3.4 Alkenes Flashcards
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)
How are carbocations classified?
By the number of alkyl groups attached directly to the carbon with the positive charge. Either primary (1°), secondary (2°), or tertiary (3°)
Positive inductive effect
Alkyl groups releasing electrons (this helps to stabilise the positive charge of carbocations).
Which carbocations are the most stable?
Tertiary (3°)
What determines which is the major product in electrophilic addition of asymmetrical alkenes?
The stability of the carbocation intermediate.
Major product
Minor product
Produced in the largest amount
Produced in the smallest amount
Hydrogenation
Addition of hydrogen, therefor a reduction reaction
What is formed in the hydrogenation of alkenes?
Alkanes
Catalyst for the hydrogenation of alkenes
Nickel
Use of hydrogenation of alkenes
Producing margarine from long chain fats or oils
Repeat unit (polymers)
A specific arrangement of atoms that occurs in the structure over and over
What determines the properties of poly(alkenes)?
Structure and intermolecular force
Is LDPE branched?
Yes
Is HDPE branched?
No
What are plasticisers?
Small molecules used to increase flexibility in polymers
How do plasticisers work?
They force neighboring chains apart and reduce intermolecular forces.
