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Organic chemistry is the study of the molecules which, among other things, form the basis of life. Which element is found in all the molecules which are included in the field of organic chemistry?

Carbon. Carbon atoms have a number of unusual properties that give them the chemical flexibility to form the variety of molecules needed to create life. Their valency of four means that they can combine with a large range of other elements; their capacity for joining with other carbon atoms to form larger molecules allows a multiplicity of molecular forms. Life on earth is based on carbon compounds, although it has been speculated that other elements, particularly silicon, could form the basis of life under different conditions. The other elements that form the building blocks of living organisms are hydrogen, nitrogen, oxygen, sulfur and phosphorus (although a bacteria which uses arsenic instead of phosphorus was discovered in 2010, making all such assertions subject to challenge).

The simplest organic molecules contain only two elements. The very smallest organic molecule is methane, which consists of molecules that have a single carbon atom and four atoms of what other element?

Hydrogen. Methane, CH4, is the smallest member of the organic family called the alkanes. These molecules consist of chains (or branched chains) of carbon atoms joined on to each other, with hydrogen atoms bonding to the unused valence electrons. Their general formula is C(n)H(2n+2). Cycloalkanes have a ring shape, and therefore two less hydrogen atoms than the alkane with the same number of carbon atoms. Systematic names for the members of the alkane family describe the number of carbon atoms in the longest chain of carbon atoms, then have extra information to describe how any branches are attached to this chain.

You have probably heard about saturated and unsaturated fats in relation to dietary issues. The term unsaturated refers to molecules which have one or more double (or triple) bonds between atoms of which element?

Carbon. An unsaturated compound is so called because it has at least one double or triple bond between adjacent carbon atoms. These bonds are less stable than single bonds, so the molecules are likely to react with other substances in a type of reaction called an addition reaction, to make new bonds and allow the double bond to be changed into a single bond.

To keep peanut butter from separating and forming an oily layer as it stands on the shelf, it used to be common practice to reduce the amount of unsaturated fats in the oil being used. The atoms of what element were added to increase the proportion of saturated molecules in the oil?

Hydrogen. The process of hydrogenation (adding hydrogen) was somewhat controversial - it produced a more stable food product, but debate raged over the possible health issues involved with the introduction of partially-saturated molecules (still containing some double bonds), referred to as trans fats, into the food. Hydrogenation is a type of addition reaction, in which a double bond breaks, and each of the adjacent carbon atoms forms a new bond with a hydrogen atom.

A group of organic molecules that we encounter regularly in our food is carbohydrates. Which of these elements is NOT found in a carbohydrate molecule?

Nitrogen. Carbohydrates contain carbon, hydrogen and oxygen. They usually have twice as many hydrogen atoms as carbon atoms, the same ratio as is found in water, which is the source of the -hydrate part of their name. For a chemist, carbohydrates include not only the large molecules that dieticians usually mean when they use the term, but also small molecules such as the monosaccharide glucose (blood sugar) and the disaccharide sucrose (table sugar).

The well-loved chemical ethanol (also known as drinking alcohol) contains a functional group called hydroxyl. What atom in the hydroxyl group is bonded onto one of the two carbon atoms that forms the main chain of this molecule?

Oxygen. The hydroxyl group consists of an oxygen atom and a hydrogen atom. The oxygen atom, with six valence electrons, usually forms two covalent bonds. In this case, one is to the hydrogen atom, and the other is used to form a bond with some other atom to attach the functional group onto a larger molecule. All alcohols contain a hydroxyl group. In primary alcohols, the hydroxyl group is on a carbon atom which is only bonded to one other carbon atom in the molecule (or to no others, on the unique case of methanol); a secondary alcohol has a hydroxyl group attached to a carbon atom that is also bonded to two other carbon atoms; a tertiary alcohol's hydroxyl group is attached to a carbon atom that also has bonds to three other carbon atoms. The different types of alcohol typically are involved in different reactions, especially as regards oxidation.

When an alcohol is oxidised, it typically forms a carbonyl group. A carbonyl group has a double bond between a carbon atom and another atom of which type?

Oxygen. A carbonyl group is made up of a carbon atom double-bonded to an oxygen atom, often shown as C=O. A molecule with a C=O bond and a C-H bond on a carbon atom on the end of the carbon chain is called an aldehyde. An example is formaldehyde, a highly toxic chemical formerly used as a preservative for biological specimens. If the carbonyl group is on a carbon atom in the middle of the chain, the molecule is called a ketone. Acetone is a familiar ketone to many, because of its popular use as a solvent - it is the main ingredient of most nail polish removers, and of many paint thinners.

When an alcohol is oxidised to form an aldehyde or a ketone, what kind of atom is removed from the molecule during the reaction?

Hydrogen. In this oxidation process, the alcohol reacts with an oxidising agent to form the aldehyde or ketone and water. The alcohol loses two hydrogen atoms - one from the hydroxyl group, and one that was bonded to the carbon atom, which needs to use that bonding capacity to form the carbonyl double bond with the oxygen atom.

When a primary alcohol is completely oxidised, it becomes an acid. The ethanol in wine, for example, can be oxidised to produce vinegar, formally known as ethanoic (or acetic) acid. When an organic acid dissolves in water, what kind of ion is formed?

Hydrogen. Like all acids, organic acids dissolve in water to produce hydrogen ions and a negatively-charged ion. Most organic acids are not very soluble, and also do not dissociate completely, so they are called weak acids. An organic acid has a carbon atom on the end of the molecule which has a carbonyl group and a hydroxyl group attached to it. It is the hydrogen atom in the hydroxyl group that is released when the acid reacts with other substances as an acid.

The difference between an amino acid and an ordinary organic acid is that an amino acid includes which of the following kinds of atom?

Nitrogen. Amino acids have both the carboxylic acid (-COOH) group that is found in all organic acids, and an amine group (-NH2). The most important amino acids, called alpha-amino acids, have the amine group attached to the carbon atom directly beside the carboxylic acid unit. Their structure can be shown as H2NCHRCOOH, where R stands for the rest of the molecule. The smallest amino acid, glycine, has a hydrogen atom as the R part of the molecule. Amino acids can join to form proteins; nine of the twenty standard amino acids are considered essential for the human diet, as they cannot be synthesized and must be obtained from the food we eat.