Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 226

Figure A.2. Molecular structure of more complex hydrocarbons, including alkenes such as (a) ethylene;
and cyclic alkenes such as (b) benzene. Replacing a hydrogen molecule with a hydroxyl group
(OH − ) yields alcohols such as (c) ethanol, which can also be written in alternate notation (d).
Other compounds of interest include (e) glycerol and esters, such as (f) a carboxylic acid ester.

Alcohols, such as ethanol in Figure A.2c, have similar structures, but in this case one or more
hydrogens are replaced by hydroxyl (OH − ), which is a functional group that results in a net
molecular negative charge. Functional groups are atomic subunits within a molecule that give
rise to chemical reactions. Higher carbon alcohols include ethanol, a straight-chain molecule with
a terminal hydroxyl functional unit (Figs. A.2c,d). As the complexity of the molecule increases,
notation becomes more challenging and a sort of shorthand is used. Some simplifications are
shown that represent ethanol in Figure A.2d. Carbon atoms and their supporting hydrogens are
represented by kinks and the empty terminus in the version on top. On the bottom, R is used
to designate a representative alkyl, in this case the ethyl group C 3 H 5 , while the other atoms
are explicitly shown. Glycerol (Fig. A.2e) is a reaction product from the refining process and
mimics half of a sugar molecule. Esters are present in vegetable oils and animal fats, and they
may also appear as reaction products. They come from from the reaction of an alcohol with an
acid. Carboxylic acid esters (Fig. A.2f) are based on a carbon atom and have the general form
R COO R , where R is an alkyl group such as C 3 H 5 and R is an aryl group. A moiety is a subunit
within a molecule that may itself include functional groups within it. There may be several ways
of parsing a molecule into its constituent moieties.
Other molecules that use hydrogen, carbon and oxygen atoms but in more complicated con-
figurations include lipids, fatty acids, sugars and carbohydrates (a/k/a saccharides) with formula
formC m (H 2 O) n . Smaller carbohydrates (mono- and disaccharides) are sugars. Polysaccharides are
polymeric carabohydrates, i.e., they are comprised of simpler units, such as a mono- or disaccha-
ride, in a repeating chain. Fatty acid methyl esters (FAME) are key components of vegetable oils
and animal fats. Free fatty acids are carboxylic acids with a long unbranched hydrocarbon chain
attached to it. The hydrocarbons may be saturated without any double carbon-to-carbon bonds;
monounsaturated with a single double bond; or polyunsaturated with multiple double bonds in
the hydrocarbon chain. Glycerol, shown in Figure A.2e, has three ends at which it may couple
with fatty acids. When the coupling occurs, the molecules are termed “glycerides”. A glycerol
with one fatty acid is a monoglyceride; likewise a glycerol with two fatty acids is a diglyceride.
A triglyceride (also called triacylglycerol or triacylglyceride or TAG) is comprised of three fatty
acids joined to glycerol subunit. TAGs are lipids that some algae use as its main carbon storage
molecule. These compounds will be important when we consider biorefining in section 11.5.
More nomenclature: C x : y is the lipid number, in which x is the number of carbon atoms in the
fatty acid and y is the number of double bonds in the fatty acid. C x :0 is a saturated compound.
C18:3 denotes an acid with 18 carbon atoms and 3 double bonds, but there are several molecules
that fit this description. To be precise, it is appended with notation that indicates where the
unsaturated bond is located relative to the carboxylic acid end, n

x or x (where the
x th carbon molecule has a double bond with the ( x + 1)th carbon molecule), depending on the
field of study. Palmitic acid is C16:0; stearitic acid is C18:0; oleic acid is C18:1; linoleic acid
(a/k/a ω -6 fatty acid) is C18:2; α -linoleic acid (a/k/a ω -3 fatty acid) is C18:3. Other examples
and discussion may be found in section 11.3.1.

− x , −