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Mechanisms of organic reactions
How Organic Reactions Occur
Homolytic bond breaking (radical):
A-B A + B radicals are formed
Heterolytic bond breaking (polar):
A-B A+ + :B- ions are formed
Heterolytic Reactions
Nucleophile
has an electron-rich atom (e.g Cl-, CN-, NH3) and
can form a bond by donating a pair of electrons to electron-poor atom
Electrophile
has an electron-poor atom (e.g H+, CH3+ ) and can form a
bond by accepting a pair of electrons from a nucleophile
A+ + :B- A:B
Kinds of Organic Reaction I.
Addition reactionsAddition reactions
- two reactants add together to form a single new product with no atoms „left over“
Elimination reactionsElimination reactions
- single reactant splits into two products
Kinds of Organic Reaction II.
Substitution reactionsSubstitution reactions
- two reactants exchange parts to give new products
Rearrangement reactionsRearrangement reactions
- single reactant undergoes a reorganization of bonds and atoms
Nuccleophilic Substitution reactions
SN1 & SN2SN1 & SN2
Uni-/Bi- molecular reactions SN1 & SN2
UniUnimolemolecculularar rea reactionction SN1SN1
only one of the reactant molecules is present in the transition state
R-X + Y- R+ + X- + Y-
R+ + Y- R-Y
BimoleBimolecculularar rea reactionction SN2SN2
both of reactants must be present together in transition state
Y- + R-X [Y…R…X] R-Y + X-
Electrophilic Substitution
- an electrophile (E) reacts with an aromatic ring and substitutes for one of the hydrogens
MECHANISM
Resonance forms of complex
Electrophiles Substance that is „electron-loving“
Has an electron-poor atom
Lewis acidsLewis acids (AlCl3, FeCl3) catalyze formation of electrophilic molecules
Lewis acid – accepts electron pair
Lewis base – donates electron pair
Aromatic Electrophilic Substitutions I
Halogenation MECHANISMX = Cl
Aromatic Electrophilic Substitutions II.
•Friedel-Crafts alkylation reactionFriedel-Crafts alkylation reaction
•Friedel-Crafts acylation reactionFriedel-Crafts acylation reaction
AlCl3
benzene + HCl
tert- butylchloride tert- butylbenzene
Aromatic Electrophilic Substitutions III.
•Nitration
Substituent Effects in Substituted Aromatic Rings
1. Ortho- and para-directing substituents
increase the electron
density inside the ring
Y = - NH2, -OH, -F, -Cl, alkyl
2. Meta-directing substituents
decrease the electron density
inside the ring
Y = - NO2, -CN, -COOH, -SO3H
2. Nucleophilic Substitution
Reactions of Alkyl HalidesReactions of Alkyl Halides
Nu:- + CH3-Br CH3 -Nu + Br -
Nucleophile = HS- > CN- > I- > HO- > Cl-
3. Radical Substitution
InitiationInitiation Cl2 2Cl
PropagationPropagation Cl + H3C-CH3 H3C -CH2 + HCl
CH3 -CH2 + Cl2 H3C -CH2Cl + Cl
TerminationTermination H3C -CH2 + Cl H3C -CH2Cl
UV
Electrophilic additionMarkovnikov´s Rule:Markovnikov´s Rule: In the addition of HX to an alkene, the H attaches to the carbon with more H.
cis addition – both groups attach to the same side of the double bond
trans addition –groups attach to the opposite side of the double bond
Elimination ReactionsDehydration
Dehydrohalogenation
CH3 C C CH3
H
Br
H
H
CH3 CH CH CH3KOH
CH3CH2OHKBr OH2+ +
Organic halide SN1
Remember the reactivity order
Benzylic > allylic > 3° RX> 2° RX> 1° RX
SN2
Remember the reactivity order
1° RX > 2° RX > 3° RX > allylic >Benzylic
SN2 example
E1
Remember the reactivity order
Benzylic > allylic > 3° RX> 2° RX> 1° RX
E1 & SN1
E2
Remember the reactivity order
1° RX > 2° RX > 3° RX > allylic >Benzylic
ALCOHOLS & PhenolsForming the phenoxide ion
Remember that the negative charge here is localized, therefore this ion more stable than the alkoxide ion (from alcohol)
Mechanism of the Kolbe-Schmitt Reaction
Ethers
Williamson synthesis
Aldehydes & Ketones
IDOFORM REACTION OF THE METHYL KETONE
Aldol condensation
Amines HOFMANN rearrangement