Alkenes Flashcards

Active recall practice for electrophilic addition, carbocation intermediate stability, and polymer chemistry.

Structure & Reactivity: read all 5 questions and answers

What is the general formula of alkenes?
CnH2n
Describe the bonding in a carbon-carbon double bond (C=C).
It consists of one sigma bond (formed by end-on orbital overlap) and one pi bond (formed by sideways overlap of adjacent p-orbitals above and below the plane of the carbon atoms).
Why are alkenes more reactive than alkanes?
The pi bond has relatively low bond enthalpy and represents a high electron density area above and below the C=C bond, making it susceptible to attack by electrophiles.
What is the shape and bond angle around the carbon atoms in a double bond?
Trigonal planar, with an ideal bond angle of approximately 120 degrees (in ethene, measured H-C-C is approximately 121.3 degrees due to electron repulsion from the double bond).
How do you test for the presence of a double bond (unsaturation)?
Add bromine water. The orange/brown solution decolourises (turns colourless) in the presence of an alkene.

Electrophilic Addition: read all 7 questions and answers

What is electrophilic addition?
A reaction where a pi bond is broken and two new sigma bonds are formed by the addition of an electrophile across the double bond.
Write the products when propene reacts with sulfuric acid (H2SO4).
Isopropyl hydrogensulfate (major product) and propyl hydrogensulfate (minor product).
State the reagent and conditions to convert alkyl hydrogensulfates into alcohols.
Warm with water (hydrolysis).
State Markovnikov's rule.
When an unsymmetrical reagent adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom that already has the greater number of hydrogen atoms.
Describe the order of stability of carbocations.
Tertiary (most stable) > Secondary > Primary (least stable).
Why are tertiary carbocations more stable than primary ones?
They have more alkyl groups attached to the positive carbon. Alkyl groups have an inductive effect, pushing electron density towards the carbon atom and spreading the positive charge.
How is industrial ethanol produced from ethene?
By direct hydration of ethene with steam at 300 degrees C, 60-70 atm pressure, using a concentrated phosphoric acid (H3PO4) catalyst.

Addition Polymers: read all 5 questions and answers

What is addition polymerisation?
A reaction where many small unsaturated monomer molecules join together to form a long saturated polymer chain, with no other products.
Are addition polymers biodegradable?
No, because the carbon-carbon backbone consists of strong, non-polar single bonds that are chemically inert and cannot be broken down by micro-organisms.
What is a plasticiser?
A small molecule added to polymers that gets between polymer chains, pushing them apart and weakening intermolecular forces, making the polymer more flexible.
How does the flexibility of PVC change when plasticisers are added?
Unplasticised PVC (uPVC) is rigid and used for window frames/pipes. Plasticised PVC is flexible and used for aprons, cable insulation, and clothing.
What is a major environmental problem associated with disposing of polymers?
They fill landfill sites due to non-biodegradability, and burning them can release toxic gases (e.g. HCl from PVC) unless incinerated at high temperatures.