INTRODUCTION TO ORGANIC CHEMISTRY

Contents for this page	Related topics
What is organic chemistry? Structure of organic compounds Functional groups Isomerism Additional questions	Systematic naming of organic compounds Addition, elimination, and substitution reactions	Data Glossary
Learning Outcomes
After studying this section, you will (a) understand what is meant by the "structure of an organic compound" and the term "functional group", (b) be able to recognize the structures of alkanes, alkenes, alkynes, haloalkanes, alkanols, carboxylic acids and esters, and (c) understand how a given empirical formula may apply to a number of different isomers.

What is organic chemistry?

Organic chemistry is the chemistry of the compounds of CARBON, which, in combination with many other elements (in particular H, N, O, S, P and the halogens) form over 5 000 000 compounds.

Many of these compounds are of immense importance, as the list below shows:

Apart from water, living things are largely made up of organic compounds, notably proteins, fats, carbohydrates and nucleic acids. The study of these compounds, and the role they play in all life processes, is a special area of study called BIOCHEMISTRY.

Historical note: Wohler and the synthesis of urea.

Structure of organic compounds:

Organic compounds tend to be built according to a general scheme as follows: The carbon atoms form a "skeleton", in this example, shown on the right, a chain of six C atoms.

Hydrogen atoms are the most common atoms found linked to carbon atoms. In HYDROCARBONS, the major constituents of petroleum, they are the only atoms found linked to carbon.

One or more reactive chemical groups of atoms (known as FUNCTIONAL GROUPS) are attached to the carbon chain (in this case the functional group is OH).

Hydrogen atoms are bound to the carbon skeleton by means of covalent bonds.

There are other ways of representing the structure of organic compounds:

represents the same molecule, without bonds being shown.

Sometimes only bonds are shown, or one uses models known as BALL-AND-STICK or FILLED (the colours black, red and white represent atoms of carbon, oxygen and hydrogen respectively):

The arrangement in space of all the atoms of a molecule is called the STRUCTURE of the molecule. The complexity of the structure will clearly depend on the size of the molecule.

For example, the structure of the particular substance with formula C6H14 might be described in terms of a STRUCTURAL FORMULA such as the one shown on the left.

As shown above, carbon skeletons are immensely varied: they may consist of straight chains of various lengths, branched chains, rings of various sizes and combinations of these.

Why is it that carbon can form stable, extended chains? (Click here for a discussion)

Functional groups:

FUNCTIONAL GROUPS are groups of atoms that have special chemical properties and which define the chemistry of an organic compound.

The following are common functional groups:

Functional group	Found in
(none)	alkanes
C=C	alkenes
CºC	alkynes
R is hydrogen or a group of atoms derived from alkanes or other hydrocarbons	arenes
C-X (X is a halogen atom)	haloalkanes
C-OH	alkanols
	aldehydes
	ketones
	carboxylic acids
	esters
	ethers
C-N	amines
	amides

Isomerism:

Have a good look at the two structural formulae shown below. Both of these are structural formulae applying to molecules with molecular formulae C₆H₁₄.

These two molecules are said to be ISOMERS. Isomers are compounds which have the same molecular formula, but different structural formulae. Isomers normally also differ in their chemical and physical properties.

Isomers do not have to belong to the same class of organic compounds. For example, the compounds whose structural formulae are shown below, are isomers. The one on the left is a CARBOXYLIC ACID, while the one on the right is an ESTER:

Verify that they both have the molecular formula C₅H₁₀O₂.

Additional questions

Alkanes:

Alkanes are hydrocarbons (compounds containg only C and H) that have single covalent bonds joining the carbon atoms. The carbon atoms form open chains, which may have branches. The molecular formula of all alkanes fits the expression C_nH_2n+2, where n is the number of carbon atoms.

Check that the formula is C6H14!

Alkanes are of tremendous importance, as the are the major constituents of petroleum.

Alkenes:

Alkenes are hydrocarbons (compounds containg only C and H) that have one or more C=C double bonds (two C atoms are linked by 4 shared electrons). The general formula is C_nH_2n, which is two hydrogen atoms less than the corresponding alkane.

Alkenes are said to be UNSATURATED, (since they do not have their full complement of hydrogen atoms).

Alkynes:

These are hydrocarbons (compounds containg only C and H) that have one or more CºC triple bonds (two C atoms are joined by 6 shared electrons). The molecular formula fits the formula C_nH_2n-2 and has therefore four hydrogen atoms less than the corresponding alkane.

Alkynes are said to be UNSATURATED, (since they do not have their full complement of hydrogen atoms). The simplest alkyne, HCºCH, is commonly known as acetylene.

Arenes:

Arenes are hydrocarbons that are built on the carbon skeleton of BENZENE, C₆H₆, a hydrocarbon whose structure is shown above in three different representations. The two structures on the right are simplified structural formulae, the one on the extreme right being the one that is most commonly used nowadays ().

Arenes all contain what is known as the BENZENE RING and are also known as AROMATIC HYDROCARBONS. They encompass a huge variety of organic compounds, some of which are shown below.

In turn, arenes provide us with a huge variety of carbon skeletons that can carry functional groups.

Haloalkanes:

If one or more hydrogen atoms of an alkane is replaced by a halogen atom (chemists use the term SUBSTITUTION for this process), the compound is a haloalkane.

Alkanols:

These compounds arise when one or more hydrogen atoms of an alkane are substituted by the -OH (hydroxyl) functional group.

Carboxylic acids:

These are compounds that have the carboxyl -COOH functional group, whose structure contains a C=O bond:

Esters:

Esters are compounds that have the CO-O-C functional group. This group, derived from the carboxyl group, also contains a C=O double bond:

Ethers:

Ethers are compounds where an oxygen atom is joined to two carbon atoms by single bonds. They are isomeric with alkanols with the same number of carbon atoms.

Ketones and aldehydes

Both ketones and aldehydes have the >C=O (carbonyl) group. In ketones, that group is "sandwiched" between two carbon atoms. This means that the simplest ketone must have 3 carbon atoms. In aldehydes, the >CO group is attached to one carbon atom and one hydrogen atom. The exception is formaldehyde, HCOH, where the carbonyl group is attached to two hydrogen atoms. A couple of examples are shown below.

Amines:

AMINES are compounds that are derived from ammonia. They all have at least one carbon atom attached to the nitrogen atom. This carbon atom is not bound to oxygen (see under amides, below). If only one carbon atom is attached to the nitrogen atom, the amine is said to be PRIMARY. If the nitrogen atom is attached to two carbon atoms, it is a SECONDARY amine, and with three carbon atoms, it is a TERTIARY amine.

Amines can be considered to be alkyl derivatives of ammonia. As such, they behave as bases.

Amides:

AMIDES are compounds that are derived from ammonia or amines AND a carboxylic acid. They all have the >C=O (carbonyl) group of the acid linked to the nitrogen atom.