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Alkenes

AQA A Level Chemistry

QuestionAnswer
What is an alkene? A hydrocarbon with a carbon-carbon double bond
Why are alkenes described as unsaturated? They have a carbon-carbon double bond
Describe the electron density of the double bond It is a region of high electron density
Draw the shape of the carbon-carbon double bond orbitals Draw
What type of isomerism can alkenes exhibit? Stereoisomerism (E-Z or cis-trans)
What physical feature of alkenes causes their stereoisomerism? That the double bond cannot rotate
What two type of reactions do alkenes typically undergo? Electrophilic addition or polymerisation
Which three reagents do alkenes typically react with? HBr, H₂SO₄ and Br₂
What type of reagents do alkenes react with? Electrophiles
What are the products of an electrophilic addition reaction with Br₂? A dibromoalkane
Draw a reaction mechanism for a given alkene with Br₂ Draw
Why is bromine water used as a test for alkenes? The Br₂ reacts with the alkene so its reddish-brown colour is decolourised
What are the products of an addition reaction with HBr? A bromoalkane
What is a carbocation intermediate? An intermediate molecule in a reaction which has a positive charge on a carbon atom
Describe the variation in stabilities of the different orders of carbocation Tertiary > secondary > primary
How does stability of carbocations effect the products of an addition reaction with HBr? More stable carbocation is formed resulting in a major and minor product
Draw a mechanism for an addition reaction of an alkene and HBr. Draw and label the major and minor products. Draw
What is the product of a reaction with an alkene and concentrated H₂SO₄? An alcohol
Draw a mechanism for the reaction between a given alkene and concentrated sulphuric acid Draw
Why can sulphuric acid be considered a catalyst in hydration of alkenes? It is regenerated by the end of the reaction.
What are addition polymers formed from? Alkenes and substituted alkenes
How are addition polymers named? Poly(name of original alkene)
What are the chemical properties of addition polymers? Generally unreactive
Why are addition polymers unreactive? Strong C-C and C-H bonds
What is PVC? Poly(chloroethene)
What is a plasticiser? A small molecule that can get inbetween polymer chains resulting in them being able to slide over one another more easily
Created by: JBearfield