STRUCTURAL, CONDENSED, AND LINE FORMULAS

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

CHAP. 18]

ORGANIC CHEMISTRY

formulas or graphical formulas. The structural formulas for compounds (a) to (e) described in Example 18.1 may be written as follows:

(a) H C H (b) H C O H (c) H C N H

(d ) H C O

(e) F C C F

As the number of carbon atoms per molecule increases, the structures are written with significantly less effort using condensed formulas rather than structural formulas. Condensed formulas are like structural formulas except that in a condensed formula, the hydrogen atoms are written to the right of the larger atom to which they are attached. (Since it can form only one bond, a hydrogen atom cannot be attached to another hydrogen atom and still be part of the organic molecule.) For example, a molecule with a carbon atom connected (1) to

a nitrogen atom bonded to two hydrogen atoms and also (2) to three other carbon atoms, each bonded to three hydrogen atoms, may be represented as

CH 3

CH 3 C NH 2

CH 3

To see the efficiency of this representation, write out both this condensed formula and the structural formula for this compound and compare the effort that it takes.

For even greater convenience in representing the structures of organic compounds, particularly in printed material, line formulas are used, so-called because they are printed on one line. In line formulas, the symbol for each carbon atom is written on a line adjacent to the symbols for the other elements to which it is bonded. Line formulas show the general sequence in which the carbon atoms are attached, but to interpret them properly, the permitted total bond orders for all the respective atoms must be kept in mind. Again referring to compounds (a) to (e) described in Example 18.1, the line formulas are as follows:

( e) CF CF or FC CF If the permitted total bond orders of the respective atoms are remembered, it is apparent that the line formula

( a) CH 4 ( b) CH 3 OH

( c) CH 3 NH 2 ( d) CH 2 O

CH 4 cannot represent such structures as

H C H H (incorrect)

which has a total order of 2 for one of the hydrogen atoms and a total bond order of only 3 for the carbon atom. Similarly, the formula CH 2 O cannot represent the structure H C O H or H C O H, because in either of these cases, the total bond order of the carbon atom is less than 4 and in H C O H the total bond order of the oxygen is greater than 2. Accordingly, line formulas must be interpreted in terms of the permitted total bond orders.

EXAMPLE 18.2. Write structural and condensed formulas for the molecules represented by the following formulas:

(a) C 2 H 4 and (b) CH 3 COCH 3 .

Ans. (a) Since the hydrogen atoms can have only a total bond order of 1, the two carbon atoms must be linked together. In order for each carbon atom to have a total bond order of 4, the two carbon atoms must be linked to each

ORGANIC CHEMISTRY

[ CHAP. 18

other by a double bond and also be bonded to two hydrogen atoms each.

H C C C H CH 2 CH 2

(b) The line formula CH 3 COCH 3 implies that two of the three carbon atoms each has three hydrogen atoms attached. This permits them to form one additional single bond, to the middle carbon atom. The middle carbon atom, with two single bonds to carbon atoms, must complete its total bond order of 4 with a double bond to the oxygen atom.

H C C C H CH 3 C CH 3