Introduction to halogenoalkanes

StarterMatch 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 halogenoalkanesHalogen 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

ExampleFive carbons in the

longest chainThe name is based

on pentaneThere is a bromo

group on the third carbon and a chloro group on the second carbon

The name is 3-bromo-2-chloropentane

QuestionsComplete question 1 on page 157

Reactivity of the halogenoalkanesHaloagenoalkanes 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 halogensDecreases down the

groupDecrease 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 reactionsNucleophile replaces

the halogen

Hydrolysis of halogenoalkanesKey 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 refluxThe product is an alcoholFor example:

hydrolysis of 1-choropropaneCH3CH2CH2Cl (aq) + OH-(aq) CH3CH2CH2OH(aq) +

Cl-(aq)1-chloropropane propan-1-ol

QuestionsComplete questions 2 and 3 on page 157