Chemistry · Ch 15 — Hydrocarbons
Chemical properties of alkanes
Chemical properties of alkanes
Alkanes are relatively unreactive toward acids, bases, and most common oxidising or reducing agents, but they do undergo four characteristic reactions under specified, fairly forcing conditions. (1) Halogenation is a substitution reaction: a hydrogen atom (or several) is replaced by a halogen atom X (X = F, Cl, Br or I) when the alkane is exposed to the halogen under UV light, diffused sunlight, or heat (573-773 K), giving a mixture of alkyl halides. The reactivity of the halogens themselves toward alkanes follows F2 > Cl2 > Br2 > I2, while the ease with which a hydrogen atom on the alkane is actually replaced follows the opposite-looking but complementary order, tertiary > secondary > primary carbon -- so even though a molecule may have far more primary hydrogens than tertiary ones, the tertiary position often still contributes a disproportionately large share of the product mixture, as seen in the bromination of 2-methylpropane giving over 99% of the tertiary bromide. This selectivity, together with the overall substitution outcome, follows from a free-radical chain mechanism: initiation is the UV-driven homolysis of the halogen molecule into two halogen radicals; propagation is a carbon radical (formed when a halogen radical abstracts a hydrogen from the alkane) reacting with another halogen molecule to form the alkyl halide product plus a fresh halogen radical that carries the chain forward. (2) Combustion is complete oxidation by dioxygen on heating, giving carbon dioxide and water and releasing a large amount of heat -- exactly why alkanes are used as fuels; methane's heat of combustion is -890 kJ/mol and butane's is -2875.84 kJ/mol, and the same pattern generalises to any alkane CnH2n+2 reacting with (3n+1)/2 O2 to give n CO2 and (n+1) H2O plus heat. (3) Pyrolysis (also called cracking) is thermal decomposition in the absence of air: heating an alkane strongly enough breaks its C-C and C-H bonds statistically at various points, yielding a mixture of smaller alkanes, alkenes and hydrogen gas -- hexane at 773 K, for instance, can crack several different ways depending on exactly which bonds break. (4) Reforming (aromatization) converts a straight-chain alkane with 6 to 10 carbons …
Worked out. Halogenation replaces alkane hydrogens by X (Cl, Br, I or F) under UV light, diffused sunlight, or 573-773 K heat, giving a mixture of alkyl halides; reactivity of the halogens follows F2 > Cl2 > Br2 > I2, and ease of replacing a hydrogen follows tertiary > secondary > primary. The reaction is a free-radical chain process: initiation is homolysis of Cl-Cl by UV light into two chlorine radicals; propagation is a methyl radical (from CH4 losing H to a Cl radical, forming HCl) reacting with Cl2 to give CH3-Cl plus a fresh Cl radical, which keeps the chain going. Successive chlorination of methane gives chloromethane, then dichloromethane, then trichloromethane, then tetrachloromethane; excess chlorine favours the fully substituted product while excess methane favours chloromethane. Because the tertiary C-H bond is weakest, tertiary hydrogens are replaced fastest even though there are fewer of them: chlorinating propane gives about 45% 1-chloropropane and 55% 2-chloropropane, while chlorinating 2-methylpropane gives about 36% 1-chloro-2-methylpropane and 64% 2-chloro-2-methylpropane. Bromination shows even sharper selectivity for the more substituted position: br …
Worked out. Combustion: methane burns as CH4 + 2O2 -> CO2 + 2H2O with heat of combustion -890 kJ/mol; butane burns as C4H10 + 13/2 O2 -> 4CO2 + 5H2O with heat of combustion -2875.84 kJ/mol; the general equation for any alkane CnH2n+2 is CnH2n+2 + (3n+1)/2 O2 -> n CO2 + (n+1) H2O + heat. Pyrolysis (cracking): heating an alkane strongly in the absence of air breaks C-C and C-H bonds to give a mixture of smaller alkanes, alkenes and hydrogen; for example hexane (C6H14) at 773 K can crack to C6H12 + H2, or to C4H8 + C2H6, or all the way to C3H6 + C2H4 + CH4, depending on which bonds break. Reforming (aromatization): a straight-chain alkane with 6 to 10 carbons, passed over a V2O5/Cr2O3/Mo2O3-type catalyst supported on alumina at about 773 K and 10-20 atm, undergoes simultaneous dehydrogenation and cyclisation to an aromatic ring; n-hexane converts this way to benzene plus 4 H2, and t …