Organic Reaction Mechanism.pdf
Transcript of Organic Reaction Mechanism.pdf
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Mechanism of
Organic Reactions
S D Samant
Institute of Chemical Technology, Mumbai
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To understand organic chemistry it is necessary to know:
• What occurs in a reaction?
• Why and how chemical reaction takes place?
• How a reaction can be described?
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Formation of molecule
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ATOM Nucleus (p+n)
Electrons Kernel electrons
A
Electronic configuration (shells , orbital)
ATOMIC ORBITALS (AO) (outermost)
Overlap of Aos Hybridization
Overlap
B
Molecular orbitals
(Mos)
Bond formation (ionic , covalent , coordinate )
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MOLECULE
3D structure stereochemistry Electronic structures
(i) Polarization effect –I C(inductive effect)
(ii) polarizability effect (electromeric effect )
(iii) Resonance
MOLECULE + MOLECULE
Bond breaking Concentrated reactions(pericyclic reactions)
Homolytic Heterolytic D
Radicals Cations Anions other species
REACTION MECHANISM 5
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Atoms Molecule
Nuclei Framework
Electrons Orbitals
3D Structure Configurations, conformations, isomerism …..
Prerequisite: Electronic configuration
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MOLECULE
3D Structure Electronic structure
Reactivity
Reaction
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Atomic structure versus
Molecular structure
AO vs MO
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n A.O.s n M.O.s (s,p,d,f )
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Approximations
Linear combination of atomic orbitals
Consider only the valence orbitals
Consider only that part of your interest
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Electronic Structure
Qualitative description of M. O. s of simple acyclic and monoclinic system
(a) Molecular Orbital Energy Diagrams (MOED)
- Explanation of reactivity
(b) Molecular Orbital energies
(c) Orbital overlap
(d) Coefficient of A.O.s in M.Os
(e) Frontier Molecular Orbitals (FMOs)
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How is a CHEMICAL BOND formed?
Ionic bond
Covalent bond
Coordinate bond
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Hybridization
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- - - - - - - - - - - - - - - - - - -
LUMO (acceptor orbital ) Nu
HOMO (donor orbital) E
Important orbital
Electrophile LUMO Nucleophile HOMO
Acid (lewis) LUMO Base (lewis ) HOMO
FMO
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FMOs in :
Electrophilicity & Basicity
Acidity & Basicity (HSAB)
Simple reactions
Pericyclic reactions
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ORGANIC REACTION MECHANISM Desired reaction :
Reactant T, Catalyst, solvent Product
product By product Kinetic Stereochemistryspread
Reaction Mechanism
TESTING
Stereochem kinetic trap. Isotropic Solvent Substrate intermediate studies effect effect
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Mechanism
confirmed
1) explanation of other reaction
2) further development
3) predictions
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Important Symbols describing
Reaction Mechnaism
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Irreversible reaction to product
Reaction which does not processd
Forward & Backward reaction
Reaction with more than one step
Reversible reaction
Reversible reaction ; Equilibrium favours products
Reversible reaction ; Equilibrium favours reactant
Reaction with inversion of cofiguration
Indication of resonance
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Migration Migration Half headed
origin terminus arrow
shift of 2e- 1 e- shift
Curly Arrow
Direction of
migration
Use appropriate symbols20
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Indicating Steps in Mechanisms
• Curved arrows indicate breaking and forming of bonds
• Arrowhead with a “half” head (“fish-hook”) indicates homolytic and homogenic steps (called ‘radical processes’)
• Arrowhead with a complete head indicates heterolytic and heterogenic steps (called ‘polar processes’)
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Using Curved Arrows in Polar Reaction Mechanisms
• Curved arrows are a way to keep track of changes in bonding in
polar reaction
• The arrows track “electron movement”
• Electrons always move in pairs
• Charges change during the reaction
• One curved arrow corresponds to one step in a reaction mechanism
• The arrow goes from the nucleophilic reaction site to the
electrophilic reaction site
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Rules for Using Curved Arrows
• The nucleophilic site can be neutral or negatively charged
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• The electrophilic site can be neutral or positively charged
• Don’t exceed the octet rule (or duet)
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Examination of a molecule
with respect to its electronic
structure
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Formal charge
Formal charge =
Valence electrons – (1/2 bonding e- s + All lone pair e-s)
= Valence e- s – (lines + dots )
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Polarization effect – Inductive effect
Polarizability effect – Electromeric effect
Localized and Delocalized bonds
– Resonance and related phenomena
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Polarizability
• The ability of the surroundings (solvent or other polar molecules) to make a molecules polar. It is like making something magnetic by pass a magnet over it.
• It starts out neutral but when the right atom or molecule comes close it becomes partial positive or partial negative.
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• Polarization - change in electron distribution as a response to change in
electronic nature of the surroundings
• Polarizability is the tendency to undergo polarization
• Polar reactions occur between regions of high electron density and regions
of low electron density
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Resonance
A way to indicate delocalization of electron
Localized vs Delocalized Bonds
Unique Structure vs Resonance structures
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Resonance Structures
Hypothetical BUT Useful
Hypothetical means taken to explain REAL
object
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Drawing of resonance structures
Nuclei do not move – Geometry of the molecule does not change
Electrons move
Follow standard rules of valence
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Each resonance structure
contributes to some extent to
the actual structure of the
molecule
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Why do Organic
Reactions Happen?
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Energy change = Coulombic interaction + Orbital interaction
Orbital interaction = FMO interaction + Other interactions
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How do the organic reactions occur: Mechanism
• 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
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Steps in Mechanisms
• 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
concerted
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Understanding unit steps
Acid – Base concept
Nucleophile and Electrophile
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Energy Frofile Diagram
• schematic representation of the energy changes that take place as reactants
are converted to products.
• plots the energy on the y axis versus the progress of reaction, often labeled
as the reaction coordinate, on the x axis.
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• For the general reaction:
• The energy diagram would be shown as:
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• Consider the following two step reaction:
• An energy diagram must be drawn for each step.
• The two energy diagrams must then be combined to form an energy
diagram for the overall two-step reaction.
• Each step has its own energy barrier, with a transition state at the energy
maximum.
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Figure 6.6Complete energy diagram for
the two-step conversion of
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Ea is the energy barrier that must be exceeded for reactants to be
converted to products.
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Describing a Reaction: Intermediates
• If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product
• These are called reaction intermediates or simply “intermediates”
• Each step has its own free energy of activation
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Acids and Bases
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An electrophile
– is “electron-loving”
– is an electron-poor species
– can form a bond by accepting a pair of electrons
– may be either neutral or positively charged
– is a Lewis acid
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A nucleophile
– is “nucleus-loving”
– is an electron-rich species
– can form a bond by donating a pair of electrons
– may be either neutral or negatively charged
– is a Lewis base
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Generalized Polar Reactions
• An electrophile, an electron-poor species, combines with a
nucleophile, an electron-rich species
• The combination is indicated with a curved arrow from nucleophile to electrophile
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Ambident Nucleophiles
O N O O N O (NO2)
O N OONO (NO2)
H2C C O H2C C O (enolate)
o o o o o
(phenolate ion)
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• Some species can act as an electrophile or a nucleophile
depending on the circumstances
• Example – Water acts as a nucleophile when it donates a
pair of electrons, and acts as an electrophile
when it donates H+
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Bond Breaking –
in anticipation of better bonds
HOW and What it Results into
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Types of Steps in Reaction Mechanisms
• Bond formation or breakage can be symmetrical or unsymetrical
• Symmetrical - homolytic
• Unsymmetrical - heterolytic
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C X
C X
C X
C X
C
C
C
Carbenium ion
Carbanion
Radical
X
X
X
General C-X Bond Breaking processes
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C X C X
Self dissociation
X Leaving group
OR
C X + Z C XZ C XZ
assisted dissociation
Y C C Y C C
Addition
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C H B C HB deprotonation
A B CB CA
C X M C MX metallation
X C C C CX addition
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Intermediates
What are they?
Transition State and Intermediates
How are they formed?
How do they look like?
Are they stable?
What are their typical reactions?
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Structures
Carbocation: Trivalent, Planar, Trigonal
Carbanion: Trivalent, Tetrahedral
Carbon radical: Tetrahedral or Trigonal
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General principles of stability:
Increase charge Unstable
Decrease charge Stable
Localise charge Unstable
Disperse, delocalise change Stable
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RADICAL REACTION A] Propagation processes :
1) Abstraction . .R + H—C≡ R—H + C≡
2) Addition . .R + C=C R—C—C
3) Decompostion . R—C=O R + O= C=O
.O . . . .
R + Mol R + Mol . ..
R Mol + R
B] Termination process :
1) Combination : . .
R + R R—R
2) Disproportion : . . ..
R Mol + Mol
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C] Rearrangements : Not common
Characteristics
* Susceptible to radicals
* Chain mechanisms
* Conc. Of radicals is low
- Termination processes less frequent
- termination process have negligible E act
* Susceptible to inhibitors
* Ionic mechanism may compete.
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General Reaction of Free Radicals
Propagation processes Termination processes
Abstraction Addition Disproportionation Combination
reaction reaction reaction reaction
. . . . . .
X + H-C≡ X + C=C R + R R + R
. . . ..
X-H + C≡ X—C—C R + R R—R
. . . . . .
Cl + H—C≡ Br + C=C CH3-CH2 + CH2-CH3 Cl + Cl
. .
HCl + C≡ Br—C—C CH3—CH3 + CH2 =CH2 Cl—Cl
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Radical Reactions
• Not as common as polar reactions
• 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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Photohalogenation of alkanes (∆H in kJ/mol) )
Steps F Cl Br I
. .X X X + X +150.6 +242.7 +188.3 +150.6
. . X + CH4 CH3 + HX -146.4 -12.55 +50.21 +117.2
. .CH3 + X2 H3C X + X -334.7 -83.68 -96.23 -62.76
Overall -481.1 -96.23 -46.23 +54.44
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Act. E : . . Cl + CH4 CH3 + HCl 16.11 Kj
. .CH3 + Cl2 CH3Cl + Cl < 21 kj
Alternative mechanism : . .
Cl + CH4 H3C-- Cl + H . .
H + Cl2 HCl + Cl
Both steps endothermic (only at High T ) Act . E : . .
Br + CH4 CH3 + HBr (∽74.48 kj)
(∽77)
H—CH3 H—CH2—CH3 HCHMe2 HCMe3
C—H BondE (kcal/mol) 102 96 92 89
C H & Ar CH3
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Radical addition to (C=C)
Radicals always attack less substituted (terminal) carbon atom of a double bond
R + HX R H + X
+ HXX
X X
H
X+ HX
+ Br Br
Br + HBr Br
H
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Radical mechanism effectively competes with ionic mechanism in the case
of HBr
X in HX F Cl Br I (Kcal/mol)
-45 -26 -5 +7
+37 +5 -11 -27
X + H2C CH2 XH2C
CH2
+ H-X + XX
H2C
CH2X
H2C
CH2
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General Reaction of Carbenion Ion
Nucleophilic Elimination Rearrangement Additon to
attack of H+ from (common) C=C
≡C+ + : Nu β –position less stable (C+ ) ≡C+ + C=C
H
≡C –Nu Cβ –Cα+ more stable (C+ ) ≡ C—C—C+
C=C Wagner –
olefins Meerwein
SN 1 (II)
AE1 (II) E1
(A) (B) (C) (D)
A or B A or B D
(polymn)
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CARBOCATION
Carbonium ion Carbenium ion
C+, C - pentavalent C+, C - trivalent ,
CH5+ CH3
+
Carbonium ion (onium) Carb-enium ion (enium)
Carbanion
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Racemate
*R+ Cl-
R+ Cl-: :: :
Nu-R
Inversion of conf iguration
Inversion > Retension
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Self Dissociation:
R-Cl R+Cl- ionization
Tight ion pair
R+Cl-
K1
Solvent, K2R+ Cl-
: :: :
Solvent separated ion pair
R+ Cl-: :: :
K3
R+ + Cl-
ions
dissociation
Stereochemical outcome:
Nu-R
(d)
R+Nu Nu
free carbonium ion
R-Nu
(l)
*
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PROTONATION PROCESSES
1. Protonation of alchols :
2. Protonation of ethers :
R OH Hk1 R O
H
H
protonium ion
R OH2
k2 R OH2
R OH2k3
R + H2O dissociation
+
k1
R O R + H R O R
H
+
R O R
H
+k2 R
+HO R
R+
HO R R+
+ HO R81
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3. Protonation of a carbonyl group :
4. Protonation of mercaptans & sulphides :
R SH + H R S R
H
R S R + H R S R
H
C
R
R
O + H C
R
R
OH
C
R
R
OH C
R
R
OH
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5. Protonation of an olefin :
R CH
CH
R1+ H R C C R1
H
HH
R C C R1
H
HH
RHC
H2
C R1 + RH2
CHC R1
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An Example of a Polar Reaction: Addition of HBr to Ethylene
• HBr adds to the part of C-C double bond
• The bond 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
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First Step in Addition
• In the addition of HBr the (conceptual) transition-state structure for the first step
• The bond between carbons begins to break
– The C–H bond begins to form
– The H–Br bond begins to break
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* With Acid halide:
R
O
Cl : +..
.. AlCl3R
+O:
Cl:..
..AlCl3
-
R
+O:
Cl:
AlCl3-
R
+O:
Cl:
AlCl3-
* With Acid anhydride:
R
O
O
O
R+ AlCl3
R
O
O
:O-AlCl3-
R
+
R
O
O
:O-AlCl3-
R
+
R
O
O
O : -AlCl3
+R
..
R
O
O
O : -AlCl3
+R
..
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Complexation of alkyl halide with a Lewis acid
R-Cl :..
..+ AlCl3
K1
..
..
R-Cl-AlCl3
+ -
Complex
..
..
R-Cl-AlCl3
+ - K2
R+(AlCl4)-
tight ion pair
R+(AlCl4)-K3, solvent
R+ (AlCl4)-
Solvent separted ion pair
R+ (AlCl4)-
K4
R+ + (AlCl4)-
ions (dissociation)
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General Raction of Carbanion
Protonation Attack at e – Elimination Rearrangement
≡C: - + H deficient carbon of nucleofuge (rare)
from β position
≡C—H
sp3 carbon sp2 carbon
C- alkylation
At C=C At C=O
polymerisation Tetrahedral mechanism
(Anionic) (Aldol type reaction)
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Tetrahedral Mechanism:
R
O
Cl + -OH R
O
OH+
-Cl ?
(SN2 type)
(a)
R
O
X + -Nu RX
Nu
O-
SP3
tetrahedral
RX
Nu
-O
R
O
NuSP2
Planar
Substitution
Nu= -OH, H2O, NH3, R-NH2
X= Cl, OH, OCOR, NH2
RX
Nu
O-
OR
+ H+
RX
Nu
OH
X= H, akyl , NH2
Addition
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ELECTRON BOOK KEEPING
(1) CH3 + CH3
(2) CH3-CH2 + Cl
Homogenic bond formations
Heterogenic bond formations
(3) CH3 + OH
(4) (C2H5)3N + H
(5) (CH3)3N + O
CH3-CH3
CH3-OH
(C2H5)3NH
(CH3)3N-O
CH3-CH2-Cl
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(1) CH3-O-CH3 + H
(2) CH3-O-N=O + O
Heterogenic bond formations
(3) CH3-CH2-Cl + AlCl3
(4) C=O + H
(5) X-CH2-H + OH
CH3-O-CH3
CH3-CH2-Cl-AlCl3
CH3-O-N=O
H
O
H-C=O
X-CH2 + H-O-H91