Structural formulas of organic molecules

Organic chemists have devised a number of shortcuts to speed the writing of structural formulas. Sometimes we leave out unshared electron pairs, but only when we are sure enough in our ability to count electrons to know when they are present and when they re not. We ve already mentioned representing covalent bonds by dashes. In condensed structural formulas we leave out some, many, or all of the covalent bonds and use subscripts to indicate the number of identical groups attached to a particular atom. These successive levels of simplification are illustrated as shown for isopropyl alcohol ( rubbing alcohol ). 

PROBLEM 1.12 Expand the following condensed formulas so as to show all the bonds and unshared electron pairs. 

SAMPLE SOLUTION (a) The molecule contains two carbon atoms, which are bonded to each other. Both carbons bear two hydrogens. One carbon bears the group HO— the other is attached to —NH2. 

When writing the constitution of a molecule, it is not necessary to concern yourself with the spatial orientation of the atoms. There are many other correct ways to represent the constitution shown. What is important is to show the sequence OCCN (or its equivalent NCCO) and to have the correct number of hydrogens present on each atom. 

The molecular formula and the connectivity are determined experimentally and are included among the information given in the statement of the problem. 

The method described for calculating formal charge has been one of reasoning through a series of logical steps. It can be reduced to the following equation  

Formal charge = Group number in periodic table - Electron count 

Electron count = Number of shared electrons) + Number of unshared electrons 

Formal charges are based on Lewis formulas in which electrons are considered to be shared equally between covalently bonded atoms. Actually, polarization of N—H bonds in ammonium ion and of B—H bonds in borohydride leads to some transfer of positive and negative charge, respectively, to the hydrogens. 

Calculate the formal charge on each nitrc n in the following Lewis formula (azide ion) and the 

Most organic compounds are more complicated than the examples we ve seen so far and require a more systematic approach to writing structural formulas for them. The approach outlined in Table 1.5 begins (step 1) with the molecular formula that tells us which atoms and how many of each are present in the compound. From the molecular formula we calculate the number of valence electrons (step 2). 

TABLE 1.5 A Systematic Approach to Writing Lewis Structures 

Based on the molecular formula, count the number of valence electrons. 

Given the connectivity, connect bonded atoms by a shared electron pair bond ( ) represented by a dash (—). 

Ethanol and dimethyl ether both have the molecular formula C2H6O. 

---

You should be able to deduce structural formulae of organic molecules given data obtained from ... 

---