Hydrocarbons

Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms. They are broadly classified into saturated hydrocarbons (alkanes, containing only C-C and C-H single bonds), unsaturated hydrocarbons (alkenes with a C=C double bond and alkynes with a C≡C triple bond), and aromatic hydrocarbons (benzene and its derivatives, containing a stable ring of delocalised pi electrons). Saturated rings that are not aromatic are called alicyclic hydrocarbons (cycloalkanes).

IUPAC nomenclature of hydrocarbons follows a set procedure: (i) select the longest continuous carbon chain containing the maximum number of multiple bonds (if any) as the parent chain; (ii) number the chain from the end that gives the lowest locants to the multiple bond and/or substituents; (iii) name substituents as prefixes in alphabetical order with their locants; (iv) use the suffix -ane for alkanes, -ene for alkenes and -yne for alkynes.

Worked example: Name the compound CH₃-CH(CH₃)-CH(CH₃)-CH₂-CH₃.

• The longest chain has 5 carbons (pentane).
• Numbering from the left gives methyl groups at C2 and C3 (locants 2,3); numbering from the right would give locants 3,4. Since {2,3} is lower, we number from the left.
• Two identical substituents are named using the multiplying prefix di: 2,3-dimethyl.
• The IUPAC name is therefore 2,3-dimethylpentane.

Alkanes can be prepared by several methods. In the Wurtz reaction, an alkyl halide is treated with sodium metal in dry ether to give a higher alkane with double the number of carbon atoms; a single alkyl halide always gives an alkane with an even number of carbon atoms. Alkanes are also obtained by decarboxylation of the sodium salt of a carboxylic acid heated with soda lime, by Kolbe's electrolytic method (electrolysis of an aqueous solution of a sodium/potassium carboxylate), and by catalytic hydrogenation of alkenes or alkynes over finely divided Ni, Pd or Pt.

Alkanes are largely unreactive towards common acids, bases and oxidising agents because of their strong, non-polar C-C and C-H sigma bonds, but they undergo free-radical substitution with halogens in sunlight or UV light, and they burn completely in excess oxygen (combustion) to give carbon dioxide and water with release of heat. They also undergo controlled oxidation, isomerisation, aromatisation and pyrolysis (cracking) under suitable conditions.

Worked example (mechanism of free-radical chlorination of methane):

• Initiation: Cl₂ absorbs UV light and undergoes homolytic fission to give two chlorine free radicals.
• Propagation step 1: A chlorine radical abstracts a hydrogen atom from methane, generating a methyl free radical and HCl.
• Propagation step 2: The methyl free radical reacts with another Cl₂ molecule to give chloromethane and regenerate a chlorine radical, which continues the chain.
• Termination: Two radicals combine (Cl• + Cl•, CH₃• + Cl•, or CH₃• + CH₃•) to stop the chain; excess Cl₂ allows further substitution to give CH₂Cl₂, CHCl₃ and CCl₄.

Alkenes are commonly prepared by dehydration of alcohols using concentrated H₂SO₄ with heating, and by dehydrohalogenation of alkyl halides using alcoholic KOH, in which the more substituted alkene is usually the major product (Saytzeff's rule). Alkenes can also be obtained by partial hydrogenation of alkynes: Lindlar's catalyst (Pd on BaSO₄, poisoned with quinoline) gives the cis alkene, while sodium in liquid ammonia gives the trans alkene.

The C=C double bond makes alkenes reactive towards electrophilic addition. Addition of hydrogen halides (HX) follows Markovnikov's rule: the halide ion attaches to the carbon already carrying fewer hydrogen atoms, because the reaction proceeds through the more stable carbocation intermediate. In the presence of peroxides, HBr alone adds in the opposite sense (anti-Markovnikov), known as the peroxide (Kharasch) effect, since the mechanism switches from ionic to free-radical. Alkenes also add H₂SO₄, water (acid-catalysed hydration) and halogens; they decolourise cold, dilute, alkaline KMnO₄ (Baeyer's test, forming a vicinal diol) and undergo ozonolysis, which cleaves the double bond to give two carbonyl compounds and is used to locate the position of the double bond.

Worked example (Markovnikov addition): Propene reacts with HBr in the absence of peroxide.

• H⁺ adds first to the terminal (less substituted) carbon, generating the more stable secondary carbocation CH₃-C⁺H-CH₃ rather than the less stable primary carbocation.
• Br^- then attacks this secondary carbocation.
• The major product is 2-bromopropane, CH₃-CHBr-CH₃, consistent with Markovnikov's rule.

The most important preparation of ethyne (acetylene) is the reaction of calcium carbide with water. Alkynes are also prepared by double dehydrohalogenation of vicinal (or gem) dihalides using excess alcoholic KOH.

A unique feature of terminal alkynes (R-C≡C-H) is the acidic character of the hydrogen attached to the triple-bonded carbon, because the sp-hybridised carbon holds its bonding electrons closer to the nucleus. This hydrogen can be replaced by sodium (using Na or NaNH₂) to form sodium acetylides, and terminal alkynes give a characteristic white precipitate with ammoniacal silver nitrate (and a red precipitate with ammoniacal cuprous chloride), which serves as a test to distinguish terminal from non-terminal alkynes. Alkynes undergo addition of H₂, halogens, and hydrogen halides (following Markovnikov's rule for unsymmetrical alkynes) across the triple bond, and controlled acid-catalysed hydration in presence of Hg²⁺/H₂SO₄ converts them into carbonyl compounds via an unstable enol intermediate.

Worked example (hydration of ethyne):

• Ethyne reacts with water in the presence of 40% H₂SO₄ and HgSO₄ catalyst.
• Markovnikov addition of water across the triple bond first gives the unstable enol, CH₂=CH-OH.
• The enol immediately tautomerises (keto-enol tautomerism) to the more stable carbonyl form.
• The final product is acetaldehyde (ethanal), CH₃CHO; higher terminal alkynes give methyl ketones by the same route.

Benzene, C₆H₆, is represented by two equivalent Kekulé structures with alternating single and double bonds; the true structure is a resonance hybrid of these (and other minor) structures. All six carbon-carbon bonds in benzene are of equal length (139 pm), intermediate between a pure C-C single bond (154 pm) and a pure C=C double bond (133 pm), because the six p-orbitals on the ring carbons overlap sideways to form a continuous, delocalised pi electron cloud above and below the planar hexagonal ring. This delocalisation makes benzene about 150 kJ mol^-1 more stable than the hypothetical Kekulé structure with localised double bonds (its resonance/delocalisation energy), which explains why benzene prefers substitution reactions over the addition reactions typical of alkenes.

A cyclic, planar compound is called aromatic if it satisfies Hückel's rule: it must have a continuous, complete cyclic conjugation of p-orbitals and possess (4n+2) pi electrons, where n = 0, 1, 2, .... Benzene has 6 pi electrons (n = 1), so it is aromatic; other examples include naphthalene and the cyclopentadienyl anion.

Worked example (testing aromaticity):

• Benzene: cyclic, planar, fully conjugated, with 6 pi electrons. Setting 4n+2 = 6 gives n = 1 (a whole number), so benzene is aromatic.
• Cyclobutadiene: cyclic and conjugated, but has 4 pi electrons. Setting 4n+2 = 4 gives n = 0.5, which is not a whole number, so cyclobutadiene does not satisfy Hückel's rule and is not aromatic (it is in fact anti-aromatic and highly unstable).

Because of its stable, delocalised pi system, benzene reacts with electrophiles by substitution rather than addition, so that aromaticity is preserved in the product. The general mechanism has three steps: (i) generation of the electrophile; (ii) attack of the electrophile on the pi cloud of the ring to form a resonance-stabilised, non-aromatic carbocation intermediate called the arenium ion (or sigma complex); and (iii) loss of a proton from the arenium ion, which restores the aromatic sextet of electrons.

In nitration, benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (the nitrating mixture) at about 333 K; the electrophile is the nitronium ion, NO₂⁺. In halogenation, benzene reacts with chlorine or bromine in presence of a Lewis acid catalyst such as anhydrous FeCl₃ or AlCl₃, which polarises the halogen molecule to generate the electrophile. In Friedel-Crafts alkylation, an alkyl halide with anhydrous AlCl₃ introduces an alkyl group (though the reaction suffers from over-alkylation and carbocation rearrangement, and fails on strongly deactivated rings such as nitrobenzene); Friedel-Crafts acylation, using an acyl halide with anhydrous AlCl₃, avoids rearrangement and over-substitution because the acyl group formed is deactivating.

Substituents already present on the ring control where the next electrophile attacks: electron-donating groups such as -OH, -NH₂, -OR and alkyl groups activate the ring and direct incoming groups to the ortho and para positions; halogens are weakly deactivating but still ortho, para-directing; strongly electron-withdrawing groups such as -NO₂, -CN, -COOH and -SO₃H deactivate the ring and direct to the meta position.

Worked example (mechanism of nitration of benzene):

• Step 1: HNO₃ is protonated by H₂SO₄ and loses water to generate the electrophile, NO₂⁺.
• Step 2: NO₂⁺ attacks the benzene pi cloud, forming a resonance-stabilised arenium ion in which the positive charge is delocalised over three ring carbons.
• Step 3: HSO₄^- removes a proton from the sp³ carbon of the arenium ion, regenerating the aromatic ring and giving nitrobenzene.