Transcript Alkakes - Sumner

Halogenoalkanes
and Reaction Pathways
IB Chemistry Topics 10.5 & 10.6
10.5 Halogenoalkanes
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10.5.1 Describe, using equations, the
substitution reactions of halogenoalkanes
with sodium hydroxide.
10.5.2 Explain the substitution reactions of
halogenoalkanes with sodium hydroxide in
terms of SN1 and SN2 mechanisms.
10.5.1
Describe, using equations, the
substitution reactions of
halogenoalkanes with sodium
hydroxide.
Halogenoalkanes
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Halogenoalkanes consist of a carbon bonded
to an atom of fluorine, chlorine or bromine
General formula = CnH2n+1X, where X is a
halogen
These are typically oily liquids that do not mix
well with water
They are used in many products
CFCs have been renowned for their negative
impact on the ozone layer
Substitution Reactions
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In a substitution reaction, one atom or group of
atoms, takes the place of another in a molecule
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Examples
CH3CH2Br + KCN  CH3CH2CN + KBr
(CH3)3CCl + NaOH  (CH3)3 COH + NaCl
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Nucleophilic Substitution
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A nucleophile is a molecule or ion that has a high
electron density… nucleo = nucleus; phile = loving.
It is attracted to atoms in molecules with a lower
electron density.
It may replace another group in an organic molecule,
such as a halogen.
The hydroxide ion (OH-) from NaOH is an effective
nucleophile that will substitute the halogen, turning the
product into an alcohol
These reactions are known as substitution nucleophilic,
or SN reactions.
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Nucleophilic Substitution
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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
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Nucleophilic Substitution
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Nu:- + R-X  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
Nucleophile
Substrate
Product
Leaving group
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Nucleophilic Substitution
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Alkyl halides commonly undergo nucleolophilic
substitution reactions. The nucleophile (OH-)
displaces the halide leaving group from the alkyl
halide.
There are two common ways for nucleophilic
substitutions to occur.
They are known as SN1 and SN2.
Nucleophile
Substrate
Product
Leaving group
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Examples of Nucleophilic
Substitutions
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Nucleophilic substitutions may be SN1 or SN2
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10.5.2
Explain the substitution reactions of
halogenoalkanes with sodium
hydroxide in terms of SN1 and
SN2 mechanisms.
Nucleophilic Substitution
Bimolecular or SN2
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A reaction is bimolecular when the rate
depends on both the concentration 2
reactants: the substrate and the
nucleophile.
SN2 mechanisms occur most readily with
methyl compounds and primary
haloalkanes
Takes place in one step
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SN2 Mechanism
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The general form for an SN2 mechanism is shown above.
Nu:- = nucleophile
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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
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This is a one step process since both the
nucleophile and the substrate must be in a rate
determining step.
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SN2 Mechanism
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One-step mechanism with *curly arrows:
H
OH- +
H3CH2C
CH3
C
H CH3
Br
HO
C
Br
H3CH2C
HO C
CH3
H
+
CH2CH3
Transition State:
As OH- attaches,
Br- leaves
*Curly arrows represent the movement of an electron pair.
Br-
Nucleophilic Substitution
Unimolecular or SN1
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A unimolecular reaction occurs when the rate of
reaction depends on the concentration of 1
reactant: 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.
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SN1 Mechanism
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The general form for an SN1 mechanism is shown above.
Nu:- = nucleophile
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SN1 Mechanism
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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.
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SN1 Mechanism:
Heterolytic Fission
SN1 Mechanism
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In the second step, the nucleophile attaches to the carbon
intermediate (carbocation). This is a very fast step.
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SN1 Mechanism
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Two-step mechanism with curly arrows:
H
C
H3CH2C
H CH3
CH3
Br
Rate
determining
step: spotaneous
dissociation of
leaving group
C
+
+
H3CH2C
Transition State:
Formation of
Carbocation
OHBr-
HO C
Very fast step:
reaction of
nucelophile and
carbocation
CH3
H
CH2CH3
SN1 and SN2 Reactions
Rate
Carbocation
intermediate?
Number of steps
Occurs with
S N1
S N2
=k[RX]
=k[RX][Nuc:-]
Yes
No
2
1
Tertiary
halogenoalkanes
Primary
halogenoalkanes
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http://www.youtube.com/watch?v=tAjFraTw0
HM
http://www.youtube.com/watch?v=ZtnAR3uO
Abo (?)
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10.6 Reaction Pathways
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10.6.1 Deduce reaction pathways given the
starting materials and the product.
Reaction Pathways and
mechanisms
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Most organic reactions proceed by a defined sequence
or set of steps. The detailed pathway which an organic
reaction follows is called a mechanism.
Knowing a reaction mechanism is very valuable
information. It allows the chemist to predict what
products will be formed when a chemical reaction
occurs.
The organic chemist can use this information to modify
compounds and to synthesize new compounds with
certain desired characteristics.
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Diagram of common organic
reactions
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Reaction Pathway Practice
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Fill in the reaction pathway chart, showing the
necessary reactants and any other additional
conditions necessary for the reaction to take
place
You may omit trihalogenoalkanes and
poly(alkenes)