James G. Speight - Encyclopedia of Renewable Energy

See also: Alkanes, Alkenes.

Alkalinity is a measure of the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. Alkalinity is closely related to the acid neutralizing capacity (ANC) of a solution, and it is the acid neutralizing capacity that is often incorrectly used to refer to alkalinity.

The alkalinity of a system is equal to the stoichiometric sum of the bases in solution. In the natural environment, carbonate alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of carbonate rocks and presence of carbon dioxide in the atmosphere.

Alkalinity is sometimes incorrectly used interchangeably with basicity as when the pH of a solution can be lowered by the addition of carbon dioxide. This will reduce the basicity; however, the alkalinity will remain unchanged.

Alkaloids are a class of nitrogenous organic compounds of plant origin which have pronounced physiological actions on humans and include many drugs (morphine, quinine) and poisons (atropine, strychnine). Alkaloids are naturally occurring chemical compounds containing basic nitrogen atoms and are produced by a large variety of organisms, including bacteria, fungi, plants, and animals.

The alkaloids are perhaps the last class of organic compounds which originate in plants that are of any importance in the formation of the organic substance of coal. These compounds all contain basic nitrogen in the molecule with the nitrogen frequently occurring in a cyclic system; in addition, most of the alkaloids also contain oxygen functions.

Alkaloids are biologically active, organic compounds that contain a nitrogen atom which compounds have many structural frameworks, and are therefore highly variable. Alkaloids occur in the roots, bark, leaves, and within the cells of many plants. The structural chemistry of the alkaloids is variable because of the many locations in which nitrogen can occur in organic systems. However, it is generally recognized that the alkaloids may be based on any one of several individual (or even on a combination of two or more) systems.

Alkaloids are divided into the several large groups, such as pyrrolidine, pyridine, quinoline, isoquinoline, indole, and quinazoline. They are often divided into main groups including peptides and cyclopeptide alkaloids, and true-alkaloids, proto-alkaloids, polyamine-alkaloids, and pseudo-alkaloids. Although most alkaloids are pharmacologically active or poisonous in high doses, there are some alkaloids in foods that are often consumed daily, of which the common examples are caffeine, theobromine, and theophylline (members of the purine alkaloid family) which are mainly found in coffee, cocoa beans, and tea leaves.

Alkaloids have been isolated as crude extracts from plants for many millennia as part of folk medications. However, since the 20 thCentury, individual alkaloids with defined and scientifically verified pharmacological properties have been purified and produced commercially as fine chemicals.

The complexity of the chemical structures of the alkaloids makes them, in most cases, impossible to produce by chemical synthesis, so extraction from a crude plant mixture remains the most economically viable strategy. However, plants normally produce complex mixtures of alkaloids (rather than a single alkaloid) with the desirable types often at low levels, with the result that commercially produced specific alkaloids are expensive. As the genetic manipulation of plants becomes more sophisticated, research has focused on the engineering of alkaloid biosynthesis to generate transgenic or cell lines that overproduce specific alkaloids. This can be achieved by increasing the synthesis of a particular alkaloid and/or inhibiting the synthesis of related compounds to increase the ease of purification.

Alkanes (sometimes referred to as paraffins or paraffin hydrocarbons) are aliphatic hydrocarbons (non-aromatic hydrocarbon derivatives) that contain carbon and hydrogen only in which all of the binding orbitals of the carbon atoms are satisfied by bonding to another carbon atom or to a hydrogen atom.

Normal alkanes (straight-chain paraffins) consist of a chain of carbon atoms. Each carbon atom is linked to four atoms, which can be either carbon or hydrogen, their general formula C nH 2n+2( Table A-16). The carbon skeleton can be structured as straight chains as are the normal paraffin. CH3(CH 2) nCH 3. The boiling points increase with the number of carbon atoms. With the low carbon numbers, the addition of a carbon increases the boiling point by approximately 25°C (77°F). Further additions give smaller increase. At the same time, the density increases with the molecular weight 0.626 kg/L for pentane, and 0.791 kg/L for pentacosane; on the other hand, the density is always much lower than 1. The normal alkanes from C 1to C 4are colorless gases; C 5to C 17colorless liquids; and from C 18onwards, colorless solids. Other physical properties, such as melting point, density, and viscosity, also increase in the same way as boiling point. There is a relationship between physical properties and chemical composition. The variation in the boiling point of compounds is due to different intermolecular forces such as hydrogen bonding. The alkanes are insoluble in water.

Table A-16Physical properties of n-paraffins.

Isoparaffins are paraffins in which branching is present, usually at the number 2 carbon atom, although branching can take place at a different position in the chain, although such molecules are not strictly isoparaffins. Isoparaffins have a boiling point lower than normal paraffin with same number of carbon atoms, and generally, the greater branching has the lower boiling point ( Table A-17).

Table A-17Physical properties of selected branched paraffins.

The octane number is a measure of the ability of a fuel (gasoline) to avoid knocking. The test engine is adjusted to give knock from the fuel rated. Then, various mixtures of isooctane (2,2,4-trimethylpentane) and n-heptane are used to find the ratio of the two reference fuels that will give the same intensity knock as that from unknown fuel. Defining isooctane as 100 octane number and n-heptane as 0, the octane number is the volumetric percentage of isooctane in heptane that matches knock from the unknown fuel is reported as the octane number of the fuel.