Environmental Engineering Reference
In-Depth Information
APPENDIX A. BASIC TERMINOLOGY AND CONCEPTS IN
HYDROCARBON CHEMISTRY
For our purposes, we can think of molecular structure and chemical reactions as being all about
putting a stable number of electrons in atomic orbitals. Each orbital can accommodate a discrete
number of electrons; in successive rings, that number is 2, 6, 10, and 14. Hydrogen has a single
proton in its nucleus and one electron in the first orbital, so that hydrogen can share one electron
and one vacancy with another atom. Carbon has vacancies for four electrons in its outer shell, so
it can share electrons with up to four hydrogen atoms. With a series of single bonds (each set of
atoms share only two electrons and two vacancies total), chains of hydrogen and carbon can also
be built up to form stable molecules. The chemical formula for these molecules can be written
as C n H n + 2 where n is the number of carbon atoms present in the molecule. The extra hydrogen
atoms are needed to stabilize the carbon atoms at the end of a chain, as shown in Figures A.1a
and b. As the number of carbons in the molecule increase, the straight-chain hydrocarbons have
higher viscosities, boiling points, and lubricating indices. However, the molecule does not have
to follow this straight-chained structure. The molecule in Figure A.1c is an isomer of butane, i.e.,
it has the same chemical formula, but it has a different configuration, in this case a branched
structure. Hexane has a ladder-like structure with 6 carbon and 14 hydrogen atoms. Cyclohexane,
shown in Figure A.1d, is an isomer of hexane. For carbon, 6 is the magic number through which
the atoms can form a stable ring. All of the molecules fromA.1a throughA.1d are called saturated
hydrocarbons, meaning that each carbon atom still shares a single electron with one neighboring
carbon and hydrogen atoms. Such molecules consisting of single bonds are called alkanes (also
paraffins, particularly in petroleum engineering). Alkanes with a cyclic structure (Fig. A.1d) are
also called cycloalkanes or napthenes.
Alkanes react with oxygen to produce carbon dioxide (CO 2 ), water (H 2 O) and energy in a
chemical reaction of the form:
C n H 2 n + 2 + (3 n + 1) / 2O 2 n CO 2 + ( n + 1)H 2 O + energy
Note that this is the net reaction; the actual chemical reaction will almost always take a series
of steps in which intermediate species are created and consumed. Hydrocarbons with double or
triple covalent bonds between adjacent carbon atoms are termed “unsaturated”. An alkene (also
olefin) is an unsaturated hydrocarbon with one or more carbon-to-carbon double bonds. The
simplest non-cyclic alkenes have only one double bond and can be described by the chemical
formula C n H 2 n . such as ethylene, shown in Figure A.2a. There are also ring-shaped alkenes, such
as benzene (Fig. A.2b). Aromatics are very stable molecules that contain one or more benzene
rings; many of them smell good, hence the name. Their properties are sufficiently different so
that they are considered as a separate class from alkenes.
Figure A.1. Molecular structure of alkanes include straight chain hydrocarbons such as (a) methane;
(b) butane; Branched alkanes such as (c) isobutane; and Cyclic hydrocarbons such as
(d) cyclohexane.
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