Download Nucleophilic Substitution Reactions

HALOGENOALKANES
AND BENZENE
Halogenoalkanes
Nucleophilic Substitution Reactions of
Halogenoalkanes
■ The polarity in halogenoalkanes is due to the fact that the halogen atom is more
electronegative than carbon, and so exserts a stronger pull on the shared electrons in the
carbon-halogen bond.
■ As a result, the halogen gains a partial negative charge and the carbon gains a partial
positive charge, and it is said to be electron deficient.
Nucleophilic Substitution Reactions of
Halogenoalkanes
■ Nucleophiles are reactants that are electron rich, as they have a lone pair of electrons
and may also carry a negative charge. These species are therefore attracted to the
electron-deficient carbon atom in the halogenoalkane.
■ This leads to reactions in which substitution of the halogen occurs, and these are
known as nucleophilic substitution reactions.
Nucleophilic Substitution Reactions of
Halogenoalkanes
■ The hydroxide ion, OH-, is a good nucleophile.
■ So, halogenoalkanes can react with NaOH to form alcohols.
Electrophilic Substitution Reactions of
Benzene
■ As we saw earlier, the delocalized electrons in benzene give it a special stability.
■ Benzene does not undergo addition reactions, as this would lead to a loss of the
stable benzene ring to produce a product with higher energy.
■ However, benzene can undergo substitution reactions in which one or more
hydrogen atoms are replaced by an incoming group. Substitution reactions produce
products in which the benzene ring is conserved.
■ The delocalized ring of electrons, which represents an are of electron density, is the
site of reactivity.
Electrophilic Substitution Reactions of
Benzene
This electrophilic substitution reaction of benzene with an electrophile shows that the
benzene ring is conserved.
Electrophilic Substitution Reactions of
Benzene
Electrophilic Substitution Reactions of
Benzene
Types of Organic Reactions
Nucleophiles and Electrophiles
Curly Arrows
■ Curly arrows are used to show the movement of a pair electrons.
■ The tail shows where the electrons come from and the head shows where they are
going.
Nucleophilic Substitution Reactions:
Halogenoalkanes
■ Fg
Nucleophilic Substitution Reactions:
Halogenoalkanes
Primary Halogenoalkanes: SN2 Mechanism
■ Example: The reaction of chloroethane, a primary halogenoalkane, with NaOH
■ The overall reaction is:
Primary Halogenoalkanes: SN2 Mechanism
Primary Halogenoalkanes: SN2 Mechanism
Primary Halogenoalkanes: SN2 Mechanism
Tertiary Halogenoalkane: SN1 Mechanism
Tertiary Halogenoalkane: SN1 Mechanism
Tertiary Halogenoalkane: SN1 Mechanism
Tertiary Halogenoalkane: SN1 Mechanism
Tertiary Halogenoalkane: SN1 Mechanism
Secondary Halogenoalkanes
Rates of Nucleophilic Substitution
Reactions
■ The effect of the mechanism
Rates of Nucleophilic Substitution
Reactions
■ The influence of the leaving group (halogen)
■ The rate of the reaction is affected by the strength of the carbon-halogen bond.
■ Since the strength of the carbon-halogen bond decrease from fluorine to iodine, we
would expect the ease with which the bonds break to be:
C-I > C-Br > C-Cl >
C-F
■ So, we would expect the reaction rates of different halogens in the halogenoalkane
to be:
Iodoalkanes > Bromoalkanes > Chloroalkanes > Fluoroalkanes
Rates of Nucleophilic Substitution
Reactions
■ Choice of Solvents
– The SN1 mechanism is favored by polar, PROTIC solvents
– The SN2 mechanism is favored by polar, aprotic solvents..