What are isomers of hexane

In the case of organic substances, there are many substances that have the same empirical formula, but differ in their molecular structure and substance properties. Molecules with the same empirical formula but different structures or spatial arrangements are called Isomers. A distinction is made between two types of isomerism, within which there are further subdivisions:

1. At the Structure isomerism or Constitutional isomerismthe atoms within a molecule are linked differently. The isomers differ in their constitution: they have the same empirical formula, but a different structure. A transfer is only possible by breaking the bond.

2. At the Stereo isomerism the atoms are linked to one another in the same places, they only differ in their spatial position, in their configuration. The isomers have the same molecular formula and the same structure. A transfer is only possible by breaking the bond. The conformational isomers that can be converted by rotation are an exception.

1. Structure isomerism or constitution isomerism 
1.1 Positional isomerism

If the isomers differ with regard to the position of the functional groups or the position of the alkyl radicals, one speaks of positional isomerism. The material properties of the isomers differ; a transfer is only possible by breaking a bond. There are five isomers of hexane. Hexane (n-Hexane) and 2-methyl-pentane (isohexane), for example, have different boiling points. They also differ in their chemical behavior: there is a lot in car gasoline n-Hexane, the engine tends to pre-ignite. This phenomenon is known as "knocking". Knock-proof gasoline, on the other hand, contains a great deal of branched isohexane. This example shows that constitutional isomers differ chemically from one another.

Hexane (n-Hexane)
Molecular formula: C6H14
Boiling point: +69 ° C
2-methyl-pentane (isohexane)
Molecular formula: C6H14
Boiling point: +60 ° C

Further examples

n-Butane and 2-methylpropane for butane
nPentane and 2-methylbutane and 2,2-dimethylpropane for pentane
Propan-1-ol and propan-2-ol
Four isomers of butanol
1-naphthylamine and 2-naphthylamine
o-xylene, m-xylene and p-xylene
Isomerism in glucose

1.2 Function isomerism

If two substances have the same molecular formula but different functional groups, then there is a functional isomerism. This is the case, for example, with butyric acid and ethyl acetate. These two substances belong to different substance classes. They also show completely different chemical-physical properties.

Butanoic acid (butyric acid)
Molecular Formula C4H8O2
Ethyl acetate
Molecular Formula C4H8O2

Further examples
Diethyl ether and butanol

2. Stereo isomerism 
2.1 Configuration isomerism

a) Mirror image isom

In mirror image isomerism or enantiomerism, two molecules are linked in the same way, but behave like an image and a mirror image. This type of isomerism occurs in almost all amino acids or in tartaric acid. The Enantiomers cannot be converted into one another by rotation. They have the same chemical-physical properties such as the same boiling point and the same solubility. However, they differ in optically active processes; this plays a role, for example, in digestion by enzymes.

D (-) -Lactic acid
Boiling point +122 ° C
optically counter-clockwise
L (+) -Lactic acid
Boiling point +122 ° C
optically clockwise

Lactic acid, which is mainly found in sour milk, contains two mirror image enantiomers. While the left turning D (-) -Lactic acid mainly arises from the microbiological breakdown of glucose, comes the clockwise L (+) -Lactic acid in the blood, muscles and organs of animals and humans. If linearly polarized light is sent through the solutions of the two enantiomers, the plane of vibration rotates in the case of the clockwise one L (+) -Lactic acid to the right and to the left-turning D (-) -Lactic acid to the left. This phenomenon was discovered by Louis Pasteur (1822-1895). The sour milk contains a mixture of both optical enantiomers. Such a mixture will also Racemate called. Racemates also occur in tartaric acid or galactose, for example.

Further examples
L (+) - tartaric acid and D (-) - tartaric acid for tartaric acid
Malic acid
Aspartic acid

b) diastereomerism

The diastereomerism includes all isomers that are stereoisomeric but do not behave like an image and a mirror image. The D-mannitol molecule and the sorbitol molecule are not enantiomers in comparison with one another, but there is still a stereo isomerism: they are Diastereomers. They differ in their chemical-physical properties.

Density 1.489 g / cm3
Melting point +168 ° C
Density 1.489 g / cm3
Melting point +111 ° C

Further examples
meso-tartaric acid and L (+) - tartaric acid for tartaric acid

A special form of diastereomerism is that cis-trans-Isomerism. Here atoms or groups of atoms lie in double bonds or in rings on the same side (cis) or on the opposite side (trans). The isomers cannot be converted into one another by rotation. The cis-trans-Isomers of butene differ in their chemical-physical properties. A cis-trans-Isomerism also occurs in unsaturated fatty acids.

Boiling point 3.7 ° C
Density 0.621 g / cm³
Boiling point 0.9 ° C
Density 0.604 g / cm³

Further examples
Oleic acid and elaidic acid
Maleic acid and fumaric acid

2.2 Conformational isomerism

In conformational isomerism, the molecules of the isomers can be converted into one another by rotation without breaking the bond. A well-known example is that Armchair shape and the Tub shape of cyclohexane. The chair shape is the most stable shape because all atom groups are staggered to one another. With cyclohexane there is even a third conformation, the Twist shapewhere the ring is severely twisted.

Tub shape
Armchair shape

If the atoms of the ethane molecule are one behind the other, it is thehidden conformation, if they are rotated by 60 °, you get the staggered conformation. The latter is energetically more favorable and therefore more stable. At room temperature, the molecules compensate for this energy difference through constant movement.

Conformation obscured
Staggered conformation

3. Same material or different material?

An important question is whether isomers are two different substances or whether they can be considered one substance. According to IUPAC, the term chemical substancecharacterized by its entity ("Entities"), i.e. its atomic and molecular structure, and its physical properties "... such as density, refractive index, electric conductivity, melting point, etc." (IUPAC, Golden Book 2014) According to this definition, all isomers are different substances because they have different physical properties. The example of the conformational isomerism shows, however, that the transitions are fluid. At room temperature, the two conformations continuously merge. If ethane molecules were available in a hidden and staggered conformation in a pure form, these would also be two different substances.

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