Chemistry - Topic 6 Alkenes

What is meant by the term electrophile?

electron-deficient species that accepts electrons

Hydrogenation - conditions

as Hydrogen is a poor electrophile this reaction only occurs using a nickel catalyst and high temperatures + pressures

Hydrogenation

alkene reacts via electrophilic addition with hydrogen in presence of nickel catalyst to form alkane - e.g. catalytic hydrogenation of unsaturated vegetable oils to manufacture margerine

Electrophilic Addition - halogens

react to form dihalogenoalkanes

Electrophilic Addition - hydrogen halides

react to form halogenoalkanes

Electrophilic Addition - steam

react to form alcohols, in presence of an acid catalyst (e.g. H2SO4)

Why can water not just be added to form an alcohol?

Water is not a very good electrophile - therefore an acid must be added as a catalyst - this forms H3O+ which can then act as an electrophile as it is positively charged)

When bromine is added to trans-but-2-enal, the aldehyde group is oxidised and the carbon-carbon double bond reacts. The product is 2,3-dibromobutanoic acid. When trans-but-2-enal reacts with bromine mixed with a little sodium chloride, a mixture of 2,3-dibromobutanoic acid and 2-bromo-3-chlorobutanoic acid is produced. No 2,3-dichlorobutanoic acid is formed. Explain these observations.

• Br delta plus is the only electrophile

• Cl is only present as Cl- and it must add to the carbocation

Relative stability of primary, secondary and tertiary carbocation intermediates:

Tertiary>Secondary>Primary - alkyl groups are electron-donating and push the electrons in their bond towards the (+)ve charge to stabilise it - use ‘That which as gets’ (in terms of hydrogens) + Markovnikov’s rule to determine which reaction pathway will give major product

Qualitative Test for C=C double bond

decolourises bromine water as dibromoalkane is formed - however note that if using bromine water then bromoalcohol is formed + water

Other potential tests that can be used to test for C=C double bond

• burn with sooty flame (not clean)

• add potassium permanganate (VII) in acidic conditions - KMnO4 oxidises double bond and decolourises from purple solution to form alco-diol and MnO2 brown sludge

How can alkenes from polymers?

addition polymerisation

Why are the waste polymers separated into specific types:

for;

• recycling

• incineration to release energy

• use as a feedstock for cracking

What does heterolytic bond fission result in?

The formation of ions

Advantages and Disadvantages of disposing polymers in landfill

Advantages:

• quick/ easy/ convenient

Disadvantages:

• polymers are non-biodegradable so will persist for a long time in landfill.

• can form microplastics/ release harmful gases if they do break down

Advantages and Disadvantages of recycling polymers to make new plastics

Advantages:

• reduces need for use of resources such as crude oil to make plastics

Disadvantages:

• not all polymers can be recycled

• requires sorting + washing etc

Advantages and Disadvantages of Incineration of polymers

Advantages:

• quickly disposes waste

• heat energy released can be used to heat homes/ power other industrial processes

Disadvantages:

• releases toxic gases

Advantages and Disadvantages of using polymers as feedstock for cracking:

Advantages:

• reduces demand for alkanes from crude oil for cracking

Disadvantages:

• can produce a lower % yield / lower quality of reactants

How can chemists limit the problems caused by polymer disposal?

• developing biodegradable polymers

• removing toxic waste gases by incineration of plastics

Polymer Definition

a long chain of repeating monomers joined together

monomer definition

a small molecule (unit) from which a long polymer can be produced

addition polymerisation - definition

a type of polymerisation in which one product (the polymer) is formed, by adding together many monomers