Introducing Organic Chemistry: Carbon and Ways to Draw Molecules

The chemistry of carbon

Look around you. The maple syrup on your pancakes, the fleece of your winter jacket, the gasoline in a car, the ink in a pen, the DNA in your cells and the medicine in the cabinet are all built mainly from carbon compounds. Organic chemistry is the chemistry of carbon compounds. Chemists have recorded many millions of them, far more than the compounds of all the other elements put together.

The name comes from an old idea. Until the early 1800s, many scientists believed that organic compounds could only be made by living things, through a special "vital force". In 1828 the German chemist Friedrich Wöhler heated ammonium cyanate, an inorganic salt, and made urea, a compound found in urine. No living thing was involved. Today an organic compound simply means a compound of carbon, usually with hydrogen. A few simple carbon compounds, such as carbon monoxide, carbon dioxide, carbonates and cyanides, are still treated as inorganic.

Remember from Grade 11: in Matter, Chemical Trends, and Chemical Bonding you drew Lewis structures, wrote structural and condensed formulas such as CH₃CH₂OH, and met tetrahedral, bent and planar molecules. In this unit you will use those skills on whole families of carbon compounds.

Why carbon is special

Carbon is in Group 14, so it has four valence electrons. It shares them to make four covalent bonds. Three facts explain why carbon forms so many compounds:

1. Carbon bonds strongly to itself. Carbon atoms join into long chains, branched chains and rings. This is called catenation.
2. Carbon makes single, double and triple bonds. Two carbons can share one, two or three pairs of electrons.
3. Carbon bonds well to other non-metals, especially H, O, N, S and the halogens.

Every correct organic structure obeys these bonding rules (the number of bonds each atom makes when it has no charge):

• C makes 4 bonds
• N makes 3 bonds
• O makes 2 bonds
• H and the halogens (F, Cl, Br, I) make 1 bond

A double bond counts as two bonds and a triple bond as three. If a carbon in your drawing has five bonds, the drawing is wrong.

Shapes around carbon

Electron pairs repel each other, so bonds spread out as far apart as they can. (You will study this idea, VSEPR theory, in detail in the unit Structure and Properties of Matter.)

• A carbon with four single bonds is tetrahedral, with bond angles of about 109.5°. A long chain of such carbons is really a 3-D zig-zag, not a straight line.
• A carbon with one double bond is trigonal planar, about 120°. The two carbons of a C=C bond and the four atoms attached to them all lie in one flat plane.
• A carbon with a triple bond is linear, 180°.

There is one more important difference. Atoms can rotate freely around a single bond, like a wheel on an axle. A double bond cannot rotate without breaking part of the bond. You will see why this matters when you study isomers.

Hydrocarbons, saturation and functional groups

The simplest organic compounds are hydrocarbons, made of only carbon and hydrogen. Natural gas (mostly methane) and the propane in a barbecue tank are hydrocarbons.

• A hydrocarbon with only single bonds between carbons is saturated: each carbon holds as many hydrogens as it can.
• A hydrocarbon with at least one C=C double bond or C≡C triple bond is unsaturated. It could add more hydrogen atoms if the multiple bond opened up.

You may have heard of saturated and unsaturated fats. The words mean the same thing there: unsaturated fats have C=C double bonds in their long carbon chains.

Most organic molecules also contain a functional group: an atom or small group of atoms, such as –OH or –COOH, that gives the molecule its typical behaviour. Ethanol (CH₃CH₂OH) has the –OH group of an alcohol. Compounds with the same functional group form a family (or class) with similar properties. The rest of this unit is a tour of these families.

Four ways to draw the same molecule

Chemists use several kinds of formula, each with a purpose.

• Molecular formula: only the number of each kind of atom, such as C₂H₆O. It is short, but many different molecules can share it.
• Complete structural formula: every atom and every bond. Clear, but slow to draw for big molecules.
• Condensed structural formula: each carbon is written with the atoms attached to it: CH₃CH₂OH. Branches go in brackets, for example CH₃CH(CH₃)CH₃.
• Skeletal (line) structure: the fastest. The rules are:
• each line is a bond between carbons;
• every corner and every line end is a carbon atom;
• hydrogens on carbon are not drawn: you add enough to give each carbon four bonds;
• every atom other than C and H (and any H attached to them, as in OH) is written.