`color{green}("𝐀𝐥𝐤𝐞𝐧𝐞𝐬 𝐬𝐡𝐨𝐰 𝐛𝐨𝐭𝐡 𝐬𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦 𝐚𝐧𝐝 𝐠𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦.")`
`color{green}("𝐒𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦 :")` As in alkanes, ethene `color{red}((C_2H_4))` and propene `color{red}((C_3H_6))` can have only one structure but alkenes higher than propene have different structures. Alkenes possessing `color{red}(C_4H_8)` as molecular formula can be written in the following three ways:
Structures I and III, and II and III = `color{green}("𝐜𝐡𝐚𝐢𝐧 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦")`
Structures I and II = `color{green}("𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧 𝐢𝐬𝐨𝐦𝐞𝐫𝐬.")`
`color{green}("𝐆𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦 :")` Doubly bonded carbon atoms have to satisfy the remaining two valences by joining with two atoms or groups. If the two atoms or groups attached to each carbon atom are different, they can be represented by `color{green}("𝐘𝐗 𝐂 = 𝐂 𝐗𝐘")` that can be represented in space in the following two ways :
In (a), the two identical atoms i.e., both the `color{red}(X)` or both the `color{red}(Y)` lie on the same side of the double bond but in (b) the two `color{red}(X)` or two `color{red}(Y)` lie across the double bond or on the opposite sides of the double bond. This results in different geometry of (a) and (b) i.e. disposition of atoms or groups in space in the two arrangements is different. Therefore, they are `color{green}("𝐬𝐭𝐞𝐫𝐞𝐨𝐢𝐬𝐨𝐦𝐞𝐫𝐬.")` They would have the same geometry if atoms or groups around `color{red}(C=C)` bond can be rotated but rotation around `color{red}(C=C)` bond is not free. It is restricted .The stereoisomers of this type are called `color{green}("𝐠𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐬.")`
The isomer of the type (a), in which two identical atoms or groups lie on the same side of the double bond is called `color{green}("𝐜𝐢𝐬 𝐢𝐬𝐨𝐦𝐞𝐫")` and the other isomer of the type (b), in which identical atoms or groups lie on the opposite sides of the double bond is called `color{green}("𝐭𝐫𝐚𝐧𝐬 𝐢𝐬𝐨𝐦𝐞𝐫")` . Thus cis and trans isomers have the same structure but have different configuration (arrangement of atoms or groups in space). Due to different arrangement of atoms or groups in space, these isomers differ in their properties like melting point, boiling point, dipole moment, solubility etc. `color{green}("𝐆𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐨𝐫 𝐜𝐢𝐬-𝐭𝐫𝐚𝐧𝐬 𝐢𝐬𝐨𝐦𝐞𝐫𝐬")` of but-2-ene are represented below :
Cis form of alkene is found to be more polar than the trans form. For example, dipole moment of cis-but - 2-ene is 0.33 Debye, whereas, dipole moment of the trans form is almost zero or it can be said that transbut- 2-ene is non-polar. This can be understood by drawing geometries of the two forms as given below from which it is clear that in the trans-but-2-ene, the two methyl groups are in opposite directions, Therefore, dipole moments of `color{red}(C-CH_3)` bonds cancel, thus making the trans form non-polar.
In the case of solids, it is observed that the trans isomer has higher melting point than the cis form. `color{green}("𝐆𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐨𝐫 𝐜𝐢𝐬-𝐭𝐫𝐚𝐧𝐬 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦")` is also shown by alkenes of the types `color{green}("𝐗𝐘𝐂 = 𝐂𝐗𝐙")` and `color{green}("𝐗𝐘𝐂 = 𝐂𝐙𝐖")`
`color{green}("𝐀𝐥𝐤𝐞𝐧𝐞𝐬 𝐬𝐡𝐨𝐰 𝐛𝐨𝐭𝐡 𝐬𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦 𝐚𝐧𝐝 𝐠𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦.")`
`color{green}("𝐒𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦 :")` As in alkanes, ethene `color{red}((C_2H_4))` and propene `color{red}((C_3H_6))` can have only one structure but alkenes higher than propene have different structures. Alkenes possessing `color{red}(C_4H_8)` as molecular formula can be written in the following three ways:
Structures I and III, and II and III = `color{green}("𝐜𝐡𝐚𝐢𝐧 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦")`
Structures I and II = `color{green}("𝐩𝐨𝐬𝐢𝐭𝐢𝐨𝐧 𝐢𝐬𝐨𝐦𝐞𝐫𝐬.")`
`color{green}("𝐆𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦 :")` Doubly bonded carbon atoms have to satisfy the remaining two valences by joining with two atoms or groups. If the two atoms or groups attached to each carbon atom are different, they can be represented by `color{green}("𝐘𝐗 𝐂 = 𝐂 𝐗𝐘")` that can be represented in space in the following two ways :
In (a), the two identical atoms i.e., both the `color{red}(X)` or both the `color{red}(Y)` lie on the same side of the double bond but in (b) the two `color{red}(X)` or two `color{red}(Y)` lie across the double bond or on the opposite sides of the double bond. This results in different geometry of (a) and (b) i.e. disposition of atoms or groups in space in the two arrangements is different. Therefore, they are `color{green}("𝐬𝐭𝐞𝐫𝐞𝐨𝐢𝐬𝐨𝐦𝐞𝐫𝐬.")` They would have the same geometry if atoms or groups around `color{red}(C=C)` bond can be rotated but rotation around `color{red}(C=C)` bond is not free. It is restricted .The stereoisomers of this type are called `color{green}("𝐠𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐢𝐬𝐨𝐦𝐞𝐫𝐬.")`
The isomer of the type (a), in which two identical atoms or groups lie on the same side of the double bond is called `color{green}("𝐜𝐢𝐬 𝐢𝐬𝐨𝐦𝐞𝐫")` and the other isomer of the type (b), in which identical atoms or groups lie on the opposite sides of the double bond is called `color{green}("𝐭𝐫𝐚𝐧𝐬 𝐢𝐬𝐨𝐦𝐞𝐫")` . Thus cis and trans isomers have the same structure but have different configuration (arrangement of atoms or groups in space). Due to different arrangement of atoms or groups in space, these isomers differ in their properties like melting point, boiling point, dipole moment, solubility etc. `color{green}("𝐆𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐨𝐫 𝐜𝐢𝐬-𝐭𝐫𝐚𝐧𝐬 𝐢𝐬𝐨𝐦𝐞𝐫𝐬")` of but-2-ene are represented below :
Cis form of alkene is found to be more polar than the trans form. For example, dipole moment of cis-but - 2-ene is 0.33 Debye, whereas, dipole moment of the trans form is almost zero or it can be said that transbut- 2-ene is non-polar. This can be understood by drawing geometries of the two forms as given below from which it is clear that in the trans-but-2-ene, the two methyl groups are in opposite directions, Therefore, dipole moments of `color{red}(C-CH_3)` bonds cancel, thus making the trans form non-polar.
In the case of solids, it is observed that the trans isomer has higher melting point than the cis form. `color{green}("𝐆𝐞𝐨𝐦𝐞𝐭𝐫𝐢𝐜𝐚𝐥 𝐨𝐫 𝐜𝐢𝐬-𝐭𝐫𝐚𝐧𝐬 𝐢𝐬𝐨𝐦𝐞𝐫𝐢𝐬𝐦")` is also shown by alkenes of the types `color{green}("𝐗𝐘𝐂 = 𝐂𝐗𝐙")` and `color{green}("𝐗𝐘𝐂 = 𝐂𝐙𝐖")`