Title: Lesson 6 Halogenoalkanes and
Benzene Substitution (SL) and SN1 SN2 (HL)
Learning Objectives:
– Understand why the nature of the halogenoalkane effects the mechanism of
nucleophilic substitution reactions
– Understand why OH- is a better nucleophile than H2O
– Understand the effect of the halogen on the rate of nucleophilic substitution
reactions
– Complete a short investigation into the factors affecting the rate of
nucleophilic substitution
Refresh
a)
Draw four structural isomers of
molecular formula C4H10O which
contain the –OH group.
b)
On reaction with acidified potassium
dichromate(VII), two of the isomers
are oxidized in two steps to produce
different products. Draw the
structural formula of the two
products formed from one of the
isomers.
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Reviewing Your Notes
You should spend 60
seconds reviewing your
notes from last lesson
before attempting this.
Halogenoalkanes
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General structure CnH2n+1X (where X is the halogen)
Contain an atom of fluorine, chlorine, bromine, or iodine
bonded to the carbon skeleton
Saturated, so reactions involve substitution (like alkanes)
But halogenoalkanes have polar bonds so are more
reactive
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Nucleophilic Substitution

One of the most important reactions undergone by
halogenoalkanes is nucleophilic substitution

A nucleophile is a ‘nucleus-loving’ species that is attracted to
positive charges.


Nucleophiles have either full negative charges or delta-negative charges
Water and hydroxide are both nucleophiles


In this case we can also call the reaction ‘hydrolysis’
The carbon in the carbon-halogen bond has a + charge due to
the greater electronegativity of the halogen

This makes it susceptible to attack by nucleophiles
The carbon is
‘electron deficient’
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Halogenoalkanes and strong bases

A substitution reaction takes place, where the halogen atom is displaced by the
hydroxide ion (good nucleophile)
halogenoalkane + sodium hydroxide  alcohol + sodium chloride
H
H
Br H
OH H
H
H

Conditions:



Aqueous base
Gently warmed (can at room temperature, but may be quite slow)
This is a nucleophilic substitution


The C attached to the halogen is + due to the high electronegativity of the halogen
The OH- ion (our nucleophile) is attracted to the + carbon

A nucleophile is a species with a negative charge or a lone pair that is attracted to
positive/delta-positive atoms
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Electrophilic substitution reactions: benzene

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Benzene does not behave like alkenes
Highly stable aromatic ring determines substitution reactions, not addition takes place
But like alkenes, benzenes delocalized cloud of pi electrons is attractive to electrophiles
Electron density seeks electron deficient species and substitutes the hydrogen atom it is bonded to…
Electrophilic substitution requires high activation energy so proceeds slowly
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Benzene

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Delocalised electrons give it special stability
Addition reactions are not favourable as it would lead to the loss of the stable
arene ring
Substitution reactions occur instead to preserve the arene ring
Delocalised ring is the site of reactivity
E+ is the generic electrophile

Benzene will undergo electrophilic substitution
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Electrophilic substitution reactions of benzene


Electrophiles are reactants that are electron deficient, have a positive charge or a partial positive
charge
They are attracted to the electron rich benzene ring
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Solutions
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Nucleophilic substitution reactions: halogenoalkanes
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Nucleophiles are electron rich and attack areas of electron deficiency.
Act as Lewis bases and donate a pair of electrons in forming a new covalent bond.
Common nucleophiles include OH- (from dilute NaOH sol’n), CN-, NH3 and H2O.
Curly (double headed) arrow convention represents the motion of the electron pair. Tail shows where
the pair comes from and head shows where it is going.
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Shorthand notation SN – Substitution nucleophilic
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The polar carbon-halogen bond means the carbon atom is electron deficient, and is attacked by
nucleophiles such as OHThe carbon—halogen bond breaks and the halogen atom is released as a negative ion (the halide)
This type of bond breakage, where both the shared electrons go to one of the products, is called
hetereolytic fission
The halogen, because it becomes detached, is called the leaving group
The exact mechanism
depends on whether
the halogenoalkane is
primary, secondary or
tertiary…
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Primary halogenoalkanes: SN2 mechanism

Primary halogenoalkanes have at least two hydrogen atoms attached to the carbon on
the carbon-halogen bond. E.g. Chloroethane…

Overall reaction with NaOH:

Hydrogen atoms are small so Carbon is open to attack by the nucleophile
The carbon-halogen
bond then breaks
heterolytically,
releasing Cl- and
forming alcohol
Dotted lines show weak bonds
between carbon and the
halogen and the nucleophile
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SN2 Substitution
Princess Halogen
realizes after the
nucleophile docks
there is no hope!
So she leaves…..
The nucleophile
attacks from the
back!!
H
–
H
H
δ+
H
δ–
Cl
SN2 and steric hindrance

Alkyl groups are physically bulky, and make it difficult for a nucleophile to
attack the carbon: this is called steric hindrance

1o halogenoalkanes only have one surrounding alkyl group so steric
hindrance is low and SN2 is favourable

3o halogenoalkanes have three surrounding alkyl groups so steric hindrance
is high and SN2 is unfavourable

The black arrows on the diagram are supposed to show possible avenues
of approach by the nucleophile, red crosses show where they are blocked
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Nucleophilic substitution reactions
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© Boardworks Ltd 2009
Nucleophilic substitution
17 of 31
© Boardworks Ltd 2009
SN2 – Bimolecular nucleophilic substitution – animation
here
Another example:

Bimolecular because two molecules are involved in the rate determining step

In the rate determining step, the nucleophile (OH-) attacks at the same time as the
carbon-halogen bond breaks.

The reaction passes through a negative transition state where the carbon has a
‘half-bond’ to both the –OH and the –Br with an overall negative charge

The rate is dependent on both the concentration of the halogenoalkane and the
nucleophile

Rate = k[halogenoalkane][nucleophile]
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Nucleophiles attack from opposite side of the leaving
group
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Inversion of the arrangement of the atoms around the carbon atom
Like an umbrella blowing inside out
The SN2 mechanism is favoured by polar, aprotic solvents. Aprotic solvents are those
which are not able to form hydrogen bonds as they do not contain –OH or –NH
bonds (although they may have strong dipoles)
The lack of hydrogen bonding means they dissolve the metal cation (e.g. Na+ from the
NaOH ) rather than the nucleophile (OH-).
The undissolved, bare nucleophile has a higher energy state and this increases reaction
rate. Examples of these solvevnts are: Propanone (CH3)2CO, and ethanitrile CH3CN
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Tertiary halogenoalkanes: SN1 mechanism
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Tertiary halogenoalkanes have 3 alkyl groups attached to the carbon of the
carbon-halogen bond. E.g. 2-chloro-2-methylpropane:
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The overall reaction with NaOH:
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Three bulky alkyl groups causes steric hindrance, making it difficult
for the incoming nucleophile to attack.
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SN1 Substitution
Princess Halogen
escapes before
the nucleophile
attacks!
Attack of the
nucleophile!
RH
–
H
H
R
δ–
δ+
HR
Cl
–
R
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Richard Thornley - Halogenoalkane Substitution Reaction Video
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Richard Thornley - SN1 SN2 Substitution Short Video
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Richard Thornley - Describe and Explain Reaction By Identity of Halogen
Video
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Richard Thornley - Halogeno Substitutions with NH3 and KCN Video
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How to deal with steric hindrance
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1st step involves the halogenoalkane ionizing by breaking its carbon-halogen
bond heterolytically

As the halide ion is detached it leaves the carbon atom with a temporary
positive charge – carbocation intermediate

2nd step involves the carbocation being attacked by the nucleophile leading to
the new bond
The presence of the 3 alkyl
groups has a positive
inductive effect (shown by
the blue curly arrows) and
stabilizes the carbocation
to persist long enough for
the second step to occur.
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SN1 – Unimolecular nucleophilic substitution – animation
here
Another example:

The rate is only dependent on the concentration of the
halogenoalkane:
 Rate = k[halogenoalkane]

SN1 mechanism is favoured by polar, protic solvents that contain
–OH or –NH so can form hydrogen bonds.

They stabilise the carbocation by dissolving involving ion-dipole
interactions. Examples are: water, alcohols and carboxylic acids
Water has permanent dipole.
Oxygen Is slightly –ve so surround the +ve carbocation.
This cancels out ions charge or effectively shields it from the –ve ion (halide).
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Secondary halogenoalkanes
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The mechanism of nucleophilic substitution in secondary halogenoalkanes is a
mixture of both SN1 and SN2
SN1 SN2 Animation Video
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SN1 or SN2?
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1o halogenoalkanes predominantly
undergo SN2
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2o halogenoalkanes undergo a mix
of SN1 and SN2
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3o halogenoalkanes predominantly
undergo SN1
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SN1 and SN2 Recap
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Comparison of the rates of nucleophilic substitution
reactions
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3 factors to consider:
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The effect of the mechanism
The influence of the leaving group (halogen)
Choice of solvent
1 The effect of the mechanism
 Tertiary (SN1) reactions only depend on the concentration of the
halogenoalkane only
 Primary (SN2) reactions depend on the concentrations of both the
halogenoalkane and the nucleophile
 Secondary reactions will show a mix of both SN1 and SN2
 Rate of reactivity as shown:
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2 The influence of the leaving group (halogen)
 2 opposing factors here:
(a) The polarity of the carbon-halogen bond
 As the electronegativity of the halogens decreases down the group, the
carbon of the carbon-halogen bond becomes progressively less electron
deficient  less vulnerable to nucleophilic attack
 So we would expect the rate to reactivity to be:
(b) The strength of the carbon-halogen bond
 carbon-halogen bond decreases in strength down the group
 Substitution reactions involves breaking bonds so we expect ease of bond
breaking to be:

Reaction rata data shows that (b) dominates the outcome here,
hence the rate of reactivity should be:
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3 Choice of solvent
 SN1 favoured by polar, protic solvents
 SN2 favoured by polar, aprotic solvents
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Therefore, the fastest reactions will be tertiary iodoalkanes in polar,
protic solvents
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Nucleophilic substitution reactions of halogenoalkanes can be followed by the
appearance of the halide ion
Silver nitrate AgNO3 can be added and a precipitation of a silver halide of a
distinct colour will be formed
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Changing the Nucleophile
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Water can act as our nucleophile:
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Halogenoalkane + water  alcohol + hydrogen halide
However, hydroxide is much better. Why? (Think about what
you know from the bonding unit…)
Any species can act as a nucleophile as long as it has an unshared pair of
electrons. (this means that it can act as a Lewis base)
The hydroxide ion has a charge on it where as water doesn't. This means
that the hydroxide ion has a greater affinity for the slightly positive carbon
atom.
Richard Thornley - Why OH- Better
than Water As a Nucleophile
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Summary of SN2 and SN1 mechanisms of nucleophilic
substitution
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Solutions
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Exploring Nucleophilic Substitution
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In this experiment you will explore the effect on the rate of nucleophilic
substitution of:
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The halogen atom involved
The position of the halogen atom
The nucleophile used
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Since these reactions produce halide ions, we can use their precipitation
reaction with silver ions to follow the reaction

Follow the instructions here
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Key Points
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The substitution mechanism followed depends on:
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Stabilisation of the carbocation by surrounding alkyl groups
Steric hindrance of the carbocation by surrounding alkyl groups
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Hydroxide ions are better nucleophiles than water due to their strong
negative charge

Iodoalkanes react faster than chloroalkanes due to the C-I bond being
weaker than the C-Cl bond
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