How many isomers are possible for butane?

Isomerism - structure and properties


Two compounds with the same empirical formula but different molecular structures are called isomeric compounds or isomers for short. They have the same type and number of atoms, but their atoms are arranged differently. In the case of butane with the empirical formula C.4H10 As the first in the homologous series of alkanes, two different structures are conceivable, and indeed: We know two types of butane with the same empirical formula but different properties. This phenomenon of isomeric molecules is called isomerism. The term comes from the Greek "isos" for equal and "meros" for part.

Alkanes with an unbranched, stretched carbon chain are called n-alkanes (from normal alkanes). Opposite them are the isomeric alkanes with the same number of atoms but with a branched carbon chain. They are called i-alkanes (from iso-alkanes). A molecular formula with 4 or more carbon atoms can no longer be clearly assigned to a specific molecular structure due to the possibility of isomerism. In structural formulas (the preferred representation in organic chemistry) a spatial representation is dispensed with and the tetrahedron angle is simply drawn as a right angle. Since the structural formulas of large molecules can be quite confusing, the clear semi-structural formulas are often used. Here the carbon atoms with their directly bonded hydrogen atoms become CH2- Groups or CH3- Groups combined, for example CH3-CH2-CH2-CH3.

The number of isomers in the homologous series of alkanes increases dramatically with longer carbon chains. While in the simplest case from the molecular formula C4H10 can derive two possible isomers, it is with C10H22 already 75. In the case of Tetracontane with the empirical formula C40H82 the number of possible isomers has risen to more than 62 billion. Few of these compounds were ever made or occur naturally. The isomerism shows the most important peculiarity of the carbon atom: It is able to form branched chains. The isomerism is thus one of the explanations for the Variety of organic compounds.

Different behavior of isomeric compounds

Isomers differ in at least one chemical or physical property. For example, a comparison of butane and isobutane (2-methyl-propane), which both have the empirical formula C4H10 have that both have different melting and boiling temperatures. The melting temperature of butane is -135 ° C, that of isobutane is -165 ° C.

In molecules with branched chains, there are more atoms inside the molecule, which reduces the surface area and thus fewer nuclei and electrons can interact with one another. In the case of branched hydrocarbons, the van der Waals forces act less than in the case of branched hydrocarbons, which is expressed in lower melting and boiling points.

Naming of isomers

Given the large number of possible organic compounds, the scientists had to develop a uniform and understandable system for naming isomeric compounds. The following must be observed:

  1. First you look for the longest continuous carbon chain. The root name of the alkane follows from the number of carbon atoms in this chain. The tribe name is derived from the longest carbon chain.
  2. The side chains are also named after the number of their carbon atoms and are placed in front of the stem name. These alkyl groups are derived from the formal splitting off of a hydrogen atom from the alkane molecules. Instead of the ending -an you get the ending -yl (e.g. methyl for -CH3, Ethyl for -C2H5, Propyl for -C3H7 etc.) If there are different side chains in the molecule, they are sorted alphabetically.
  3. If the same side chains appear several times, the corresponding Greek numeral (di-, tri-, tetra-, penta-) is used as a prefix. They are ignored in the alphabetical order of the side chains.
  4. To identify the connection points between the main and side chains, the carbon atoms of the main chain are numbered consecutively. It should be noted that the connection points are given the smallest possible numbers. If there are two side chains from a carbon atom in the main chain, the number of this carbon atom appears twice in the isomer name.