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SME Chemistry Topic 20 Notes
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SME Chemistry Topic 20 Notes
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H e a d t o s a v e m y e x a m s . c o . u k f o r m o r e a w e s o m e r e s o u r c e sPage 1 of 48
© 2 0 1 5 − 2 0 2 3 S a v e M y E x a m s , L t d . · R e v i s i o n N o t e s , T o p i c Q u e s t i o n s , P a s t P a p e r sIB Chemistry DP
20. HL Organic Chemistry
CONTENTS
20.1 Types of Organic Reactions
20.1.1 Nucleophilic Substitution Reactions
20.1.2 Electrophilic Addition Reactions
20.1.3 Electrophilic Substitution Reactions
20.1.4 Reduction Reactions
20.2 Synthetic Routes
20.2.1 Synthesis
20.3 Stereoisomerism
20.3.1 Stereoisomers
20.3.2 Cis-Trans & E/Z Isomers
20.3.3 Optical IsomersYOUR NOTES H e a d t o s a v e m y e x a m s . c o . u k f o r m o r e a w e s o m e r e s o u r c e s
Page 2 of 48
© 2 0 1 5 − 2 0 2 3 S a v e M y E x a m s , L t d . · R e v i s i o n N o t e s , T o p i c Q u e s t i o n s , P a s t P a p e r sNucleophilic Substitution Reactions
In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is
replaced by a nucleophile
The strength of any nucleophile depends on its ability to make its lone pair of electrons
available for reaction
The hydroxide ion, OH, is a stronger nucleophile than water because it has a full negative
charge
This means that it has a readily available lone pair of electrons
A water molecule only has partial charges, δ + and δ -
This means that its lone pair of electrons is less available than the hydroxide ions
The lone pairs of electrons in a water molecule are still available to react
Lewis structures of the hydroxide ion and water molecule - illustrating the lone pairs of
electrons and charges within their structures
Exam Tip
In general:
A negatively charged ion will be a stronger nucleophile than a neutral molecule
A conjugate base will be a stronger nucleophile than its corresponding
conjugate acid
e.g. the hydroxide ion is a stronger nucleophile than water
20.1 Types of Organic Reactions
20.1.1 Nucleophilic Substitution Reactions-
YOUR NOTES H e a d t o s a v e m y e x a m s . c o . u k f o r m o r e a w e s o m e r e s o u r c e s
Page 3 of 48
© 2 0 1 5 − 2 0 2 3 S a v e M y E x a m s , L t d . · R e v i s i o n N o t e s , T o p i c Q u e s t i o n s , P a s t P a p e r sSN1 Mechanism
Nucleophilic substitution reactions can occur in two di erent ways (known as
S2 and S1 reactions) depending on the structure of the halogenoalkane involved
S1 reactions
In tertiary halogenoalkanes, the carbon that is attached to the halogen is also bonded to
three alkyl groups
These halogenoalkanes undergo nucleophilic substitution by an S1 mechanism
‘S’ stands for ‘substitution’
‘N’ stands for ‘nucleophilic’
‘1’ means that the rate of the reaction (which is determined by the slowest step of the
reaction) depends on the concentration of only one reagent, the halogenoalkane
The S1 mechanism is a two-step reaction
In the first step, the C-X bond breaks heterolytically and the halogen leaves the
halogenoalkane as an X ion (this is the slow and rate-determining step)
As the rate-determining step only depends on the concentration of the
halogenoalkane, the rate equation for an S1 reaction is rate = k[halogenoalkane]
In terms of molecularity, an S1 reaction is unimolecular
This forms a tertiary carbocation (which is a tertiary carbon atom with a positive
charge)
In the second step, the tertiary carbocation is attacked by the nucleophile
For example, the nucleophilic substitution of 2−bromo-2−methylpropane by hydroxide
ions to form 2−methyl-2−propanol
The mechanism of nucleophilic substitution in 2−bromo-2−methylpropane which is a
tertiary halogenoalkaneN N
N
N
N
-
N
NYOUR NOTES H e a d t o s a v e m y e x a m s . c o . u k f o r m o r e a w e s o m e r e s o u r c e s
Page 4 of 48
© 2 0 1 5 − 2 0 2 3 S a v e M y E x a m s , L t d . · R e v i s i o n N o t e s , T o p i c Q u e s t i o n s , P a s t P a p e r sExam Tip
You are expected to know the di erence between the heterolytic fission that
features in S1 reactions and homolytic fission in other reactions:
Heterolytic fission forms anions and cations and uses double headed arrows to
show the movement of both electrons from the covalent bond
Homolytic fission forms free radicals and uses single headed arrows,
sometimes called fish hooks, to show the movement of a single electron as the
covalent bond breaks
NYOUR NOTES
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