Problem 111 - General Chemistry: Principles and Modern Applications

Question

Draw the structures of (a) \((E)-3\) -benzyl- 2,5 -dichloro- 4 -methyl- 3 -hexene (b) 1 -ethenyl- 4 -nitrobenzene (c) trans-1-(4-bromophenyl)-2-methylcyclohexane.

Step-by-Step Solution

Answer

The chemical structure of each compound can be drawn based on its name. By understanding the naming of organic compounds, the functional groups and the number of atoms in each molecule, and by following the rules of IUPAC nomenclature, each molecule can be drawn correctly. The drawn structures would show location of all the substituents as well as the correct stereochemistry where necessary (E, trans).

Step 1: Structure of (E)-3-benzyl-2,5-dichloro-4-methyl-3-hexene

Firstly, identify that this compound is a hexene, meaning it has six carbon atoms and contains a double bond. The double bond is located at the 3rd carbon atom. At the 4th carbon position, there is a methyl group (CH3). There are two chloro molecules at the 2nd and 5th carbon atoms. At the 3rd position, there's a benzene ring (C6H5) attached, which represents a benzyl group. The (E) part of the name indicates the stereochemistry about the double bond, i.e., the priority groups (by atomic number, in this case, the benzyl group and ethyl part of the molecule) are on the opposite sides of the double bond.

Step 2: Structure of 1-ethenyl-4-nitrobenzene

Here, you need to draw a benzene ring. At the 1st position, there is an ethenyl group attached, which is a vinyl group (CH=CH2). At the 4th position, a nitro group (NO2) is attached. So you draw these groups accordingly at the specified positions on the benzene ring.

Step 3: Structure of trans-1-(4-bromophenyl)-2-methylcyclohexane

This compound is a cyclohexane, so you have to draw a cyclohexane ring. At the 1st carbon, a phenyl group (a benzene ring) is attached, which also has a bromine atom at the 4th position of the benzene ring. This means that '4-bromophenyl' is attached at the 1st position of cyclohexane. At the 2nd position of the cyclohexane, a methyl group (CH3) is attached. As the prefix 'trans-' suggests, the bromine atom of the phenyl group and the methyl group are on the opposite sides of the ring.

Key Concepts

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms within molecules. It's important for understanding how different isomers, or molecules with the same formula but different spatial arrangements, behave in chemical reactions. In the exercise, we encountered the concept of stereochemistry in the molecule

• (E)-3-benzyl-2,5-dichloro-4-methyl-3-hexene. The "E" designation refers to the "entgegen" configuration, indicating the large groups on the double bond are on opposite sides. It's like saying the big parts of the molecule are holding a wide stance.
• There's also the molecule trans-1-(4-bromophenyl)-2-methylcyclohexane, where "trans" means the substituents at the 1st and 2nd positions on the cyclohexane ring are opposite. This affects how the molecule fits or reacts with other molecules.

Understanding stereochemistry is crucial because even small changes in the arrangement can make huge differences in a compound's properties, such as their smell, taste, or effectiveness as drugs.

Hexene

Hexene refers to any alkene with six carbon atoms and at least one carbon-carbon double bond. In organic chemistry, it is essential to know that alkenes are unsaturated hydrocarbons, meaning they have fewer hydrogen atoms than alkanes. The presence of a double bond allows for geometric isomerism, as seen in the exercise's (E)-3-benzyl-2,5-dichloro-4-methyl-3-hexene.

The double bond in hexenes makes them more reactive than their alkane counterparts. This is because the pi bond, a part of the double bond, is weaker and more easily broken. This reactivity allows hexenes to participate in various reactions, such as hydrogenation or polymerization.

In the presented problem, the hexene is part of a more complex structure with additional substituents that alter its physical and chemical properties. Recognizing the structure of hexene and how it might react with other chemicals is key to mastering organic chemistry.

Benzene Ring

The benzene ring is a fundamental structure in organic chemistry, composed of six carbon atoms arranged in a hexagon with alternating double bonds. Its delocalized electron cloud provides stability and makes it a building block for many complex molecules.

• In our problem, we saw the benzene ring in the 3-benzyl-2,5-dichloro-4-methyl-3-hexene, where it formed part of a benzyl group. Here, a phenyl group (a benzene without one hydrogen) is attached to a CH2 group, showing the versatility of benzene in forming different structures.
• In 1-ethenyl-4-nitrobenzene, the benzene ring has an ethenyl group attached at the 1st position and a nitro group at the 4th position. These substitutions influence the ring's reactivity, showing how benzene serves as a dynamic framework for various compounds.

The unique properties of benzene, such as resonance stability, account for its widespread use in a plethora of chemical reactions, making it a cornerstone of aromatic chemistry.

Functional Groups

Functional groups are specific groupings of atoms within molecules that dictate the chemical properties and reactivity of those molecules. Understanding these groups helps predict behaviors in chemical reactions. In the exercise, consider the following groups:

• The chloro groups in (E)-3-benzyl-2,5-dichloro-4-methyl-3-hexene are halogens that often make the molecule more reactive and polar.
• The nitro group in 1-ethenyl-4-nitrobenzene is recognized for its electron-withdrawing nature, altering the electron density of the benzene ring and increasing reactivity towards electrophilic substitution reactions.
• The methyl group seen in several compounds, including trans-1-(4-bromophenyl)-2-methylcyclohexane, contributes significantly to the alkane characteristics when present.

Functional groups play defining roles in determining the molecule's properties and how they interact with the environment or other substances. Each offers unique attributes that are utilized extensively in synthesizing pharmaceuticals, plastics, and other chemicals.