Page 273 - Theory and Problems of BEGINNING CHEMISTRY

262 ORGANIC CHEMISTRY [CHAP. 18

In order to complete its octet, each carbon atom must share a total of four electron pairs. The order of a bond
is the number of electron pairs shared in that bond. The total number of shared pairs is called the total bond
order of an atom. Thus, carbon must have a total bond order of 4 (except in CO). A single bond is a shar-
ing of one pair of electrons; a double bond, two; and a triple bond, three. Therefore, in organic compounds,
each carbon atom forms four single bonds, a double bond and two single bonds, a triple bond and a single bond,
or two double bonds. As shown in the table below, each of these possibilities corresponds to a total bond order of 4.

Number and Types of Bonds Total Bond Order

Four single bonds 4 × 1 = 4
One double bond and two single bonds (1 × 2) + (2 × 1) = 4
One triple bond and one single bond (1 × 3) + (1 × 1) = 4
Two double bonds 2 × 2 = 4
A hydrogen atom has only one electron in its outermost shell, and can accommodate a maximum of two
electrons in its outermost shell. Hence, in any molecule, each hydrogen atom can form only one bond—a single
bond. The oxygen atom, with six electrons in its outermost shell, can complete its octet by forming either two
single bonds or one double bond, for a total bond order of 2 (except in CO). The total bond orders of the other
elements usually found in organic compounds can be deduced in a similar manner. The results are given in
Table 18-1.

Table 18-1 Total Bond Orders in Organic Compounds

Element Symbol Total Bond Order Periodic Group Number

Carbon C 4 4
Nitrogen N 3 5
Phosphorus P 3 5
Oxygen O 2 6
Sulfur S 2 6
Halogen X 1 7
Hydrogen H 1 1

With the information given in Table 18-1, it is possible to write an electron dot structure for many organic
molecules.

EXAMPLE 18.1. Write an electron dot structure for each of the following molecular formulas: (a)CH 4 ,(b)CH 4 O,
(c)CH 5 N, (d)CH 2 O, and (e)C 2 F 2 .
H H H H
Ans. (a) H C H (b) H C O H (c) H C N H (d) H C O (e) F C C F
H H HH
Each atom in this example is characterized by a total bond order corresponding to that listed in Table 18-1.

18.3. STRUCTURAL, CONDENSED, AND LINE FORMULAS
Electron dot formulas are useful for deducing the structures of organic molecules, but it is more convenient
to use simpler representations—structural or graphical formulas—in which a line is used to denote a shared pair
of electrons. Because each pair of electrons shared between two atoms is equivalent to a bond order of 1, each
shared pair can be represented by a line between the symbols of the elements. Unshared electrons on the atoms
are often not shown in this kind of representation. The resulting representations of molecules are called structural