ORGANIC CHEMISTRY: INTRODUCTION - PETROLEUM Petroleum, which was formed over millions of years by the compaction of dead organisms and the anaerobic decomposition of their biological molecules, is usually found trapped beneath impermeable cap rock and above a lower dome of sedimentary rock. This oil, a non-renewable 'fossil fuel', is a mixture of solid, liquid, and gaseous hydrocarbons (mostly alkanes). In refineries, crude oil is fractionally distilled - a physical process which separates the various components on the basis of their different boiling points:
Some of these hydrocarbon fractions are used directly; e.g., as fuels (natural gas), or to manufacture petrol (gasoline), or to surface roads (tar). But, as this double bar-graph exemplifies, the demand for these fractions rarely matches their relative amounts in typical crude oil.
So, to overcome this economic problem, industry takes certain fractions and subjects them to catalytic cracking - a chemical process whereby large molecules are broken down by heat and by catalysis into smaller (and more useful) molecules; e.g.,
Mixtures of cracked products are separated by fractional distillation, and the fractions used as fuels, solvents, or synthetic intermediates.
ORGANIC CHEMISTRY: INTRODUCTION - REACTION TYPES The biological and industrial importance of organic compounds, together with their structural diversity, has resulted in the development of hundreds of synthetic methods. Happily, most of these can be classified into just eight reaction types: Addition (A), Condensation (C), Elimination (E), Hydrolysis (H), Isomerization (I), Oxidation (O), Reduction (R), and Substitution (S).
Two points are worth keeping in mind when considering the exemplars of each reaction type shown above. First, more than one description can be in common use for a given type; e.g., what is, formally, the addition of hydrogen to ethene will also be described as the reduction or the hydrogenation or the saturation of ethene. And second, each type can be usefully viewed as one of a 'pair', as exemplified explicitly above by addition-elimination and by condensation-hydrolysis.
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