Representing Organic Compounds: Structural Formulas.
Because of the complexity of the structure of many organic compounds, as well as the
importance of structure in organic chemistry, the development of shorthand methods for
representing the structures of organic compounds has been almost mandatory in order to allow
the drawing of structures in a reasonable amount of space and time. There are several different
ways of representing organic compounds: we will illustrate each by using tributylcarbinol
(Figure 1.8) as an example.
[CH3 (CH2 )3 ]3 C–OH
H
H H H H
O
H H H H
H C C C C
C
C C C C H
or
(CH3 CH2 CH2 CH2 )3 C–OH
H H H H H C H H H H H
or
H C H
OH
H C H
H C H
CH3CH2CH2CH2
C CH2CH2CH2CH3
H
CH2CH2CH2CH3
(a)
(b)
OH
C C C C C C C C C
OH
C
C
C
C
(c)
(d)
Figure 1 The different representations of tributylcarbinol found in organic chemistry are the
complete structural formula (a), the condensed structural formula (b), the connectivity formula
(c), and the line formula (d). Of these, the line formula has found the most widespread use in
modern organic chemistry.
The molecular formula of this compound is C13H28O, and its full structure is shown in Figure
1 as (a). This particular representation, in which every atom of the molecule is explicitly drawn,
is often referred to as a complete structural formula. Even in a molecule as small as
tributylcarbinol, however, it takes considerable space to draw this structure. The first
simplification, therefore, is to condense the various groups of atoms about each carbon to a
simple molecular formula for each carbon atom. This gives rise to formula (b) in Figure 1,
which is usually referred to as a condensed structural formula, or simply a condensed formula.
Such formulas are most frequently encountered in printed works because they are very amenable
to typesetting, although they convey little or no information about molecular shapes. An
alternative simplification of formula (a) can be made by omitting the hydrogen atoms. While
such formulas (c) are very good for showing connectivity, they are also very poor at showing
molecular shape. By omitting the symbols for the carbon atoms, one can simplify the formula
(c) even further to give (d), which is known as a line formula, or, if a ring of atoms is involved, a
polygon formula.
In a line formula, the end of each line, as well as each vertex, is occupied by a carbon atom
bearing sufficient hydrogen atoms to complete its octet (unless it is charged). Where less than
four lines meet at a vertex, the remainder of the four valencies of the carbon atom are assumed to
be occupied by hydrogen atoms. Atoms which are neither carbon nor hydrogen are called
heteroatoms, and all heteroatoms are explicitly shown in line formulas. In general, only nonhydrogen and non-carbon atoms are explicitly indicated on a line formula, although hydrogen
atoms bonded to heteroatoms are frequently shown, as illustrated in Figure 1(d).
Early in the nineteenth century, Jöns Jakob Berzelius, the great Swedish chemist, proposed a
theory of compound formation which was essentially ionic in nature. While this theory worked
well for inorganic compounds, its extension to organic chemistry has not been as successful.
Nevertheless, Berzelius did coin the term radical (or radicle, from the Latin radix, a root), a term
which was used during the nineteenth century for the part of a compound which remains
unchanged during the course of a reaction. One of the first to criticize this idea was the French
chemist Jean-Baptiste Dumas, who proposed instead that organic compounds can be viewed as
belonging to families. His "type" theory of organic compounds first appeared in 1826, and is
remarkable modern in some of its concepts.
The term "radical" still remains part of the organic chemist's vocabulary today, although it is
now used in a different sense than originally intended. The original meaning can still be traced,
however, to the use of the generic symbol R to designate the unchanging part of an organic
compound undergoing a reaction.
The part of the molecule which undergoes a change during the course of a reaction is called
the functional group. A functional group is any part of a molecule which is not a carbonhydrogen or carbon-carbon single bond. In other words, all multiple bonds and all bonds
involving heteroatoms (atoms other than carbon or hydrogen) are functional groups. Practically
all of the chemistry of organic compounds occurs at functional groups because the electrons in
the carbon-carbon and carbon-hydrogen single bonds of the carbon skeleton are seldom directly
involved in reactions – they are tightly held between the two nuclei, and they are seldom
accessible to external reagents or respond to their influences. In contrast to this, the electrons of
the functional groups are usually fairly accessible to an outside reagent. Indeed, it is because of
this that organic chemistry is a manageable study at all – because the number of functional
groups is limited, the possible reaction types known today is limited, and can be systematized.
Several of the most commonly occurring functional groups are given in Table 1.
We have just seen that heteroatoms are the only atoms explicitly shown in line formula
representations of molecules, and that carbon-hydrogen bonds are not shown at all. Thus, by
using line formulas to represent organic molecules, one immediately focuses the reader's
attention on the functional groups. It is this effect of drawing the reader's attention to the most
important parts of the molecule immediately on seeing the structure that is one of the strengths of
the line formula representation of organic compounds.
Table 1.5 Some Common Functional Groups
Functional
Group
R
alkane
H
R
Compound
Type
R
C
alkene
C
R
R
Functional
Group
Compound
Type
R
C
O
aldehyde
C
O
ketone
O
acid chloride
C
O
carboxylic acid
C
O
ester
C
O
amide
C
N
nitrile
H
R
aromatic ring
R
R
R
C
R
X
alkyl halide
OH
alcohol
R
R
O
R
alkyne
C
Cl
R
HO
ether
R
C
R
R
amine
N
R
R
R
NO2
R
nitro compound
O
R2N
R
R
R
RO
epoxide
R
R
Sample Problem 1.1 What is the molecular formula of each of the following compounds?
(a)
Answers: (a) C10H18.
(b)
(b) C4H8O.
O
(c)
O
(c) C10H14O.
Sample Problem 1.2. Draw the line or polygon representations of each of the following
compounds:
(a) (CH2)4. (b) (CH3)3C(CH2)3CH(CH3)2. (c) HOCH2C(CH3)2[CH(CH2)2].
Answers:
(a)
(b)
(c)
HO
Sample Problem 1.3. What is the condensed formula of each of the following compounds?
F
OH
(a)
(b)
OH
F
F
CH2OH
F
F
Answers: (a) (CH3CH2)2CHCH(CH3)C(OH)(CH3)CH2C(OH)(CH3)CH2CH3.
(b) [(CF2)2CF]CH2OH.
Problem 1.1. What is the molecular formula of each of the following compounds?
(a)
(b)
(c)
N
H
O
Problem 1.8. Draw line or polygon representations of each of the following compounds.
(a) (CH3)2C=CHCH2CH2C(CH3)=CHCH2OH.
(c) [(CH2)3N]CH2CH3.
(b) [(CH2)5CH]OH.
Problem 1.9. What is the condensed formula of each of the following compounds?
(a)
Br
(b)
(c)
HO