Editing Cis–trans isomerism (section)

== Organic chemistry ==
Very often, ''cis''–''trans'' stereoisomers contain [[double bond]]s or ring structures. In both cases the rotation of bonds is restricted or prevented.<ref name="Reusch10">{{cite web|first=William |last=Reusch |date=2010 |title=Stereoisomers Part I |website=Virtual Textbook of Organic Chemistry |publisher=Michigan State University |url=http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/sterisom.htm#start |access-date=7 April 2015}}</ref> When the [[substituent|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''. An example of a small hydrocarbon displaying ''cis''–''trans'' isomerism is [[2-butene|but-2-ene]]. 1,2-Dichlorocyclohexane is another example.

{| class="wikitable" style="margin: 1em auto 1em auto"
|[[Image:Trans-1,2-dichlorocyclohexane-2D-skeletal.svg|class=skin-invert-image|100px]] [[Image:Trans-1,2-dichlorocyclohexane-3D-balls.png|150px]] || [[Image:Cis-1,2-dichlorocyclohexane-2D-skeletal.svg|class=skin-invert-image|100px]] [[Image:Cis-1,2-dichlorocyclohexane-3D-balls.png|150px]]
|- style="text-align:center;"
| ''trans''-1,2-dichlorocyclohexane || ''cis''-1,2-dichlorocyclohexane
|}

=== Comparison of physical properties ===
''Cis'' and ''trans'' isomers have distinct physical properties. Their differing shapes influences the [[Bond dipole moment|dipole moment]]s, boiling, and especially melting points.
{| class="wikitable skin-invert-image"  style="text-align:center; margin: 1em auto 1em auto"
|[[File:Cis-2-pentene.svg|150px]]||[[File:Trans-2-pentene.svg|150px]]
|- 
|''cis''-2-pentene||''trans''-2-pentene
|-
|[[File:Cis-1,2-dichloroethene.png|110px]]||[[File:Trans-1,2-dichloroethene.png|110px]]
|- 
|''cis''-1,2-dichloroethene||''trans''-1,2-dichloroethene
|}
These differences can be very small, as in the case of the boiling point of straight-chain alkenes, such as [[Pentene|pent-2-ene]], which is 37&nbsp;°C in the ''cis'' isomer and 36&nbsp;°C in the ''trans'' isomer.<ref>{{cite web|url=http://www.chemicalland21.com/info/Alkene%20Compound%20Boiling%20Points.htm |title=Chemicalland values |publisher=Chemicalland21.com |access-date=2010-06-22}}</ref> The differences between ''cis'' and ''trans'' isomers can be larger if polar bonds are present, as in the [[1,2-dichloroethene]]s. The ''cis'' isomer in this case has a boiling point of 60.3&nbsp;°C, while the ''trans'' isomer has a boiling point of 47.5&nbsp;°C.<ref>{{cite book|title=CRC Handbook of Chemistry and Physics|edition=60th|date=1979–1980|page=C-298}}</ref> In the ''cis'' isomer the two polar C–Cl [[bond dipole moment]]s combine to give an overall molecular dipole, so that there are intermolecular [[intermolecular forces#Dipole-dipole interactions|dipole–dipole forces]] (or Keesom forces), which add to the [[London dispersion forces]] and raise the boiling point. In the ''trans'' isomer on the other hand, this does not occur because the two C−Cl bond moments cancel and the molecule has a net zero dipole moment (it does however have a non-zero [[quadrupole moment]]).
{| class="wikitable"  style="text-align:center; margin: 1em auto 1em auto"
|[[File:Maleic-acid-3D-balls-A.png|150px]]||[[File:Fumaric-acid-3D-balls.png|150px]]
|-
|''cis''-butenedioic acid <br> ([[maleic acid]])|| ''trans''-butenedioic acid <br> ([[fumaric acid]])
|-
|[[File:Oleic-acid-3D-vdW.png|270px]]||[[File:Elaidic-acid-3D-vdW.png|270px]]
|- style="text-align:center;"
|''cis''-9-octadecenoic acid <br> ([[oleic acid]]) || ''trans''-9-octadecenoic acid <br> ([[elaidic acid]])
|}
The differing properties of the two isomers of butenedioic acid are often very different. 
{| class="wikitable"
|+ Properties of isomers of ''cis''- and ''trans''-{{chem2|HO2CH\dCHCO2H}}
|-
! !! maleic acid !! fumaric acid
|-
| color || white || white
|-
| melting point, °C || 130 || 286
|-
| water solubility, g/L || 788 || 7
|-
| [[Acid dissociation constant]], pK<sub>a1</sub> || 1.90 || 3.03
|}

Polarity is key in determining relative boiling point as strong intermolecular forces raise 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, even if it does not alter the polarity of the molecule.  Another example of this is the relationship between [[oleic acid]] and [[elaidic acid]]; oleic acid, the ''cis'' isomer, has a melting point of 13.4&nbsp;°C, making it a liquid at room temperature, while the ''trans'' isomer, elaidic acid, has the much higher melting point of 43&nbsp;°C, due to the straighter ''trans'' isomer being able to pack more tightly, and is solid at room temperature.

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 <ref>{{cite book |doi=10.1016/B978-0-12-802444-7.00004-5 |chapter=Alkenes and Alkynes |title=Principles of Organic Chemistry |year=2015 |last1=Ouellette |first1=Robert J. |last2=Rawn |first2=J. David |pages=95–132 |isbn=978-0-12-802444-7 }}</ref> due to being on opposite sides of the molecule. ''Trans'' isomers also tend to have lower densities than their ''cis'' counterparts.{{Citation needed|date=February 2007}}

As a general trend, ''trans'' alkenes tend to have higher [[melting point]]s and lower [[solubility]] in inert solvents, as ''trans'' alkenes, in general, are more symmetrical than ''cis'' alkenes.<ref name="march">{{cite book|title=Advanced Organic Chemistry, Reactions, Mechanisms and structure |edition=3rd |page=111 |first=Jerry |last=March |year=1985 |isbn=978-0-471-85472-2 }}</ref>

[[Vicinal (chemistry)|Vicinal]] [[J-coupling|coupling constant]]s (<sup>3</sup>''J''<sub>HH</sub>), measured by [[NMR spectroscopy]], are larger for ''trans'' (range: 12–18&nbsp;Hz; typical: 15&nbsp;Hz) than for ''cis'' (range: 0–12&nbsp;Hz; typical: 8&nbsp;Hz) isomers.<ref>{{cite book|title=Spectroscopic Methods in Organic Chemistry|first1=Dudley H.|last1=Williams|first2=Ian|last2=Fleming|edition=4th rev.|publisher=McGraw-Hill |date=1989|chapter=Table 3.27|isbn=978-0-07-707212-4}}</ref>

==== Stability ====

Usually for acyclic systems ''trans'' isomers are more stable than ''cis'' isomers. This difference is attributed to the unfavorable [[Steric effects|steric interaction]] of the substituents in the ''cis'' isomer. Therefore, ''trans'' isomers have a less-exothermic [[heat of combustion]], indicating higher [[thermochemistry|thermochemical]] stability.<ref name="march"/> In the Benson [[heat of formation group additivity]] dataset, ''cis'' isomers suffer a 1.10&nbsp;kcal/mol stability penalty. Exceptions to this rule exist, such as [[1,2-difluoroethylene]], [[1,2-difluorodiazene]] (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, the ''cis'' isomer is more stable than the ''trans'' isomer.<ref>{{cite journal|title=The stereochemical consequences of electron delocalization in extended π systems. An interpretation of the ''cis'' effect exhibited by 1,2-disubstituted ethylenes and related phenomena|first=Richard C.|last=Bingham|journal=[[J. Am. Chem. Soc.]]|date=1976|volume=98|issue=2|pages=535–540|doi=10.1021/ja00418a036}}</ref> This phenomenon is called the ''[[cis effect (organic chemistry)|cis effect]]''.<ref>{{Cite journal | last1 = Craig | first1 = N. C. | last2 = Chen | first2 = A. | last3 = Suh | first3 = K. H. | last4 = Klee | first4 = S. | last5 = Mellau | first5 = G. C. | last6 = Winnewisser | first6 = B. P. | last7 = Winnewisser | first7 = M. | title = Contribution to the Study of the Gauche Effect. The Complete Structure of the ''Anti'' Rotamer of 1,2-Difluoroethane | journal = [[J. Am. Chem. Soc.]] | volume = 119 | issue = 20 | pages = 4789 | year = 1997 | doi = 10.1021/ja963819e}}</ref>

=== ''E''–''Z'' notation ===
{{Main article|E–Z notation}}
[[File:(Z)-1-Bromo-1,2-dichloroethene.svg|class=skin-invert-image|thumb|Bromine has a higher [[Cahn–Ingold–Prelog priority rules|CIP priority]] than chlorine, so this alkene is the ''Z'' isomer]]

In principle, ''cis''–''trans'' notation should not be used for alkenes with two or more different substituents. Instead the ''E''–''Z'' notation is used based on the priority of the substituents using the [[Cahn–Ingold–Prelog priority rules|Cahn–Ingold–Prelog (CIP) priority rules]] for absolute configuration. The IUPAC standard designations ''E'' and ''Z'' are unambiguous in all cases, and therefore are especially useful for tri- and tetrasubstituted alkenes to avoid any confusion about which groups are being identified as ''cis'' or ''trans'' to each other.

''Z'' (from the German {{lang|de|zusammen}}) means "together". ''E'' (from the German {{lang|de|entgegen}}) means "opposed" in the sense of "opposite". That is, ''Z'' has the higher-priority groups ''cis'' to each other and ''E'' has the higher-priority groups ''trans'' to each other. Whether a molecular configuration is designated ''E'' or ''Z'' is determined by the CIP rules; higher atomic numbers are given higher priority. For each of the two atoms in the double bond, it is necessary to determine the priority of each substituent. If both the higher-priority substituents are on the same side, the arrangement is ''Z''; if on opposite sides, the arrangement is ''E''.

Because the ''cis''–''trans'' and ''E''–''Z'' systems compare different groups on the alkene, it is not strictly true that ''Z'' corresponds to ''cis'' and ''E'' corresponds to ''trans''. For example, ''trans''-2-chlorobut-2-ene (the two methyl groups, C1 and C4, on the [[2-Butene|but-2-ene]] backbone are ''trans'' to each other) is (''Z'')-2-chlorobut-2-ene (the chlorine and C4 are together because C1 and C4 are opposite).

==== Undefined alkene stereochemistry ====
Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry or a mixture of isomers (as with tetrahedral stereocenters). A crossed double-bond has been used sometimes; it is no longer considered an acceptable style for general use by [[IUPAC]] but may still be required by computer software.<ref name="IUPAC-2006">{{cite journal |first=Jonathan |last=Brecher |title=Graphical representation of stereochemical configuration (IUPAC Recommendations 2006) |journal=[[Pure and Applied Chemistry]] |year=2006 |volume=78 |issue=10 |pages=1897–1970 |url=http://www.iupac.org/publications/pac/pdf/2006/pdf/7810x1897.pdf |doi=10.1351/pac200678101897 |s2cid=97528124 }}</ref>

[[File:E-Z notation in alkenes.svg|class=skin-invert-image|thumb|400px|center|Alkene stereochemistry]]