Hydrocarbons — Study Notes

1. Introduction

Hydrocarbons are compounds composed solely of carbon and hydrogen, and they are the simplest organic molecules. They form the foundation of organic chemistry and the petrochemical industry, serving as fuels, solvents and the raw materials for polymers, pharmaceuticals and countless other products. Their study introduces the three great families of organic compounds: alkanes, alkenes and alkynes, together with aromatic hydrocarbons.

Alkanes are saturated hydrocarbons with only single bonds, alkenes contain carbon-carbon double bonds, alkynes contain triple bonds, and aromatic hydrocarbons contain benzene-like rings with delocalised electrons. The presence of these bonds and rings determines the reactivity of each family: alkanes are relatively inert, alkenes and alkynes are reactive at their multiple bonds, and aromatic compounds react by substitution rather than addition.

Hydrocarbons are obtained primarily from petroleum and natural gas, which are fractionally distilled into usable fractions. This chapter covers the structure, preparation, properties and reactions of each class of hydrocarbon, providing the mechanistic framework, including Markovnikov's rule and the aromaticity of benzene, that will be applied throughout the rest of organic chemistry.

2. Alkanes: Structure and Sources

Alkanes have the general formula CnH2n+2 and contain only single bonds. The carbon atoms are sp3 hybridised with tetrahedral geometry and bond angles of about 109.5 degrees. The simplest alkanes are methane (CH4), ethane (C2H6) and propane (C3H8). Alkanes show chain isomerism beyond the fourth member, but because carbon forms only single bonds, alkanes are comparatively unreactive.

The main commercial source of alkanes is petroleum and natural gas. Petroleum is a complex mixture of hydrocarbons that is separated by fractional distillation into fractions such as gasoline, kerosene and diesel based on boiling point ranges. Natural gas consists mainly of methane with smaller amounts of ethane, propane and butane.

Alkanes with 1 to 4 carbons are gases at room temperature, those with 5 to 17 carbons are liquids, and the higher members are waxy solids. The boiling points increase regularly with molecular mass because of increasing van der Waals forces, while branched alkanes boil at lower temperatures than their straight-chain isomers because of reduced surface area.

3. Preparation of Alkanes

Alkanes can be prepared by several laboratory and industrial methods. Hydrogenation of alkenes and alkynes over a metal catalyst such as nickel, palladium or platinum gives alkanes. Reduction of alkyl halides with zinc and acid gives alkanes, and Wurtz reaction couples two alkyl halides with sodium metal in dry ether:

$$2\text{RX} + 2\text{Na} \rightarrow \text{R-R} + 2\text{NaX}$$

The Wurtz reaction gives a symmetrical alkane with double the number of carbon atoms of the halide. The decarboxylation of the sodium salt of a carboxylic acid with soda lime (NaOH and CaO) gives an alkane with one less carbon. The Kolbe's electrolytic method produces alkanes by electrolysing a concentrated solution of the sodium salt of a carboxylic acid.

The reduction of alkyl halides by nascent hydrogen, the reduction of alcohols, and the reaction of Grignard reagents with water also produce alkanes. Industrial sources, however, are dominated by petroleum refining, including cracking, which breaks large hydrocarbons into smaller ones, and reforming, which converts straight chains into branched or cyclic hydrocarbons.

4. Properties and Reactions of Alkanes

Alkanes are relatively inert because their C-H and C-C bonds are strong and non-polar. They are insoluble in water but soluble in non-polar organic solvents, and they are good fuels, burning in excess oxygen to give carbon dioxide and water:

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

The most characteristic reaction of alkanes is halogenation, especially chlorination, which occurs by a free radical chain mechanism. The mechanism has three steps: initiation, in which chlorine molecules split into chlorine radicals by UV light; propagation, in which a chlorine radical abstracts a hydrogen to form HCl and an alkyl radical, which then reacts with Cl2 to give the alkyl chloride; and termination, in which radicals combine.

Chlorination gives a mixture of products because each hydrogen can be substituted. The relative rates of substitution follow the stability of the intermediate radicals: tertiary > secondary > primary. Controlled oxidation of alkanes gives alcohols, aldehydes and acids, and alkanes also undergo thermal cracking and pyrolysis.

5. Alkenes: Preparation and Properties

Alkenes have the general formula CnH2n and contain one carbon-carbon double bond, which consists of a sigma bond and a pi bond. The carbon atoms of the double bond are sp2 hybridised with trigonal planar geometry and a bond angle of 120 degrees. Because rotation about the double bond is restricted, alkenes can show geometrical (cis-trans) isomerism.

Alkenes are prepared by the dehydrohalogenation of alkyl halides with alcoholic potassium hydroxide, by the dehydration of alcohols with concentrated sulphuric acid, and by the dehalogenation of vicinal dihalides with zinc. Dehydration of alcohols follows Zaitsev's rule, which states that the alkene formed is the more substituted one, with the double bond at the carbon that has fewer hydrogens.

Alkenes are much more reactive than alkanes because of the pi bond, which is a region of high electron density. They undergo addition reactions with hydrogen (hydrogenation), halogens (halogenation), hydrogen halides (hydrohalogenation) and water (hydration). The addition of hydrogen halides to unsymmetrical alkenes follows Markovnikov's rule.

6. Markovnikov's Rule and Addition Reactions

Markovnikov's rule states that when a hydrogen halide adds to an unsymmetrical alkene, the hydrogen attaches to the carbon that already has more hydrogen atoms, and the halogen attaches to the more substituted carbon. For example, the addition of HBr to propene gives 2-bromopropane as the major product:

$$\text{CH}_3\text{CH}=\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CHBrCH}_3$$

The rule is explained by the stability of the intermediate carbocation: the more substituted carbocation (secondary, tertiary) is formed preferentially because it is more stable. In the presence of peroxides, however, HBr adds against Markovnikov's rule (anti-Markovnikov) through a free radical mechanism, giving the less substituted product.

Other addition reactions include hydration to give alcohols, addition of water in the presence of an acid catalyst following Markovnikov's rule, and ozonolysis, which cleaves the double bond to give carbonyl compounds. Polymerisation of alkenes produces polymers such as polyethylene and polyvinyl chloride, making alkenes the monomers of the plastics industry.

7. Alkynes: Preparation and Properties

Alkynes have the general formula CnH2n-2 and contain a carbon-carbon triple bond consisting of one sigma bond and two pi bonds. The triple bond carbons are sp hybridised with linear geometry and a bond angle of 180 degrees. Ethyne (acetylene), C2H2, is the simplest alkyne.

Alkynes are prepared by the dehydrohalogenation of geminal dihalides or vicinal dihalides with alcoholic KOH, and by the reaction of calcium carbide with water:

$$\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_2 + \text{Ca(OH)}_2$$

Like alkenes, alkynes undergo electrophilic addition reactions. The addition of hydrogen halides follows Markovnikov's rule, and addition of halogens occurs stepwise. Water adds to ethyne in the presence of mercuric sulphate and sulphuric acid to give acetaldehyde through a process called hydration. Terminal alkynes have acidic hydrogen and react with sodium or ammoniacal silver nitrate to form acetylides, a reaction used to distinguish terminal from internal alkynes.

8. Aromatic Hydrocarbons: Benzene

Aromatic hydrocarbons contain benzene rings with delocalised pi electrons. Benzene (C6H6) is a planar molecule in which six sp2 hybridised carbons form a ring, and the six p orbitals overlap sideways to form a delocalised pi system. All C-C bonds are equal in length, between a single and a double bond, and the aromatic ring is exceptionally stable.

Benzene undergoes electrophilic substitution reactions rather than addition, because substitution preserves the aromatic stability. The most important reactions are nitration (with nitric acid and sulphuric acid giving nitrobenzene), halogenation (with a halogen and a Lewis acid catalyst), sulphonation (with oleum giving benzenesulphonic acid), and Friedel-Crafts alkylation and acylation.

The directing effects of substituents on the benzene ring are important. Electron-donating groups like methyl activate the ring and direct further substitution to the ortho and para positions, while electron-withdrawing groups like nitro deactivate the ring and direct to the meta position. These concepts explain the products of aromatic substitution reactions and form the basis of aromatic synthesis.

Quick Revision Tables

Table 1: Families of Hydrocarbons

Table 2: Reactions of Alkenes and Alkynes

Table 3: Benzene Substitution Reactions

Mind Map

Important Diagrams (SVG)

Diagram 1: Free Radical Chlorination of Methane

UV light PROPAGATION CH4 + Cl -> CH3 + HCl CH3 + Cl2 -> CH3Cl + Cl TERMINATION CH3 + Cl* -> CH3Cl MIXTURE OF PRODUCTS Continued substitution gives CH2Cl2, CHCl3, CCl4 Relative rate: tertiary H > secondary > primary GOLDEN RULE Halogenation of alkanes is a free radical chain reaction: initiation, propagation, termination.

Diagram 2: Benzene Structure and Electrophilic Substitution

Common Mistakes

1. Applying Markovnikov's rule without checking for peroxides; HBr adds anti-Markovnikov in the presence of peroxides.
2. Forgetting that alkanes are inert to addition reactions; their characteristic reaction is free radical substitution.
3. Writing the formula of alkynes as CnH2n; alkynes are CnH2n-2 and alkenes are CnH2n.
4. Assuming benzene readily undergoes addition; it undergoes substitution to preserve its aromatic stability.
5. Confusing Zaitsev's rule with Markovnikov's rule; Zaitsev governs the alkene formed in elimination, Markovnikov governs addition.
6. Forgetting that only terminal alkynes have acidic hydrogen and form acetylides.
7. Using the wrong mechanism for alkane halogenation; it is a free radical chain mechanism, not electrophilic addition.

Exam Tips

1. Memorise the general formulas: CnH2n+2 (alkanes), CnH2n (alkenes), CnH2n-2 (alkynes).
2. Remember Markovnikov's rule and its carbocation stability explanation, plus the peroxide anti-Markovnikov exception for HBr.
3. Know the preparation of ethyne from calcium carbide: CaC2 + 2H2O gives C2H2 + Ca(OH)2.
4. Recall that alkenes decolourise bromine water, a test for unsaturation.
5. For benzene, remember the substitution reactions and their reagents (nitration, halogenation, sulphonation, Friedel-Crafts).
6. Remember that o- and p-directing groups are electron-donating (like methyl) while m-directing groups are electron-withdrawing (like nitro).
7. Know that branched alkanes have lower boiling points than their straight-chain isomers due to reduced surface area.

Conclusion

Hydrocarbons form the structural backbone of organic chemistry and the basis of the modern petrochemical economy. Alkanes, with their strong single bonds, are relatively inert and react mainly by free radical substitution. Alkenes and alkynes, with their pi bonds, are reactive and undergo electrophilic addition following Markovnikov's rule, making them the building blocks for alcohols, halides and polymers. Aromatic hydrocarbons, with their delocalised pi systems, display exceptional stability and react by electrophilic substitution, governed by the directing effects of substituents. Together, these four families illustrate the full range of structure-reactivity relationships that organise organic chemistry, from the simplest fuels to the sophisticated molecules of medicine and materials science.