This article is about the chemical concept. For "isomerism" of atomic nuclei, see nuclear isomer. A nuclear isomer is a Metastable state of an Atomic nucleus caused by the excitation of one or more of its Nucleons A nuclear isomer occupies

In chemistry, isomers are compounds with the same molecular formula but different structural formulae^[1] Isomers do not necessarily share similar properties unless they also have the same functional groups. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties A chemical formula is a way of expressing information about the Atoms that constitute a particular Chemical compound, and how the relationship between those atoms changes The structural formula of a Chemical compound is a graphical representation of the molecular structure showing how the atoms are arranged In Organic chemistry, functional groups are specific groups of Atoms within Molecules that are responsible for the characteristic Chemical reactions This should not be confused with a nuclear isomer, which involves a nucleus at different states of excitement. A nuclear isomer is a Metastable state of an Atomic nucleus caused by the excitation of one or more of its Nucleons A nuclear isomer occupies There are many different classes of isomers, like stereoisomers, enantiomers, geometrical isomers, et cetera (see graph below).

A simple example of isomerism is given by propanol: it has the formula C₃H₈O (or C₃H₇OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)

Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first isomer, and to the center carbon in the second. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Hydroxyl in Chemistry stands for a molecule consisting of an Oxygen atom and a Hydrogen atom connected by a Covalent bond. Propan-1-ol is a primary Alcohol with the formula CH3CH2CH2OH Isopropyl alcohol (also isopropanol, iso, isopro, Rubbing alcohol, or the abbreviation IPA) is a common name for Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6

There is, however, another isomer of C₃H₈O which has significantly different properties: methoxyethane (methyl-ethyl-ether; III). Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Methoxyethane, also known as methyl ethyl ether, is a colorless gaseous Ether with a medicine-like odor Unlike the isomers of propanol, methoxyethane has an oxygen atom that is connected to two carbons rather than to one carbon and one hydrogen. This makes it an ether, not an alcohol, as it lacks a hydroxyl group, and has chemical properties more similar to other ethers than to either of the above alcohol isomers. Ether is a class of Organic compounds which contain an ether group — an Oxygen Atom connected to two (substituted Alkyl Hydroxyl in Chemistry stands for a molecule consisting of an Oxygen atom and a Hydrogen atom connected by a Covalent bond.

An example of isomers having more subtly different properties can be found in certain xanthines. Xanthine (ˈzænθiːn ˈzænθaɪn] (37-dihydro-purine-26-dione is a Purine base found in most body tissues and fluids and in other organisms Theobromine, found in chocolate, is a vasodilator with some effects in common with caffeine, but if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is theophylline, which has a variety of effects, including bronchodilation and anti-inflammatory action. Theobromine, also known as xantheose, which also includes the similar compounds Theophylline and Caffeine. Chocolate ( pronounced or /-ˈələt/ comprises a number of raw and processed foods that are produced from the seed of the tropical Cacao tree Caffeine is a bitter white crystalline Xanthine Alkaloid that acts as a Psychoactive Stimulant Drug and a mild Diuretic In Chemistry, a methyl group is a Hydrophobic Alkyl Functional group named after Methane (4 Theophylline, also known as dimethylxanthine, is a Methylxanthine drug used in therapy for Respiratory diseases such as COPD or Asthma A bronchodilator is a substance that dilates the bronchi and Bronchioles decreasing airway resistance and thereby facilitating airflow Anti-inflammatory refers to the property of a substance or treatment that reduces Inflammation.

Allene and propyne are examples of isomers containing different bond types. An allene is a Hydrocarbon in which one atom of Carbon is connected by double bonds with two other atoms of carbon Methylacetylene ( propyne) is an Alkyne with the Chemical formula H 3C≡CH Allene contains two double bonds, while propyne contains one triple bond.

Classification

The different types of isomers

There are two main forms of isomerism: structural isomerism and stereoisomerism. Structural isomerism, or constitutional isomerism, is a form of isomerism in which Molecules with the same Molecular formula have Atoms Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution but which differ in the three dimensional orientations

In structural isomers, the atoms and functional groups are joined together in different ways, as in the example of propyl alcohol above. In Organic chemistry, functional groups are specific groups of Atoms within Molecules that are responsible for the characteristic Chemical reactions This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism which deals with the position of a functional group on a chain; and functional group isomerism in which one functional group is split up into different ones. In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon.

In stereoisomers the bond structure is the same, but the geometrical positioning of atoms and functional groups in space differs. This class includes enantiomers where different isomers are non-superimposable mirror-images of each other, and diastereomers when they are not. In Chemistry, an enantiomer ( from the Greek ἐνάντιος opposite and μέρος part or portion is one of two Stereoisomers that are nonsuperimposable Erythro redirects here For the fictional planet see Erythro (Asimov. Diastereomerism is again subdivided into conformational isomerism (conformers) when isomers can interconvert by chemical bond rotations and cis-trans isomerism when this is not possible. In Chemistry, conformational isomerism is a form of Stereoisomerism in which Molecules with the same Structural formula (same connectivity Trans-2-butenesvg|right|thumb|Trans-2-butene]] In Chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism is a form of Note that although conformers can be referred to as having a diastereomeric relationship, the isomers over all are not diastereomers, since bonds in conformers can be rotated to make them mirror images.

In skeletal isomers the main carbon chain is different between the two isomers. This type of isomerism is most identifiable in secondary and tertiary alcohol isomers.

Tautomers are structural isomers of the same chemical substance that spontaneously interconvert with each other, even when pure. Tautomers are Organic compounds that are interconvertible by a Chemical reaction called tautomerization. They have different chemical properties, and consequently, distinct reactions characteristic to each form are observed. If the interconversion reaction is fast enough, tautomers cannot be isolated from each other. An example is when they differ by the position of a proton, such as in keto/enol tautomerism, where the proton is alternately on the carbon or oxygen.

In food chemistry, medicinal chemistry and biochemistry, cis-trans isomerism is always considered. Food chemistry is the study of Chemical processes and interactions of all biological and non-biological components of foods Biochemistry is the study of the chemical processes in living Organisms It deals with the Structure and function of cellular components such as Trans-2-butenesvg|right|thumb|Trans-2-butene]] In Chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism is a form of In medicinal chemistry and biochemistry, enantiomers are of particular interest since most changes in these types of isomers are now known to be meaningful in living organisms. Biochemistry is the study of the chemical processes in living Organisms It deals with the Structure and function of cellular components such as In Chemistry, an enantiomer ( from the Greek ἐνάντιος opposite and μέρος part or portion is one of two Stereoisomers that are nonsuperimposable Pharmaceutical and academic researchers have found chromatographical methods to reliably separate these from each other. Chromatography (from Greek χρώμα chroma, color and γραφειν"graphein" to write is the collective term for a family of Laboratory On an industrial scale, however, these methods are rather costly and are mostly used to filter out the potentially harmful or biologically inactive enantiomer.

While structural isomers typically have different chemical properties, stereoisomers behave identically in most chemical reactions, except in their reaction with other stereoisomers. Enzymes however can distinguish between different enantiomers of a compound, and organisms often prefer one isomer over the other. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Some stereoisomers also differ in the way they rotate polarized light. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations

Other types of isomerism exist outside this scope. Topological isomers called topoisomers are generally large molecules that wind about and form different shaped knots or loops. Topology ( Greek topos, "place" and logos, "study" is the branch of Mathematics that studies the properties of Topoisomers or topological isomers are Molecules with the same Chemical formula and stereochemical bond connectivities but different topologies Molecules with topoisomers include catenanes and DNA. A catenane is a mechanically-interlocked molecular architecture consisting of two or more interlocked Macrocycles The interlocked rings cannot be separated Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Topoisomerase enzymes can knot DNA and thus change its topology. Topoisomerases (type I, type II) are Isomerase Enzymes that act on the topology of DNA. There are also isotopomers or isotopic isomers that have the same numbers of each type of isotopic substitution but in chemically different positions. Isotopomers (isotopic isomers are Isomers having the same number of each Isotopic atom but differing in their positions In nuclear physics, nuclear isomers are excited states of atomic nuclei. Nuclear physics is the field of Physics that studies the building blocks and interactions of Atomic nuclei. A nuclear isomer is a Metastable state of an Atomic nucleus caused by the excitation of one or more of its Nucleons A nuclear isomer occupies Spin isomers have differing distributions of spin among their constituent atoms. Each Hydrogen Molecule (H2 consists of two Hydrogen atoms linked by a Covalent bond. In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin

History

Isomerism was first noticed in 1827, when Friedrich Woehler prepared cyanic acid and noted that although its elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different. Friedrich Wöhler (31 July 1800 - 23 September 1882 was a German Chemist, best-known for his synthesis of Urea, but also the first to isolate several Fulminic acid is a compound with a Molecular formula HCNO This substance was discovered in 1824 by Justus von Liebig. Justus von Liebig ( May 12, 1803 &ndash April 18, 1873) was a German Chemist This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be different only when they had different elemental compositions. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. After additional discoveries of the same sort were made, such as Woehler's 1828 discovery that urea had the same atomic composition as the chemically distinct ammonium cyanate, Jöns Jakob Berzelius introduced the term isomerism to describe the phenomenon. Urea is an Organic compound with the Chemical formula ( N[[hydrogen H]]22 C[[oxygen O]] Friherre Jöns Jacob Berzelius (20 August 1779 &ndash 7 August 1848 was a Swedish chemist

In 1849, Louis Pasteur separated tiny crystals of tartaric acid into their two mirror-image forms. Louis Pasteur (27 December 1822 – 28 September 1895 a French Chemist and Microbiologist, is best known for remarkable breakthroughs in the causes and Tartaric acid is a white crystalline Organic acid. It occurs naturally in many plants particularly Grapes Bananas and Tamarinds and is "Mirror Image" is an episode of the Television series The Twilight Zone. The individual molecules of each were the left and right optical stereoisomers, solutions of which rotate the plane of polarized light in opposite directions. Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution but which differ in the three dimensional orientations Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700

See also

• Structural isomerism
• Cis-trans isomerism
• Isotopomers
• Isotopologue
• Chirality (chemistry)

References