HALOGENOALKANES

By Andrew Kropp, Hannah Couture, Mohamed Bounaim, and Amy Smith

What is a Halogenoalkane?

 a chemical compounds derived from an alkane containing one or more halogens

Examples: Chlorofluorocarbons (ex. CH2ClF)

Methyl bromide CH3Br

Ethyl chloride C2H5Cl

How to name Halogenoalkanes Functional group = halogen

Ex. Fluorine = fluoro Number by which carbon attached

to, put in alphabetical order Ex.

Bromoethane

More examples of Naming

1,1 dibromoethane

2 fluoropropane

1-bromo 2-methylpropane

Classification

Halogenoalkanes can be classified as primary, secondary or tertiary depending on their structure Primary (1°) – carbon carrying halogen

is attached to only one carbon alkyl group

Secondary (2°)– carbon carrying halogen is attached to two other alkyl groups

Tertiary (3°) – carbon carrying halogen is attached to three alkyl groups

Examples of Primary (1°) Halogenoalkanes

** Can you name these halogenoalkanes? (10 bonus points)

Examples of Secondary(2°) Halogenoalkanes

Examples of Tertiary (3°) Halogenoalkanes

Nucleophilic Substitution Reactions Halogenoalkanes react with substances such

as sodium hydroxide to produce an alcohol through a reaction called nucleophilic substitution

A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else.

Nucleophiles are either fully negative ions, or else have a strongly negative charge somewhere on a molecule.

Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

Nucleophilic Substitution Reactions Cont. Example using sodium hydroxide: The hydroxide ion acts as a nucleophile and

is thus attracted to where the halogen has left the alkane and takes the place of the halogen

These reactions are called Sn reactions (substitution nucleophilic) and there are two mechanisms for them: Sn 1 and Sn 2

The type of mechanism depends on whether the halogenoalkane is a primary, secondary, or tertiary alkane

Sn2 Mechanism – For primary halogenoalkanes Substitution nucleophilic bimolecular

(called bimolecular because there are 2 reactants at the slow step)

1. slow step: OH¯ attacks Carbon 2. transition step: OH¯ and Cl¯ are partially

bonded to C 3. fast step: alkane releases chloride ion,

alcohol is formed

Sn1 Mechanism – For tertiary halogenoalkanes Substitution Nucleophilic Unimolecular

(because one reactant at slow step) *since there are 3 alkyl groups around carbon

it creates a steric hindrance = the 3 bulky groups make it hard for OH¯ to attack the C

1. Halogenoalkane ionizes – the C-Halogen bond is broken, Halide ion takes electrons – there is a + charge on the carbon = carbocation

2. Alkyl groups donate electrons to keep it stable, OH¯ fills the open spot and it becomes an alcohol

Step 1:

Step 2:

Mechanism of Secondary Halogenoalkanes They can use either Sn1 or Sn2

mechanism depending on the reaction conditions

Many times they use a mixture of the two mechanisms, with the Sn1 being the majority

Reactivity of Halogenoalkanes The relative reactivity of

halogenoalkanes depends on the Carbon-halogen bond strength and decreases down the group

Examples: Iodoalkanes= weakest bond, most

reactive Fluoroalkanes = strongest bond, least

reactive

THE END