Ch 13Hydrocarbons — Class 11 Chemistry, concept-first.

The first three alkanes -- methane (CH4), ethane (C2H6) and propane (C3H8) -- have only one possible structure each.

A combustion reaction is the reaction of a substance with oxygen, releasing heat and (usually) light as a flame. With sufficient oxygen, alkanes burn completely to carbon dioxide and water, e.g.

A halogenation reaction substitutes one or more hydrogens of an alkane with halogen atoms. Chlorination and bromination are the two widely used halogenation reactions -- fluorination is too violent an…

Alkanes with six to ten carbon atoms convert into homologues of benzene at high temperature over a catalyst, a process called aromatisation, proceeding by simultaneous cyclisation and dehydrogenation.…

Methane reacts with steam at 1273 K over a nickel catalyst, decomposing to carbon monoxide and hydrogen gas: CH4(g) + H2O(g) --Ni, 1273K-- CO(g) + 3H2(g).

Pyrolysis is the thermal decomposition of an organic compound into smaller fragments in the absence of air, under heat ('pyro' = fire, 'lysis' = separating); for alkanes this is also called cracking.

Isomerisation converts a compound into one of its isomeric forms. Normal (straight-chain) alkanes can be converted into their branched isomers in the presence of AlCl3 and HCl at 298 K (e.g.

The strongly exothermic nature of alkane combustion explains their extensive use as fuels: methane in natural gas is used for home heating; a propane-butane mixture (LPG) is used for domestic cooking;…

Alkenes are IUPAC-named by the same rules used for alkanes (Unit 11), replacing the '-ane' ending with '-ene' and giving the double bond the lowest possible locant.

The double bond in alkenes allows both structural and geometrical isomerism. STRUCTURAL ISOMERISM: the first two members, ethene (C2H4) and propene (C3H6), have no isomers, since their carbon atoms ca…

Hydrogen adds to an alkene in the presence of a metal catalyst (Ni, Pd or Pt) to give the corresponding alkane -- catalytic hydrogenation.

Treating an alkene with chlorine or bromine causes rapid addition, forming a 1,2-dihaloalkane (vicinal dihalide), e.g. ethene + Cl2 -- 1,2-dichloroethane.

MARKOVNIKOV'S RULE: when an unsymmetrical alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that already carries more hydrogens, and the halogen adds to the carbon carrying fewer h…

Adding HBr to an alkene in the presence of an organic peroxide gives the ANTI-Markovnikov product -- Br ends up on the carbon with MORE hydrogens instead of fewer.

Water does not normally react with alkenes, but in the presence of concentrated H2SO4 it adds across the double bond to give an alcohol -- a reaction that follows a carbocation mechanism and obeys Mar…

Alkenes react with cold, concentrated sulphuric acid to form an alkyl hydrogen sulphate, again following Markovnikov's rule (e.g.

With cold, dilute, alkaline KMnO4 (Baeyer's reagent): an alkene is oxidised to a vicinal diol (glycol); the purple permanganate colour is discharged, first turning dark green (Mn6+) and then depositin…

Ozonolysis oxidatively cleaves an alkene's (or alkyne's) C=C bond using ozone, giving two carbonyl compounds; it is widely used to locate the exact position of a double or triple bond in an unknown al…

A polymer is a large molecule built by combining a large number of small molecules (monomers); the process is polymerisation.

Alkynes are IUPAC-named by the same rules used for alkanes/alkenes (Unit 11), replacing the ending with '-yne' and giving the triple bond the lowest possible locant.

An alkyne is acidic ONLY if it carries a terminal (triple-bond) hydrogen. This follows from the hybridisation of the triple-bonded carbon: the s-character of an sp orbital (50%) exceeds that of an sp2…

ADDITION OF HYDROGEN: an alkyne adds two moles of H2 over a platinum catalyst, first to the alkene and then to the alkane -- e.g. acetylene + H2 --Pt-- ethylene, ethylene + H2 --Pt-- ethane.

Alkynes undergo hydration on warming with mercuric sulphate and dilute H2SO4 at 333 K, adding water across the triple bond by Markovnikov's rule to give an unstable enol that immediately tautomerises…

Ozone adds across the C-C triple bond of an alkyne to form an ozonide, which hydrolyses with water to give carbonyl compounds directly (rather than needing the Zn/H2O reductive workup used for alkenes…

Alkynes undergo two distinct types of polymerisation. LINEAR POLYMERISATION: passing ethyne into a solution of cuprous chloride and ammonium chloride links two molecules into vinylacetylene (CH2=CH-CC…

Aromatic hydrocarbons are classified by how many rings they contain. Monocyclic aromatic hydrocarbons (MAH) have a single ring, e.g. benzene (C6H6) and toluene (C7H8).

Two lines of experimental evidence establish that benzene is cyclic. (i) SUBSTITUTION: benzene reacts with bromine in the presence of AlCl3 to give only ONE monobromobenzene product (C6H6 + Br2 --AlCl…

In 1865, August Kekulé proposed that benzene is a cyclic, planar ring of six carbons with ALTERNATING single and double bonds. This structure faced two objections.

Resonance is the phenomenon where two or more valid structures can be drawn for a substance that has identical atomic positions but different electron-pair placements; the molecule's real structure is…

The molecular-orbital picture gives the fullest description of benzene's structure. All six carbons are sp2 hybridised: the six sp2 orbitals overlap linearly with six hydrogen 1s orbitals to form six…

Coal tar, a viscous liquid obtained by pyrolysing coal, is fractionally distilled to separate its many volatile aromatic components by boiling-point range; benzene, toluene and xylene distil off toget…

FROM ACETYLENE: passing acetylene through a red-hot iron tube trimerises it directly to benzene (the same cyclic-polymerisation reaction already met under alkynes), 3 CHCH --873K-- C6H6.

NITRATION: heating benzene at 330 K with a nitrating mixture (conc. HNO3 + conc. H2SO4) substitutes one ring hydrogen with the nitronium ion (NO2+, the electrophile) to give nitrobenzene, C6H6 + HNO3…

Benzene undergoes electrophilic substitution, rather than addition, because it is an electron-rich system thanks to its delocalised pi electrons, and is readily attacked by electrophiles -- but the ar…

HYDROGENATION: benzene adds three moles of H2 over platinum or palladium to give cyclohexane, C6H6 + 3H2 --Pt/Pd-- C6H12 (the same reaction used earlier as structural evidence for benzene's ring).

VAPOUR-PHASE OXIDATION: although benzene strongly resists ordinary oxidising agents, its vapour, mixed with oxygen and passed over V2O5 at 773 K, undergoes ring-opening oxidation to maleic anhydride (…