Introduction to halogenoalkanes

Starter Match the definitions

Learning objectives • Describe the structure, general formula and uses of halogenoalkanes. • Describe and explain the nature of the carbon– halogen bond and its susceptibility to nucleophilic attack. • Define the term nucleophile as an electron pair donor.

Halogenoalkanes Compound where a halogen has replaced one of the hydrogen atoms in the alkane chain Were very widely used as refrigerants, aerosol propellants, degreasers, dry-cleaning solvents, however their use has changed (we will look at this in later lessons) They are useful in organic synthesis and to prepare many useful materials

Halogenoalkanes What is the general formula for a halogenoalkane? (where X represents the halogen) CnH2n+1X Prefixes for naming halogenoalkanes Halogen Prefix F Fluoro- Cl Chloro- Br Bromo- I Iodo-

Naming the halogenoalkanes Only contain single bonds, therefore the names are based on the ALKANE homologous series Appropriate prefix is added to the name of the longest alkane chain Position of the halogen is indicated If there is more than one halogen, they are listed alphabetically

Example Five carbons in the longest chain The name is based on pentane There is a bromo group on the third carbon and a chloro group on the second carbon The name is 3-bromo-2-chloropentane

Questions Complete question 1 on page 157

Reactivity of the halogenoalkanes Haloagenoalkanes have a polar bond between the carbon and halogen atoms. Why is this bond polar? It is polar because of the different electronegativities 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 result is a polar bond

Polarity in a molecule of bromomethane

Electronegativity of halogens Decreases down the group Decrease in polarity of the carbon-halogen bond from fluorine to iodine The electron deficient carbon atom attracts nucleophiles such as H2O, OH- and NH3 Substitution reactions Nucleophile replaces the halogen

Hydrolysis of halogenoalkanes Key definition – hydrolysis A reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds In halogenoalkanes a nucleophilic substitution reaction occurs Reaction is carried out under reflux The product is an alcohol For example: hydrolysis of 1-choropropane CH3CH2CH2Cl (aq) + OH-(aq) CH3CH2CH2OH(aq) + Cl-(aq) 1-chloropropane  propan-1-ol

Questions Complete questions 2 and 3 on page 157