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Very often, cis–trans stereoisomers contain double bonds or ring structures. In both cases the rotation of bonds is restricted or prevented. [4] 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 the cis isomer the two polar C−Cl bonds are on the same side of the C=C double bond and the molecular dipole moment is 1.90 D. In the trans isomer, the dipole moment is zero because the two C−Cl bonds are on opposite sides of the C=C and cancel (and the two bond moments for the much less polar C−H bonds also cancel).
The cis isomer has C 2v symmetry and the trans isomer has C 2h symmetry. These isomers can interconvert, but the process is slow enough at low temperature that the two can separated by low-temperature fractionation. [clarification needed] The trans isomer is less thermodynamically stable [2] but can be stored in glass vessels
The cis and trans isomers interconvert via the open isomer, which has no bridging ligands between iron atoms. Instead, it is formulated as (η 5-C 5 H 5)(OC) 2 Fe−Fe(CO) 2 (η 5-C 5 H 5) — the metals are held together by an iron–iron bond. At equilibrium, the cis and trans isomers are predominant.
The two compounds are isomers, each being colorless liquids with a sweet odor. It can exist as either of two geometric isomers, cis-1,2-dichloroethene or trans-1,2-dichloroethene, but is often used as a mixture of the two. They have modest solubility in water.
The trans isomer is more stable by approximately 50 kJ/mol, and the barrier to isomerization in the ground state is approximately 100 kJ/mol. Azobenzene photoisomerization. The trans form (left) can be converted to the cis form (right) using a UV wavelength of 300–400 nm. Visible illumination at >400 nm converts the molecule back to the trans ...
The two isomers are extremely difficult to separate by distillation because of the proximity of their boiling points (~4 °C for cis and ~1 °C for trans [5]). However, separation is unnecessary in most industrial settings, as both isomers behave similarly in most of the desired reactions.
trans-Dichlorodiammineplatinum(II) is the trans isomer of the coordination complex with the formula trans-PtCl 2 (NH 3) 2, sometimes called transplatin. [1] It is a yellow solid with low solubility in water but good solubility in DMF. The existence of two isomers of PtCl 2 (NH 3) 2 led Alfred Werner to propose square planar molecular geometry. [2]