Question

Draw the following. a. cis- 2 -hexene b. trans-2-butene c. cis-2,3-dichloro-2-pentene

Short answer

The short answer consists of three drawn structures. a. Cis-2-hexene: A chain of six carbon atoms numbered 1 to 6, with a double bond between carbon 2 and 3, and hydrogens placed in the cis conformation as explained above. b. Trans-2-butene: A chain of four carbon atoms numbered 1 to 4, with a double bond between carbon 2 and 3, and hydrogens placed in the trans conformation as explained above. c. Cis-2,3-dichloro-2-pentene: A chain of five carbon atoms numbered 1 to 5, with a double bond between carbon 2 and 3, two chlorines on the same side of the double bond, and hydrogens placed as explained above.

Step-by-step solution

1. Identifying the structure for cis-2-hexene

In order to draw cis-2-hexene, we need to know that hexene signifies an alkene with 6 carbon atoms, with the double bond beginning at the 2nd carbon (C2). The cis conformation means that the two groups attached to the double bonded carbons are on the same side.

2. Drawing cis-2-hexene

First, draw a linear chain of six carbon atoms and number them from left to right (1 to 6). Place a double bond between carbon atoms 2 and 3. Now, arrange the hydrogen atoms such that the higher priority groups are on the same side of the double bond, i.e., put two hydrogen atoms on carbons 1 and 4. Add two hydrogens to carbons 5 and 6, and one hydrogen at carbons 2 and 3. This conformation represents a cis-2-hexene molecule.

3. Identifying the structure for trans-2-butene

To draw trans-2-butene, we need to understand that butene signifies an alkene with 4 carbon atoms, with the double bond beginning at the 2nd carbon (C2). The trans conformation means that the two groups attached to the double bonded carbons are on opposite sides.

4. Drawing trans-2-butene

First, draw a linear chain of four carbon atoms and number them from left to right (1 to 4). Place a double bond between carbon atoms 2 and 3. Now, arrange the hydrogen atoms such that one hydrogen atom is attached at each end (carbons 1 and 4) of this alkene molecule and it is diagonally opposite to another hydrogen atom on the double bonded carbons (2 and 3). This conformation represents a trans-2-butene molecule.

5. Identifying the structure for cis-2,3-dichloro-2-pentene

To draw cis-2,3-dichloro-2-pentene, we need to know that pentene signifies an alkene with 5 carbon atoms and the double bond beginning at the 2nd carbon (C2). The cis conformation indicates that the two chlorine atoms are on the same side of the double bond.

6. Drawing cis-2,3-dichloro-2-pentene

First, draw a linear chain of five carbon atoms and number them from left to right (1 to 5). Place a double bond between carbon atoms 2 and 3. Attach one chlorine atom to carbon atom 2 and another to carbon atom 3, on the same side of the double bond. Add two hydrogen atoms to carbons 1 and 4. Finally, put two hydrogen atoms on carbons 5. This conformation represents a cis-2,3-dichloro-2-pentene molecule.

Key concepts

Alkene Structures

Alkenes are a fascinating group within organic chemistry, characterized by having at least one carbon-carbon double bond. This double bond is what sets alkenes apart from other hydrocarbons like alkanes, which only have single bonds. The presence of the double bond adds a level of complexity to their structure, allowing for different shapes and behaviors.

In alkenes, the double bond is a result of the overlap of p orbitals, forming a pi-bond capable of restricting rotation. This restriction creates unique structural possibilities not present in single bonded carbon chains.

An essential feature in drawing alkene structures is identifying where the double bond is located in the carbon chain. For example, in hexene, the name 'hex' stands for six carbon atoms, while the number '2' indicates the double bond starts at the second carbon. Placing the double bond correctly is crucial in forming accurate alkene structures.

When designing alkene structures, achieving the correct arrangement of surrounding atoms is vital for maintaining the integrity and chemical properties of the molecule.

Cis-Trans Isomerism

Cis-trans isomerism is a critical concept in organic chemistry, particularly concerning alkenes. It relates to the geometric arrangement of atoms around the double bond. The rigidity of the double bond prevents free rotation, allowing for distinct geometries which are crucial when considering molecular properties.

In cis isomers, similar or higher priority groups attached to the double-bonded carbons are on the same side. This closeness can affect how the molecule interacts with its environment, affecting properties like boiling point and solubility. These isomers often have a distinct "U" shape.

In contrast, trans isomers have these groups on opposite sides of the double bond, resulting in a more stretched or linear configuration. This spatial arrangement can lead to differences in molecular interactions and often results in different physical properties than its cis counterpart.

Recognizing these small yet significant differences in isomerism can help understand and predict the behaviors of different organic molecules.

Carbon Chains

Carbon chains are the backbone of organic chemistry. They form the skeletal structure from which different functional groups branch out. Alkanes, alkenes, and alkynes are all types of carbon chain compounds, differing primarily in the type of bonds between carbon atoms.

In these chains, the length and branching of the carbon atoms greatly influence the physical and chemical properties of the molecule. Longer chains typically have higher boiling points, while branched chains might have lower boiling points due to differences in molecular interactions.

In alkene structures specifically, the carbon chain length and layout help define the location of double bonds, impacting the molecule's overall behavior. For example, identifying the carbon chain as pentene or hexene helps determine where to place double bonds, as illustrated in exercises involving cis and trans isomers.

Understanding carbon chains is fundamental in accurately drawing structures, predicting properties, and synthesizing new organic compounds.