Open-Chain and Ring Compounds

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In organic chemistry, there are a few basic structural shapes that you will encounter. They are chains and rings.
There are also two types of chains, a straight chain, and a branched chain. In a straight chain, one carbon atom holds no more than two other carbon atoms. As its name implies, the straight chain is a straight link of carbon, sometimes oxygen or nitrogen, atoms, in structural formula that is. Because of twisting and contouring, they chain may have several conformations
Branched chains have at least one carbon holding more than two other carbon atoms. It will, as its name implies, have branches of other chains coming off another chain. Branching is one of the reasons why there are so many isomers for each compound. 
  H H H H H H
  | | | | | |
H-C-C-C-C-C-C-H
  | | | | | |
  H H H H H H
Straight Chain
 
              H
              |
    H   H H H-C-H H H H
    |   | |   |   | | |
H---C---C-C---C---C-C-C-H
    |   | |   |   | | |
  H-C-H H H   H   H H H
    |
  H-C-H
    |
    H
Branched Chain
Rings (or cyclic compounds) are composed of rings of carbon and sometimes oxygen or nitrogen. For example, cyclohexane has a ring of six carbon atoms.
cyclohexane
cyclohexane (fully condensed)
In the fully condensed version of the structural formula of cyclohexane, or any cyclic compound for that matter, the conventions for drawing them are:
  1. A C occurs at every corner unless O or N (or another atom) is explicitly written at the corner.
  2. A line connecting two corners is a covalent bond between adjacent ring atoms.
  3. Remaining bonds are understood to hold H atoms (C normally has 4 total bonds).
  4. Double bonds are always explicitly shown.
To illustrate the rules, here are some types of cyclic compounds:
cyclopropane
cyclobutane
cyclopentane
cyclohexane
cyclohexene
bromocyclohexane
There is no theoretical upper limit to the size of the ring.
Heterocyclic compounds are those which have an atom other than carbon in the ring. Examples are:
pyrrole
piperidine
tetrahydropyran
The atom that is not carbon is called the heteroatom. The heteroatom's bonds must be shown or clearly understood. For example, nitrogen normally forms three bonds, so three bonds must be indicated for it. Oxygen normally forms two, so two bonds must be shown. Rings may have more than one heteroatom.
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