An Introduction to Organic Reactions and Their Mechanism

Acids and Bases

Acid-Base propertiesAcid Properties:When dissolved in water,

acids Conduct electricity Change blue litmus to

red Have a sour taste React with bases to

neutralize their properties

React with active metals to liberate hydrogen.

Base Properties:When dissolved in

water, bases Conduct electricity Change red litmus to

blue Have a slippery

feeling React with acids to

neutralize their properties.

3.1 Reactions and Their MechanismChemical equation:

Reactants productsAtoms are neither created nor destroyed.

Four categories: substitution, additions, eliminations and rearrangement

SubstitutionSubstitution: one group is replaced with

anotherGeneral reaction

A + B B + A

H

ClH

H

+ Na+ -OH

H

OHH

H

+ Na+ Cl-

EliminationAtoms or groups are removed from adjacent

atoms to form a double or triple bondGeneral reaction

A B

+ A B

C C

H

HH

H

+ H BrKOH

H

CH

H

C

H

H

Br

AdditionAtoms or groups add to adjacent atoms of a

multiple bondGeneral reaction

+ A B

BA

C C

H

H

H

H

+ Br Br

H

C C H

H

BrBr

H

CCl4

RearrangementA molecule undergoes a reorganization of its

constituent parts.Positions of the double bond and a hydrogen

atom changed

C

C C

H

HH

CH3

H3C

H3C

Acid Cat.

H3C

H3C CH3

CH3

3.1Homolysis and Heterolysis of Covalent BondsWhen an organic reaction occurs, some

covalent bonds must be break and a new bond must form.A covalent bond can be broken in two ways:Heterolysis

Bond cleavage in which bonding of electrons are unevenly divided between the two parting atoms

Homolysis Bond cleavage in which bonding of electrons are

evenly divided between the two parting atoms

A B A+ + B-

ions

A B A + B

Heterolysis

Homolysis

radicals

full arrow = transferred 2 e-

half arrow = transferred 1e-

3.2 Acid-Base ReactionsBronsted-Lowry acids: an acid is a substance

that can donate a proton (H+)Bronsted-Lowry acids: an base is a substance

that can accept a proton (H+)

H O

H

+ H Cl H O

H

H + Cl

Bronsted Base Bronsted acid conjugated acid of water

conjugated base of HCl

The Lewis Definition of Acids and BasesLewis acid is (electrons acceptor)

electrophilesLewis base is (electrons donor)

nucleophiles

Cl H + NH3 Cl + H NH3

Lewis acid Lewis Base

HCl + H2O H3O+ + Cl-

Combination of Site TypeThe reaction site is

A polar bond because oxygen is more electronegative than carbon

A Lewis base since there are unshared pairs of electrons to the oxygen that can be shared with a Lewis acid

Multiple bond with a π bond in which electrons are available to incoming speciesO

C

ExamplesWhich of the following are potential Lewis

acids and which are potential Lewis base?Identify Lewis Base site

CH3CH2 N CH3

CH3

H3C C

CH3

CH3

a.b.

c.(CH3)3B

d.O

ExamplesWrite equations showing the Lewis acid-base

reaction that take place when:Methanol (CH3OH) reacts with BF3

C L

heterolysisC + L L= leaving group

carbocation

C Lheterolysis

C + L L= leaving group

carboanion

Carbocations and ElectrophilesElectrophiles are reagent which in their

reactions seek the extra electrons that will give them a stable valence of electrons

C + B

carbocationLewis acid

C B

anionLewis Base

Carbocations and NucleophilesNucleophiles are reagents that seek a proton

or some other positive center

C+ H A H

carboanionLewis base Lewis acid

A+

C + Cl ??????

ExamplesClassify the following as either electrophiles

or nucleophiles

Use the curved-arrow notation to write the reaction what would take place between dimethylamine (CH3)2NH and boron trifluoride). Identify Lewis acid and Lewis Base

C N Br CH3CH2a. b. c

16.2 Acid StrengthStrong acids:

completely ionized or completely dissociated

Forward reaction is predominated Most of HA is

dissociatedConjugated base is weak

and has low attraction for proton

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Acids StrengthWeak acids: partially ionized or dissolved Reverse reactions is predominated

Most of HA is undissociateConjugated base is strong and has high

attraction for protonHC2H3O4(aq) + H2O(l) H3O+(aq) +C2H3O4

-(aq)