Alkenes

Question 1

What is the bond enthalpy of a single carbon-carbon bond?

Answer 1

347 kJmol^-1

Question 2

What is the bond enthalpy of a double carbon-carbon bond?

Answer 2

612 kJmol^-1

Question 3

What addition reactions can alkenes go through? The three H’s

Answer 3

• HYDROGENATION

When an alkene is passed through a nickel catalyst and temp of 423K this forms an alkane. Requires two molecules of H2.

• HALOGENATION

A bromine water test, alkene discolours orange bromine water to colourless and alkane has no visible reaction with bromine water, remains the same colour.

• HYDRATION

In presence of steam and phosphoric acid catalyst an alcohol is formed from an alkene for e.g. ethanol from ethene.

Question 4

Why do alkenes react in electrophilic reactions most of the time?

Answer 4

• Double bond in an alkene represents region of high electron density because of presence of pi-electrons
• This attracts electrophiles more readily

Question 5

What is an electrophile?

Answer 5

• This an atom or group of atoms that are elctron pair acceptors, they are attracted to regions with high electron density.
• Usually delta-positive or a positive ion like H+ or a bromine molecule becoming dipolar.

Question 6

What makes a carbocation more stable?

Answer 6

• The more alkyl groups are attached to the carbon atom, the more the positive charge is spread out making the ion more stable.
• Primary carbocation is when only one R-group is attached to carbon and so forth and it increases the stability of ion the higher the levels.

Question 7

Can you suggest less harmful more environmental friendly ways to dispose of polymer waste during recycling?

Answer 7

• Sorting recycling into different parts, this helps to differentiate polymers for appropriate action to be taken to properly recycle.
• Recycling PVC can be hazardous for e.g. when burnt can release toxic gases such as HCl and other pollutants. New technology uses solvents to dissolve the polymer then the precipitate is collected and used again.
• Waste polymers can be used as fuel. Can be incinerated to produce heat, which produces steam this drives a wind turbine to generate electricity. In Sheffield this is used to work the National Grid at times and to heat buildings across city centre.
• Feedstock recycling uses chemical and thermal processes to reclaim monomers, polymers, this is useful to handle unsorted and unwashed polymers. These can be used as raw material for production in new polymers.

Question 8

Can you explain the benefits of bio/photodegradable polymers?

Answer 8

• Bioplastics that were manufactured from cellulose, plant starch/oils and proteins.
  These conserve valuable oil reserves by lessening its use.
• Biodegradable are broken down by micro-organisms into water, CO2 and biological compounds. Normally made from plant starch/cellulose or additives are added to alter polymer structure so microorganisms can break them down. Compostable polymers degrade and leave toxic residues.
• Light-absorbing additives are added to photodegradable polymers. This weakens bonds in polymer and starts degradation after light exposure.