Introductory organic chemistry and alkanes

Organic chemistry is the chemistry of carbon compounds, and there are millions of them. This topic gives you the tools to handle them all: how to draw and name organic molecules, how isomers arise, and how chemists describe the way bonds break. Then it puts those tools to work on the simplest family, the alkanes: where they come from, why they make good fuels, what pollution they cause, and how they react with halogens by a free-radical chain reaction.

Hazards and risks

Many organic compounds catch fire easily or are toxic, so before any practical you assess what could go wrong. Two words are used in a precise way, and the exam expects you to keep them apart.

• Hazard: a substance or a procedure that has the potential to cause harm. Bromine is a hazard because it is toxic and corrosive, whatever you do with it.
• Risk: the chance that harm will actually happen when the hazardous substance or procedure is used in a particular way. The risk depends on how much you use, how concentrated it is and what precautions you take.

So the hazard of a chemical is fixed, but the risk is something you can change. A dilute solution, or a small amount, carries a much lower risk than the pure substance, even though the hazard is the same. Risk can be reduced by:

• working on a smaller scale, or with more dilute solutions;
• taking precautions matched to the particular hazard (see the table);
• choosing a different method or a less hazardous substance that does the same job.

Formulae of organic compounds

Carbon forms four covalent bonds and can bond to other carbon atoms to make chains and rings of almost any length. That is why there are so many organic compounds. Start with two basic words:

• Hydrocarbon: a compound made of hydrogen and carbon only.
• Saturated: contains only single carbon–carbon bonds. Unsaturated: contains at least one C=C double bond (or a triple bond).

The same molecule can be written in several ways, each showing a different amount of detail. You need to be able to read and write all of them.

Skeletal formulae

Skeletal formulae are quick to draw and easy to read once you are used to them. Each line is a C–C bond, a carbon atom sits at every corner and at the end of every line, and you assume each carbon has enough hydrogens to make four bonds. Atoms other than C and H, and the hydrogens attached to them (as in –OH), are always written in.

Homologous series and functional groups

• Homologous series: a family of compounds with the same functional group and the same general formula. Each member differs from the next by a CH₂ group.
• Functional group: the atom or group of atoms that gives a compound its characteristic chemical reactions. Molecules with the same functional group react in the same ways.

Because the functional group decides how a molecule reacts, all members of a series have similar chemical properties. Their physical properties, such as boiling temperature, change gradually along the series, because each extra CH₂ adds eight electrons and makes the molecule a little longer, so the London forces between molecules get a little stronger each time.

Naming organic compounds

Every name is built from three parts: a stem that says how many carbons are in the longest chain, an ending (suffix) for the main functional group, and prefixes for branches and other groups, each with a number to say where it is.

The rules

1. Find the longest continuous carbon chain. It may bend round corners in the drawing. This gives the stem.
2. Number the chain from the end that gives the main functional group the lowest number. If there is no functional group needing a suffix, number from the end that gives the branches the lowest numbers.
3. Name each branch or substituent as a prefix with its position number. Use di-, tri-, tetra- when the same group appears more than once, and give every position (2,2-dimethyl, not 2-dimethyl).
4. Put different prefixes in alphabetical order, ignoring di-, tri- and so on (bromo before chloro before methyl).
5. Punctuation: commas between numbers, hyphens between numbers and words.
6. Leave out numbers that cannot vary: propanone, ethanoic acid and methylpropane need no numbers, because there is only one possible position.

Names with functional groups

• Halogenoalkanes: the halogen is always a prefix: 2-chloropropane, 1-bromo-2-methylpropane. With two different halogens, list them alphabetically: CH₂ClCHBrCH₃ is 2-bromo-1-chloropropane (numbering 1,2 beats 2,3).
• Alkenes: the position of the double bond is the lower-numbered of its two carbons: CH₃CH=CHCH₃ is but-2-ene. Two double bonds make a diene, and an "a" is added to the stem: penta-1,3-diene.
• Alcohols: the position number goes just before -ol: butan-2-ol, 2-methylpropan-2-ol. Two or three –OH groups make a diol or triol: ethane-1,2-diol, propane-1,2,3-triol.
• Aldehydes and carboxylic acids: the functional group is always at the end of the chain, so its carbon is carbon 1 and needs no number: butanal, 2-methylpropanoic acid. An acid group at both ends gives a dioic acid: ethanedioic acid, HOOC–COOH.
• Ketones: the C=O is inside the chain. From five carbons up you need a number: pentan-2-one and pentan-3-one are different compounds.

When a molecule has more than one functional group, the one highest in this order takes the suffix and sets the numbering. The others become prefixes (–OH becomes hydroxy-), except that a C=C keeps its "-en-" inside the name.

carboxylic acid > aldehyde > ketone > alcohol > alkene > halogenoalkane

Structural isomers

Structural isomers have the same molecular formula but a different structural formula: the same atoms are joined up in a different order. Isomers are different compounds with different properties. Structural isomerism can arise in three ways, although the Edexcel specification simply calls them all structural isomers.

• Chain isomers: the carbon skeleton is different (straight or branched in different ways).
• Position isomers: the same functional group sits at a different position on the same skeleton.
• Functional group isomers: the atoms are arranged to give a different functional group altogether.

Two more functional group pairs worth knowing: ethanol, CH₃CH₂OH, and methoxymethane, CH₃OCH₃, are both C₂H₆O; and hex-1-ene, CH₂=CHCH₂CH₂CH₂CH₃, and cyclohexane are both C₆H₁₂, because a ring uses up the same two hydrogens as a double bond.

A second kind of isomerism, stereoisomerism (same structural formula, different arrangement in space), turns up with alkenes in the next topic.

Types of reaction and how bonds break

Classifying reactions

Homolytic and heterolytic fission

A covalent bond is a shared pair of electrons. When it breaks, those two electrons can be shared out in two ways, and the two ways lead to completely different kinds of reaction.

• Homolytic fission: each atom takes one electron from the bond. This makes two free radicals. It is shown with half-headed ("fish-hook") arrows, each moving a single electron.
• Heterolytic fission: one atom takes both electrons. This makes a positive ion and a negative ion. It is shown with a full curly arrow, which moves a pair of electrons. The more electronegative atom usually takes the pair.

Three kinds of attacking species

• Free radical: a species with an unpaired electron, shown by a dot, such as Cl• or •CH₃. Radicals have no charge but are very reactive, because the unpaired electron "wants" a partner.
• Electrophile: an electron-pair acceptor. Electrophiles are attracted to regions of high electron density, such as a C=C bond. They are positive ions or have a δ+ atom: H⁺, Br^δ+ in a polarised Br₂ molecule.
• Nucleophile: an electron-pair donor. A nucleophile has a lone pair it can use to form a new covalent bond with a δ+ carbon: OH⁻, CN⁻, NH₃, H₂O.

Alkanes: structure and physical properties

Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2. Every carbon is tetrahedral, with bond angles of 109.5°, so a "straight" chain is really a zig-zag. Cycloalkanes, such as cyclohexane, are rings of CH₂ groups with the general formula C_nH_2n.

Carbon and hydrogen have very similar electronegativities, so C–H bonds are almost non-polar, and alkane molecules are non-polar. The only forces between alkane molecules are London forces (instantaneous dipole–induced dipole forces, see Topic 7). When an alkane boils, it is these weak forces that are overcome. The strong covalent bonds inside the molecules do not break. Two factors control how strong the London forces are:

• Chain length. Each extra CH₂ adds 8 electrons. More electrons give larger instantaneous dipoles, so the London forces are stronger and more energy is needed to separate the molecules: the boiling temperature rises steadily along the series.
• Branching. Isomers have the same number of electrons, but a branched molecule is more compact, closer to a sphere. Neighbouring molecules cannot lie alongside each other, so there is a smaller area of contact and fewer points where the molecules are close. The London forces are weaker and the boiling temperature is lower.

Alkanes are not very reactive. Their C–C and C–H bonds are strong (about 347 and 413 kJ mol⁻¹), and because the bonds are almost non-polar there are no δ+ or δ− sites to attract nucleophiles or electrophiles. Alkanes do two important things: they burn, and they react with halogens in ultraviolet light.

Crude oil and fractional distillation

Crude oil (petroleum) is a mixture of hydrocarbons, mostly alkanes, of many different chain lengths. On its own it is not much use. It is separated into fractions: mixtures of hydrocarbons with similar chain lengths and therefore similar boiling temperatures.

Cracking and reforming

Fractional distillation produces more of the long-chain fractions than anyone wants, and not enough of the short-chain fractions such as petrol, which are in high demand. Two processes fix the mismatch and also make more useful molecules.

Cracking

Cracking breaks C–C bonds in long-chain alkanes to make shorter alkanes and alkenes (sometimes hydrogen as well). Because strong covalent bonds have to be broken, cracking is a chemical change and needs a lot of energy. Industry mostly uses catalytic cracking, with a zeolite catalyst at around 450 °C; thermal cracking uses much higher temperatures and pressures instead.

Why crack? The shorter alkanes are used in petrol, and they are worth more than the long-chain fractions. The alkenes are even more valuable: unlike alkanes they are reactive, so they are the starting materials for polymers such as poly(ethene) and for chemicals such as ethanol and ethane-1,2-diol.

\[ \mathrm{C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4} \qquad \mathrm{C_{16}H_{34} \rightarrow C_8H_{18} + C_6H_{12} + C_2H_4} \]

There is no single "right" answer when you crack an alkane: the bonds break at random. In the exam you will be told one product, or the products, and asked to balance the equation. Check it by counting carbons and hydrogens on each side. The alkane products have 2n + 2 hydrogens, the alkenes 2n.

Reforming

Reforming turns straight-chain alkanes into branched alkanes and cyclic hydrocarbons (cycloalkanes and arenes, such as benzene), usually over a platinum catalyst. Branched and cyclic hydrocarbons burn more smoothly in a car engine and are less likely to ignite too early (knocking), so they give a more efficient fuel with a higher octane number. Reforming to a ring releases hydrogen:

\[ \mathrm{C_6H_{14} \rightarrow C_6H_{12} + H_2} \qquad\quad \mathrm{C_6H_{14} \rightarrow C_6H_6 + 4H_2} \]

The first equation makes cyclohexane, the second benzene. Turning heptane into 2,2,3-trimethylbutane is also reforming: both are C₇H₁₆, so nothing else is produced.

Alkanes as fuels: combustion

A fuel is a substance that releases heat energy when it burns. Alkanes make good fuels: they are easy to transport, and their combustion is very exothermic, because the bonds made in CO₂ and H₂O are much stronger than the bonds broken in the alkane and in oxygen.

Complete combustion

With plenty of oxygen, the carbon ends up as carbon dioxide and the hydrogen as water.

Incomplete combustion

If oxygen is limited, as in a poorly ventilated gas fire or inside a car engine, some carbon ends up as carbon monoxide or as carbon (soot) instead. Less energy is released per mole of fuel, because the carbon is not fully oxidised.

\[ \mathrm{CH_4(g) + 1\tfrac{1}{2}O_2(g) \rightarrow CO(g) + 2H_2O(l)} \qquad \mathrm{CH_4(g) + O_2(g) \rightarrow C(s) + 2H_2O(l)} \]

Pollution from burning fuels

Nitrogen oxides. The N≡N triple bond is very strong, so N₂ normally does not react with oxygen. In an engine the temperature is so high, and the spark supplies so much energy, that some does:

\[ \mathrm{N_2(g) + O_2(g) \rightarrow 2NO(g)} \qquad\quad \mathrm{2NO(g) + O_2(g) \rightarrow 2NO_2(g)} \]

Sulfur dioxide. Sulfur in the fuel burns to SO₂, which dissolves in rainwater and is oxidised to sulfuric acid, giving acid rain that damages buildings, forests and lakes. Most sulfur is now removed from petrol and diesel at the refinery.

\[ \mathrm{S(s) + O_2(g) \rightarrow SO_2(g)} \]

Catalytic converters

A catalytic converter in a car exhaust removes CO, NO_x and unburnt hydrocarbons by turning them into CO₂, N₂ and H₂O. It is a ceramic honeycomb coated with a thin layer of platinum, palladium and rhodium. The honeycomb gives a very large surface area, so a small mass of these expensive metals catalyses a lot of gas.

\[ \mathrm{2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)} \] \[ \mathrm{C_8H_{18}(g) + 25NO(g) \rightarrow 8CO_2(g) + 12\tfrac{1}{2}N_2(g) + 9H_2O(g)} \]

Alternative fuels and carbon neutrality

Crude oil is a finite resource that took millions of years to form, so it will run out. Biofuels are fuels made from plants, which can be grown again each year, so they are renewable.

• Bioethanol is made by fermenting sugars from crops such as sugar cane or maize with yeast: C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g).
• Biodiesel is made by reacting vegetable oils (for example from rapeseed) with methanol, using an alkali as a catalyst.

An activity is carbon neutral if it gives no overall increase in the amount of carbon dioxide in the atmosphere. Biofuels are often described as close to carbon neutral. Follow the carbon in bioethanol to see why.

Free-radical substitution of alkanes

In the dark, methane and chlorine do not react. In ultraviolet light (or sunlight) they react at room temperature, and a hydrogen atom on the methane is replaced by a chlorine atom:

\[ \mathrm{CH_4(g) + Cl_2(g) \xrightarrow{\text{UV light}} CH_3Cl(g) + HCl(g)} \]

This is a substitution, and because it happens through free radicals it is called free-radical substitution. Bromine reacts with alkanes in exactly the same way. The mechanism has three stages.

Applying the mechanism to other alkanes

The pattern is always the same: initiation splits the halogen; in propagation step 1 the halogen radical removes an H from the alkane, and in step 2 the alkyl radical takes a halogen atom from X₂. For bromine and ethane:

\[ \mathrm{Br_2 \rightarrow 2Br^{\bullet}} \] \[ \mathrm{Br^{\bullet} + CH_3CH_3 \rightarrow HBr + {}^{\bullet}CH_2CH_3} \qquad \mathrm{{}^{\bullet}CH_2CH_3 + Br_2 \rightarrow CH_3CH_2Br + Br^{\bullet}} \] \[ \mathrm{{}^{\bullet}CH_2CH_3 + Br^{\bullet} \rightarrow CH_3CH_2Br} \qquad \mathrm{{}^{\bullet}CH_2CH_3 + {}^{\bullet}CH_2CH_3 \rightarrow CH_3CH_2CH_2CH_3} \]

With propane, a hydrogen can be removed from an end carbon or from the middle carbon, so two different products form: 1-bromopropane and 2-bromopropane. To show substitution on the middle carbon, put the radical dot on that carbon:

\[ \mathrm{CH_3CH_2CH_3 + Br^{\bullet} \rightarrow HBr + CH_3\overset{\bullet}{C}HCH_3} \qquad \mathrm{CH_3\overset{\bullet}{C}HCH_3 + Br_2 \rightarrow CH_3CHBrCH_3 + Br^{\bullet}} \]

Further substitution and the limits of the reaction

The chloromethane made in the first cycle still has C–H bonds, so a chlorine radical can attack it too:

\[ \mathrm{Cl^{\bullet} + CH_3Cl \rightarrow HCl + {}^{\bullet}CH_2Cl} \qquad \mathrm{{}^{\bullet}CH_2Cl + Cl_2 \rightarrow CH_2Cl_2 + Cl^{\bullet}} \]

Repeating this can give CHCl₃ and finally CCl₄. Overall equations add up all the substitutions, with HCl always as the other product, never H₂:

\[ \mathrm{CH_4 + 2Cl_2 \rightarrow CH_2Cl_2 + 2HCl} \qquad \mathrm{CH_4 + 4Cl_2 \rightarrow CCl_4 + 4HCl} \]

Quick checks

Interactive diagrams