Introduction to Organic Carbon Chemistry

Organic chemistry is the study of carbon compounds, which are built on the ability of carbon atoms to form four strong covalent bonds, link into long chains and rings, and combine with hydrogen, oxygen, nitrogen and other elements. In brief: a carbon atom forms four bonds, so methane is CH₄, ethanol is CH₃CH₂OH and millions of different molecules can be assembled from just a few elements.

Almost everything that is alive is made of organic compounds, and so are fuels, plastics, medicines, dyes and foods. The field began as the chemistry of substances from living things, but today it includes all carbon compounds except for a few simple ones such as carbon dioxide, carbonates and cyanides. This article explains why carbon is so special, how carbon bonds and hybridizes, which elements are common in organic molecules and how organic compounds are classified.

Why is carbon special?

Carbon has four valence electrons (electron configuration 1s² 2s² 2p²), so it can form four covalent bonds. Three properties make it unique:

• Catenation: carbon atoms bond strongly to one another and form chains, branches and rings. The C–C bond has an average energy of about 346 kJ/mol, compared with only 222 kJ/mol for Si–Si.
• Multiple bonds: carbon forms stable double and triple bonds, with other carbon atoms as well as with oxygen and nitrogen.
• Moderate electronegativity: at 2.55 on the Pauling scale, carbon forms stable bonds with both metals and nonmetals, and its bonds to hydrogen (413 kJ/mol) are strong and nearly nonpolar.

Silicon is below carbon in the periodic table and also has four valence electrons, but long silicon chains are not stable, and Si–O bonds (about 452 kJ/mol) are much stronger than Si–Si bonds. For this reason silicon forms rocks and glass, not life.

How does carbon bond and hybridize?

In organic molecules the carbon atom mixes its 2s and 2p orbitals to form hybrid orbitals. The type of hybridization determines the geometry around the atom.

Which elements occur in organic compounds?

The simplest organic compounds contain only carbon and hydrogen and are called hydrocarbons. When other elements are present, they are usually bound in functional groups, which determine the properties of the molecule.

How are organic compounds classified?

• Hydrocarbons: alkanes (only single bonds), alkenes (C=C), alkynes (C≡C) and aromatic compounds (benzene rings).
• Compounds with oxygen: alcohols, ethers, aldehydes, ketones, carboxylic acids and esters.
• Compounds with nitrogen: amines, amides and nitriles.
• Halogen compounds: haloalkanes such as chloromethane.

Another distinction is between open-chain (aliphatic) compounds and those with rings (cyclic compounds). A ring that has only carbon atoms is carbocyclic, whereas a ring that includes other atoms such as N or O is heterocyclic.

How do you work out the bonds in a molecule?

Worked example: ethanol

The molecular formula of ethanol is C₂H₆O. Using the usual valences (C = 4, H = 1, O = 2):

1. Join the two carbon atoms: C–C. Each carbon has 3 remaining bonds.
2. Attach the oxygen to one carbon. That carbon now has 2 bonds left, and oxygen has 1.
3. Place the hydrogen atoms: one on oxygen (O–H), two on the carbon with oxygen and three on the other carbon.
4. The count is 1 + 2 + 3 = 6 hydrogen atoms, as the formula requires.

The result is CH₃–CH₂–OH. The formula C₂H₆O also fits another structure, CH₃–O–CH₃ (dimethyl ether), which shows that a molecular formula alone does not determine the structure.

Worked example: three hydrocarbons with three carbons

• Propane, C₃H₈: only single bonds, the maximum number of hydrogens (2 × 3 + 2 = 8).
• Propene, C₃H₆: one C=C double bond, so two fewer hydrogens.
• Propyne, C₃H₄: one C≡C triple bond, so four fewer hydrogens than propane.

What are typical properties of organic compounds?

• Low melting and boiling points: most are molecular substances held together by weak intermolecular forces.
• Flammability: most burn in oxygen to give carbon dioxide and water, for example C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
• Poor solubility in water: hydrocarbons are nonpolar and dissolve better in nonpolar solvents. Small molecules with –OH or –COOH groups are exceptions.
• Slow reactions: the strong covalent bonds often need heat, a catalyst or a reagent to react, and the reactions tend to be slower than ionic reactions.
• Many isomers: the same atoms can be joined in different ways.

How did organic chemistry begin?

In the early 19th century, chemists believed in vitalism, the idea that compounds from living organisms contained a “vital force” and could not be made from inorganic materials. In 1828 Friedrich Wöhler heated ammonium cyanate, an inorganic salt, and obtained urea, a compound that had been known from urine. This is often called the beginning of the end of vitalism, though historians point out that the idea faded gradually over several decades. In 1858 August Kekulé and Archibald Couper independently proposed that carbon is tetravalent and forms chains, which led to modern structural theory.

Why does organic chemistry matter?

• Medicine: most drugs, from aspirin to antibiotics, are organic molecules.
• Materials: plastics, rubber, fibers and paints are made of organic polymers.
• Energy: natural gas, petroleum and biofuels consist mostly of hydrocarbons.
• Biology: sugars, fats, proteins and nucleic acids are organic.
• Food and agriculture: flavors, preservatives and pesticides are organic compounds.

Which mistakes are common?

• Drawing carbon with five bonds: neutral carbon in an organic molecule has four.
• Calling CO₂ and carbonates organic: by convention they belong to inorganic chemistry.
• Thinking that organic means natural: synthetic compounds are organic compounds too, and natural substances can be toxic.
• Assuming that one formula means one compound: isomers have the same formula.
• Mixing up the chemical and the food-labeling meaning of “organic”: they are unrelated.