Question

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

Short answer

a. cis-2-hexene: \[ H_3C-CH=CH-CH_2-CH_2-CH_3 \] b. trans-2-butene: \[ H_3C-CH=CH-CH_3 \] (substituents on opposite sides of the double bond) c. cis-2,3-dichloro-2-pentene: \[ Cl-CH_2-CH=CH-CH_2-CH_3 \] (chlorine atoms on the same side of the double bond)

Step-by-step solution

Draw the base structure

Draw the unbranched carbon backbone for each compound: a. For cis-2-hexene, draw 6 carbons connected in a straight chain. b. For trans-2-butene, draw 4 carbons connected in a straight chain. c. For cis-2,3-dichloro-2-pentene, draw 5 carbons connected in a straight chain.

Add the double bond

Place a double bond between the specified carbon atoms: a. For cis-2-hexene, add a double bond between the 2nd and 3rd carbons. b. For trans-2-butene, add a double bond between the 2nd and 3rd carbons. c. For cis-2,3-dichloro-2-pentene, add a double bond between the 2nd and 3rd carbons.

Add the substituents

Add substituents based on the cis/trans notation: a. For cis-2-hexene, no additional substituents are needed, so just add the hydrogen atoms to complete the structure. b. For trans-2-butene, we have a trans configuration, which means that the substituents are on opposite sides of the double bond. Add the hydrogen atoms to complete the structure. c. For cis-2,3-dichloro-2-pentene, we have a cis configuration, which means that the substituents are on the same side of the double bond. Add two chlorine atoms on carbons 2 and 3 and complete the structure with hydrogen atoms.

Review the final structures

Verify that the structures are correct for the given compounds: a. cis-2-hexene: \[ H_3C-CH=CH-CH_2-CH_2-CH_3 \] b. trans-2-butene: \[ H_3C-CH=CH-CH_3 \mathrm{with\;substituents\;on\;opposite\;sides\;of\;the\;double\;bond} \] c. cis-2,3-dichloro-2-pentene: \[ Cl-CH_2-CH=CH-CH_2-CH_3 \mathrm{with\;chlorine\;atoms\;on\;the\;same\;side\;of\;the\;double\;bond} \] The structural formulas drawn are correct representations of the given compounds.

Key concepts

Cis-Trans Isomerism

Understanding cis-trans isomerism is crucial in the realm of organic chemistry. It's a type of stereoisomerism that occurs due to restricted rotation around the carbon-carbon double bonds in alkenes. This means that the spatial arrangement of certain atoms or groups once locked in one position cannot freely rotate to the other side. In cis-isomers, similar or identical substituents on the involved carbons are positioned on the same side of the double bond. Conversely, in trans-isomers, these substituents are placed on opposite sides.

• Cis-isomer: "Cis" is Latin for "on this side," meaning the substituents are together.
• Trans-isomer: "Trans" means "across," implying that the substituents are on opposite sides.

This difference in spatial arrangement can lead to significant variations in physical and chemical properties of the molecules, such as boiling points and solubilities. Grasping this concept aids in predicting and explaining the behavior of organic compounds in reactions and real-world applications.

Alkene Nomenclature

When naming alkenes, following the IUPAC rules is essential to achieving clarity and consistency. Remember, alkenes are hydrocarbons carrying at least one carbon-carbon double bond, and their nomenclature involves a few straightforward steps. First, identify the longest carbon chain that contains the double bond. This chain forms the base of the alkene name, replacing the "-ane" ending of alkanes with "-ene." For instance, a six-carbon chain with a double bond becomes "hexene." Next, assign the lowest possible numbers to the positions of the double bond, ensuring it receives the smallest locant (number location) designation. Prefix the chain name with this number to signify the bond's position, such as in "2-hexene." Substituents or other functional groups should also be numbered and named according to their positions on the chain, and in alphabetical order. The geometric configuration (cis/trans) can then be added at the beginning of the name. Applying these rules generates a systematized approach, making compound identification and communication much clearer in scientific discourse.

Molecular Structure Drawing

Drawing the molecular structure of organic compounds helps visualize and understand their 3D organization and potential reactivity. For alkenes with specified stereochemistry like "cis" or "trans," it is crucial to reflect these aspects in your drawings. Start by sketching the specified carbon backbone. For example, if given cis-2,3-dichloro-2-pentene, first draw a five-carbon chain. Next, locate and draw the carbon-carbon double bond. Remember that the double bond inhibits rotation, hence preserving the spatial arrangement of the bonds or groups attached to the involved carbons. When placing the substituents, ensure that you accurately represent the stated stereochemical configuration:

• For cis configurations, place the designated groups on the same side of the double bond.
• For trans configurations, alternate these groups across the double bond.

Finally, verify that each carbon adheres to the tetravalency rule, commonly with hydrogen atoms completing the structure. This careful approach to drawing ensures a correct interpretation of each molecule's spatial arrangement and characteristics.