What is Geometrical Isomerism in Alkenes Chemistry Class 12
In this Chemistry class 11 notes, we will learn What is Geometrical Isomerism in Alkenes , What are isomers? and Characteristics of isomers from unit 13.
What are Isomers?
Isomers are compounds with identical formulas but distinct structures. In other words, isomers may be defined as molecules that have the same molecular formula, but a different arrangement of the atoms in space. Two main forms of isomerism are structural isomerism (or constitutional isomerism) and stereoisomerism (or spatial isomerism).
Isomerism is a phenomenon when two or more compounds have the same chemical formula but keeps different structural formulas, and hence different properties.
Characteristics of isomers
i. They have the same molecular formula but different structural formulas.
ii. Two isomers can have different boiling points.
iii. Isomers have the same atomic weight and the same number of atoms.
iv. They exhibit similar properties only when they contain the same functional group.
STRUCTURAL ISOMERS/ ISOMERISM
A structural isomer also called as constitutional isomer, is a type of isomer in which molecules have the same molecular formula but different bonding patterns and atomic organization. Structural isomers have different IUPAC names. There are different types of structural isomerism as given in the above chart.
Tautomeric also one of the types of structural isomerism in which the molecules spontaneously interconvert with each other, even when they are in pure form. Thetautomers have different chemical properties. If the interconversion reaction is quite fast, then it would be difficult to isolate the tautomers from each other.
Alkenes show structural isomerism. These isomers are either because of the changes in the position of the double bond or in the position of their substituents. Ethene and propene have only one structure. Alkenes higher than propene have different structural isomers.
Isomers of Butene
For example, shown above the structural isomers of C4H8 (Butene). These three structural isomers contain position isomers and chain isomers.
3- methyl -1-butene (C5H10) 2- methyl-1-butene (C5H10)
Given above are the isomers of pentene and they are the position isomers as the position of the substituent group is different in both the molecules.
Alkenes also show stereoisomerism or spatial isomerism. In stereoisomers, the bond structure in the molecules is the same, but the geometrical positioning of atoms and functional groups in space is different. One of the most common types of stereoisomerism exhibited by alkenes is the Geometrical isomerism.
Geometric isomerism or configurational isomerism is found in the molecules having the restricted rotation about the double bond.Another term for geometrical isomers is “cis-trans “isomers. The prefixes “cis” and “trans” are from Latin origin, where cis means “this side of” and trans means “the other side of”.
The compounds that show geometric isomerism must have:
i. Restricted rotation (often having carbon-carbon double bond)
ii. When there are two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn’t matter if the left-hand groups are the same as the right-hand ones or not.
GEOMETRICAL ISOMERISM IN ALKENES
Each double-bonded carbon atoms make bonds with other atoms or groups to satisfy its tetravalent nature. If alkenes have two different substituents at each end of the C=C then they show stereoisomerism (as geometric isomers). This occurs because there is a restricted rotation of the double bond due to the pi bond. The pi bond prevents rotation because of the electrons overlap above and below the plane of the atoms. Because the bond is relatively strong (270 kJ/mol), rotation around the C=C double bond can only occur by breaking the bond. Thus, rotation cannot occur at room temperature.
Alkenes from stereoisomers that differ in the way as substituents are arranged around the C=C double bond. The isomer with similar substituents on the same side of the double bond is called cis, a Latin stem meaning “on this site.” The isomer in which similar substituents are across from each other is called trans, a Latin stem meaning “across.” The cis isomer of 2-butene has both CH3 groups on the same side of the double bond. In the trans isomer, the CH3 groups occupy opposite sides of the double bond.
Cis/trans isomers have similar chemical properties but different physical properties. Cis-2-Butene, for example, freezes at -138.9°C, whereas trans-2-butene freezes at -105.6°C
The carbon atoms that form the C=C double bond in 2-butene are called stereocenters or stereogenic atoms. A stereocenter is an atom around which the interchange of two groups converts one stereoisomer into another. The carbon atoms in the C=C double bond in 2-butene are the stereocenters. CH3 and H are the substituents on the carbon atom on either side of the C=C double bond which gets interchanged and convert cis-2-butene into trans-2-butene, and vice versa.
i. There are limitations for assigning the molecules as cis or trans isomers, which can be explained by considering the following compound.
The above molecule is expected to be trans 3- methyl-2- pentene because the two CH3 substituents are across the double bond from each other. Or is it cis-3-methyl-2-pentene as the two “bulky” substituents (CH3 and CH3CH2) are on the same side of the double bond?
An explicit system for describing stereoisomers of alkenes has been suggested, in which strict rules are used to assign a priority to the substituents on each end of the double bond. These rules are:
ii. The highest priority is given to the atom that has the largest atomic number. Carbon (Z = 6), for example, would have a higher priority than hydrogen (Z = 1), and bromine (Z = 35) atom will have a higher priority than chlorine (Z = 17).
iii. If the atoms directly attached to the double bond are the same, continue down the substituent chain until a difference is found. Considering the CH3 and CH2CH3 substituents in 3-methyl-2-pentene, the carbon atom on the CH2CH3 substituent would be assigned a higher priority because it is bound to two H atoms and a C atom, in contrast, the carbon atom in the CH3 substituent is bound to three H atoms.
iv. Identify highest priority substituent on each carbon atom.
v. If the substituents with the highest priority on both carbon atoms are on the same side of the double bond, then the compound is described as the Z isomer. (From the German zusammen, which means “together.”)
vi. If the substituents with the highest priority on the carbon atoms are on opposite sides of the double bond, the compound is the E isomer. (From the German entgegen, “opposite.”)
Thus, the above structure of 3-methyl- 2-pentene is E-isomer
Comparison of physical properties of geometrical isomers
i. Cis and trans isomers often have different physical properties, due to the differences in the shape of the molecule or the overall dipole moment. These differences can be very small, for example, the boiling point of straight-chain alkenes, such as pent-2-ene, is 37 °C in the cis isomer and 36 °C in the trans isomer.
ii. The differences between cis and trans isomers become larger if polar bonds are present, like the cis isomer of the 1,2-dichloroetheneshas a boiling point of 60.3 °C, while the trans isomer has a boiling point of 47.5 °C.
iii. The trans alkenes are less polar and more symmetrical, have lower boiling points and higher melting points, and cis alkenes, are generally more polar and less symmetrical, have higher boiling points and lower melting points.
iv. For acyclic compounds, trans isomers are more stable than cis isomers. This is due to the increased unfavorable steric interaction of the substituents in the cis isomer.
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Tag – What are isomers?; Characteristics of isomers; Isomers; STEREOISOMERS; Geometrical Isomerism in Alkenes