SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 160

The same is true of a large and important class of
unsaturated organic compounds based on the benzene
molecule C 6 H 6 :

Box 9.4 Notation for organic molecules

the complexity of most organic molecules necessitates
a shorthand notation. hydrocarbon molecules are often
abbreviated to 'matchstick' structures showing just the
bonds between carbon atoms: at each angle in the
symbol the appropriate Ch x group is implied.

H

H H

C

C C

C

C

C

H

H H

HH

C

H C H

Condensed notation (Box 9.4)

C

C H becomes

C

Compounds based on the benzene ring, like styrene
below (from which polystyrene is manufactured), are
called aromatic compounds, as many possess distinc-
tive odours.

H H H H H

the cyclic molecule of benzene is written as a hexagon
incorporating three double bonds. this can be written
in two alternative ways:

H

H

H

H C C

C

C C

H

C

C

C

H H

the molecule actually adopts both configurations at
once: as with graphite (Box 7.4), the π -electron density
is delocalized around the entire ring. this is often sym-
bolized as:

Condensed notation (Box 9.4)

H

Styrene

Open-chain (acyclic) and aromatic hydrocarbons are
the main, and most useful, constituents of petroleum
(Killops and Killops, 2005).

Carbohydrates
Carbohydrates are a class of organic compounds in
which C, H and O combine in the proportion C x (H 2 O) y ,
where x and y are integers. Sugars such as glucose
(C 6 H 12 O 6 ):

and ethyne (acetylene), C 2 H 2 :

HCCH - ≡ -

H H H OH H H

Hydrocarbons like ethyne that incorporate a triple
C ≡ C bond are known collectively as alkynes .
When forming double bonds (as we discovered
with graphite - Box 7.4), carbon uses the sp 2 - hybrid ,
which has 120° 'planar trigonal' geometry. All of the
atoms in the ethene molecule therefore lie in the
same plane. 2

O

H C C C CCC

OH OH OH H OH

are the simplest carbohydrates. (Like alkanes, these are
zigzag molecules in 3D.) Sugars consist of chains of a
few of these C 6 units: sucrose, for example, is C 12 H 22 O 11 .
Starches consist of longer chains. The longest-chain
carbohydrates are celluloses (with x ~ 10,000). Cellulose
is familiar as the main constituent of paper. Wood

In the triple-bonded C 2 H 2 molecule, on the other hand, carbon
uses the linear sp 1 -hybrid .

2