Alkenes Flashcards
What are alkenes and their formula?
Unsaturated hydrocarbons
Contain C=C double bonds
Formula CnH2n
What are the bonds formed in an alkene?
C-H (sigma) bonds
Formed by the overlap of electron clouds belonging to the carbon atoms
C=C (Pi bond)
Formed by the overlap of adjacent P orbitals and has a high electron density.
What are the shapes of alkenes?
The Pi bond restricts rotation around the double bond and hence alkenes have flat shapes (planar)
Ethene (trigonal planar)
Alkenes can produce E/Z isomers
Why are alkenes more reactive than alkanes?
Alkenes have a C=C double bond which is a highly electron dense area and hence attracts electrophiles.
What is an electrophile?
A species which is electron deficient and hence is attracted to an electron dense region.
What is an electrophilic addition reaction?
Where an electrophile is attracted to the electron dense Pi bond in a molecule (alkene) which is then broken and the molecule is added in across the double bond
What is the reaction and conditions for hydrogenation of alkenes?
Hydrogen is added across the double bond.
Conditions:
60 degrees
Buckle catalyst
What happens:
Gases passed over catalyst
Alkene becomes saturated
Forms an alkane
What is the reaction and conditions for the hydration of alkenes?
Water molecule is added across the double bond.
Conditions:
High pressure
Phosphoric acid catalyst
What happens:
Steam and gaseous alkene are mixed
Forms an alcohol
Why are alkenes hydrogenated to be used in industry?
Vegetable and animal oils contain a high proportion of unsaturated fats (C=C bonds do not fit well together so are liquid at room temperature)
And are difficult to spread.
Hydrogenation of these fats raises their boiling points and produces products such as margarine
What is the halogenation reaction involving alkenes?
Give the mechanism and conditions
A halogen molecule is added across the double bond. (Cl-Cl)
Conditions:
Room temperature
What happens:
Halogen and alkene are mixed
Alkene becomes saturated as the electron dense region accepts one of the halogen atoms.
Other halogen atom is attracted to the carbocation
Forms a dihaloalkane
Used to test for C=C bonds by using Br-Br (bromine water)
What is the addition reaction involving hydrogen halides?
Hydrogen halide is added across the double bond
Forms a haloalkane
The Pi bond is attracted to the slightly positive hydrogen of the hydrogen halide (due to electronegativity) forming a halide ion and a carbocations. They then react forming a haloalkane.
What is formed when a hydrogen halide is added to an unsymmetrical alkene (eg propene)?
2 products can be formed depending on where the halogen joins to the molecule.
Use markovnikovs rule to identify the most likely product
What is markovnikovs rule?
When a hydrogen halide is added to an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom with the most hydrogen atoms initially attached.
What is a carbocation and how does their stability change with structure
When a carbon atom carries a positive charge often as an intermediate in a reaction.
The hydrogen atoms attached affects the stability of the carbo cation. The more alkyl groups attached (less hydrogen’s) the more the alkyl groups release electrons towards to carbocation allowing for a great spread of the overall charge (inductive effect)
How are alkenes converted to diols?
First the alkene is oxidised and then an addition reaction occurs.
Conditions:
Acidic (usually dilute H2SO4)
Potassium manganate VII (oxidising agent)
Water
The 2 OH groups are added across the C=C double bond.
