Advanced carbon

Structure of carbon

Carbon has four valence electrons, which enables a carbon atom to form strong covalent bonds with itself and/or other elements in various structures. Its versatility has already been witnessed in studying the many formations of its allotropes. This chapter will cover carbon covalent bonding in more detail.

Covalent bonding involves two or more atoms sharing electrons. The electrons tend to stay between the nuclei due to the attraction between the positive nuclei and the negative electrons. In addition to the attraction, there are also two forces of repulsion. The nuclei both have a positive charge, therefore repel each other and the electrons both have a negative charge and repel each other. The force of attraction is greater than the force of repulsion, keeping the molecule together. In a non-polar covalent bond (where the atoms are from the same element), the charge is neutral, while in a polar covalent bond the atoms will have a slightly positive or negative charge.

To 'gain' electrons in a covalent bond, an electron must share the same number of its own, thus all covalent bonding occurs in pairs - at least one electron from each element. In the case of hydrogen, which requires one additional electron, it must share one electron. With oxygen, which requires two additional electrons, it must share two of its electrons.

A single bond is where two elements share one pair of electrons, one electron from each atom). Double or triple bonds indicate two or three pairs of shared electrons respectively. Unsaturated compounds are those where carbon has formed double or triple bonds with itself and do not have many other atoms in their structure, unlike single-bonded carbon atoms, saturated with other elements.

Representing covalent molecules

A molecular formula is the alphanumeric representation of the molecule, e.g. 'O2' to symbolise two atoms of oxygen. There are a few ways to display a molecular formula containing covalent bonds, the simplest being with an electron dot diagram where each dot represents an electron. Write the chemical symbol of the element with the electrons dots arranged such that only the electrons in the outer shell are represented. The shared electrons destined for each atom's outer shell face each other. See image 1.

The structural representation of covalent bonding is slightly different, using a line to represent each pair of electrons that have covalently bonded. The structural formula of a molecule allows the shape of a molecule to be depicted more accurately. See image 2.

If there are four or more carbon atoms in a molecule, the atoms can arrange themselves in a number of different ways. The different structures are isomers, which have different properties from standard molecules. An isomer is a different compound with the same molecular formula, often identified by whether its structure is linear or branched. As the number of carbon atoms increases, so, too, does the number of possible isomers. Naming these isomers is complex and it is not necessary to go into further detail here.

Types of carbon compounds

Hydrocarbons are simple organic compounds containing only hydrogen and carbon atoms, although there are several combinations of these two elements in different ratios. Carbon compounds are named according to the components of their molecules. In the case of hydrocarbons, the first part (prefix) of each name refers to the number of carbon atoms while the last part (suffix) refers to the number of bonds - single, double or triple.

Table 1.1: Prefixes for carbon compounds

Table 2.1: The hydrocarbon homologous series 

Alkanes

A hydrocarbon atom with all four of its electrons occupied in single bonds belongs to the alkane homologous series. The ratio of hydrogen atoms to carbon atoms in alkanes is 2n+2 where 'n' is the number of carbon atoms.

If the hydrocarbon alkane contains one carbon atom, then the number of hydrogen atoms is 2(1)+2=4 or double the number of carbon atoms plus two. The formula for this particular molecule is therefore CH4.

Drawing on knowledge of naming hydrocarbons, note that the one carbon atom indicates that the prefix should be meth- and because the atoms form only single bonds with one another, the suffix is therefore -ane, meaning the molecule represented by CH4 is methane.

Alkanes usually exist in the form of crude oil, formerly the remains of plants and animals that lived millions of years ago.

Alkenes

When a hydrocarbon lacks one hydrogen atom with which to bond, it bonds twice with itself. Molecules that belong to this double-bond group are alkenes. The easiest way to remember this formula is that the number of hydrogen atoms is double the number of carbon atoms.

The smallest stable molecule of this group is ethene, which is two carbon atoms forming a double covalent bond and surrounded by four hydrogen atoms, because a single carbon atom has no other carbon atom with which to bond twice.

Alkenes are often used in making plastics as it is possible to string them out to make long molecule chains.

Alkynes

If there are even fewer hydrogen atoms in the molecule, carbon will form triple bonds with itself. These triple-bonded hydrocarbons are alkynes. The formula 2n-2, where 'n' is the number of carbon atoms, gives the ratio of hydrogen atoms to carbon atoms. That is, to find the number of hydrogen atoms in an alkyne molecule, double the number of carbon atoms and subtract two.

Once again, the smallest possible molecule in this series has two carbon atoms but, in the case of alkynes, there are only two hydrogen atoms, meaning that the carbon atoms must form a triple bond. The result is ethyne (often called acetylene), a highly reactive compound that, when used with oxygen, burns to produce high temperatures of almost 3000^oC.

Alcohol

Hydrocarbons are not the only carbon compounds in a functional group. The hydroxy group, containing hydrogen and oxygen, operates as the alcohol family. Generally, the formula C_nH_2n + 1+ OH represents the alcohol group, showing that alcohol is an alkane where a hydroxy unit replaces one hydrogen atom.

The best-known alcohol compound is hydroxyethane, popularly referred to as ethanol and represented as C₂H₅OH or CH₃CH₂OH, which implies the following basic structure: See image 3.

Alcohol is used in drinking beverages and as a solvent in some glues, paints and inks.

Esters

Esters are organic compounds formed as the result of combining alcohol with a carboxylic acid (an acid containing carbon, hydrogen and oxygen). Esterification is the process by which these two substances chemically react to form an ester. Commonly found naturally in fruits and plants, esters are often used as fragrances or flavouring agents as they possess a sweet smell and taste.

One example of an ester is methyl butyrate, which has a fruity fragrance of pineapples. Methyl butyrate forms after combining methyl alcohol with butyric acid. In addition to creating artificial fragrances and flavourings, some esters are used in materials (such as polyester) or as solvents.