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