Question

Which of the following alkenes exhibit geometrical isomerism? (a) 2,4 -hexadiene (b) 2 -methyl-1-butene (c) 1 -methylcyclopentene (d) 1,3 -butadiene

Short answer

Answer: The alkenes (b) 2-methyl-1-butene and (c) 1-methylcyclopentene exhibit geometrical isomerism.

Step-by-step solution

Draw the structures of the given alkenes

Start by drawing the structures of the given alkenes: (a) 2,4-hexadiene, (b) 2-methyl-1-butene, (c) 1-methylcyclopentene, and (d) 1,3-butadiene.

Identify the presence of different groups attached to each carbon atom in the double bond

For each alkene, identify if there are two different groups attached to each carbon atom in the double bond. This is a requirement for geometrical isomerism to occur. (a) 2,4-hexadiene: On the first carbon of the first double bond, there are two hydrogen atoms and one ethyl group, while the second carbon has one hydrogen atom, one methyl group, and is a part of the second double bond. The second double bond contains two carbons each with one hydrogen atom and one alkyl group and are part of the first double bond. (b) 2-methyl-1-butene: On the first carbon of the double bond, there are two different groups (one hydrogen atom and one ethyl group), whereas, on the second carbon, there is one hydrogen atom, one methyl group, and the double bond is connected to the carbon carrying another methyl group. (c) 1-methylcyclopentene: On the first carbon of the double bond, there are two different groups (one hydrogen atom and one methyl group), whereas, on the second carbon, there is one hydrogen atom, the double bond is connected to the carbon that's part of the cyclopentene ring. (d) 1,3-butadiene: The first double bond has two carbons, each with one hydrogen atom and one alkyl group. Likewise, the second double bond also has two carbons, each with one hydrogen atom and one alkyl group.

Determine which alkenes exhibit geometrical isomerism

From the analysis in Step 2, we observe that: (a) 2,4-hexadiene does not exhibit geometrical isomerism because the carbons in the double bond are part of another double bond (conjugated diene). (b) 2-methyl-1-butene exhibits geometrical isomerism because each carbon in the double bond has two different groups attached. (c) 1-methylcyclopentene exhibits geometrical isomerism because each carbon in the double bond has two different groups attached. (d) 1,3-butadiene does not exhibit geometrical isomerism as it is a conjugated diene, and has carbons with double bonds that are part of another double bond. Therefore, the alkenes (b) 2-methyl-1-butene and (c) 1-methylcyclopentene exhibit geometrical isomerism.

Key concepts

2-methyl-1-butene isomerism

Geometrical isomerism is a fascinating aspect of organic chemistry where the arrangement of groups within a molecule creates distinct compounds with unique properties. For 2-methyl-1-butene, this isomerism results from the double bond's geometry. In a double bond, two carbon atoms are each attached to two groups. These groups need to be different on each carbon for geometrical isomerism to occur.

• 2-methyl-1-butene consists of two types of geometrical isomers: cis and trans.
• In the cis isomer, the methyl group (CH₃) and the hydrogen atom on each carbon are on the same side of the double bond.
• Conversely, in the trans isomer, these groups lie on opposite sides.

Such isomers have different physical and chemical properties due to the spatial arrangement. The trans isomer usually has lower energy and greater stability than the cis isomer. This difference in stability and orientation is crucial in many chemical reactions and applications.

1-methylcyclopentene structure

1-methylcyclopentene stands out in the world of alkenes with its unique cyclic structure. This compound is a cycloalkene, where a ring structure incorporates a double bond. In the case of 1-methylcyclopentene, it is a five-membered ring with one methyl group branching off from the ring.
Understanding the structure:

• It's called '1-methylcyclopentene' because the double bond begins at the carbon connected to the methyl group.
• This structure, due to the presence of a double bond, allows for geometrical isomerism.
• Since each carbon in the double bond holds two different substituents (hydrogens and methyl group or ring), geometrical isomerism is possible.

This cyclic nature imparts different chemical properties compared to its open-chain counterparts, affecting reactivity and stability.

conjugated dienes

Conjugated dienes are a special type of alkene featuring multiple double bonds separated by a single bond. This arrangement allows electrons in the double bonds to delocalize, providing unique stability and reactivity features.
The key aspects of conjugated dienes:

• The 1,3-butadiene is a classic example with two double bonds, each sharing an interlying single bond.
• This kind of setup reduces the likelihood of geometrical isomerism due to electron delocalization.
• Conjugated dienes are famed for their ability to undergo specific reactions like Diels-Alder, linking them to synthetically important pathways.

While geometric isomerism is less pronounced in conjugated systems, their importance in advanced organic chemistry cannot be overstated.

double bond stereochemistry

The concept of double bond stereochemistry revolves around the spatial arrangement of atoms around the double-bonded carbons. Double bonds restrict rotation, a key feature that leads to different spatial configurations–geometrical isomers.
In the context of stereochemistry:

• Double bonds cause a planar arrangement, where substituents are across an immobile plane.
• Stereoisomers, particularly Z (zusammen, "together") and E (entgegen, "opposite"), identify the relative positions of substituents on the double bond.
• 'Z' configuration denotes priority groups on the same side, while 'E' represents them on opposite sides.

The concepts of stereochemistry are central in determining molecular interactions, biological activity, and material properties, making them vital in diverse fields like pharmaceuticals and materials science.