HALOGENOALKANES AND BENZENE Halogenoalkanes Nucleophilic Substitution Reactions of

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 Na. OH 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

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: SN 2 Mechanism ■ Example: The reaction of chloroethane, a primary halogenoalkane, with Na. OH ■ The overall reaction is:

Primary Halogenoalkanes: SN 2 Mechanism

Tertiary Halogenoalkane: SN 1 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 SN 1 mechanism is favored by polar, PROTIC solvents – The SN 2 mechanism is favored by polar, aprotic solvents. .