Ionic Reactions
Nucleophilic Substitution and Elimination Reactions
of Alkyl Halides
Organic Halides
 Alkyl Halides: alkane molecule in which a halogen has
replaced a hydrogen
Physical Properties Of Organic
Halides
 Low solubilities in water
 Miscible with each other and other relatively nonpolar
solvent
 E.g CH2Cl2
CHCl3
CCl4
Reaction Intermediates
 3 major types
 Carbocation (C+)
 Carbanion (C-)
 Free radical (C· )
Reaction Sites
 Nucleophiles: (nucleus-loving) any species containing
electron pairs
 Electrons are (-), so Nu: are attracted to (+) site
 Charge Nu: are better than neutral one
E.g
Reaction Sites
 Electrophiles (electron-loving): any species or site in
molecule that’s deficient in electron density because it’s near
a electronegative atom or lacking of e- altogether
Multiple bonds
 The electrons are available to be donated to another species
Nucleophilic Substitution Reactions
 General Reaction
Reaction Schemes
Leaving Groups
 Relatively stable, weakly basic molecule or anion
 Halogen atom of an alkyl halide is a good leaving group
because once departed it is a weak base and table anion
A mechanism for SN2Reaction
 The nucleophile approaches the carbon bearing the leaving
group from the back side
 Directly opposite the leaving group
 As the reaction progresses, the bond between nucleophile and
the carbon strengthens, and the bond between the carbon
atom and the leaving group weakens
 Carbon atom has its configuration turned inside out 
inversion
Transition State
 Arrangement of the atoms in which nucleophile and the
leaving group are both partially bonded to the carbon atom
undegoing substitution
 Bond breaking and forming and occurred simultaneouly

Concerted reaction
Kinetics of a Nucleophilic
substitution: an SN2 reaction
 1 step reaction
 Second order
 Rate of reaction depends an alkyl halides and Nu:
 Rate Rxn = k [alkyl halide] x [Nu:]
Stereochemistry of SN2 Reactions
 Nucleophiles approaches from the back side, that is directly
opposite the leaving group.
 Consider the cis-1-chloro-3-methylclyclopentane
 When undergoes SN2, the product become trans
examples
 Give the structure of the product that would be formed
when trans-1-bromo-3-methylcyclobutane undergoes an SN2
reaction with NaI
A mechanism for SN1Reaction
The Relative stabilities of
Carbocations
 The order of stability of carbocations can be explained on
the basic of hyperconjugation.
 Involves electrons delocalization from a filled bonding orbital
to an adjacent unfilled orbital
 Any time a charge can be disperred or delocalized, a system
will be stabilized
The Relative stabilities of
Carbocations
Kinetics of a Nucleophilic
substitution: an SN1 reaction
 2 step reaction
 1st order rate determination
 Rate of reaction depends the slowest step
 Heterocleavage of halide
 Rate Rxn = k [alkyl halide]
Multistep Reactions and The
Rate-Determining Step
Multistep Reactions and The
Rate-Determining Step
 The step is intrinsically slower than all other is called the
rate-limiting step or rate determining step
Transition State
 Stabilization of leaving group
 I- > Br- > Cl- > F Weaker conjugated base  stronger leaving group
Mechanism for SN1 Reaction
 Show a complete mechanism with stereochemisty for the
following reaction
Mechanism for SN1 Reaction
 Show a complete mechanism with stereochemisty for the
following reaction
Factors Affecting the Rates of
SN1 and SN2 Reactions
 The structure of the substrate,
 The concentration and reactivity of the nucleophile
 The effect of the solvent
 The nature of the leaving group
The Effect of the Structure of
the substrate
 SN2 reaction shows the following general order of reactivity
 Methyl > primary > secondary >> (tertiary – unreactive)

steric hindrance
 SN1 reaction
 Tertiary > secondary > methyl

Hyperconjugation between p orbitals
Hammond-Leffler Postulate
 The structure of a transition state resembles the stable
species that is nearest it in free energy
The effect of the Concentration
and Strength of the Nucleophile
 A negatively-charged nucleophile is always a more reactive
nucleophile than its conjugated acid
 HO- is a better Nu: then H2O and RO- is better than ROH
 In a group of nucleophiles in which the nucleophilic atom is
the same, nucleophilicities parallel to basicities
 RO- > HO- >> RCO2- >> ROH > H2O
 equilibrium favors the side with weaker acid
Solvent Effects on SN2 Reactions:
Polar Protic and Aprotic solvent
 The effect of hydrogen bonding with the nucleophile is to
encumber the Nu: and hinder its reactivity in a substitution
reaction
Solvent Effects on SN2 Reactions: Polar
Protic and Aprotic solvent
 Hydrogen bonds to a small nucleophile atom are more
stronger than those to larger nucleophilic atoms
 Halide Nucleophilic in Protic Solvent
I- > Br- > Cl- > FLarger atoms have greater polarizability  larger nucleophile
atom can donate a greater degree of electron density to
substrate
SH- > CN- > I- > -OH > N3- > Br- > CH3CO2- > Cl- > F- > H2O
Solvent Effects on SN2 Reactions:
Polar Protic and Aprotic solvent
 Aprotic solvents are those solvents whose molecules do
not have a hydrogen that is attached to an atom of an
electronegative element
 They do the same way as protic solvents solvate cations;
by orienting their negative ends around cation and by
donating unshared electron pairs to vacant orbitals of
the cation
Solvent Effects on SN2 Reactions:
Polar Protic and Aprotic solvent
 They cannot form H-bond because their positive
centers are well shielded by the steric effects from any
interaction with anions
 Rate of SN2 reaction generally increased when they are
carried out in a polar protic solvent
Solvent Effects on SN1 Reactions: The
Ionizing Ability of the solvent
 The use of polar protic solvent will greatly increase the rate
of ionization of an alkyl halide in SN1 reaction
 Able to solvate cations and anions more affectively
 Stabilize transition state leading to the intermediate
carbocation and halide ion more than it does the reactant
 Lower activation energy
Summary of SN1 and SN2
Reactions
Factor
SN1
SN2
Substrate
3o (requires formation of a relatively Methyl > 1o > 2o (requires
stable carbocation)
unhindered substrate)
Nu:
Weak Lewis base, neutral molecule
Nu: may be the solvent
Strong Lewis Base, rate
increased by high
concentration of Nu:
Solvent
Polar Protic (e.g alcohol, water…)
Polar protic (e.g DMF,
DMSO
Leaving Group: I > Br > Cl > F for both SN1 and SN2
( the weaker the base after the group departs
the better the leaving group)
Elmination Reactions of Alkyl
Halides
 General Reaction
 If carbon next to alkyl halide has a halogen, Elimination is
possible
 Require a strong base
Dehydrohalogenation
 General Reaction
 Elements of hydrogen halide are eliminated from a
haloalkane
1,2 Elimination
 Alpha (α) carbon atom: carbon that bears the substituent
 Beta (β) hydrogen atom: hydrogen that attached to the
carbon adjacent to α carbon
Bases used in Dehydrohalogenation
Bases used in Dehydrohalogenation
Mechanism for E2 Reaction
Example
 Show a complete mechanism for E2 reaction
Mechanism for E1 Reaction
 Unimolcular reaction
Example
 Show a complete mechanism for E1 reaction
Substitution Versus Elmination
 As a rule:
 If your subsitution mechanism is SN1, E1 is the elimination
mechanism
 If your substitution mechanism is SN2, E2 is the elmination
mechanism
 2o alkyl halides: if 2o ; weak Nu: E1
if 2o; strong Nu: E2
Substitution Versus Elmination
Examples
 Prove the possible mechanisms (SN1, SN2, E1 and/or E2)
and possible products for the reaction below