Part 2.2 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

Part 2.2 Sources and synthesis, physical properties and combustion of alkenes.

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The manufacture and sources of alkenes

Unlike alkanes in crude oil, alkene hydrocarbons do not occur naturally and are manufactured in a process called cracking - a thermal degradation reaction on heating fractions from crude oil.

The naphtha fraction, from the primary fractional distillation crude oil, is heated to 700 to 900^oC under various conditions (variation in pressure and/or catalyst) to give varying % of ethene and high grade petrol.

You alter the conditions to suit the proportion of products you need.

You can also crack LPG, a mixture of propane and butane and heavier fractions from crude oil distillation.

Since I've already written two pages on cracking, I'm not going to repeat myself in detail.

See Cracking - a problem of supply and demand, other products (the basics and usefulness of products)

and Modification of alkanes by cracking, isomerisation and reforming (more advanced note on technical aspects)

A laboratory synthesis of alkenes - the elimination of HX from a haloalkane

(a) The elimination reaction between bromoethane and ethanolic potassium hydroxide

bromoethane  +  potassium hydroxide  ===>  ethene  +  potassium bromide  +  water

  +  KOH  ===>    +  KBr  + H₂O

CH₃CH₂Br  +  OH^-  ===> CH₂=CH₂  +  Br^-  + H₂O

You also get some hydrolysis to give ethanol CH₃CH₂OH.

(b) The elimination reaction between 1-bromopropane or 2-bromopropane and ethanolic potassium hydroxide

1-bromopropane or 2-bromopropane  +  potassium hydroxide  ===>  propene  +  potassium bromide  +  water

or +  KOH  ===>    +  KBr  + H₂O

CH₃CH₂CH₂Br  or  CH₃CHBrCH₃  +  OH^-  ===> CH₃CH=CH₂  +  Br^-  + H₂O

You get the same product in this case.

 

(c) In the case of 2-iodobutane, you can get two different products depending on which H leaves with the halogen e.g.

(i) CH₃CH₂CHICH₃  +  OH^-  ===> CH₃CH₂CH=CH₂  +  I^-  + H₂O

The formation of but-1-ene from 2-iodobutane, and ...

(i) (CH₃CH₂CHICH₃  +  OH^-  ===>  CH₃CH=CHCH₃  +  +  I^-  + H₂O

The formation of but-2-ene, also from 2-iodobutane, so watch out for these isomeric products.

I've also quoted this reaction (c) as ionic equations.

 

For mechanism details see Elimination of hydrogen bromide from bromoalkanes to form alkenes

Also need ref to halogenoalkanes Part 3.7 too when written

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Making ethene by dehydration of ethanol

In section 2.6 The reaction of ethene and steam to make ethanol was described

BUT, sometimes it economically an advantage to work the reaction in reverse!

Manufacturing of 'bioethene' via dehydration of bioethanol is an alternative to the fossil fuel hydrocarbon based ethene production by cracking and decreases the environmental consequences for this chemical commodity (maybe?, see last comment).

Ethanol can be produced by fermenting sugar cane and separating it by fractional distillation

The ethanol is passed over a heated alumina catalyst (~350^oC) and the ethene gas formed is separated from the water.

  ====>    

The dehydration of ethanol is an endothermic thermal decomposition.

The 'bioethene', as it is sometimes referred to, can now be used as a chemical feedstock to produce lots of products, without the need to use crude oil.  See 2.9 Uses of alkenes

However, it should be pointed out that sugar cane is often grown on deforested land and planted, grown and harvested using cheap labour!

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The physical properties of alkenes

Intermolecular forces and physical state of alkenes at room temperature

Alkenes are colourless gases, liquids or solids.

They are non-polar hydrocarbon molecules, so the intermolecular forces (intermolecular bonds) are weak between the molecules - these are transient dipole - induced dipole attractive forces.

The random partial positive charge of one dipole will attract the partial negative in the neighbouring molecule or vice versa

Consequently alkenes have relatively low melting points and boiling points.

For the same carbon skeleton, the melting points and boiling points of alkenes are similar to alkanes.

Because of the weak intermolecular forces, C₂ to C₄ alkenes are gases and from pent-1-ene C₅H₁₀ onwards, they are liquids of low density (as low as 0.62 g/cm³), so will float on water.

High molecular mass alkenes are waxy solids.

The gases and liquids have a strong 'hydrocarbon' odour.

Solubility

Alkenes, like all hydrocarbons, are virtually insoluble in water.

The non-polar alkene molecules cannot hydrogen bond with water.

Neither are the weak hydrocarbon - water interactions strong enough to disrupt the strong hydrogen bonding between water molecules.

Alkenes will dissolve in non-polar solvents like hexane, where the solute-solute, solute-solvent and solvent-solvent intermolecular forces are of a similar magnitude.

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The combustion of alkenes

Alkenes are highly flammable gases or liquids.

So, alkenes readily burn, just like alkanes, to give carbon dioxide and water if combustion is complete e.g.

complete oxidation = complete combustion with excess oxygen/air.

alkene hydrocarbon + oxygen ===> carbon dioxide + water

e.g.

ethene + oxygen ====> carbon dioxide + water

C₂H₄ + 3O₂ ====> 2CO₂ + 2H₂O

propene + oxygen ====> carbon dioxide + water

C₃H₆ + 4¹/₂O₂ ====> 3CO₂ + 3H₂O

or   2C₃H₆ + 9O₂ ====> 6CO₂ + 6H₂O

However, they are NOT used as fuels for two reasons.

1. They are far too valuable as a chemical feedstock for making plastics, anti–freeze and numerous other useful compounds.
2. They tend to burn with a more smoky flame than alkanes due to less efficient, and more polluting incomplete combustion, so the heat energy release is lower than for alkanes.

Free unburned carbon (soot) might be released, alkenes have a higher C/H ratio than alkanes.

The hydrogen will always be oxidised to water.

So, for example, pentene might partially burn (oxidised) as follows ..

C₅H₁₀  +  2.5O₂  ===>  5C  +  5H₂O

... giving two complete combustion products and two incomplete combustion products, smokey!

 

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