Fission, addition, substitution and elimination

Starter We have covered lots of material in the last few lessons, it would be really useful to know; 1. At least two things you are happy with (e.g. Drawing displayed formula) 2. At least two things you want more practise at (e.g. Naming organic compounds) 3. The best thing you’ve learnt since starting AS Chemistry (I naming organic compounds, very therapeutic)

Learning outcomes Describe the different types of covalent bond fission – homolytic and heterolytic. Define the terms: nucleophile; electrophile; radical; addition; and substitution.

Organic reactions In organic chemistry, a covalent bond can be broken in one of two ways: Homolytic fission Heterolytic fission What could these terms mean?

Homolytic fission Each bonded atom takes one of the shared pair (they are the same) Each atom has an unpaired electron and is called a radical Two species of the same type are produced E.g. X-Y  X · + Y · A single dot is often written next to the species to represent the unpaired electron This is a species with an unpaired electron

Heterolytic fission One of the bonded atoms takes both of the shared pair of electrons Two ions are produced: The atom that takes both the electrons becomes a negatively charged ion The atom that does not take the electrons becomes a positively charged ion E.g. X-Y  X + + Y -

Key definitions Can you find out what an nucleophile and an electrophile are?

Nucleophile Reactant that attacks an electron-deficient carbon atom Donates an electron pair Often a negative ion with a lone pair of electrons Often electronegative with a lone pair and a partial δ - charge Examples include :Br -, :OH -, H 2 O:, :NH 3 (if it helps nuclei are positive – nucleophiles love positive charges)

Electrophile Reactant that attacks an area of high electron density Accepts an electron pair Electrophiles are often positive ions Often include an atom with a δ + partial charge Examples include Br 2, HBr, NO 2 +

Starter for ten Have a go at sheet electrophiles and nucleophiles

Types of reaction We will be meeting three types of reaction; can you work out how the reactions might work? Addition Substitution Elimination

Addition Top tip, 2 reactants  1 product Two reactants combine to make one A molecule is added across a double bond of an unsaturated molecule to make a saturated molecule Key reaction of the alkenes

Example - addition

Substitution Top tip 2 reactants  2 products An atom, or group of atoms is replaced with a different atom or group of atoms Key reaction of the halogenoalkanes

Example - substitution

Elimination Top tip 1 reactant  2 products One reactant reacts to form two products A molecule is removed from a saturated molecule to make an unsaturated molecule Key reaction of the alcohols

Example - elimination

Plenary Questions 3, 4, 5 & 6 pg 143 Question 1a pg 144