Structural Representation of Organic Compounds

Because the source textbook prints this section's own heading as '11.2 Structural representation of organic compounds' -- reusing the number already assigned to the Classification section earlier in the chapter -- it has been renumbered 11.4 here, in its true reading position between Nomenclature and Isomerism, so that every section in this chapter carries one unique, sortable number; the content itself is unchanged.

A molecular formula (say, C3H8O) is the least informative way to describe a compound -- it fixes only the ratio of atoms present, saying nothing about how they are connected. Chemists build up from there through an increasingly compact hierarchy of structural representations. A Lewis (dot) structure shows every atom and every bonding/lone electron pair explicitly. Replacing each shared electron pair with a dash gives the dash (line-bond) structure: a single line is one sigma bond, a double line is a double bond (one sigma, one pi), a triple line is a triple bond (one sigma, two pi), and lone pairs on heteroatoms may or may not be drawn in. Abbreviating that dash structure further -- dropping some or all of the bond lines and using a subscript to show how many identical groups sit on one atom -- gives the condensed structural formula (CH3-CH2-CH2-OH for n-propanol). The most compact representation of all is the bond-line (skeletal, zig-zag) structure, used almost universally by working organic chemists: carbon and hydrogen atoms are not written at all, carbon-carbon bonds are drawn as a zig-zag of plain lines, and only heteroatoms (O, N, Cl, and so on …