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The most important elements in nature are carbon and silicon. Carbon compounds consist mainly of biological molecules that are responsible for life itself on Earth. We have currently defined many million carbon compounds, and the number is still increasing. Carbon has the unique quality of being able to create strong bonds to itself and thus also to create carbon chains. Carbon can also create strong bonds with other non-metals such as hydrogen, nitrogen, oxygen, sulphur and halogens. This leads to formation of compounds called hydrocarbon derivatives. Below, we provide more information on a selection of carbon compounds.
Hydrocarbons | Alcohols | Carboxylic acids | Esters
1. HYDROCARBONS
We are well acquainted with hydrocarbons in everyday life. We can find them in fuels such as oil and gas. They are also used as lubricants in many kinds of machinery, and may also be used as propellant in spray cans.
Hydrocarbons are made up exclusively of carbon and hydrogen atoms and can be divided into three classes: alkanes, alkenes and alkynes. The so-called aromatic compounds constitute a special class. Hydrocarbons are non-polar molecules, because carbon and hydrogen have approximately the same electronegativity – and this minimises charge polarisation, i.e. the charge is evenly distributed throughout the molecule. This means that hydrocarbons are stable molecules that do not react easily with other molecules. They are also relatively insoluble in polar solvents such as, for example, water. “Like dissolves like” is a saying in the world of chemistry and means that polar solvents are best at dissolving polar substances and non-polar solvents are best at dissolving non-polar substances.
The chemical bonds between atoms in a hydrocarbon molecule are covalent bonds. In a covalent bond, two bonded neighbouring atoms will share one or more electron pairs. The electrons are attracted electrostatically by the oppositely charged nuclei. Between the nuclei there are repulsion. Altogether, the nuclei are located in an equilibrium distance, kept together by the electron pairs. An electron pair shared by two atoms, corresponds to a single bond.
There are also forces between molecules. They are called van der Waals forces, and are also basically of electrical nature. Van der Waals forces are weak, but increase by increasing size of the molecules. Therefore, hydrocarbons with the smallest molecules are gasses at room temperature (e.g. the components of natural gas), while hydrocarbons with larger molecules are liquids (to be found in gasoline and diesel fuel).
Below is a short explanation of the three subclasses.
Alkanes
The simplest alkane is methane, CH4. The formulas for alkanes containing several C-atoms can mentally be derived by successive replacement of H-atoms by C-atoms. Since carbon is in Group 4 of the periodic table, it has four valence electrons, or outer electrons, that create the bonds. All four valence electrons must participate in bonds for the alkane to be stable. Alkanes are made up purely of single bonds. The general formula for alkanes is CnH2n+2. A special type of alkanes is cycloalkanes. These are ring-shaped. The simplest cycloalkane consists of three C atoms and is called cyclopropane. Due to the presence of a ring, cycloalkanes have two less H atoms than open-chain alkanes in order to satisfy the valence electron rule.
IUPAC, the International Union of Pure and Applied Chemistry, has compiled a set of rules for naming hydrocarbons and their derivatives. The prefix of the name reflects the number of C atoms. From one to ten C-atoms the prefixes are meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-. The suffix is determined by whether the substance is an alkane, an alkene, or an alkyne (-ane, -ene, or –yne, respectively). An example of an alkene with four hydrocarbons is thus butene.
In a hydrocarbon with a branched carbon chain, the carbon atoms of the longest chain are numbered starting from one end. These numbers define the position of side chains, halogen atoms, OH- groups or whatever might substitute H in the major chain. The numbers must be assigned to be as small as possible.
Alkenes
What is special about alkenes is that they contain one or more double bonds. In one double bond, two C atoms share two electron pairs instead of one – which occurs with single bonds. Mentally, to facilitate the creation of double bonds, two H atoms must be removed in order for the valency electron rule to be satisfied. The general formula for alkenes is thus CnH2n.
In double bonds, the atoms are in closer proximity, reflecting the fact that the double bond is stronger than the single bond. This means that the C atoms that participate in the double bond have less chance of rotating than C atoms in single bonds. At room temperature such rotations are impossible, and the geometry around a double bond becomes planar (see Figure 1.1).
This leads to the possible existence of two structural isomers of alkenes: cis and trans. Isomers are molecules that consist of like numbers of like atoms, but the atoms are bonded differently to each other in the two different isomers. The double-bonded C-atoms must be bonded to different kinds of atoms or atomic groups for the structural isomerism to occur (see also Figure 1.1).
According to the naming conventions for alkenes, the number of the C-atom where the double bond starts is placed before the suffix -en.
Aromatic compounds
A particularly stable type of hydrocarbon is that of aromatic hydrocarbons. These substances seem to contain double bonds, but they do not react like alkenes. They contain a special number of C atoms and are cyclical. Perhaps the most famous example of an aromatic compound is benzene C6H6. It is convenient to write the structural formulas of benzene and other aromatic compounds as if they contain double bonds between every other carbon pair. Then one can interchange single and double bonds in the formula without violation of the valence electron rule, a "phenomenon" with the rather misleading name "resonance" (see Figure 1.2 demonstrating the resonance structures of the inorganic phosphate ion). It is nevertheless an empirical fact that many molecules for which different resonance structures can be written down are very stable. These molecules contain non-localised electrons - electrons participating in chemical bonds between more than two atoms. Such bonds can increase the stability of the molecule according to quantum mechanics.
Alkynes
Alkynes contain one or more triple bonds. In triple bonds, two C atoms share three electron pairs, and the general formula is: CnH2n-2. Triple bonds render alkynes even more robust and less flexible. Each of the two triply bonded C-atoms is bonded to some other atom by a single bond. These single bonds are in-line (collinear), and the overall geometry of the triple bond becomes linear.
2 ALCOHOLS
Alcohols contain a hydroxyl group – the OH group. The simplest alcohol is methanol, CH3OH. Alcohols are named following the same convention as hydrocarbons – according to the number of C atoms in the carbon chain. However, they are given the suffix “-ol” to indicate that they are alcohols. Alcohols are more stable than hydrocarbons because OH groups are polar. This allows them to create hydrogen bonds identical to the bonds found in water.
In general, alcohols are used as solvents for organic substances. Even though there are many important alcohols, it is the two simplest alcohols (methanol and ethanol) that have the greatest usefulness.
Methanol is found, inter alia, in fibres and plastics, and is being used more and more in motor fuel. Methanol is a hazardous substance. Swallowing it causes intoxication and it is highly poisonous. It may also lead to blindness and, ultimately, to death by poisoning. Ethanol, on the other hand, exists in a diluted form in a number of useful applications, such as beer, wine and whisky. It is ethanol that we actually mean when we talk of alcohol or spirits in everyday language. Ethanol is produced by fermentation of glucose or other sugars from grain, grapes and other fruit. During the process, the glucose ferments without the presence of oxygen and produces CO2 gas and ethanol.
3 CARBOXYLIC ACIDS
If you exchange the OH group in alcohols with a COOH group, you obtain a group of molecules called carboxylic acids. These acids function as weak acids in water because they are able to split off H+ from the functional group – the COOH group. To distinguish the carboxylic acids, the word “acid” is added to the end of the name, for example: methanoic acid and ethanic acid. These names are not used very much, as the common names have worked their way into the language and are easy to remember. The common names of methanoic acid and ethanic acid are formic acid and acetic acid, respectively. The common names are thus everyday terms that derive, for example, from the substances’ colour, taste, or from the name of the person who discovered them.
Formic acid exists, in its natural state, in ants, nettles and sweat. It is a common misconception that formic acid is the cause of the skin-irritating effect of nettles, which is rather due to histamine. Formic acid is highly corrosive on skin, but also acts as an antiseptic, thus making it useful as a preservative in fruit juice and green fodder. Concentrated acetic acid is also corrosive on skin and mucous membranes. In diluted form, however, it is widely used in the food industry.
Many carboxylic acids can be synthesised from alcohols with a strong oxidation agent, so that the alcohol oxidises, taking up oxygen and splitting off hydrogen. Ethanol, for example, can be oxidised to ethane acid by using potassium permanganate as an oxidation agent.
The carboxylic acids with few C-atoms have a sharp or detestable smell.
4 ESTERS
A number of other organic compounds resemble the carboxylic acids and can be derived from these. Esters are examples of such compounds. They are formed through the reaction of one carboxylic acid, or an inorganic acid containing oxygen, and an alcohol. During the process water is split off. Esters are given the suffixes “-oate” and one example is methyl ethanoate.
Esters encompass a number of useful and important substances. They often have a pleasant scent of flowers and fruit. This is the case even for esters formed from very unpleasantly smelling carboxylic acids. For example, the smell of banana comes from n-amyl acetate.
Nitro-glycerine (common name), which is derived from inorganic nitric acid and the alcohol glycerine, is an important example of an ester that is used both in explosives and in medicine.
Various types of fat also belong to the ester group. Fats are esters from glycerol and carboxylic acids. Esters are also easy to break down further. The classic method of manufacturing soap is, in fact, based on a resolution of fats using alkali that creates soap and glycerine.
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