Question

Draw the structural formula for at least one bromoalkene with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{Br}\) that shows: (a) Neither \(E, Z\) isomerism nor chirality. (b) \(E, Z\) isomerism but not chirality. (c) Chirality but not \(E_{1} Z\) isomerism. (d) Both chirality and \(E, Z\) isomerism.

Short answer

Question: Identify and draw the structural formulas of bromoalkene molecules that exhibit (a) neither chirality nor E/Z isomerism, (b) only E/Z isomerism, (c) only chirality, and (d) both chirality and E/Z isomerism. Answer: (a) \(H_{3}\mathrm{C}-\mathrm{C}(\mathrm{Br})= \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) (b) \(H_{3}\mathrm{C}-\mathrm{C}(\mathrm{Br})= \mathrm{C}-\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (c) \(\mathrm{C}(\mathrm{Br})(\mathrm{CH}_{3})-\mathrm{C}(\mathrm{H}_{3}\mathrm{C})=\mathrm{C}(\mathrm{H}_{2}\mathrm{C}\mathrm{H}_{3})\) (d) \(H_{3}\mathrm{C}-\mathrm{C}(\mathrm{Br})= \mathrm{C}(\mathrm{CH}_{3})-\mathrm{CH}_{2}-\mathrm{CH}_{3}\)

Step-by-step solution

(a) Neither E, Z isomerism nor chirality

One option for a bromoalkene that does not exhibit chirality nor E/Z isomerism is: \(H_{3}\mathrm{C}-\mathrm{C}(\mathrm{Br})= \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) The double bond does not allow for E/Z isomerism since both attached carbon atoms have hydrogen atoms bonded to them, and the molecule has no carbon atoms connected with four different groups, which eliminates chirality.

(b) E, Z isomerism but not chirality

One option for a bromoalkene with E/Z isomerism but not chirality is: \(H_{3}\mathrm{C}-\mathrm{C}(\mathrm{Br})= \mathrm{C}-\mathrm{CH}_{3}-\mathrm{CH}_{3}\) In this case, there is a possibility for E/Z isomerism around the carbon-carbon double bond, but there are no chiral centers as none of the carbon atoms have four different groups attached to them.

(c) Chirality but not E, Z isomerism

One option for a bromoalkene with chirality but not E/Z isomerism is: \(\mathrm{C}(\mathrm{Br})(\mathrm{CH}_{3})-\mathrm{C}(\mathrm{H}_{3}\mathrm{C})=\mathrm{C}(\mathrm{H}_{2}\mathrm{C}\mathrm{H}_{3})\) Here, the carbon atom bonded to a bromine, a hydrogen, and two different alkyl groups is a chiral center. However, the molecule does not show E/Z isomerism because the double bond involves two carbon atoms each bonded to two hydrogen atoms.

(d) Both chirality and E, Z isomerism

One option for a bromoalkene exhibiting both chirality and E/Z isomerism is: \(H_{3}\mathrm{C}-\mathrm{C}(\mathrm{Br})= \mathrm{C}(\mathrm{CH}_{3})-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) This molecule can have E/Z isomerism around the carbon-carbon double bond, and the carbon atom connected with the bromine atom, a hydrogen, and two other different alkyl groups becomes a chiral center.

Key concepts

E,Z Isomerism

Understanding E,Z isomerism is essential when studying the complexity of chemical compounds, particularly alkenes. It refers to a form of stereoisomerism occurring due to the restricted rotation of the double bonds in alkenes.

Imagine each side of the double bond holding two different substituents. If the two higher priority substituents are on opposite sides of the double bond, the isomer is termed as E (from German 'Entgegen', meaning 'opposite'). Conversely, if these substituents are on the same side, the isomer is called Z (from German 'Zusammen', meaning 'together'). The priorities are determined by the Cahn-Ingold-Prelog rules, which consider the atomic numbers of the atoms directly attached to the carbon atoms of the double bond.

When drawing bromoalkenes, recognizing the possibility of E,Z isomerism is a step towards accurately depicting their molecular diversity. For example, in the provided exercise, (b) demonstrated a molecule having different substituents on each carbon of the double bond generating the potential for E or Z isomers.

Chirality in Organic Compounds

Chirality is a geometric property in organic chemistry where a molecule is not superimposable on its mirror image, much like our hands. A molecule is considered chiral if it has a chiral center, typically a carbon atom with four different substituents.

For a molecule to be chiral, it must lack an internal plane of symmetry. In the context of bromoalkenes, a molecule can exhibit chirality if it has a carbon atom with four diverse substituents, ensuring it has a non-superposable mirror image. In the exercise, (c) illustrates such a case, which has a chiral center but does not show E,Z isomerism because the double bond doesn't have the required substituent variations.

Structural Formula

The structural formula of a compound is a graphic representation of how atoms are arranged and bonded together in a molecule. It provides a visual illustration of molecular structure, showing the connections between atoms and the specific types of bonds – single, double, or triple.

When drawing structural formulas for bromoalkenes, we carefully examine the connectivity of atoms to represent attributes such as E,Z isomerism and chirality accurately. Structural formulas are vital for predicting the physical and chemical properties of molecules, as the arrangement of atoms can significantly influence the behavior of the compound.

Alkene Stereochemistry

Alkene stereochemistry involves the spatial arrangement of atoms around the carbon-carbon double bond in alkenes. This aspect of stereochemistry defines how atoms are oriented in three-dimensional space, which can result in vastly different physical and chemical properties for stereoisomers.

In alkenes, the rigidity of the double bond gives rise to E,Z isomerism, and the possibility of steric interactions between substituents influences the stability and reactivity of the molecules. The example (d) from the exercise showcases an alkene with both chirality and E,Z isomerism, highlighting the intricate stereochemical considerations necessary when studying and drawing these organic compounds.