HL only
Learning outcomes
 Understand:
 Nucleophilic unimolecular substitution reactions are represented by
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
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SN1 and nucleophilic bimolecular substitution reactions are
represented by SN2.
Carbocation intermediates are formed in SN1 reactions whereas SN2
reactions involve a concerted reaction with a transition state.
The predominant mechanism for tertiary halogenoalkanes is SN1 and
for primary halogenoalkanes it is SN2. Secondary halogenoalkanes react
by both mechanisms.
The rate determining step (slow step) for SN1 reactions depends only on
the concentration of the halogenoalkane, rate = k[R−X]. For SN2
reactions, rate = k[R−X][Nu]. SN2 is stereospecific and involves
inversion of configuration at the carbon atom.
SN2 reactions are favoured by aprotic, polar solvents and SN1 reactions
are favoured by protic, polar solvents
Learning outcomes
 Apply knowledge to:
 Explain why a hydroxide ion is a better nucleophile than a
water molecule.
 Deduce the mechanism of the nucleophilic substitution
reactions of halogenoalkanes with aqueous sodium
hydroxide in terms of SN1 and SN2 reaction mechanisms.
 Explain how the rate depends upon the identity of the
leaving group (i.e. the halogen), whether the
halogenoalkane is primary, secondary or tertiary and on the
choice of solvent.
 Outline the difference between protic and aprotic solvents.
Substitution Reactions
 In a substitution reaction, one atom or group of
atoms, takes the place of another in a molecule
 Examples
CH3CH2Br + KCN  CH3CH2CN + KBr
(CH3)3CCl + NaOH  (CH3)3 COH + NaCl
5
Nucleophilic Substitution
 A nucleophile is a molecule or ion that has a
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
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
high electron density.
It is attracted to atoms in molecules with a
lower electron density.
It may replace another group in an organic
molecule.
The molecule to which the nucleophile is
attracted is called the substrate
The group that the nucleophile replaces is
called the leaving group
These reactions are known as nucleophilic
substitutions.
6
Nucleophilic Substitution
 One covalent bond is broken as a new
covalent bond is formed
 The general form for the reaction is
Nu:- + R-X  R-Nu +
X:
Nucleophile
Substrate
Product
Leaving group
7
Nucleophilic Substitution
 R-Nu +
X:
 The bond to the leaving group is broken
 The leaving group takes both electrons that
formed the bond with it
 The nucleophile provides the electrons to form
the new bond
 Nu:- + R-X
Nucleophile
Substrate
Product
Leaving group
8
Nucleophilic Substitution
 halogenoalkanes commonly undergo
nucleolophilic substitution reactions. The
nucleophile displaces the halide leaving group
from the halogenoalkane.
 There are two common ways for nucleophilic
substitutions to occur. They are known as SN1
and SN2.
Nucleophile
Substrate
Product
Leaving group
9
Examples of Nucleophilic
Substitutions
 Nucleophilic substitutions may be SN1 or SN2
10
Nucleophilic Substitution
Bimolecular or SN2
 A reaction is bimolecular when the rate depends
on both the concentration of the substrate and
the nucleophile.
 SN2 mechanisms occur most readily with methyl
compounds and primary haloalkanes
11
SN2 Mechanism

The general form for an SN2 mechanism is shown above.
Nu:- = nucleophile
12
An Example of a SN2 Mechanism
The nucleophilic substitution of ethyl bromide is shown
above. This reaction occurs as a bimolecular reaction.
The rate of the reaction depends on both the concentration
of both the hydroxide ion and ethyl bromide
 This is a one step process since both the
nucleophile and the substrate must be in a rate
determining step.
13
Nucleophilic Substitution
Unimolecular or SN1
 A unimolecular reaction occurs when the
rate of reaction depends on the
concentration of the substrate but not the
nucleophile.
 A unimolecular reaction is a two step process
since the subtrate and the nucleophile
cannot both appear in the rate determining
step
 SN1 mechanisms occur most readily with
tertiary haloalkanes and some secondary
haloalkanes.
14
SN1 Mechanism

The general form for an SN1 mechanism is shown above.
Nu:- = nucleophile
15
SN1 Mechanism

The first step is the formation of the carbocation. It is the
slow step. The rate of the reaction depends only on the
concentration of the substrate.
16
SN1 and SN2 Reactions
SN1
SN2
=k[RX]
=k[RX][Nuc:-]
Carbocation
intermediate?
Yes
No
Stereochemistry
mix
Inversion of
configuration
Rearrangement
~H, ~ CH3
possible
No
rearrangements
Rate
17
Aprotic polar solvent and protic solvent
 POLAR PROTIC
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SOLVENTS (polar and ability to
be H-bond donor)
have dipoles due to polar bonds
can H atoms that can be
donated into a H-bond
examples are the more common
solvents like H2O and ROH
remember basicity is also usually
measured in water
anions will be solvated due to Hbonding, inhibiting their ability
to function as Nu
 POLAR APROTIC
SOLVENTS (polar but no
ability to be H-bond donor)have
dipoles due to polar bonds
 don't have H atoms that can be
donated into a H-bond
 examples are acetone,
acetonitrile
 anions are not solvated and
are "naked" and reaction is not
inhibited
https://www.youtube.com/watch?v=FSxaox
Pu124
Overall
 All nucleophiles will be more
reactive in aprotic than protic solvents
 Those species that were most strongly solvated
in polar protic solvents will "gain" the most
reactivity in polar aprotic (e.g. F-).
 Polar aprotic solvents are typically only used when
a polar protic solvent gives poor results due to
having a weak Nu, (esp. F-, -CN, RCO2-)
Learning outcomes
 Understand:
 Electrophiles are electron-deficient species that can
accept a pair of electrons from a nucleophile. All
electrophiles are Lewis acids.
 The major product in electrophilic addition reactions
involving asymmetrical alkenes with hydrogen halides
and interhalogens can be predicted using
Markovnikov’s rule.
 The formation of the major product can be explained
by the relative stability of the possible intermediate
carbocations in the reaction mechanism.
Learning outcomes
 Apply their knowledge to:
 Deduce the mechanism of the electrophilic addition
reactions of alkenes with hydrogen halides and with
halogens and or interhalogens.
Addition Mechanisms
 Electrophilic addition occurs in reactions involving
containing carbon-carbon double bonds - the
alkenes.
 An electrophile is a molecule or ion that is
attracted to electron-rich regions in other
molecules or ions.
 Because it is attracted to a negative region, an
electrophile carries either a positive charge of a
partial positive charge
24
Electrophilic Addition II
Electrophilic addition occur in molecules where there are delocalized
electrons. The electrophilic addition to alkenes takes the following
general form:
25
Electrophilic Addition II
The electrophilic addition of alkanes occurs in two stages
First there is the formation of a carbocation
Followed by the attack the chloride ion to form the addition product
26
Markovnikoff’s Rule
Actually there are two possible carbocations that could be formed.
In may cases this would result in two possible products. However
only one form is preferred
“Birds of a feather flock together!”
The hydrogen ion will tend to migrate to the side with the
greater number of hydrogen atoms. This preference is known
as Markovnikoffs Rule.
27
Electrophilic Additions
 An addition reaction is a reaction in which two
molecules join together to make a larger molecule.
There is only one product. All the atoms in the original
molecules are found in the single product molecule.
 An electrophilic addition reaction is an addition
reaction which happens because what we think of as
the "important" molecule is attacked by an electrophile.
The "important" molecule has a region of high electron
density which is attacked by something carrying some
degree of positive charge.
28
Exercise
Write a mechanism for the electrophilic addition of HBr to 1butene.
29
Solution
Write a mechanism for the electrophilic addition of HBr to 1butene.
Solution
30
Condensation Reactions
 The condensation of an acid and an alcohol results in
the formation of an ester and water.
The carbon chain from the alcohol is attached to the single
bonded oxygen of the acid. The hydrogen lost from the acid
and the –OH from the alcohol combine to form a water
molecule.
31
Exercises Condensation Reactions
Write chemical reactions for the following
esterification reactions:
1.
Ethanol and ethanoic acid
2.
Methanol and butanoic acid
3.
2-Pentanol and ethanoic acid
4.
Methanol and 2 hydroxybenzoic acid
5.
Ethanoic acid and 2-hydroxybenzoic acid
32
Solutions to exercises
33
Learning outcomes
 Understand:
 The simplest arene (aromatic hydrocarbon compound)
is benzene.
 Benzene has a delocalized structure of π bonds around
its six-membered ring.
 Each carbon to carbon bond has a bond order of 1.5.
 Benzene undergoes electrophilic substitution
reactions.
Learning outcomes
 Apply knowledge to:
 Deduce the mechanism for the nitration of benzene
using concentrated nitric acid and a catalyst of
concentrated sulfuric acid.
Orbital model of Benzene Structure
 Benzene is built from hydrogen atoms (1s1) and carbon
atoms (1s22s22px12py1).
 Promotion of electron
 Hybridization
2
sp
hybrid orbital
2
sp
hybrid orbital
Benzene undergoes electrophilic substitution reactions
 Electrophilic substitution reactions involving
positive ions
 Benzene and electrophiles
 Because of the delocalised electrons exposed above
and below the plane of the rest of the molecule,
benzene is obviously going to be highly attractive to
electrophiles - species which seek after electron rich
areas in other molecules.
 The electrophile will either be a positive ion, or the
slightly positive end of a polar molecule
The general mechanism
 The first stage
 The second stage
The electrophilic substitution reaction
between benzene and nitric acid
 The facts
 Benzene is treated with a mixture of concentrated
nitric acid and concentrated sulphuric acid at a
temperature not exceeding 50°C. As temperature
increases there is a greater chance of getting more than
one nitro group, -NO2, substituted onto the ring.
 Nitrobenzene is formed.
The electrophilic substitution reaction
between benzene and nitric acid
 The formation of the electrophile
 The electrophile is the "nitronium ion" or the "nitryl
cation", NO2+. This is formed by reaction between the
nitric acid and the sulphuric acid
The electrophilic substitution reaction
between benzene and nitric acid
 The electrophilic substitution mechanism
 Stage one
 Stage two
Learning outcomes
 Understand:
 Aldehydes can be reduced to primary alcohols.
 Carboxylic acids can be reduced first to aldehydes then
to primary alcohols.
 Ketones can be reduced to secondary alcohols.
 Typical reducing agents are sodium borohydride,
NaBH4, and lithium aluminium hydride, LiAlH4, (used
to reduce carboxylic acids
Learning outcomes
 Apply knowledge to:
 Write equations for the reduction reactions of
aldehydes to primary alcohols, ketones to secondary
alcohols and carboxylic acids to aldehydes, using
suitable reducing agents.
 Describe the two-stage conversion of nitrobenzene to
phenylamine
REDUCTION OF ALDEHYDES AND KETONES
 Reducing agents
 lithium tetrahydridoaluminate(III) (also known as lithium
aluminium hydride) LiAlH4
 Sodium tetrahydridoborate(III) (sodium borohydride).
NaBH4
REDUCTION OF ALDEHYDES AND KETONES
 The overall reactions
The reduction of an aldehyde
Reduction of an aldehyde leads to a primary alcohol.
The reduction of a ketone
 Reduction of a ketone leads to a secondary alcohol
REDUCTION OF ALDEHYDES AND KETONES
 Using lithium tetrahydridoaluminate (lithium
aluminium hydride)
 Lithium tetrahydridoaluminate is much more reactive
than sodium tetrahydridoborate. It reacts violently
with water and alcohols, and so any reaction must
exclude these common solvents.
 The reactions are usually carried out in solution in a
carefully dried ether such as ethoxyethane (diethyl
ether). The reaction happens at room temperature,
and takes place in two separate stages.
REDUCTION OF ALDEHYDES AND KETONES
Using lithium tetrahydridoaluminate (lithium
aluminium hydride)
 In the first stage, a salt is formed containing a complex
aluminium ion.
 In the second stage, the product is then treated with a
dilute acid (such as dilute sulphuric acid or dilute
hydrochloric acid) to release the alcohol from the
complex ion
REDUCTION OF ALDEHYDES AND KETONES
Using sodium tetrahydridoborate (sodium
borohydride)
 Sodium tetrahydridoborate is a more gentle (and therefore
safer) reagent than lithium tetrahydridoaluminate. It can
be used in solution in alcohols or even solution in water provided the solution is alkaline.
 Solid sodium tetrahydridoborate is added to a solution of
the aldehyde or ketone in an alcohol such as methanol,
ethanol or propan-2-ol. It is either heated under reflux or
left for some time around room temperature. This almost
certainly varies depending on the nature of the aldehyde or
ketone
REDUCTION OF ALDEHYDES AND KETONES
 First Stage – Salt is formed
 Second Stage In the second stage of the reaction, water
is added and the mixture is boiled to release the
alcohol from the complex.
Nitrobenzene to phenylamine
 The conversion is done in two main stages:
 Stage 1: conversion of nitrobenzene into phenylammonium
ions
 Nitrobenzene is reduced to phenylammonium ions using a
mixture of tin and concentrated hydrochloric acid.
 The electron-half-equation for this reaction is:
 The electrons come from the tin, which forms both tin(II) and
tin(IV) ions
Nitrobenzene to phenylamine
 Stage 2: conversion of the phenylammonium ions
into phenylamine
 Sodium hydroxide solution is added to the product of
the first stage of the reaction.

 Summary

Summary
 Benzene to Phenylamine