Reactions of Alkenes
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What are the reactions that alkenes undergo based on this specific area
• Combustion • Electrophilic addition with hydrogen halides • Electrophilic addition with halogens • Electrophilic addition with sulfuric acid (H₂SO₄) • Electrophilic addition with water
Outline the combustion of ethene
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
What is an electrophile?
An electron deficient species that accepts an electron pair
Draw the electrophilic addition reaction between ethene and hydrogen bromide to form a halogenoalkane
Draw/Search up answer, do your own and check if correct
Draw the electrophilic addition reaction between ethene and hydrogen chloride to form a halogenoalkane
Draw/Search up answer, do your own and check if correct
What determines which product is favoured in asymmetrical alkenes, when undergoing electrophilic addition?
The number of alkyl groups surrounding the carbocation
What causes differences in carbocation stability?
The number of alkyl groups attached to the positively charged carbon - more alkyl groups increase stability
What is the positive inductive effect?
When alkyl groups release electrons towards the carbocation, to stabilise it’s positive charge
Rank the carbocation stability: primary, secondary, tertiary (1 - best/ 3 - least stable)
Tertiary > Secondary > Primary
EXAM QUESTION: Why is the secondary carbocation more stable than the primary carbocation
It is more stable because it has a greater positive inductive effect making the carbocation more stable, so more product is likely to be formed from there
What do alkenes form when they react with halogens?
Dihalogenoalkanes
What is the name of the mechanism when alkenes reacts with halogens?
Electrophilic addition
Draw and outline the electrophilic addition reaction between an alkene (ethene) and a bromine molecule
Find the correct mechanism and then do your own + compare
What chemical test is used to detect a carbon - carbon double bond?
You add a few drops of bromine water (which is reddish - brown) to an alkene
What is the positive result of the bromine water test for alkenes?
A positive test the solution becomes decolourised (going from reddish - brown to colourless)
What does concentrated sulfuric acid do to alkenes, what does it form?
Concentrated sulfuric acid adds across the double bond, forming an alkyl hydrogensulfate
Draw and outline the electrophilic addition reaction between an alkene (ethene) and sulfuric acid (H2SO4)
Find the correct mechanism and then do your own + compare
What happens when water is added to the alkyl hydrogensulfate?
An alcohol is formed and sulfuric acid reforms
What is the product of the steam hydration of an alkene?
An alcohol
What type of reaction occurs when water is added to an alkene?
Electrophilic addition - water adds across the double bond to form an alcohol
What is the general equation for the hydration of an alkene?
CnH₂n + H₂O → CnH₂n₊₁OH
What are the industrial conditions needed to hydrate alkenes to alcohols?
Steam (H2O gas) Temperature: 300 degrees celcius Pressure: 60 atm Acid catalyst: phosphoric acid
What is the name of the alcohol formed when ethene reacts with steam?
Ethanol
Why is ethanol made by hydration of ethene industrially useful?
It is a cheap method of production The process is continuous - more made in small amount of time There are no waste products