Question

Draw the structure for each of the following. (a) \((Z)-1,3,5\) -tribromo- 2 -pentene (b) \((E)-1,2\) -dibromo- 3 -methyl- 2 -hexene (c) (S)-1-bromo-1-chlorobutane (d) \((R)-1,3\) -dibromohexane (e) (S)-1-chloro-2-propanol.

Short answer

The structures of the given molecules can be drawn by following the rules of IUPAC nomenclature and considering the stereochemistry denoted by 'E', 'Z', 'R', and 'S' terminology. The location and priority of atoms or groups around a double bond (as in 'E' and 'Z') or a chiral center (as in 'R' and 'S') will determine the 3D structure of the molecule.

Step-by-step solution

(a) (Z)-1,3,5-tribromo-2-pentene

The molecule (Z)-1,3,5-tribromo-2-pentene is a five-carbon alkene, indicating the molecule is a pentene. The molecule also has three bromine atoms located on the first, third and fifth carbon. The '2' before 'pentene' suggests there is a double bond starting at the second carbon atom. 'Z' indicates that the higher priority groups on both sides of the double bond are on the same side.

(b) (E)-1,2-dibromo-3-methyl-2-hexene

Here, the molecule (E)-1,2-dibromo-3-methyl-2-hexene is a six-carbon alkene, indicating it is a hexene, with a double bond starting at the second carbon atom. The molecule also has two bromine atoms located on the first and second carbon atom. There is also a methyl group at the third carbon. 'E' indicates that the higher priority groups on both sides of the double bond are diagonally opposed.

(c) (S)-1-bromo-1-chlorobutane

This refers to a four-carbon straight-chain alkane, which is a butane, with bromine and chlorine substituents at the first carbon. 'S' indicates that, if we view the molecule such that the lowest priority group is away from us, the other groups are arranged in a counter-clockwise order of priority.

(d) (R)-1,3-dibromohexane

Here, the molecule (R)-1,3-dibromohexane is a six-carbon alkane, meaning it's a hexane, with bromine substituents at the first and third carbon. 'R' denotes that the groups around the carbon atom with the bromine substituents, when arranged in decreasing order of priority, go in a clockwise direction.

(e) (S)-1-chloro-2-propanol

The last molecule (S)-1-chloro-2-propanol is a three carbon alkane, which means it's a propane. It has a chloro substituent at the first carbon atom and alcohol group (-OH) substituent at second. 'S' indicates that the groups around the carbon atom with the alcohol group, when arranged in decreasing order of priority, go in a counter-clockwise direction.

Key concepts

Stereochemistry

Stereochemistry is one of the most intricate subjects in organic chemistry. It refers to the study of the spatial arrangement of atoms in molecules and how this arrangement affects their chemical properties and reactions. In the provided exercise, stereochemistry involves understanding the configuration of molecules with respect to their chiral centers. Chiral centers are atoms that have four different substituents and can therefore exist in two forms, which are mirror images of each other; these forms are not superimposable, much like left and right hands.

This exercise touches on enantiomers with the designation of (R) and (S), which stand for the Latin terms 'rectus' and 'sinister', meaning 'right' and 'left', respectively. By using the Cahn-Ingold-Prelog priority rules, one can assign the configuration to chiral centers, giving insight into the molecule's three-dimensional shape, which is crucial for understanding its reactivity and interactions with biological systems.

Alkene Structure

Alkene structure is essential in understanding various properties and reactions in organic chemistry. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, which imparts specific chemical characteristics. These double bonds are areas of high electron density, making alkenes reactive toward electrophiles. Additionally, alkenes can exhibit cis-trans or E-Z isomerism, which significantly affects their physical properties and reactions.

In the examples given, we examine different alkene structures like pentene and hexene. The position of the double bond is indicated by a number immediately preceding the name; for example, '2-pentene' indicates the double bond starts at the second carbon. It is crucial to understand that the double bond not only defines the alkene's basic skeleton but also acts as a pivotal functional group for its chemistry.

Functional Groups in Organic Compounds

Functional groups are specific groups of atoms within molecules responsible for the characteristic chemical reactions of those molecules. One of the core features of organic chemistry nomenclature is the ability to identify and name these functional groups. They are the reactive parts of molecules, such as hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), and halogens (such as -Cl, -Br), among others.

For instance, in (S)-1-chloro-2-propanol, we need to identify the alcohol group (-OH) and the chlorine (-Cl) substituent. Functional groups can significantly change the chemical behavior of a compound even if the carbon backbone remains unchanged. Knowing the functional groups helps predict reactivity and solubility and is foundational for understanding complex organic synthesis and biological function.

Geometric Isomerism

Geometric isomerism, or cis-trans isomerism, is a form of stereoisomerism in which molecules with the same structural formula have a different spatial orientation due to the presence of a double bond or a ring structure that prevents rotation. In compounds with double bonds, like alkenes, the atoms or groups attached to the carbons can either be on the same side (cis or Z) or opposite sides (trans or E) of the bond.

The exercise showcases geometric isomerism through the (Z)-1,3,5-tribromo-2-pentene and (E)-1,2-dibromo-3-methyl-2-hexene. The 'Z' and 'E' descriptors come from the German words 'zusammen' and 'entgegen', meaning 'together' and 'opposite', respectively. They are part of the Cahn-Ingold-Prelog naming system and used globally to denote the relative positions of higher priority groups in alkenes. Geometric isomers often have different physical and chemical properties, which make them an important study in drug design and material science.