Chemistry of Hydrocarbons notes

MDCAT Chemistry

Hydrocarbons are compounds made only of carbon and hydrogen. This chapter covers saturated and unsaturated hydrocarbons, their preparation, reactions, naming, isomerism, cracking and reforming, and the structure and reactions of benzene and other aromatic compounds.

Classification and Nomenclature of Hydrocarbons

Hydrocarbons are divided into saturated and unsaturated compounds. Saturated hydrocarbons contain only carbon-carbon single bonds and are called alkanes. Unsaturated hydrocarbons contain at least one carbon-carbon double or triple bond. Alkenes contain double bonds, while alkynes contain triple bonds.

The presence of a double or triple bond is called unsaturation. Hydrocarbons may also be open-chain or cyclic. Aromatic hydrocarbons contain benzene-like rings with special stability.

In IUPAC nomenclature, the longest carbon chain is selected and numbered from the end nearest the multiple bond or substituent. Halogen prefixes include fluoro, chloro, bromo and iodo.

points are written here as exact facts

Alkanes are saturated hydrocarbons with the general formula CnH2n+2.
Alkenes contain a carbon-carbon double bond and have the general formula CnH2n.
Alkynes contain a carbon-carbon triple bond and have the general formula CnH2n-2.
The common naming suffix of alkenes is -ylene, as in ethylene and propylene.
A double or triple bond indicates unsaturation.
Propanal and acetone are functional group isomers because both have the molecular formula C3H6O but different functional groups.
In a name containing several different halogens, prefixes are written alphabetically. For example, 2-bromo-4-chloro-3-iodopentane is an acceptable IUPAC name.

Alkanes and Their Sources

Alkanes are also called paraffins. The name paraffin means low affinity because alkanes are comparatively unreactive. They contain carbon atoms joined by single covalent bonds, and each carbon is sp3 hybridised with approximately tetrahedral geometry.

The first members are methane, ethane, propane and butane. As molecular size increases, boiling point and melting point generally increase. Lower alkanes are gases, medium members are liquids, and higher members are solids at ordinary conditions.

Petroleum and natural gas are important natural sources of alkanes. Petroleum is separated into useful fractions by fractional distillation according to differences in boiling points.

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The general formula of open-chain alkanes is CnH2n+2.
Alkanes contain only carbon-carbon and carbon-hydrogen single sigma bonds.
Methane is the simplest alkane, with formula CH4.
Alkanes are called paraffins because of their low chemical reactivity.
Complete combustion of an alkane produces carbon dioxide and water: hydrocarbon + O2 gives CO2 + H2O.
Incomplete combustion may produce carbon monoxide or carbon.
Petroleum fractions are separated by fractional distillation, which depends on different boiling points.

Preparation and Reactions of Alkanes

Alkanes can be prepared by several laboratory methods. In the soda lime method, a sodium salt of a carboxylic acid is heated with soda lime. The alkane formed has one carbon atom fewer than the original acid.

Kolbe's electrolytic method involves electrolysis of a concentrated aqueous solution of a sodium or potassium salt of a carboxylic acid. Decarboxylation occurs at the anode and an alkane with twice the alkyl group is formed.

Alkanes undergo substitution reactions rather than ordinary addition reactions. In halogenation, light or heat helps produce free radicals. Chlorination and bromination are common examples.

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Soda lime is a mixture of sodium hydroxide and calcium oxide.
Heating sodium ethanoate with soda lime gives methane: CH3COONa + NaOH gives CH4 + Na2CO3.
Kolbe's electrolytic method takes place in the presence of electrolysis.
Electrolysis of sodium ethanoate gives ethane at the anode.
Methane reacts with chlorine in ultraviolet light by free radical substitution.
The overall chlorination reaction is CH4 + Cl2 gives CH3Cl + HCl in the presence of light.
Alkanes generally do not decolourise bromine water because they do not readily undergo addition.

Cracking, Reforming and Octane Number

Cracking is the breakdown of large hydrocarbon molecules into smaller molecules. It produces more useful fuels and smaller alkenes, which are raw materials for polymers. Cracking may be thermal or catalytic.

Reforming changes the structure of hydrocarbon molecules without necessarily changing their molecular formula. Straight-chain hydrocarbons may be converted into branched-chain hydrocarbons, cyclic compounds or aromatic compounds.

The octane number measures the anti-knock quality of petrol. A higher octane number means that the fuel resists knocking more effectively in an engine.

points are written here as exact facts

Large hydrocarbons are converted into smaller hydrocarbons by cracking.
Cracking commonly produces a mixture of smaller alkanes and alkenes.
Catalytic cracking uses a catalyst and a suitable high temperature.
Reforming produces fuels with higher octane numbers.
Branching generally increases the octane number of a hydrocarbon.
The octane number of 2,2,4-trimethylpentane, called iso-octane, is assigned the value 100.
The octane number of n-heptane is assigned the value 0.
Fuels with higher octane numbers can be produced by reforming.

Alkenes and Their Addition Reactions

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. The two carbon atoms of a double bond are sp2 hybridised. The double bond consists of one sigma bond and one pi bond, and the pi bond makes alkenes more reactive than alkanes.

The double bond prevents free rotation. Therefore, some alkenes show geometrical isomerism. Cis isomers have similar groups on the same side, while trans isomers have them on opposite sides.

Alkenes mainly undergo electrophilic addition. The pi electrons attract an electrophile. In the addition of hydrogen chloride to propene, H+ attacks the double bond first and the more stable carbocation is formed. Chloride ion then completes the reaction.

points are written here as exact facts

Alkenes have the general formula CnH2n for open-chain compounds with one double bond.
The common suffix for an alkene is -ene, as in ethene and propene.
Ethene decolourises bromine water by addition of bromine across the double bond.
Hydrogenation of an alkene produces an alkane in the presence of a suitable catalyst such as nickel.
Hydration of an alkene produces an alcohol.
Addition of HCl to CH3CH=CH2 begins with addition of H+ to the double bond.
Markovnikov's rule states that, in addition of HX to an unsymmetrical alkene, hydrogen generally adds to the carbon already bearing more hydrogen atoms.
Hydrogenation of vegetable oil converts it into vegetable ghee, usually using hydrogen and a nickel catalyst.

Polymerisation and Oxidation of Alkenes

Alkenes can form addition polymers. In this process, many small alkene molecules called monomers combine repeatedly without eliminating a small molecule. The product is a polymer with a high molar mass.

Polyethene is formed from ethene. Other alkenes form polymers with different side groups and properties. Industrial polymerisation may use coordination catalysts.

Oxidation reactions help identify the position of a double bond. Ozonolysis breaks the double bond and forms carbonyl compounds. The structures of these products show the original location of the double bond.

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Addition polymerisation involves repeated combination of monomers to form a polymer.
Ethene is the monomer of polyethene.
The double bond of the monomer changes into a single bond in the polymer chain.
The Ziegler-Natta catalyst system contains TiCl4 and Al(C2H5)3.
Ozonolysis is used to locate the position of a double bond between carbon atoms.
Oxidation of alkenes may produce aldehydes, ketones or acids depending on the alkene structure and reaction conditions.
Alkenes are generally more reactive than alkanes because of their pi bond.

Alkynes and Ethyne

Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond. The triple bond consists of one sigma bond and two pi bonds. The bonded carbon atoms are sp hybridised and have a linear arrangement.

Ethyne, also called acetylene, is the first member of the alkyne series. It is used in oxyacetylene welding because its combustion produces a very hot flame. Alkynes undergo addition reactions at the triple bond.

Ethyne is prepared industrially from calcium carbide and water. It can also be manufactured from natural gas by partial oxidation at a high temperature.

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The general formula of open-chain alkynes containing one triple bond is CnH2n-2.
The common suffix of an alkyne is -yne, as in ethyne and propyne.
Calcium carbide reacts with water to produce ethyne: CaC2 + 2H2O gives C2H2 + Ca(OH)2.
Ethyne is prepared on the industrial scale by the reaction of calcium carbide with water.
Acetylene can be produced from natural gas by partial oxidation.
The carbon atoms in ethyne are sp hybridised and the molecule is linear.
Ethyne burns with a luminous flame and is used in welding and cutting metals.
Alkynes undergo addition of hydrogen, halogens and hydrogen halides.

Benzene and Aromatic Compounds

Benzene is the prime member of aromatic compounds. Its molecular formula is C6H6. Each carbon atom is sp2 hybridised, and the six carbon atoms form a planar hexagonal ring.

Kekule proposed a structure with three alternate single and double bonds. These are called conjugate bonds. However, this structure could not explain the equal lengths of all carbon-carbon bonds, the unusual stability of benzene, or its resistance to ordinary addition reactions.

The modern description of benzene has delocalised pi electrons spread over the ring. Benzene therefore undergoes substitution reactions more readily than addition reactions, while retaining its aromatic stability.

points are written here as exact facts

Benzene is the prime member of aromatic compounds.
Benzene has the molecular formula C6H6.
Three alternate single and double bonds in benzene are called conjugate bonds.
Kekule's structure failed to explain the equal carbon-carbon bond lengths, unusual stability and preference for substitution reactions.
The pi electrons in benzene are delocalised over the entire ring.
Benzene is prepared in the laboratory by distillation of phenol with zinc dust.
Benzene is not usually considered an alkene, despite having three apparent double bonds.
A benzene derivative is formed when one or more hydrogen atoms of benzene are replaced by other atoms or groups.

Electrophilic Substitution and Directing Effects in Benzene

Benzene commonly undergoes electrophilic substitution. In these reactions, an electrophile replaces a hydrogen atom on the benzene ring. Important reactions include halogenation, nitration, sulphonation and Friedel-Crafts reactions.

In halogenation, chlorine reacts with benzene in the presence of FeCl3 or AlCl3. Nitration uses concentrated nitric acid and concentrated sulphuric acid. The sulphuric acid helps generate the nitronium ion, NO2+, which attacks the ring.

Substituents already present on the ring affect the position of a second substitution. Ortho and para directors usually direct the new group to positions 2 and 4. Meta directors direct it mainly to position 3.

points are written here as exact facts

Benzene reacts with chlorine in the presence of FeCl3 to form chlorobenzene and HCl.
Nitration of benzene uses HNO3 and H2SO4 and forms nitrobenzene.
The electrophile in nitration is the nitronium ion, NO2+.
Chlorine is an ortho-para directing group, although it deactivates the ring overall.
The amino group, NH2, is an ortho-para directing group and is not a meta directing group.
The nitro group, NO2, is a meta directing group.
If benzene is first chlorinated and then nitrated, 2-chloronitrobenzene is one possible product.
Benzene reacts with acetyl chloride in the presence of AlCl3 to form acetophenone.

Key terms

Hydrocarbon: A compound made up only of carbon and hydrogen.
Alkane: A saturated open-chain hydrocarbon containing only carbon-carbon single bonds.
Alkene: An unsaturated hydrocarbon containing at least one carbon-carbon double bond.
Alkyne: An unsaturated hydrocarbon containing at least one carbon-carbon triple bond.
Saturation: The condition in which a hydrocarbon has only carbon-carbon single bonds.
Unsaturation: The presence of a carbon-carbon double or triple bond.
Isomerism: The existence of compounds with the same molecular formula but different structures or arrangements.
Functional group isomerism: Isomerism in which compounds have the same molecular formula but different functional groups.
Cracking: The conversion of large hydrocarbon molecules into smaller hydrocarbons.
Reforming: The rearrangement of hydrocarbon structures to produce fuels with improved octane quality.
Octane number: A number that expresses the anti-knock quality of petrol.
Electrophilic addition: An addition reaction in which an electron-seeking species attacks the electron-rich pi bond.
Polymerisation: The repeated joining of small monomer molecules to form a large polymer.
Ozonolysis: Oxidative cleavage of a carbon-carbon double bond using ozone to form carbonyl compounds.
Aromatic compound: A stable cyclic compound containing a delocalised pi-electron system, such as benzene.
Electrophilic substitution: A reaction in which an electrophile replaces a hydrogen atom of an aromatic ring.
Conjugation: The arrangement of alternating single and double bonds that permits electron delocalisation.
Directing group: A substituent on a benzene ring that influences the position of a new substituent.

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