Information about Geometric Isomerism
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Cis-2-butene
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Trans-2-butene
In chemistry, cis-trans isomerism or geometric isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. Such isomers typically contain double bonds which cannot rotate, but they can also arise from ring structures where the rotation of bonds is greatly restricted.
The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by IUPAC. It is sometimes used as a synonym for general stereoisomerism (e.g. optical isomerism being called geometric isomerism); the correct term for non-optical stereoisomerism is diastereomerism.
There are two forms of a cis-trans isomer, the cis and trans versions. When the substituent groups are oriented in the same direction the diastereomer is referred to as cis, while when the substituents are oriented in opposing directions the diastereomer is referred to as trans. An example of a small hydrocarbon displaying cis-trans isomerism is 2-butene.
Alicyclic compounds can also display cis-trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:
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| trans-1,2-dichlorocyclohexane | cis-1,2-dichlorocyclohexane |
Cis vs trans physical properties
Cis isomers and trans isomers often have different physical properties. Differences between isomers generally arise from the differences in the shape of the molecule or the overall dipole moment. These differences can be very small as in the case of the boiling point of straight chained alkenes, such as 2-pentene where the trans isomer has a boiling point of 36°C and cis isomer is 37°C [1]. The differences between cis and trans isomers can also be quite large as in the case of cyclooctene. The cis isomer in this case has a boiling point of 145°C [2], while the trans isomer has a boiling point of 75°C.[3] The large difference between the two isomers of cyclooctene arise from the large ring strain for trans-cyclooctene, which also makes it significantly less stable than the cis isomer. In fact, the two isomers of 2-butenedioic acid have such large differences in properties and reactivities that they were actually given completely different names. The cis isomer is called maleic acid and the trans is named fumaric acid.In the case of geometric isomers that are a consequence of double bonds and in particular when both substituents are the same some general trends usually hold. These trends can be attributed to the fact that the dipoles of the substituents will add when together for a cis isomer to give an overall molecular dipole while for trans isomer the dipoles of the substituents will cancel out . Trans isomers also tend to have lower densities than their cis counterparts.
March [4] observes that as trans alkenes in general have more symmetry that cis alkenes, the trans alkenes also tend to have higher melting points and lower solubility in inert solvents.
Vicinal coupling constants (3JHH), measured by NMR spectroscopy, are larger for trans- (range: 12-18 Hz, typical: 15 Hz) than for cis- (range: 0-12 Hz, typical: 8 Hz) isomers.[5]
Stability
According to March trans isomers also have a lower heat of combustion, indicating higher thermochemical stability. In the Benson Heat of formation group additivity dataset cis isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist. For instance for 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF) and several other halogen and oxygen substituted ethylenes the cis isomer is more stable than the trans isomer.[6] This phenomenon is called the cis effect.[7]E/Z notation
The cis/trans system for naming isomers breaks down when there are more than two different substituents on a double bond. The E/Z notation should then be used. Z (from the German zusammen) means together and corresponds to the term cis; E (from the German entgegen) means opposite and corresponds to the term trans.Whether a molecular configuration is designated E or Z is determined by the Cahn Ingold Prelog priority rules (higher atomic numbers are given higher priority). For each of the two atoms in the double bond, individually determine which of the two substituents is of a higher priority. If both of the substituents of higher priority are on the same side, the arrangement is Z; if they are on opposite sides the arrangement is E.
External links
- The IUPAC definition of "stereoisomerism"
- The IUPAC definition of "geometric isomerism"
- The IUPAC definition of "cis-trans isomers"
See also
- Important publications in organic chemistry
References
1. ^ Chemicalland values
2. ^ Akros data
3. ^ Organic Syntheses, Coll. Vol. 5, p.315 (1973); Vol. 49, p.39 (1969) Link
4. ^ Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. page 111 Jerry March ISBN 0-471-85472-7
5. ^ "Spectroscopic Methods in Organic Chemistry," Dudly H. WIlliams and Ian FLeming, 4th ed. revised,McGraw-Hill Book COmpany (UK) Limited, 1989.Table 3.27
6. ^ The stereochemical consequences of electron delocalization in extended .pi. systems. An interpretation of the cis effect exhibited by 1,2-disubstituted ethylenes and related phenomena Richard C. Bingham J. Am. Chem. Soc.; 1976; 98(2); 535-540 Abstract
7. ^ Contribution to the Study of the Gauche Effect. The Complete Structure of the Anti Rotamer of 1,2-Difluoroethane Norman C. Craig et al. J. Am. Chem. Soc.; 1997; 119 p 4789 doi:10.1021/ja963819e
2. ^ Akros data
3. ^ Organic Syntheses, Coll. Vol. 5, p.315 (1973); Vol. 49, p.39 (1969) Link
4. ^ Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. page 111 Jerry March ISBN 0-471-85472-7
5. ^ "Spectroscopic Methods in Organic Chemistry," Dudly H. WIlliams and Ian FLeming, 4th ed. revised,McGraw-Hill Book COmpany (UK) Limited, 1989.Table 3.27
6. ^ The stereochemical consequences of electron delocalization in extended .pi. systems. An interpretation of the cis effect exhibited by 1,2-disubstituted ethylenes and related phenomena Richard C. Bingham J. Am. Chem. Soc.; 1976; 98(2); 535-540 Abstract
7. ^ Contribution to the Study of the Gauche Effect. The Complete Structure of the Anti Rotamer of 1,2-Difluoroethane Norman C. Craig et al. J. Am. Chem. Soc.; 1997; 119 p 4789 doi:10.1021/ja963819e
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Stereoisomers are isomeric molecules whose atomic connectivity is the same but whose atomic arrangement in space is different.
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Enantiomers
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In organic chemistry, functional groups (or moieties) are specific groups of atoms within molecules, that are responsible for the characteristic chemical reactions of those molecules.
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Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.
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The International Union of Pure and Applied Chemistry (IUPAC) (IPA: [aɪ ju pæk]) is an international non-governmental organization established in 1919 devoted to the advancement of chemistry.
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The term chiral (pronounced /ˈkaɪɹ(ə)l̩/) is used to describe an object that is non-superimposable on its mirror image. In terms of chemistry, such objects are usually molecules.
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Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (mirror images of each other). Diastereomers can have different physical properties and different reactivity.
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For lower-case cis, see .
CIS usually refers to:
- Commonwealth of Independent States, a modern-day political entity consisting of 11 former Soviet Union Republics
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Trans is a Latin noun or prefix, meaning "across", "beyond" or "on the opposite side [of]" . It is the opposite of cis, which means "on the same side [of]".
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In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl (meaning "attached to") is used when naming organic compounds that contain a substituent.
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1-butene α-butylene
cis-2-butene cis-β-butylene
trans-2-butene trans-β-butylene
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alicyclic compound is an organic compound that is both aliphatic and cyclic. They contain one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character.
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The bond dipole moment is a measure for the polarity of a chemical bond within a molecule. The bond dipole μ is given by:
The bond dipole is modeled as +δ — δ- with a distance d
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- .
The bond dipole is modeled as +δ — δ- with a distance d
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Maleic acid (ionised maleate in biology) or (Z)-butenedioic acid or cis-butenedioic acid or malenic acid or maleinic acid or toxilic acid
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Fumaric acid is the chemical compound with the formula HO2CCH=CHCO2H. This colorless crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid wherein the carboxylic acid groups are cis.
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The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds
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Solubility is a physical property referring to the ability for a given substance, the solute, to dissolve in a solvent.[1] It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution.
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vicinal (from Latin vicinus = neighbour) stands for any two functional groups bonded to two adjacent carbon atoms. For example the molecule 2,3-dibromobutane carries two vicinal bromine atoms and 1,3-dibromobutane does not.
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J-coupling (also called indirect dipole dipole coupling) is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei.
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Nuclear magnetic resonance spectroscopy most commonly known as NMR spectroscopy is the name given to the technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on Nuclear magnetic resonance.
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The Heat of combustion (ΔHc0) is the energy released as heat when a compound undergoes complete combustion with oxygen. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat.
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In thermodynamics and physical chemistry, thermochemistry is the study of the heat evolved or absorbed in chemical reactions. Thermochemistry, generally, is concerned with the heat exchange accompanying transformations, such as mixing, phase transitions, chemical reactions, etc.
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Heat of formation group additivity methods in thermochemistry enable the calculation and prediction of heat of formation of organic compounds based on additivity. This method was pioneered by S. W. Benson [1].
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Cahn-Ingold-Prelog priority rules, CIP system or CIP conventions are a set of rules used in organic chemistry to name the stereoisomers of a molecule. A molecule may contain any number of stereocenters and any number of double bonds, and each gives rise to two
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Organic Syntheses is a scientific journal that since 1921 has provided the chemistry community with annual collections of detailed and checked procedures for the organic synthesis of organic compounds.
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Journal of the American Chemical Society (usually abbreviated as J. Am. Chem. Soc., or JACS), is a peer-reviewed scientific journal, published since 1879 by the American Chemical Society.
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Journal of the American Chemical Society (usually abbreviated as J. Am. Chem. Soc., or JACS), is a peer-reviewed scientific journal, published since 1879 by the American Chemical Society.
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digital object identifier (or DOI) is a permanent identifier given to a document, which is not related to its current location. A typical use of a DOI is to give a scientific paper or article a unique identifying number that can be used by anyone to locate details of the paper, and
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