Halogenoalkanes

1. Halogenoalkanes By Andrew Kropp , Hannah Couture, Mohamed Bounaim , and Amy Smith

2. What is a Halogenoalkane ?  a chemical compounds derived from  an alkane containing one or more  halogens Examples: Chlorofluorocarbons (ex. CH 2 ClF) Methyl bromide CH 3 Br Ethyl chloride C 2 H 5 Cl

3. How to name Halogenoalkanes Functional group = halogen Ex. Fluorine = fluoro Number by which carbon attached to, put in alphabetical order Ex. Bromoethane

4. More examples of Naming 1,1 dibromoethane 2 fluoropropane 1-bromo 2-methylpropane

5. 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

6. Examples of Primary (1°) Halogenoalkanes ** Can you name these halogenoalkanes ? (10 bonus points)

7. Examples of Secondary (2°) Halogenoalkanes

8. Examples of Tertiary (3°) Halogenoalkanes

9. 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.

10. 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 S n reactions (substitution nucleophilic ) and there are two mechanisms for them: S n 1 and S n 2 The type of mechanism depends on whether the halogenoalkane is a primary, secondary, or tertiary alkane

11. S n 2 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

12. S n 1 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

13. Step 1: Step 2:

14. Mechanism of Secondary Halogenoalkanes They can use either S n 1 or S n 2 mechanism depending on the reaction conditions Many times they use a mixture of the two mechanisms, with the S n 1 being the majority

15. 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

16. The End