Alkanes and alkenes
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Alkanes are saturated hydrocarbon chains with the general formula
Ethane has the formula .
An alkane with carbons will have hydrogens.
Alkanes can be branched but will still conform to the same general formula.
Cyclic alkanes, despite similarities in reactivity and properties to alkanes, form their own homologous series, with the same general formula as alkenes, .
Alkanes contain only single covalent bonds called σ bonds, which can freely rotate.
σ is pronounced ‘sigma’.
All carbon atoms in an alkane form four single covalent bonds. The eight electrons are arranged as four pairs.
The electrons in the bonds around carbon repel each other. They orientate to form a tetrahedral shape, according to electron pair repulsion theory, to minimise repulsion between the electron pairs.
The angle between covalent bonds in an alkane is approximately 109.5°.
The 3D structure of an alkane is typically displayed with two bonds on the plane of the page, (solid lines), one bond receding into the page, (dashed line), and one bond coming out from the plane of the page (bold line).
Alkane molecules only interact through London forces, (induced dipole–dipole interactions).
The impact of structure on boiling points should only be considered for molecules of the same size.
Longer chain alkanes have higher boiling points since they are larger. They have a larger electron cloud leading to greater London forces requiring more energy to break.
Alkanes have lower boiling points than equivalently sized molecules containing polar groups, such as alcohols, carboxylic acids, and haloalkanes.
Linear alkanes have higher boiling points than their branched isomers. There is greater surface contact between linear molecules, resulting in closer packing and stronger London forces that require more energy to break.
Alkanes are the least reactive of all organic compounds.
Alkanes are composed of strong covalent bonds, which require a large amount of energy to break.
The similar electronegativity values of hydrogen and carbon means there is very low polarity in the carbon-hydrogen bonds. There are no electron-rich areas to attract electrophiles nor electron-deficient areas to attract nucleophiles.
The lack of molecular polarity in alkanes makes them insoluble in polar solvents, such as water.
The density of alkanes increases with chain length, and is always lower than water. When combined with an aqueous solution, the alkane will always form the upper layer.
Alkanes undergo free radical substitution reactions with chlorine or bromine in the presence of UV radiation.
The radical substitution of an alkane with a halogen involves the homolytic fission of bonds and proceeds via three types of reaction;
- Initiation – the formation of radicals from a non radical, in this case the halogen,
- Propagation – formation of a radical and non radical from a radical and non radical
- Termination – combination of two radicals to form a non radical product.
A free radical or radical is a species with an unpaired electron and is formed by the homolytic fission of covalent bonds. This is where the bond pair of electrons are split evenly so both participating atoms receive one electron only.
The radical is symbolised by a dot .
UV light has sufficient energy content to be able to split chlorine molecules homolytically forming radicals.
The table below shows how to write mechanisms for free radical substitution of alkanes.
Methane reacts with chlorine in the presence of UV light to form a mixture of chlorinated alkanes.
The products formed depend on the concentration of the reactants present.
If excess chlorine is used with methane as the limiting reagent, then further substitution reactions can occur, forming di, tri, and tetrachloro products.
An excess of methane and limiting reagent of chlorine ensures that the monosubstituted chloromethane is the major product.
Radical substitution has limited use in organic synthesis as a mixture of products are formed.
Haloalkanes formed during free radical substitution are susceptible to repeated radical attack when the halogen is in excess. This leads to multiple substitutions .
During radical substitution carbon-halogen bonds can form in multiple positions on the carbon chain. This leads to a range of structural isomers.
Alkanes burn readily in oxygen, producing carbon dioxide and water.
Making bonds releases energy to the environment; it is exothermic.
The high bond enthalpy of the carbon-oxygen bond, in the carbon dioxide product, results in a highly exothermic combustion reaction.
Alkanes are commonly used as fuels.
With excess oxygen gas, alkanes burn to exclusively produce carbon dioxide and water.
This is complete combustion.
To produce a balanced equation for the complete combustion of a hydrocarbon, balance the carbons and hydrogens first and then consider the oxygens.
When there is insufficient oxygen available, the carbon from the alkane is not fully oxidised.
This is incomplete combustion.
Incomplete combustion also results in the formation of carbon monoxide and/or particulate carbon (soot) alongside carbon dioxide and water.
The ratio or carbon based products will depend on the amount of oxygen available.
During the practical combustion of alkane fuels, a variety of products are produced.
Alongside complete combustion products, other pollutants can be found within the exhaust fumes.
Additional pollutants including carbon monoxide, oxides of nitrogen and sulfur, carbon particulates, and unburned hydrocarbons.
Combustion of hydrocarbons that contain sulfur compounds as impurities, leads to sulfur dioxide formation.
Sulfur dioxide causes air pollution.
is linked to the formation of acid rain. It is very soluble and forms an acidic solution in rainwater.
Sulfur dioxide can be removed from flue gases by scrubbing. Scrubbing involves passing the waste gases through a suspension of bases, such as a limestone slurry made from and (lime).
The sulfur dioxide dissolves in the suspension and is then converted to non-toxic calcium sulfite and calcium sulfate salts.
This is flue gas desulfurisation.
All carbon based products of fuel combustion have negative environmental impacts.
Carbon dioxide emissions are linked to the greenhouse effect and global warming, and are present in both complete and incomplete combustion.
Carbon monoxide and particulate carbon are formed in incomplete combustion.
Carbon monoxide is a colourless, odourless, and toxic gas. It binds to haemoglobin in the blood more strongly than oxygen, reducing the capacity of the blood to transport oxygen around the body. Carbon monoxide poisoning is first noticeable through dizziness and can eventually lead to death.
Particulate carbon is linked to global dimming. It can settle on the leaves of plants, reducing photosynthesis, and can be inhaled, causing irritation to the lungs.
Combustion products are largely removed from vehicle exhaust emissions by passing the hot gases through a catalytic converter. This is a ceramic honeycomb-like material coated with catalytic metals, such as platinum, palladium, and rhodium.
The catalytic converter is situated in the exhaust train just after the engine.
There is a combination of oxidation and reduction reactions to convert the waste gases containing , , and unburnt hydrocarbons into less harmful products such as , , and .
Biofuels are alternative fuels to petroleum-based hydrocarbons. They are from renewable sources, such as crop plants, or from fermentation of plant-based carbohydrates, such as starch.
Ethanol and methanol are fermentation products and biodiesel from vegetable oils is widely produced.
A comparison of biofuels with fossil fuels is shown below.
Crude oil is a complex mixture of mainly alkanes that can be separated by fractional distillation.
The mixture is heated and fed into a fractionating column with a temperature gradient; the temperature decreases with height. This separates the mixture into groups of similar sized hydrocarbons with similar boiling points called fractions.
The fractions produced can be used directly, as fuels, lubricants, or bitumen, or processed further.
Certain less useful fractions from fractional distillation of crude oil are modified by industrial processing to increase the supply of in-demand products, such as petrol.
This makes use of the heavier (higher boiling point) fractions, where supply exceeds demand.
Processing involves cracking and/or reforming.
Cracking involves breaking up larger hydrocarbon molecules into smaller, more useful ones. This can be done by direct heating, which breaks carbon-carbon bonds. It can also be done by using catalysts.
Thermal cracking involves heating the petroleum vapour to a high temperature, 700 oC, under a high pressure ~70 MPa.
Thermal cracking produces mainly shorter chain alkanes and also some alkenes.
Catalytic cracking involves passing heated petroleum vapour over aluminium silicate-based zeolite catalyst, at a lower temperature than thermal cracking (450 oC) and a much lower pressure of 20 MPa.
The products of catalytic cracking are also smaller alkanes, but contain a greater proportion of alkenes compared to thermal cracking
Reforming is a catalytic process that converts straight chain alkanes into branched and cyclic forms, as well as aromatics. The process works by breaking the alkane chains, and allowing them to rejoin. Much hydrogen is also produced during the reformation as more unsaturated and cyclic products are formed.
Cyclic, aromatic and branched alkanes burn more efficiently and make better fuels than straight chain alkanes.
Alkenes are a type of unsaturated hydrocarbon.
They contain at least one double bond, which are centres of high electron density.
The general formula for a straight chain alkene with one double bond is .
Note that cyclic alkanes have the same general formula as straight chain alkenes.
A carbon-carbon double bond contains one bond and one bond.
A bond forms when two orbitals overlap directly along the axis between the two atoms.
The sideways overlap of the two p orbitals above and below the bonding atoms forms a bond.
There is no free rotation around bonds.
Electron pairs in bonds repel each other. Molecules adopt an orientation to maximise the distance between electron dense areas, and therefore reduce repulsion.
In an alkene, the carbon within a double bond has three areas of electron density, all of which are bonding. It will have trigonal planar geometry, with bond angles based around 120°.
Increased repulsion from the , which contains two electron pairs, results in the angle between the double bond and the single bonds slightly exceeding the angle between two single bonds.
All four groups attached to the must be on the same plane to allow overlap of the p orbitals within the bond.
The bond within in alkenes has a much lower enthalpy than that of the bonds present in both alkanes and alkenes.
As a result the energetic barrier to break the bond within is lower and reactions occur more readily in alkenes than in alkanes.
Typical bond enthalpies for , , and are given in the table below.
The increase in the bond enthalpy compared to the bond enthalpy is due to the bond; this is weak at only .
In an addition reaction, two or more reactants combine to make a single product.
Catalytic hydrogenation of alkenes is an addition reaction.
Alkenes react with hydrogen in the presence of a suitable catalyst to form alkanes.
The most commonly cited catalysts for this reaction are nickel, palladium, and platinum.
Alkenes react spontaneously with halogens to form dihaloalkanes in an electrophilic addition reaction.
One bromine adds to each carbon in the double bond.
The electrophilic addition of bromine to alkenes is used as a test for unsaturation.
is orange and bromoalkanes are colourless. When the substances are mixed they react and as the reaction progresses the orange colour fades.
To complete the test, bromine water is shaken with a test sample, and the results are observed.
Alkenes react spontaneously with hydrogen halides to form haloalkanes in an electrophilic addition reaction.
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Where the alkene is not symmetrical, the reaction can result in a mixture of two structural isomers.
or
Alkenes react, via electrophilic addition, with steam in the presence of an acid catalyst to form alcohols.
The acid catalyst forms a hydronium ion with water, making it a better electrophile. The reaction completes with the release of a ion, meaning that the total amount of in the system does not change. The acid is reformed and so is catalytic.
A common catalyst is phosphoric acid, .
Note that halide, sulfur, and nitrogen based acids are not suitable as they provide competing nucleophiles.
Where the alkene is not symmetrical the reaction can result in a mixture of two structural isomers.
or
An electrophile is a species able to accept a pair of electrons.
Electrophiles can be:
- Positively charged ions, such as
- Polar molecules, such as
- Non-polar compounds with empty p orbitals, such as or , which only contain six electrons in their outer shells.
The mechanism for any electrophilic addition to an alkene features heterolytic fission of the bond.
Electrons from the bond attack an electrophile, forming a carbocation intermediate.
In the mechanism, the curly arrow should be drawn from the centre of the bond to the atom on the electrophile.
The newly formed carbocation is a lone pair acceptor (an electrophile) and is therefore susceptible to nucleophilic attack.
A curly arrow is drawn from the electron source in the nucleophile towards the positive charge on the carbocation.
The electron source on the nucleophile could be a lone pair, if in a compound, or a negative charge on an ion.
Where an asymmetrical alkene reacts with a haloalkane or steam, a mixture of structural isomers is formed.
Propene is an asymmetrical alkene.
Two different carbocations are possible after the initial addition of an electrophile to propene.
This leads to the formation of two different products.
In 1865, Vladimir Markownikoff recorded the observation:
When a protic acid () is added to an asymmetric alkene, the acidic hydrogen attaches itself to the carbon having a greater number of hydrogen substituents whereas the halide group attaches itself to the carbon atom which has a greater number of alkyl substituents.
This was later linked to an increased stability of carbocations formed on more heavily substituted carbons.
As the number of residual groups (R groups) on a carbocation decreases, the stability of the carbocation decreases.
The ability of alkyl groups to stabilise carbocations is NOT an inductive effects. This has been disproved following 75 years of acceptance. In reality, electron donation is caused by a combination of polarisability and hyperconjugation. Exam boards are currently awarding marks for inductive donation, but this is likely to change in due course.
The major product in an electrophilic addition reaction of an alkene is the one formed via the most stable carbocation.
The impact of carbocation stability on electrophilic addition of an alkene with a hydrogen halide or steam is a major product, with the or group on the most substituted carbon.
Cold, concentrated, sulfuric acid, () can react with alkenes in an electrophilic addition to form alkyl hydrogen sulfates.
The in the bond in the sulfuric acid has a partial positive charge and acts as an electrophile.
The electrons in the double bond attack the leading to the formation of a carbocation and breaking the bond in sulfuric acid.
The nucleophile then attacks the carbocation to form an alkyl hydrogensulfate.
The alkyl hydrogen sulfate product is susceptible to nucleophilic substitution reactions and will form an alcohol in aqueous conditions.
The manufacture of margarine involves catalytic hydrogenation of vegetable oils, which are naturally unsaturated fats.
In this process, hydrogen gas, , is added across the carbon-carbon double bonds () to make the oil more saturated and increase its melting point. The resulting fat is solid.
A nickel catalyst is used to speed up the reaction, typically at 150–200 °C under moderate pressure.
Partial hydrogenation is preferred to achieve the right consistency without making the fat too hard. This process can, however, create trans fats, which have health risks, so their formation is minimised.
The result is a spreadable margarine with a texture similar to butter.
Manganate (VII) ions are a strong oxidising agent.
Alkenes are oxidised into diols in a redox reaction.
The double bonds in the alkenes are areas of high electron density and react with the .
The alkene undergoes oxidation and two hydroxyl groups are added to adjacent carbon atoms. This is a diol.
The pale purple solution becomes colourless as the permanganate ions, , are reduced to manganese(II) ions, .
