Introduction to halogenoalkanes Starter: Test your partner on the homework questions Keywords: Halogenoalkane Nucleophile Substitution reaction

Learning Objectives A Explain the nature of the carbon– halogen bond and its susceptibility to nucleophilic attack. B Describe the structure, general formula and uses of halogenoalkanes. C Define the term nucleophile as an electron pair donor.

Halogenoalkanes A compound in which a halogen has replaced at least one of the hydrogen atoms in an alkane chain Used to be used in aerosols but now they are not due to their damaging effect on the o-zone layer Important in organic synthesis and can be used to prepare many materials General formula C n H 2n+1 X (where X represents the halogen)

Halogenoalkanes contain only single bonds so their names are based on alkanes The appropriate prefix is added to the name of the longest chain The position of the halogen on the chain is indicated If there is more than one halogen atom they are named alphabetically F Fluoro- Cl Chloro- Br Bromo- I Iodo-

Draw structural formulae for the following: 2-chloropropane 2,3-dibromopentane 2-bromo-4-chlorohexane

Reactivity of the halogens Halogenoalkanes contain a polar carbon-halogen bond. The polarity arises from the different electronegativity of the carbon and halogen atoms Halogen atoms are more electronegative than carbon atoms. The bonded electron pair is attracted more towards the halogen atom than towards the carbon atom

The electronegativity of the halogens decreases down the group, resulting in a decrease in polarity of the carbon-halogen bond from fluorine to iodine

The electron deficient carbon atom in halogenoalkanes attracts nucleophiles such as H 2 O, OH- and NH 3 Halogenoalkanes react with nucleophiles in substitution reactions The nucleophile replaces the halogen atom in the halogenoalkane, forming a compound containing a different functional group

The hydrolysis of halogenoalkanes When halogenoalkanes react with an aqueous solution of hot hydroxide ions, a nucleophilic substitution reaction occurs to produce alcohol. Hydrolysis is a reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds

Plenary Pg 157 Questions 1, 2 and 3