Chemistry is a physical science dedicated to studying the substances of which matter is composed, what properties arise from how they interact, and the changes they undergo. Organic chemistry is the chemistry of compounds that contain carbon. Inorganic chemistry is the study of the properties and reactions of molecules built with the other hundred and something atoms in the periodic table.
Why does carbon get its own sub-dicipline? Originally, it was because organic chemicals were naturally derived from plants or animals and it was believed that they could never be synthesized from inorganic minerals. But that hasn’t been true in a couple hundred years.
So why does this distinction persist? Most atoms either are not inclined to react with other molecules (like noble gases) or only react to form small molecules, like sodium chloride (NaCl) or sulfuric acid H2SO4). Additionally, it is unusual and typically strongly reactive for many of one type of atom to hang out with itself. For example, sodium chloride is common table salt, but both sodium and chlorine by themselves are quite reactive and dangerous. Neither of these things are true for carbon. Carbon plays nicely with itself, as in graphite or diamonds, or with a few other molecules like hydrogen and oxygen. Carbon can also form large molecules, like cellulose, with molecular weights well into the hundreds of thousands, which sets it apart from most inorganic molecules.
As chemists, we believe that structure is important because physical properties of molecules arise from them, or, conversely, what physical properties give us clues as to what structure or structure(s) are consistent with those properties. Organic chemists like to look at polarity and dipole moment because of their effect on solubility, and melting and boiling point.
Why is carbon so friendly? To answer that question, we need to have some concept of an atom as having a nucleus with protons and neutrons and electrons that orbit around that nucleus in a certain path. The atomic orbitals where the electrons are likely to be are described by quantum mechanics. Orbital hybridization, or changes that happen to the shape of likely electron locations when two different atomic orbitals share electrons in some hybrid pattern rather than one original pattern or the other to reach a lower energy state, determine the shapes of the molecules that atoms are likely to make when they get together. Now we are into science that is less than a hundred years old, as It was Louis Pauling who had this insight back in the 1930s. He won the nobel prize for this work in 1954. He realized that methane (CH4) would have 4 single bonds of equal length and strength all separated by 109° if instead of having a full s orbital and 2 half full and one empty p orbital that instead the s and p orbitals shared electrons equally in a configuration now known as sp3 hybridization.
When atoms join together to form molecules, the shared electron cloud surrounds both nuclei in covalent bonds. But the atoms do not always share the electrons equally. If the electron cloud is denser around one atom in a bond than the other, the atom getting a larger share of the electron will be slightly negative and the other atom will be slightly positive. The degree to which this occurs is called the polarity of the bond. Electronegativity is the affinity an atom has for its electrons, and the difference in electronegativity between two atoms predicts the polarity of their covalent bond. Carbon and hydrogen have close enough electronegativities that it is difficult to say which is larger. Polarity of not just a bond but of a whole molecule (when the positive partial charge is not geometrically balanced by the negative partial charge) is called a dipole moment. Dipole moments are measured by how big the charge imbalance is and how long of a distance the imbalance acts over.
In organic chemistry, it is convenient to group similar molecules into families with similar structures and properties and prone to similar types of reactions. The most basic family is hydrocarbons. Hydrocarbons are molecules that consist of only carbon and hydrogen atoms, and there are thousands of unique hydrocarbons.
Hydrocarbons
Hydrocarbons are either aliphatic or aromatic.
Alkanes are saturated hydrocarbons, which means that they only have single bonds. Therefore, the chemical formula for all alkanes is CnH2n+2. Methane (CH4) is the simplest alkane.
Alkenes are unsaturated hydrocarbons, in that they have a carbon-carbon double bond. For a typical alkene, the formula is CnH2n. Ethene (C2H4) is simplest alkene.
Alkynes are unsaturated hydrocarbons. They have a carbon-carbon triple bond. For a typical alkene, the formula is CnH2n-2. Acetylene (C2H2) is the simplest alkyne.
Cyclic aliphatic
Aromatic
Arenes are aromatic-aliphatic compounds.
Adding Oxygen
Alcohols
Phenols
Ethers
Aldehydes
Ketones
Carbanions
Carboxylic acids
Adding Nitrogen
Amines
Adding Halides
Alkyl halides
Biomolecules
Lipids
Carbohydrates
Proteins