Question

Which of the following compounds will show geometrical isomerism? 1\. 2 -butene 2\. propene 3\. 1 -phenylpropene 4\. 2-methylbut-2-ene (a) 1,2 (b) 3,4 (c) \(1,2,3\) (d) 1,3

Short answer

Choice (d) 1,3 is correct, as 2-butene and 1-phenylpropene exhibit geometrical isomerism.

Step-by-step solution

Understand Geometrical Isomerism

Geometrical isomerism occurs in compounds with restricted rotation around a bond, typically around double bonds, and requires different groups attached to the bonded carbons. The isomers differ in the spatial arrangement of these attachments.

Analyze 2-butene

2-butene has a double bond between the second and third carbon atoms. It has the required different groups attached on each of these carbons, allowing for geometrical isomers - cis (same side) and trans (opposite side).

Analyze Propene

Propene has a double bond between the first and second carbon. With one hydrogen always attached to the primary carbon, propene does not have distinct different groups on both sides of the double bond. Hence, it does not show geometrical isomerism.

Analyze 1-phenylpropene

1-phenylpropene has a phenyl group attached to the double-bonded carbon atoms. This structure allows different arrangements (cis and trans) around the double bond, thus showing geometrical isomerism.

Analyze 2-methylbut-2-ene

2-methylbut-2-ene has a double bond between the second and third carbon atoms. However, the presence of identical methyl groups on both ends of the double bond restricts the possibility of cis or trans isomers, preventing geometrical isomerism.

Conclude the Analysis

From the analysis, only 2-butene and 1-phenylpropene have the required conditions for geometrical isomerism. This conclusion allows us to select the compounds which show such isomerism.

Key concepts

2-butene

2-butene is an alkene with the chemical formula C\(_4\)H\(_8\). Understanding 2-butene is key to using geometrical isomerism as it perfectly exemplifies this concept. This molecule has a double bond located between the second and third carbons in the chain. The existence of this double bond restricts the rotation of atoms around it, leading to different spatial arrangements of substituent groups.

2-butene mainly exists in two isomers:

• Cis-2-butene: In this arrangement, the two higher priority groups attached to the carbon atoms of the double bond are on the same side. This arrangement gives the molecule specific physical and chemical properties, such as higher boiling and melting points compared to its trans counterpart.
• Trans-2-butene: Here, the substituent groups are on opposite sides of the double bond. This form typically shows lower boiling and melting points but can be more stable under certain conditions.

Geometrical isomerism in 2-butene is a classic example of how double bonding in hydrocarbons can lead to significant differences in molecular function.

1-phenylpropene

1-phenylpropene is an intriguing compound displaying geometrical isomerism due to its unique structure. This molecule contains a benzene ring, referred to as the phenyl group, attached to propene. This attachment introduces a remarkable feature in the form of a fixed spatial arrangement around the double bond.

The double bond is formed between the first carbon of the propene chain and the next carbon, allowing for the different geometrical states:

• Cis-1-phenylpropene: The phenyl group and a methyl group are on the same side of the double bond. This orientation gives specific advantages in reactions where steric factors (the arrangement of atoms in space) play a crucial role.
• Trans-1-phenylpropene: The phenyl and methyl groups lie on opposite sides of the double bond. This formation is often more stable due to reduced steric hindrance, which can lead to different reactivity or physical properties.

The ability to exist in these forms makes 1-phenylpropene an exciting molecule for studying the effects of spatial configurational changes in organic chemistry.

Double Bonds

Double bonds are a fundamental feature in chemistry that define many classes of compounds, including alkenes like 2-butene and 1-phenylpropene. Composed of one sigma bond and one pi bond, double bonds result from the overlap of p orbitals. This structure introduces resistance to rotation, locking substituent groups in place and creating potential for different spatial orientations.

Key characteristics of double bonds include:

• Restricted rotation: The presence of a pi bond prevents free rotation, allowing for the fixed spatial arrangement necessary for geometrical isomerism.
• Reactivity: Double bonds are reactive sites in a molecule, as they are typically more exposed and prone to participate in chemical reactions, such as addition reactions, where other atoms can add to the carbons connected by the double bond.

Double bonds are central to understanding geometrical isomerism, as they enable the fixed positions of atoms that lead to significant chemical diversity.

Cis-Trans Isomers

Cis-trans isomers are a specific type of geometrical isomerism. This phenomenon is most pronounced in compounds with double bonds and is vital in distinguishing organic molecules based on their spatial configurations around such bonds.

What makes cis-trans isomerism intriguing are the differences in physical and chemical properties that arise from the spatial arrangement:

• Cis Isomers: Groups of highest priority are aligned on the same side. This setup influences properties like boiling point, solubility, and molecular interactions due to the distribution of mass and potential dipole moments. Cis configurations can also highlight steric hindrances that affect reactivity.
• Trans Isomers: These have groups on opposite sides of the double bond, leading to generally more balanced molecules. Trans isomers tend to have lower boiling points as they typically do not have permanent dipole moments, making them less polar.

Understanding cis-trans isomerism is critical in organic chemistry and pharmaceuticals, where the geometry can influence biological function, efficacy, and selectivity of compounds.