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10 Introduction to Organic Chemistry
Organic chemistry comprises the chemistry of carbon compounds. Carbon atoms exhibit a unique
property called catenation, which means that they can form covalent bonds with other carbon atoms.
There exist a variety of carbon chains of different lengths (simple, branched, cyclic), to which hydro-
gen atoms are nearly always bonded (hydrocarbons); besides carbon and hydrogen, the compounds in
human body include atoms of oxygen, nitrogen, sulfur, phosphorus, and a few others. Carbon com-
pounds can be classified according to the carbon skeleton or the groups attached to it. According to the
carbon skeleton, there are three main classes:
Acyclic compounds: aliphatic, open chain structures either straight-chain or branched, both saturated
and unsaturated (i.e. having double or triple bonds), e.g. ethane, ethanol, butyric acid.
Carbocyclic compounds: closed rings of carbon atoms that may have branches attached to them and
may contain multiple bonds, they are divided into two subgroups - alicyclic, and aromatic compounds
(i.e., those with a ring stabilized by the fully conjugated system of double bonds) e.g. cyclohexane,
benzene, naphthalene.
Heterocyclic compounds: at least one atom in the ring is a heteroatom, an atom that is not carbon,
e.g. pyridine, pyrimidine, pyrrole.
Classification according to the functional groups: hydrocarbons, halogen derivatives, compounds with
oxygen group (alcohols, phenols, aldehydes, ketones, carboxylic acid), sulfur (thiols, sulfides), nitro-
gen (e.g. amines, amides, nitro compounds), and various combinations.
Common properties of organic compounds
Most of the bonds in organic compounds are covalent. Consequently, organic compounds are mostly
typical covalent compounds exhibiting various degrees of polarity; only some of them are ionic (salts
of organic acids and bases). The polarity of a covalent bond depends on the difference in electro-
negativities of the two bonded atoms. Typical non-polar bonds are C–C and C–H, hydrocarbons do not
exhibit any substantial dipole moment. With the increasing difference in electronegativities of bonded
atoms, the polarity of a bond increases and the atoms acquire partial electric charges ( +, -).
For example, the molecule of ethanol CH3-CH2-OH is polar because the C–O bond is qu ite polar and
the O–H bond is even more polar than the C–H bonds.
Note that a polar bond is a prerequisite for the polarity of a molecule, but not all molecules with polar
bonds must necessarily be polar. Dipole moments of individual bonds are vectors. When more polar
bonds are present in a molecule, the resulting dipole moment of the molecule depends on the spatial
arrangement of polar bonds. In some symmetrical molecules, internal cancelling of partial dipoles
results in an overall absence of polarity. For example, tetrachloromethane CCl4 is non-polar because of
the symmetrical tetrahedral geometry of four polar C–Cl bonds.
