INTRODUCTION TO ORGANIC CHEMISTRY

Organic chemistry is the study of “living” things—not in the same way that biology is the study of life. Rather, organic chemistry takes a look at what composes the living things, and how they’re structured. Organic chemistry breaks down living things not only into organs seen in organisms, but goes a step further to break down those organs into atoms and molecules. It focuses mainly on carbon, which is highly essential to maintaining life, and particularly zeroes in on the hydrocarbon, which is a molecule composed of hydrogen and carbon. Hydrocarbons not only compose what we’re made of, but also what we consume, including carbohydrates, proteins, steroids, fats, and more! As a matter of fact, you may be surprised to know that everyday things, such as caffeine, plastic, and paint are all composed of hydrocarbons!

When we look at organic chemistry, we’re mainly looking at molecules composed of carbon and hydrogen, however we may also see nitrogen, sulfur, oxygen, phosphorous, silicon, and the halogens (F, Cl, Br, I, and At) taking a part in our compounds and reactions. Organic compounds are generally composed of long carbon chains displaying covalent bonds.

For our purposes, we’ll be giving a general overview, or introduction, to organic chemistry, as you might find in a high school chemistry course or an AP/IB chemistry course. We’ll take you through organic structures, alcohols, and combustion of a hydrocarbon.

Basic Vocabulary.

alkane: a chemical compound made up of only hydrogen and carbon. It consists of only single bonds (no double or triple bonds) so all bonds have to be H-C or C-C. Compounds that are alkanes end in –ane (like methane, ethane, and so on).

alkene: an unsaturated chemical compound that contains at least one C-C double bond. Compounds that are alkenes end in –ene, like ethylene.

alkyl group: a functional group that consists only of hydrogen and carbon atoms. It is commonly abbreviated with R when drawing chemical structures. Methyl and ethyl groups are both alkyl groups.

alkyne: a chemical compound made up of only hydrogen and carbon. It consists of at least one triple bond between two carbon atoms. Compounds that are alkynes end in –yne, like ethyne. Alkynes are also known commonly as acetylenes.

aromatic hydrocarbon: a hydrocarbon that has a cyclic structure (ring) instead of a carbon chain. It is more stable than the chain structure, due to the alternating bonding system.

hydrocarbon: an organic compound that consists of carbon and hydrogen.

isomer: a chemical compound that has the same composition, but can vary in structure. The two main types of isomers are structural isomers and stereoisomers, which are discussed later on this page.

Structure of Organic Molecules

The most basic unit in organic chemistry is CH₄, or methane. It is the simplest alkane, and is the basic functional unit for natural gas. Methane looks like this:

As you can see in this picture, methane has a central atom of carbon, with four surrounding hydrogen atoms. The bonds are covalent and single.

Generally speaking, in organic molecules, bonds are either single, double, or triple covalent bonds between atoms. Organic molecules usually have central atoms of carbon, or a central carbon chain, or backbone. Hydrogen atoms are always bonded using a single bond, because hydrogen only has one electron to share in a bond. Carbon atoms can be singly, doubly, or triply bonded to each other, because each carbon atom has four valence (bonding) electrons to share.

As you continue your studies, you may run across compounds that do not possess a straight carbon chain, but rather have a ring of carbon atoms. This compound is called benzene, which is a highly flammable liquid that contributes to the production of many types of plastic (including rubber) as well as serving as an additive in gasoline. The structure of benzene is a hexagonal ring consisting of alternating single and double bonds between the carbon atoms. The formula for benzene is C₆H₆. Benzene is classified as an aromatic hydrocarbon, and looks like this:

Notice that here, the carbon chain is a circle (ring) and that the double bonds alternate with the single bonds. This means that more than one structure is possible for benzene—both of them are pictured here.

Isomers

Isomerism results when two or more molecules that have the same molecular makeup have different structures and orientations. There are two basic types of isomers: structural isomers and stereoisomers. Structural isomers indicate a differing placement in atoms oriented around the central atom. Stereoisomers indicate a rotation or an inversion of the molecule—for example, stereoisomers can be either cis-trans isomers (geometric isomers) or can be “mirror images,” which are called optical isomers. The chart below shows the different types of isomers and gives examples of each.

Knowing how to determine isomers is important in showing the correct configuration of organic compounds. As you can see, many compounds have the same makeup (number of carbon, hydrogen, oxygen, and so on) but they are arranged differently, which affects not only their names (as a name is what scientists use to indicate structure) but also their configuration, which is what you would see when looking at the structure.

Alcohols

Alcohols are compounds containing the hydroxyl group, which is –OH. The most simple alcohol is methyl alcohol. The formula for methyl alcohol (also known as methanol) is CH₃OH. The structure for methanol is as follows:

This is the most simple of the alcohols, since it contains only one carbon atom that is singularly bonded to four other atoms (three hydrogen, one oxygen). We can tell that this is an alcohol by locating the hydroxyl group (–OH) at one end of the compound. All alcohols are identifiable by looking for this hydroxyl group as part of the atomic structure.

A few other common alcohols are:

Ethanol: C₂H₅OH

Ethylene glycol (Ethane-1, 2-diol): C₂H₄(OH)₂

Glycerin (Propane-1, 2, 3-triol): C₃H₅(OH)₃

Isopropyl alcohol (propan-2-ol): C₃H₇OH

Obviously, there are many more different alcohols (each with a unique configuration) that we are not going to list here. However, these are some of the most common ones, and this will help you recognize future alcohols by knowing their structure (especially the hydroxyl group(s) that stem from a carbon atom).

Combustion of a Hydrocarbon

Combustion of a hydrocarbon involves, quite simply, a reaction between a hydrocarbon and oxygen, resulting in the production of CO₂ (gas) and H₂O (water). These reactions are easily spotted, because they will always involve the same reactants and products with varying coefficients and subscripts. The general format for a combustion reaction is:

C_xH_y + O₂ –> CO₂ (g) + H₂O

It can also be noted that energy, released as heat, is a product of this reaction—sometimes heat is emitted as light, which can be seen as a glowing appearance, or as a flame.

Many combustions do not reach completion because there is not enough O₂ to complete the reaction. In the case of an incomplete combustion, there is excess hydrocarbon (because all the oxygen will be used). These reactions typically burn with a “smoky” flame. Under ideal conditions (and for homework problems) it is generally assumed that the reaction reaches completion. This means that there is an excess of O₂ gas. These reactions typically burn with a “clean” flame—there is no smoke produced. When dealing with combustion reactions, you can find the amounts of reactants and/or products, as well as balance the equation (using Stoichiometry skills). Most of the time, you will be asked to do one of these two problems when dealing with combustion of hydrocarbons.

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