1. AH
CHEMISTRY
Unit 2
Alkenes

2. LEARNING INTENTIONS AH Part 1 Preparation Dehydration of alcohols
CHEMISTRY
LEARNING INTENTIONS
Part 1 Preparation
Dehydration of alcohols
Base induced elimination of hydrogen halides from monohalogenoalkanes

3. Dehydration of alcoholsAH
CHEMISTRY
Dehydration of alcohols
Concentrated sulphuric acid is often used but phosphoric acid and aluminium oxide are alternatives.

4. AH
CHEMISTRY
Base-induced elimination of hydrogen halides from monohalogenoalkanes
Note: The strong base must be dissolved in ethanol – if the base is dissolved in water the substitution occurs.

5. AH Reactions of alkenes
CHEMISTRY
Reactions of alkenes
Alkenes have a double bond. This means a high concentration of negative charge and hence will react with positively charged species.
The characteristic reaction of alkenes is electrophilic addition.

6. AH
CHEMISTRY
Reactions: Addition of hydrogen

7. AH Reactions: Addition of bromine
CHEMISTRY
Reactions: Addition of bromine
The bromine molecule is polarised by the double bond
A cyclic bromonium ion is produced
The bromide ion adds on to form the product
As the initial attack is by the Br+ ion this is an electrophilic addition

8. AH
CHEMISTRY
Reactions: Addition of hydrogen halides

9. Reactions: Addition of hydrogen halides
Markovnikoff’s Rule
If we consider the addition of HCl to propene there are two possible products yet we obtain virtually 100% of one and none of the other. Why?

10. To explain this we must consider the carbonium ion formed and its stability.
The more alkyl groups attached, the greater the stability.

11. The more stable carbocation will be formed and lead to the final product.
Simplified version : the Hydrogen goes to the carbon containing the most hydrogens already attached.

12. Note: this follows Markovnikoff’s rule
Reactions: Addition of water (hydration)
Note: this follows Markovnikoff’s rule

13. Mechanism