Decane, C10H22, obtained from the fractional distillation of crude oil, is in lower demand than the shorter, more volatile hydrocarbons used as fuels. It is converted into more useful products by cracking. (a) State the two general types of cracking, and give typical conditions…

Answer: ((a)) Thermal cracking: very high temperature (roughly 700-1200°C) and high pressure, no catalyst, via a free-radical mechanism, giving a higher proportion of alkenes. Catalytic cracking: a zeolite catalyst, a lower temperature (roughly 450°C) and only a slight pressure, via a carboca… ((b)) C10H22 → C8H18 + C2H4 ((c)) Homolytic fission of a C-C bond, forming two free radicals that react further to give a stable alkane and alkene; cracking is important because fractional distillation alone produces a surplus of longer-chain fractions (e.g. heavy fuel oil) and a shortage of shorter-chain fracti…. Part (a): Types of cracking and their conditions There are two general types of cracking: - Thermal cracking: very high temperature (roughly 700–1200°C) and high pressure, with no catalyst; it proceeds via a free-radical mechanism and tend…

AS Chemistry 9701: Thermal and Catalytic Cracking of Decane

Part (a): Types of cracking and their conditions

There are two general types of cracking:

Thermal cracking: very high temperature (roughly $700$ – $1200°C$ ) and high pressure, with no catalyst; it proceeds via a free-radical mechanism and tends to produce a higher proportion of alkenes (useful as feedstock for the polymer industry).
Catalytic cracking: a lower temperature (roughly $450°C$ ) and only a slight pressure, using a zeolite catalyst; it proceeds via a carbocation mechanism and tends to produce branched-chain and cyclic/aromatic hydrocarbons useful as high-octane petrol.

Part (b): The cracking equation

One possible equation for the thermal cracking of decane into octane and a small alkene is:

$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$

Check (atom balance): C: $8+2=10$ on the right, matching $10$ on the left. H: $18+4=22$ on the right, matching $22$ on the left. Both balance.

(Many other splits of decane into a shorter alkane plus an alkene, or into two smaller alkenes plus hydrogen, are also chemically valid. This is just one example.)

Part (c): Bond fission and industrial importance

A C-C bond breaks by homolytic fission during thermal cracking, each carbon atom keeping one electron from the shared pair to form two free radicals; these radicals then react further (for example, by a hydrogen atom transferring from one radical to another) to give a stable, closed-shell alkane and alkene as the final products.

Cracking is industrially important because fractional distillation of crude oil alone produces a surplus of longer-chain, less volatile fractions (such as heavy fuel oil and bitumen) and a shortage of shorter-chain fractions that are in high demand (such as petrol, and small alkenes needed as feedstock for the plastics industry). Cracking converts some of the excess long-chain alkanes into a mixture of shorter alkanes and alkenes, better matching the supply of hydrocarbon products to market demand.

Final answers

(a) Thermal cracking (very high temperature/pressure, free-radical mechanism, more alkenes); catalytic cracking (zeolite catalyst, lower temperature, carbocation mechanism, more branched/cyclic products).
(b) $\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$ (one valid example).
(c) Homolytic fission of a C-C bond; cracking converts a surplus of long-chain alkanes into more useful, higher-demand shorter alkanes and alkenes.