3.4 Alkenes
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General formula of alkenes
C₂H₂ₙ
Functional group of alkenes
C=C
Shape around C atom in a C=C double bond in alkenes
Trigonal planar
What bonds are in a C=C bond?
σ-bond and a π-bond
Diene
2 C=C bonds
Triene
3 C=C bonds
General formula of cycloalkenes
CₙH₂ₙ₋₂
What causes E/Z isomerism in alkenes
Restricted rotation about the planar C=C bond
What condition has to be met for an alkene to have E/Z isomerism?
Each carbon atom in the C=C bond must have 2 different atoms/groups bonded to it
Which rules determine priority in E/Z isomerism?
The Cahn-Ingold-Prelog Priority Rules (CIP rules)
What are the CIP rules?
• The atom with the highest atomic number has priority • If atoms have the same atomic number, the next atoms along the chain should be compared
Why are alkenes reactive?
The C=C has a high electron density
Electrophile
An electron pair accepter
Why are electrophiles attracted to alkenes?
The high electron density in the C=C
Curly arrow
Shows the movement of a pair of electrons
Mechanism when alkenes react with halogens/hydrogen halides
Electrophilic addition
What is the reactive intermediate in electrophilic addition? (alkenes)
A carbocation
Heterolytic fission
Where both electrons remain with one atom when a covalent bond is broken
Why are halogen molecules attracted to alkenes?
The high electron density in the C=C induces a temporary dipole in the halogen molecule
Chemical test for C=C group
Bromine water orange —> colourless
Conditions for electrophilic addition
Room temperature
Hydrolysis
The breaking of bonds using water
What is formed when a hydrogensulfate is hydrolysed with warm water?
An alcohol
What is the catalyst for : alkene → hydrogensulfate (+water) → alcohol
Sulfuric acid (reacts with alkene, formed with alcohol)