Cis-2-butene

Trans-2-butene

In chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties 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 Organic chemistry, functional groups are specific groups of Atoms within Molecules that are responsible for the characteristic Chemical reactions In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted.

The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by IUPAC. The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization 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. The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image Erythro redirects here For the fictional planet see Erythro (Asimov.

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, whereas, when the substituents are oriented in opposing directions, the diastereomer is referred to as trans. 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 An example of a small hydrocarbon displaying cis-trans isomerism is 2-butene. Isomers Among the molecules which have the Chemical formula 48 four Isomers are Alkenes.

Alicyclic compounds can also display cis-trans isomerism. An alicyclic compound is an Organic compound that is both Aliphatic and cyclic As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:

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, in general, arise from the differences in the shape of the molecule or the overall dipole moment. The bond dipole moment uses the idea of Electric dipole moment to measure the polarity of a chemical bond within a Molecule. These differences can be very small, as in the case of the boiling point of straight-chained alkenes, such as pent-2-ene, wherein 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. Cyclooctene is a Cycloalkene with an eight-membered ring It can exist as either the ''cis'' - or trans - Isomer with the cis -isomer 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 arises 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. Maleic acid or ( Z)-butenedioic acid or cis -butenedioic acid or malenic acid or maleinic acid or toxilic Fumaric acid is the Chemical compound with the formula HO2CCH=CHCO2H Polarity is key in determining relative boiling point as it causes increased intermolecular forces, thereby raising the boiling point. In the same manner, symmetry is key in determining relative melting point as it allows for better packing in the solid state. Thus, trans-alkenes which are less polar and more symmetrical have lower boiling points and higher melting points and cis-alkenes, which are generally more polar and less symmetrical have higher boiling points and lower melting points.

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 in a cis isomer will add up to give an overall molecular dipole. In a trans isomer, the dipoles of the substituents will cancel out due to their being on opposite site of the molecule. Trans isomers also tend to have lower densities than their cis counterparts.

March^[4] observes that, as trans alkenes, in general, have more symmetry than cis alkenes, the trans alkenes also tend to have higher melting points and lower solubility in inert solvents. The melting point of a solid is the temperature range at which it changes state from solid to Liquid. Solubility is the characteristic Physical property referring to the ability of a given substance the Solute, to dissolve in a Solvent.

Vicinal coupling constants (³J_HH), 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. In Chemistry vicinal (from Latin vicinus = neighbour stands for any two Functional groups bonded to two adjacent Carbon atoms 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 Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei ^[5]

Stability

Usually, trans isomers are more stable than the cis isomer. This is partly due to their shape, the straighter shape of the trans isomer leads to hydrogen intermolecular forces that make the isomer more stable. According to March, trans isomers also have a lower heat of combustion, indicating higher thermochemical stability. The heat of combustion (ΔHc0 is the Energy released as Heat when a compound undergoes complete Combustion with Oxygen In Thermodynamics and Physical chemistry, thermochemistry is the study of the Heat evolved or absorbed in Chemical reactions Thermochemistry In the Benson Heat of formation group additivity, dataset cis isomers suffer a 1. Heat of formation group additivity methods in Thermochemistry enable the calculation and prediction of Heat of formation of Organic compounds based on 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. In this case, the cis isomer is more stable than the trans isomer. ^[6] This phenomenon is called the cis effect. ^[7]

E/Z notation

Main article: E-Z notation

Bromine has a higher CIP priority than chlorine, so this alkene is the Z isomer. The more complex Stereochemistry of highly substituted Alkenes (especially 3 or 4 substituents cannot be defined using the Cis-trans notation

The cis/trans system for naming isomers is not effective 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, it is necessary to 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

• Isomer
• Structural isomerism
• Chirality (chemistry)
• Important publications in organic chemistry

References

1. ^ Chemicalland values
2. ^ Akros data
3. ^ Organic Syntheses, Coll. This article is about the chemical concept For "isomerism" of atomic nuclei see Nuclear isomer. Structural isomerism, or constitutional isomerism, is a form of isomerism in which Molecules with the same Molecular formula have Atoms The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image Some criteria for editing this page have been debated and are displayed Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures 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 Journal of the American Chemical Society (usually abbreviated as J 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