Problem 33 - Chemistry: The Central Science in SI Units, Global Edition

Question

Indicate whether each of the following molecules is capable of geometrical isomerism. For those that are, draw the structures: (a) 2 -methylpropene, (b) 1,3-diiodobenzene, (c) 1,2-dichloropent-1-ene, (d) 3-methyl-1-butyne.

Step-by-Step Solution

Answer

(a) 2-methylpropene is capable of geometrical isomerism. The two possible isomers are E-2-methylpropene (methyl groups on opposite sides of the double bond) and Z-2-methylpropene (methyl groups on the same side of the double bond). (b) 1,3-diiodobenzene is not capable of geometrical isomerism. (c) 1,2-dichloropent-1-ene is capable of geometrical isomerism. The two possible isomers are E-1,2-dichloropent-1-ene (chlorine atoms on opposite sides of the double bond) and Z-1,2-dichloropent-1-ene (chlorine atoms on the same side of the double bond). (d) 3-methyl-1-butyne is not capable of geometrical isomerism.

Step 1: (a) 2-methylpropene

2-methylpropene has the formula \(CH_3C(CH_3)=CH_2\). It has a double bond between the second and third carbon atoms. For geometrical isomerism, we need to consider if there are different groups attached to each end of the double bond. In this case, one side of the double bond has a methyl group and hydrogen atom attached, and the other side has a hydrogen atom and another carbon with two hydrogen atoms and a methyl group attached. Since both sides of the double bond have different groups attached, 2-methylpropene is capable of geometrical isomerism. The two possible geometrical isomers are: 1. E-2-methylpropene: The methyl groups are on opposite sides of the double bond. 2. Z-2-methylpropene: The methyl groups are on the same side of the double bond.

Step 2: (b) 1,3-diiodobenzene

1,3-diiodobenzene has a benzene ring with two iodine atoms replacing hydrogen atoms in positions 1 and 3. In this case, the presence of aromatic ring \(C_6H_6\) does not allow for any restricted rotation of bonds. Thus, 1,3-diiodobenzene is not capable of geometrical isomerism.

Step 3: (c) 1,2-dichloropent-1-ene

1,2-dichloropent-1-ene has the formula \(CH_2=CH(Cl)CH_2CH_2Cl\). It has a double bond between the first and second carbon atoms. For geometrical isomerism, we need to pay attention to the attachments to the double bond. In this case, one side of the double bond has a hydrogen atom and a chlorinated carbon atom, while the other side has a hydrogen atom and a carbon atom with chlorine. Since both sides of the double bond have different groups attached, 1,2-dichloropent-1-ene is capable of geometrical isomerism. The two possible geometrical isomers are: 1. E-1,2-dichloropent-1-ene: The chlorine atoms are on the opposite sides of the double bond. 2. Z-1,2-dichloropent-1-ene: The chlorine atoms are on the same side of the double bond.

Step 4: (d) 3-methyl-1-butyne

3-methyl-1-butyne has the formula \(CH_3C \equiv CCH_2CH_3\). It has a triple bond between the second and third carbon atoms. No geometrical isomerism is expected due to the presence of a triple bond and absence of a repeated functional group on one side of the triple bond. Thus, 3-methyl-1-butyne is not capable of geometrical isomerism.

Key Concepts

Organic Chemistry

Organic chemistry is the branch of chemistry that studies the structure, properties, and reactions of organic compounds, which contain carbon. Carbon is unique because it can form stable, complex molecules using covalent bonds. This makes it the backbone of many molecules in living organisms, such as DNA, proteins, and lipids.
One fascinating aspect of organic chemistry is the study of isomerism, where molecules have the same molecular formula but different structures. This leads to different physical and chemical properties.

• Structural Isomerism: Molecules with the same molecular formula but different bonding patterns and atomic organization.
• Stereoisomerism: Isomers with the same sequence of bonded atoms but different three-dimensional orientations.

Geometrical isomerism, a type of stereoisomerism, is further explored in the study of alkenes. Organic chemistry is essential for understanding a vast variety of natural and synthetic substances.

Alkenes

Alkenes are a class of hydrocarbons that contain at least one carbon-to-carbon double bond (C=C). This double bond is what sets them apart from alkanes, which only contain single bonds. The presence of the double bond in alkenes leads to fascinating chemical properties and reactions.
An essential characteristic of the double bond in alkenes is restricted rotation. This means that atoms attached to the carbon atoms in a double bond cannot freely rotate around it. This restricted rotation grants alkenes the ability to exhibit geometrical isomerism.

• E/Z isomerism: Depending on the position of substituents around the double bond.
• Cis/trans isomerism: A subset of E/Z isomerism where larger groups are on the same (cis) or opposite (trans) sides.

Understanding the structure of alkenes and their isomers is crucial for predicting their behavior in chemical reactions.

Molecular Structures

Molecular structures help us understand the arrangement of atoms in a molecule and the spatial orientation of these atoms. This is crucial for predicting the molecule's reactivity, color, phase of matter, polarity, biological activity, etc.
The geometrical isomerism observed in molecules like alkenes arises from specific spatial arrangements produced by double bonds. Double bonds require a planar structure which limits rotational movement and allows for different geometric configurations.

• Structural Representation: Shows the connectivity of atoms within the molecule using lines (bonds) between atoms (symbols).
• 3D Molecular Models: Provides insight into the actual spatial arrangement of atoms, important for understanding isomerism.

Geometrical isomerism cannot occur if the molecular structure allows free rotation or if identical groups are attached to both ends of the double bond. By understanding molecular structures, chemists can manipulate and predict compound behaviors in various applications, from material science to pharmacology.