Answer true or false. (a) Both ethylene and acetylene are...

Answer true or false. (a) Both ethylene and acetylene are planar molecules. (b) An alkene in which each carbon of the double bond has two different groups bonded to it will show cis-trans isomerism. (c) Cis -trans isomers have the same molecular formula but a different connectivity of their atoms. (d) $C i s-2$ -butene and $t r a n s-2$ -butene can be inter converted by rotation about the carbon-carbon double bond. (e) Cis-trans isomerism is possible only among appropriately substituted alkenes. (f) Both 2-hexene and 3-hexene can exist as pairs of cis-trans isomers. (g) Cyclohexene can exist as a pair of cis-trans isomers. (h) 1 -Chloropropene can exist as a pair of cis-trans isomers.

Answer true or false.
(a) Both ethylene and acetylene are planar molecules.
(b) An alkene in which each carbon of the double bond has two different groups bonded to it will show cis-trans isomerism.
(c) Cis -trans isomers have the same molecular formula but a different connectivity of their atoms.
(d) $C i s-2$ -butene and $t r a n s-2$ -butene can be inter converted by rotation about the carbon-carbon double bond.
(e) Cis-trans isomerism is possible only among appropriately substituted alkenes.
(f) Both 2-hexene and 3-hexene can exist as pairs of cis-trans isomers.
(g) Cyclohexene can exist as a pair of cis-trans isomers.
(h) 1 -Chloropropene can exist as a pair of cis-trans isomers.

Key Concepts

Connectivity vs. Stereochemistry

Geometric or cis-trans isomers share the same molecular formula and connectivity of atoms; however, they differ in the three-dimensional arrangement of these atoms. This distinction is crucial in stereochemistry, as it defines stereoisomers from constitutional isomers, which differ in connectivity.

Molecular Geometry

Alkenes possess a double bond that features a planar arrangement of atoms due to the sp2 hybridization of the carbons involved. This planar geometry is critical because it fixes the positions of substituents, influencing the spatial orientation that leads to isomeric forms.

Restricted Rotation in Double Bonds

The presence of a pi bond in double bonds restricts rotation around the bond axis. This lack of free rotation prevents interconversion between different spatial arrangements of substituents, which is fundamental to the formation and stability of stereoisomers.

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism where compounds with the same connectivity have different spatial arrangements of substituents relative to a double bond or ring system. This phenomenon arises because the restricted rotation of the bond allows for two distinct configurations that are not interconvertible under normal conditions.

Substitution Requirements for Geometric Isomerism

For an alkene to exhibit cis-trans isomerism, each carbon of the double bond must be attached to two different substituents. This requirement ensures that the molecule has distinct sides, allowing the formation of two non-superimposable isomers with different spatial arrangements.

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