Your goal should be to understand, not memorize organic chemistry. The following rules should be learned at the beginning of this semester. Please come back here and review these rules a few times during the semester. Understanding these rules of thumb is the starting point to developing your chemical intuition.
1. Atoms prefer filled valence shells. This rule not only explains why Li is positive and chlorine has a negative charge, but more importantly explains why atoms make bonds, and the type of bonds they make. A corollary is that centers of electron density (bonds and lone pairs of electrons) repel each other so they stay as far apart as possible. This latter rule, the basis for the so-called VSEPR model, explains the 3-dimensional (3D) molecular structure.
2. The most important question in chemistry is "Where are the electrons?" The answer is that electrons are generally in higher amounts around the more electronegative atoms (e.g. F, Cl, O, N). The electronegative atoms pull or suck the electron density away from the less electronegative atoms (e.g. C, H) to which they are bonded. Thus, understanding electronegativities provides a simple method of deciding which portions of a molecule have a relatively high electron density, and which portions have a relatively low electron density. You should also think about where are the electrons in terms of orbitals (i.e. are they in spn hybrid orbitals, HOMO or LUMO etc.)
3. Nature hates unpaired electrons so you should avoid them too. If a molecule must have an unpaired electron (a.k.a. radical), it is better to have the unpaired electron distributed over as many atoms as possible through resonance, inductive effects, and hyperconjugation.
4. Nature hates localized charges. If a molecule must have a charge, it is better to have the charge distributed over as many atoms as possible through resonance, inductive effects, and hyperconjugation. In addition, when given the choice, it is better to have a more negative charge on a more electronegative atom (e.g. O), and a more positive charge on a less electronegative atom (e.g. C).
5. Most reactions involve nucleophiles (molecules with a location of particularly high electron density) attacking electrophiles (molecules with a location of particularly low electron density). Thus, simply understanding where electrons are provides you with the best way of analyzing new molecules so that you will be able to PREDICT how they will react. When in doubt, transfer a proton!
6. Steric interactions (atoms bumping and banging into each other) control the structures of molecules but more importantly can prevent reactions by keeping the reactive atoms away from each other.
7. π electrons prefer to be delocalized over as many adjacent sp2 hybridized atoms (or sp hybridized atoms in some cases) as possible, and aromaticity is the most stable form of π electron delocalization. Pi electrons cannot delocalize onto or through sp3 hybridized atoms since an sp3 atom has no 2p orbital available. This delocalization always results in a stabilizing effect. For example, we will learn about the stability associated with conjugated dienes and aromatics.
1. In mechanisms, proton transfers are generally the fastest possible reaction, so this usually happens before other possible processes such as nucleophilic attack. The exception is the deprotonation of non-acidic carbon atoms such as in alkanes, these can be slow.
2. If a five or six-membered ring can be formed, intramolecular reaction will predominate if a molecule has two functional groups that can react with each other. Other rings can sometimes be formed, but when five or six-membered rings are not possible, intermolecular reactions become important competing reactions.