Part 2. The chemistry of ALKENES - unsaturated hydrocarbons

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

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My advanced A level organic chemistry index of notes on alkenes

Index of GCSE level Oil - Useful Products Chemistry Revision Notes

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2.9 The uses of alkenes (an important chemical feedstock) and the occurrence of the alkene functional group in 'biological' molecules and their uses

Alkenes from the petrochemical oil industry are rarely used directly, but they are important starting materials to make lots of useful chemicals in the chemical and pharmaceutical industries.

EXAMPLES of the industrial importance of alkenes

Apart from industrial manufacturing processes, a direct use of ethene gas itself

The only one I can think of is the use of ethene gas to control the ripening of fruit.

It gets a mention in Hormone control of plant growth and uses of plant hormones (school biology notes)

Manufacture of addition polymers - poly(alkenes)

Already dealt with on three other of my web pages.

Addition polymers - plastics e.g. poly(ethene) and PVC, manufacture and uses

Problems with using polymers, recycling, biodegradable plastics

The polymerisation of alkenes to form addition polymers - structure, properties, uses of poly(alkene) polymers (extra notes for advanced level chemistry students)

Stereoregular polymers -  isotactic/atactic/syndiotactic poly(propene)  (extra advanced notes)

Alkenes are used as the monomer unit from which addition polymers are manufactured e.g.

Ethene makes poly(ethene).

Ethene is converted to phenylethene (styrene) to make poly(phenylethene), polystyrene.

Ethene is converted to chloroethene (vinyl chloride) to make poly(chloroethene) PVC.

Chloroethene, the monomer for producing poly(chloroethene), PVC, is made in two stages from ethene, which originates from cracking oil fractions. In this case the haloalkane is an intermediate compound.

ethene  +  chlorine  == (1) =>  1,2-dichloroethane  == (2) ==>  chloroethene  +  hydrogen chloride

H₂C=CH₂  +  Cl₂  ===>  ClH₂CCH₂Cl  ===> H₂C=CHCl  +  HCl

Stage (1) This addition reaction catalysed by iron(III) chloride (FeCl₃) and is exothermic.

Stage (2) Is a thermal decomposition and elimination reaction, 500^oC and at high pressure (1.5 to 3.0 MPa (15 - 30 atm) and is a very endothermic reaction.

The anhydrous hydrogen chloride formed can be used to make other chloroalkanes or chloroalkenes, or dissolved in water to make hydrochloric acid.

Ethane-1,2-diol (see below - antifreeze manufacture)  is also used to manufacture Terylene, a polyester and a condensation polymer.

For details see The manufacture, molecular structure, properties and uses of polyesters

Manufacture of alcohols

See Part 2.6 The reaction of alkenes with steam synthesis of alcohols

Alcohols: including ethanol production and uses of alcohols

Ethanol as a fuels

Alcohols are used solvents and manufacturing esters - the latter uses range from flavourings, odours and drugs in the pharmaceutical industry.

Antifreeze

Anti-freeze liquid for motor vehicles contains ethane-1,2-diol (ethylene glycol) which is made via a two stage synthesis from ethene.

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Stage 1: Ethene is mixed with air or oxygen and passed over a catalyst (finely divided silver on an inert support such as alumina) at 520-550 K and under 15-20 atmospheres pressure to make epoxyethane

Note another example of a transition metal catalyst, silver is in the 4d block below copper.

Stage 2: Epoxyethane is catalytically hydrolysed with water to give ethane-1,2-diol

Ethane-1,2-diol is also used to manufacture Terylene, a polyester and a condensation polymer.

For details see The manufacture, molecular structure, properties and uses of polyesters

Carotenes

Now this is what you call a 'real' unsaturated hydrocarbon molecule with 11 C=C double bonds!

The middle 9 C=C bonds are connected in the E E/Z stereoisomerism conformation (trans positions).

Beta-carotene is a red-orange pigment found in plants and fruits, especially carrots and colourful vegetables.

The name beta-carotene comes from the Greek “beta” and Latin “carota” (carrot).

Beta-carotene is the yellow/orange pigment that gives vegetables and fruits their rich colours.

The human body converts beta-carotene into vitamin A (retinol), so beta carotene is a precursor of vitamin A.

We all need vitamin A for healthy skin and mucus membranes, our immune system, and good eye health and vision.

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Unsaturated oils and fats - hydrogenation and margarine

The term 'unsaturated' when applied to oils and fats tells you the molecule contains at least one C=C double bond.

Unsaturated vegetable oils and more saturated fats (e.g. butter or lard) are important sources of energy in our diet.

In the two diagrams above, you can see the basic difference between an unsaturated vegetable oil and a saturated fat molecule, due to the presence or absence of C=C double bonds.

They illustrate the general structure for all naturally occurring triglyceride esters.

The manufacture of margarine

Margarine is manufactured as a butter substitute and is made from animal or vegetable fats mixed with skimmed milk and salt.

Animal fats and vegetable oils a re primarily made up of triglyceride molecules illustrated above with different degrees of unsaturation due to the presence one or more C=C double bonds in the long chains of the glycerol ester molecules.

Margarine has become more popular because it is supposed to be more healthy than the animal fats in butter which is much higher in saturated fats.

Margarine has high content of mono and polyunsaturated fats which are supposed to be more healthy than saturated fats, and they are often obtained from olive, rapeseed and sunflower oils

However, unsaturated vegetable oils are liquid at room temperature and not very spreadable on foods such as bread.

Using hydrogenation and a nickel catalyst, adding hydrogen across a C=C double bond, unsaturated oils/fats can be converted to more saturated molecules that are soft and spreadable solid at room temperature.

The hydrogenation produces more saturated molecules between which the intermolecular forces are slightly increased - sufficient to increase the softening point by a few degrees.

By varying the degree of hydrogenation, varying the number of C=C bonds remaining, you can produce a range of margarines with different softening points - the point at which the margarine hardens and solidifies.

However, in the process of hydrogenation, some of the cis/Z linkages are 'isomerised' into trans/E linkages as a by-product and 'trans-fats' are supposed to be less healthy in our diet (see the first triglyceride diagram).

For more on oils, fats and hydrogenation see alkenes section 2.5

Test for unsaturation in fats and oils

If you shake a vegetable oil or saturated animal fat with bromine water or bromine in a solvent like hexane, the unsaturated vegetable oil will decolourise the bromine water and a saturated fat will not.

For more details of this reaction see section 2.4)

Chemically, cholesterol, which contains the alcohol group –OH, is classed as a sterol, a sub–group of organic molecules called steroids.

Cholesterol is an essential steroid–sterol to humans but if too much is produced or ingested in food, it can cause heart disease e.g. hardening of arteries.

The image on the left gives the skeletal formula structure of cholesterol and shows it to contain one C=C alkene double bond as well as the hydroxy group.

[Cholesterol image from NIST]

The opiate drugs morphine, codeine and heroin contain an alkene group to the bottom of the molecule.

Don't confuse with the presence of the benzene ring (written in Kekule style) at the top of the molecule with the alkene functional group.