Chemical nomenclature

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Chemical nomenclature

Contents

Inorganic Compounds

Inorganic compounds consist of the remaining compounds divided in to two groups; Ionic compounds and Molecular compounds

Ionic Compounds

The names of ionic compounds are based on the names of the ion which they are composed. For example, NaCl is called sodium chloride after the Na+ or sodium ion, and the Cl- or chloride ion. The positive ion is always named first and listed first in writing the formula for the compound. The negative ion is named and written last. To see how the names of these ions arise, consider first the naming of positive ions, also called cations. Ions may be monatomic (composed of a single atom) or polyatomic (formed from two or more atoms). Monatomic cations are most commonly formed from metallic elements.
These ions take the name of the element itself:

Na+ Sodium ion
Zn+ Zinc ion
Al3+ Aluminum ion

If an element can form more than one positive ion, the positive charge of the ion is indicated by a Roman numeral in parentheses following the name of the metal:

Fe2+ Iron(II) ion
Fe3+ Iron(III) ion
Cu+ Copper(I) ion
Cu2+ Copper(II) ion

An older method still widely used for distinguishing between two differently charged ions of a metal is to apply the endings –ous or –ic. These endings represent the lower and higher charged ions, respectively, They are added to the root of the Latin name of the element:

Fe2+ Ferrous ion
Fe3+ Ferric ion
Cu+ Cuprous ion
Cu2+ Cupric ion

Negative ions are called anions. Monatomic anions (those derived from a single atom) are most commonly formed from atoms of the nonmetallic elements. They are named by dropping the ending of the name of the element and adding the ending -ide:

H- Hydride ion
F- Fluoride ion
O2- Oxide ion
S2- Sulfide ion

Many polyatomic anions contain oxygen. Anions of this kind are referred to as oxyanions. A particular element such as sulfur may form more than one oxyanion. When this occurs, there are rules for indicating the relative numbers of oxygen atoms in the anion. When an element forms only two oxyanions, the name of the one that contains more oxygen ends in -ate; the name of the one with less oxygen ends in -ite:

NO2- Nitrite ion
SO32- Sulfite ion
NO3- Nitrate ion
SO42- Sulfate ion

When the series of anions of a given element extends to three or four members, as with the oxyanions of the halogens, prefixes are also employed. The prefix hypo- indicates less oxygen, and the prefix per- indicates more oxygen:

ClO- Hypochlorite ion
ClO2- Chlorite ion
ClO3- Chlorate ion
ClO4- Perchlorate ion

Molecular Compounds

The procedures for naming binary (two element) molecular compounds are similar to those for naming ionic compounds. In these molecular compounds it is possible to associate a more positive nature with one element in the molecule and a more negative nature with the other element. The element with the more positive nature is named first and also appears first in the chemical formula. The second element is named with an -ide ending. For example, the name for HCl is hydrogen chloride. (this is the name used when referring to the pure compound; water solutions of HCl are referred to as hydrochloric acid.)

Often a pair of elements can form several different molecular compounds. For example, carbon and oxygen form CO and CO2. To distinguish these compounds from one another, the prefixes given below are used to denote the numbers of atoms of each element present.

Prefix Meaning
Mono- 1
Di- 2
Tri- 3
Terta- 4
Penta- 5
Hexa- 6
Hepa- 7
Octa- 8

Thus CO is called carbon monoxide, and CO2 is called carbon dioxide. When the prefix ends in a and the name of the anion begins with a vowel (such as oxide), the a is often dropped.

Organic Compounds

Organic compounds contain carbon, usually in combination with hydrogen, oxygen, nitrogen, or sulfur. Organic compounds were first associated only with plants and animals. However, a great number of organic compounds have now been prepared that do not occur in nature.

The prefix for an organic molecule depends on the number of carbon atoms it contains in the longest continuous chain.

Prefix Meaning
Meth- 1
Eth- 2
Prop- 3
But- 4
Penta- 5
Hexa- 6
Sept- 7
Oct- 8

Alkanes

Alkanes are the simplest organic molecules, containing only c-c single bonds. An alkane has the suffix -ane. For example Pentane has 5 carbon and 12 Hydrogen atoms with only single bonds connecting them all together.

Alkenes

Alkenes are organic molecules which contain at least one C=C bouble bond. An alkene has the suffix -ene. Ethene is the simplest alkene possible with 2 carbon atoms double bonded to each other and 4 hydrogen atoms. Methene (1 carbon atom) is an impossible chemical because there is nowhere to put a double bond between carbon atoms

Alkenes longer than ethene have an added complication. It is essential to state the relative position of a double bond. Butene for example can have the double bond in two different positions (Hydrogen atoms ommitted): C=C-C-C or C-C=C-C (C-C-C=C is identical to C=C-C-C). The first is still called Butene, the second is But-2-ene. The 2 comes from the double bond being attached to the second carbon in the chain.

Using butene as an example again it can be seen that there is enough space for more than one double bond: C=C=C-C, C=C-C=C or even C=C=C=C. These are respectively called But-1,2-diene, But-1,3-diene and but-triene. Again the numbers 1,2 and 3 signify the position the carbon bond. but-triene does not need any positioning numbers because there is no other way of arranging the double bonds. The 'di' and 'tri' infixes signify that there is more than one double bond in the chain. The sum of the positioning numbers should always be as low a possible ie. although But-3,4-diene is understandable as C-C=C=C, the sum of the numbers (3+4=7) is more than the sum of But-1,2-diene (1+2=3) which is an identical compound.

Alkynes

Alkynes are unsaturated organic compounds with at least one carbon-carbon triple bond. In nature, alkynes are found in numerous plants and fruits, and most compounds can be readily synthesized industrially or in a laboratory.

Physical Properties

Overall the physical properties of alkynes are somewhat similar to those of alkenes. Alkynes have higher boiling points than the corresponding alkanes and alkenes:

Compound: Butane < Butene < Butyne
Boiling point: Low → High

Most alkynes have low density and low solubility in water; conversely, alkynes are nonpolar and are readily dissolved in organic colvents. At room temperature, compounds with lower molecular weights are gases.

Aromatic Hydrocarbons

The best know aromatic compound is Benzene (C6H6)

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