Chapter 13 - Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons
Contain at least 1 carbon to carbon double bond
How is a pi bond formed?
The sideways overlap of two p-orbitals, one from each carbon atom of the bond
What does the pi bond do?
It locks the two carbon atoms in position and prevents them from rotating around the double bond
What is the shape and bond angle around the double bond?
Trigonal planar
120 degrees
Why are alkenes more reactive than alkanes
Due to the presence of the pi bond:
- C=C is made up of a sigma bond and a pi bond
- the pi bond electron density is concentrated above and below the plane of the sigma bond
- being on the outside of the double bond, the pi electrons are more exposed than the electrons in the sigma bond
- the pi bond readily breaks and alkenes under undergo addition reactions relatively easily
What are the addition reactions of alkenes?
- hydrogenation
- halogens
- hydrogen halides
- hydration
What are the conditions for hydrogenation of alkenes?
- nickel catalyst
- 423K
What is the hydrogenation of alkenes?
Reaction of alkenes with hydrogen to form an alkane
How can you identify alkenes from alkanes?
By adding bromine water
If it turns clear it indicates the presence of a C=C
What is produced from the addition reaction of alkenes with hydrogen halides?
A haloalkane
What is produced when alkenes react with steam?
Produces alcohols
Used widely on the industry to price shampoo from ethene
What catalyst is used for the hydration of alkenes?
Phosphoric acid catalyst
What is an electrophile?
An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair
Usually a positive ion or molecule containing an with a partial positive charge
How does the mechanism for electrophilic addition?
- Br is more electronegative than hydrogen so hydrogen bromide is polar and contains the H+ dipole and Br- dipoles
- Electron pair in the pi bond is attracted to the partially positive hydrogen atom causing the double bond to break
- A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond
- The H-Br bond breaks by heterolytic fusion with the electron pair going to the Br atom
- A bromide ion and a carbonation are formed
- The Br- ion reacts with the carbocation to form the addition product
How is the minor and major product formed?
Minor - from the primary carbocation (less stable due to less positive inductive effect)
Major - front the secondary carbocation (greater positive inductive effect)
What determines the stability of a carbocation?
The electron donating ability of alkyl groups:
- each alkyl group donates and pushes electrons towards the positive charge of the carbocation
- positive charge is spread over the alkyl groups
- more alkyl groups attached to the positively charged carbon atom, the more charge of spread out making the ion more stable
What are polymers?
Extremely large molecules formed from many thousands of repeat units of smaller molecules known as monomers
What is a repeat unit?
The specific arrangement of atoms in the polymer molecule that repeats over and over again
Always written in square brackets with the letter n (large number of repeats)
What conditions does polymerisation occur under?
- high temperature
- high pressure
What is an environmental concerns of polymers?
Disposing of waste polymers (killing marine life)
How can we reduce waste polymers?
- recycling
- PVC recycling
- using waste polymers as fuel
- feedstock recycling
- bio/photodegradable polymers
How can polymers be recycled?
- sorted by type
- chopped into flakes, washed, dried and melted
- recycled polymer is cut into pellets and used by manufacturers
Why is PVC hazardous?
- high chlorine content
- range additives present in the polymer
- unsustainable to dump into landfill
- releases hydrogen chloride (corrosive) when burnt
How does PVC recycling work?
Uses solvents to dissolve the polymer
High grade PVC is recovered by precipitation from the solvent and the solvent is used again
How can waste polymers be used as fuel?
Can be incinerated to produce heat, generating steam to drive a turbine producing electricity
What is feedstock recycling?
Describes the chemical and thermal process that can reclaim monomers, gases or oil from waste polymers
What is the biggest advantage of feedstock recycling?
Able to handle unsorted and unwashed polymers
What are bioplastics and why are they useful?
Produced from plant starch, cellulose, plant oils and proteins offer a renewable and sustainable alternative
Protect our environment and conserve valuable oil reserves
What are biodegradable polymers?
Usually made from starch or cellulose or contain additives that alter the structure of traditional polymers so that microorganisms can break them down
Broken down by microorganisms into water, carbon dioxide and biological compounds
Why are biodegradable polymers useful?
Degrade and leave no visible or toxic residues
How do photodegradable polymers work?
Contain bonds that are weakened by absorbing light to start the degradation
What are plasticisers?
Inserted between gaps of polymer chains which pushes the chains apart so they can slide over each other and become more flexible (weaker London forces)
What are the differences between high and low density polythene?
HDPE:
- stronger
- higher MP
Made by low temperature, pressure, nickel catalyst
LDPE:
- flexible
- stretch
Made by high temperature, pressure and free radical mechanism
