Markovnikov`s rule:
“In addition of HX to asymmetrical alkenes, the H+ of HX goes to the double-bonded carbon that already has the greatest number of hydrogens”
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EXAMPLE:
Addition of HCl to 1-Propene.
It is a regioselective reaction, follow Markovnikov`s rule.
Cont.Addition of H-X
Anti-Markovnikov addition
EXAMPLE
Addition of HBr to 1-Propene in presence of peroxideperoxide.
In the presence of peroxides (chemicals containing the general structure ROOR'), HBr adds to a given alkene in an anti-Markovnikovfashion
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WWU -- ChemistryWWU -- Chemistry
MarkovnikovMarkovnikov
Markovnikov
Not Markovnikov
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WWU -- ChemistryWWU -- Chemistry
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WWU -- ChemistryWWU -- Chemistry
MARKOVNIKOV RULEMARKOVNIKOV RULE
CH2
+ HCl
CH3
Cl
When adding HX to a double bond thehydrogen of HX goes to the carbonwhich already has the most hydrogens
..... conversely, the anion X adds to the mosthighly substituted carbon ( the carbon withmost alkyl groups attached).
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WWU -- ChemistryWWU -- Chemistry
REGIOSELECTIVEREGIOSELECTIVE REACTION REACTION
CCH3
CH3
CH2 CCH3
CH3
CH3
Cl
+ CHCH3 CH2
Cl
CH3HCl
major minor
one of the possible products is formedin larger amounts than the other one
Compare
REGIOSPECIFICREGIOSPECIFIConly one of the possible products isformed (100%).
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WWU -- ChemistryWWU -- Chemistry
Mechanism (Markovnikov)Mechanism (Markovnikov)
R CH CH2 R CH-CH2
R CH CH2 H
Br
1)+
+
slow
2)_ fast
+
+
H
Br
H
R CH-CH2
+H
Electrophile
Nucleophile
Secondary C+
Major product
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WWU -- ChemistryWWU -- Chemistry
Mechanism (anti-Mechanism (anti-Markovnikov)Markovnikov)
R CH CH2R CH CH2
H
R CH CH2
H
R CH CH2 Br
H
1)+ slow
+2)
_ fast
+
+
H
Br
+
Minor!
Primary carbocation
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WWU -- ChemistryWWU -- Chemistry
Markovnikov Addition to Markovnikov Addition to an Alkenean Alkene
C CH2
CH3
CH3
H X CH3 C CH2
CH3
X
H CH3 C CH2
CH3
H
X+ +
major product minor productX = Cl, Br, I
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ADDITION OF H2O
• ADDITION OF H2O to alkene
• ADDITION OF H2O to aldehyde
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HBr and HCl easily add to alkenes. Since water also is a molecule of the type HX which can donate a proton, H2O should be able to add to alkenes in the same way as HBr, for example, resulting in the hydration of an alkene. However, for the addition of H2O to alkenes to occur acid catalysts acid catalysts are required.
ADDITION OF H2O to alkene
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ADDITION OF H2O to aldehyde
Form 1,1-diol (hydrate) what is the type of the reaction?????what is the type of the reaction?????
EXAMPLE: Formation of chloral hydrate.
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2.2. Nucleophilic Addtion Nucleophilic Addtion
It is the most common reaction of aldehydes (RCHO) and ketones (RCOR)It is the most common reaction of aldehydes (RCHO) and ketones (RCOR)
e.g. e.g. The reaction of aldehydes and ketones with hydrogen cyanideThe reaction of aldehydes and ketones with hydrogen cyanide
hydroxynitriles.hydroxynitriles.
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WWU -- ChemistryWWU -- Chemistry
MechanismMechanism
C CH21)+
+
slow
2)_ fast
+
+
H
Electrophile
Nucleophile
CH3
CH3
CH CH2
CH3
CH3 H
+C CH2
CH3
CH3 H
C CH2CH3
CH3 H
X
X
3 o C+
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37
Classify each of the following as either substitution, elimination or addition reactions.
a)OH
Br
substitution
b)
addition
c)
OH
elimination
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SOME ADDITIONAL EXAMPLESSOME ADDITIONAL EXAMPLES
CH3
+ HCl
CH3
Cl
CH2
+ HCl
CH3
Cl
CH CH2 CH CH3
Cl+ HCl
only major product is shown