Question

Name and draw structural formulas for all alkenes with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10}\) - \(\mathrm{As}\) you draw these alkenes, remember that cis and trans isomers are different compounds and must be counted separately.

Short answer

Question: Name and draw all the possible alkenes with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10}\). Answer: There are three possible alkenes: 1-pentene (\(\mathrm{CH}_{2}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\)), (Z)-2-pentene (\(\mathrm{CH}_{3}\mathrm{CH}(\mathrm{H})=\mathrm{CH}(\mathrm{CH}_{3})\mathrm{CH}_{2}\mathrm{CH}_{3}\)), and (E)-2-pentene (\(\mathrm{CH}_{3}\mathrm{CH}(\mathrm{CH}_{3})=\mathrm{CH}(\mathrm{H})\mathrm{CH}_{2}\mathrm{CH}_{3}\)).

Step-by-step solution

Identify the alkenes with a \(\mathrm{C}_{5}\) backbone

Firstly, we arrange the five carbon atoms in a straight chain. There are two options to place a double bond in this chain: 1. At the first carbon: \(\mathrm{CH}_{2}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\) 2. At the second carbon: \(\mathrm{CH}_{3}\mathrm{CH}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{3}\) These two arrangements will be our first two main groups of alkenes.

Determine cis and trans isomers

In each of the above-mentioned groups, we will check if cis and trans isomers can be formed. Cis and trans isomers arise when there are two different groups on adjacent carbon atoms around the double bond. 1. \(\mathrm{CH}_{2}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\): There is no possibility for cis and trans isomer for this structure because it does not have two different groups on adjacent carbon atoms around the double bond. 2. \(\mathrm{CH}_{3}\mathrm{CH}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{3}\): In this case, we have the possibility to form cis and trans isomers since there are two different groups on adjacent carbon atoms of the double bond. Cis isomer will have both the \(\mathrm{H}\)'s on the same side of the double bond and trans isomer will have them on opposite sides: a. Cis isomer: \(\mathrm{CH}_{3}\mathrm{CH}(\mathrm{H})=\mathrm{CH}(\mathrm{CH}_{3})\mathrm{CH}_{2}\mathrm{CH}_{3}\) b. Trans isomer: \(\mathrm{CH}_{3}\mathrm{CH}(\mathrm{CH}_{3})=\mathrm{CH}(\mathrm{H})\mathrm{CH}_{2}\mathrm{CH}_{3}\)

Name the alkenes according to IUPAC nomenclature

We will now name the alkenes obtained in steps 1 and 2 according to IUPAC rules: 1. \(\mathrm{CH}_{2}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\): This is a straight chain alkene with a double bond at the first carbon. The IUPAC name for this alkene is 1-pentene. 2. \(\mathrm{CH}_{3}\mathrm{CH}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{3}\): a. Cis isomer: The double bond is at the second carbon and both \(\mathrm{H}\) atoms are on the same side of the double bond. So, the IUPAC name for this isomer is (Z)-2-pentene. b. Trans isomer: The double bond is at the second carbon and the \(\mathrm{H}\) atoms are on the opposite sides of the double bond. The IUPAC name for this isomer is (E)-2-pentene. In conclusion, there are three alkenes with the molecular formula \(\mathrm{C}_{5}\mathrm{H}_{10}\): 1. 1-pentene: \(\mathrm{CH}_{2}=\mathrm{CH}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\) 2. (Z)-2-pentene: \(\mathrm{CH}_{3}\mathrm{CH}(\mathrm{H})=\mathrm{CH}(\mathrm{CH}_{3})\mathrm{CH}_{2}\mathrm{CH}_{3}\) 3. (E)-2-pentene: \(\mathrm{CH}_{3}\mathrm{CH}(\mathrm{CH}_{3})=\mathrm{CH}(\mathrm{H})\mathrm{CH}_{2}\mathrm{CH}_{3}\)

Key concepts

Understanding Structural Formulas

In organic chemistry, structural formulas play a crucial role in visualizing the arrangements of atoms within a molecule. Alkenes, as a subset of hydrocarbons, have at least one double bond between carbon atoms, and their structural formulas provide information about the location of these bonds as well as the distribution of hydrogen atoms. For instance, alkenes with the molecular formula C₅H₁₀ can be depicted with different structural arrangements or isomers.

When drawing structural formulas, it’s essential to consider all possible variations of the carbon backbone and the positioning of the double bond. For C₅H₁₀, we see that the double bond can either be at the end or internal within the carbon chain, leading to distinct molecules such as 1-pentene or 2-pentene, respectively. Remember, a different positioning of the double bond results in a different structural formula, hence a different compound.

Cis and Trans Isomers

Cis and trans isomers are types of stereoisomers, where molecules have the same molecular formula but differ in the orientation of groups attached to the double-bonded carbon atoms. This occurs only when each carbon of the double bond has two different groups attached to it. Let's use the molecular formula C₅H₁₀ as an example.

In our case, the (Z)-2-pentene and (E)-2-pentene isomers represent the cis and trans forms, respectively. The cis (Z) isomer has the larger groups (in this case, CH₃) on the same side of the double bond, while the trans (E) isomer has them on opposite sides. The distinction between these isomers is crucial because they have different physical and chemical properties despite having identical molecular formulas.

IUPAC Nomenclature of Alkenes

The International Union of Pure and Applied Chemistry (IUPAC) nomenclature provides a standardized method for naming organic compounds. To name alkenes using IUPAC rules, we identify the longest carbon chain containing the double bond and number the chain from the end nearest the double bond. The location of the double bond is indicated by the number of the first carbon in the bond, followed by the suffix '-ene' to indicate the presence of a double bond.

For the molecular formula C₅H₁₀, we use 'pentene' as the base name, with the 1 or 2 prefix to indicate the position of the double bond (1-pentene or 2-pentene). Additionally, for isomers with the same double bond location, we use (Z) and (E) prefixes to designate cis and trans configurations. Thus, we have 1-pentene, (Z)-2-pentene, and (E)-2-pentene, which provide us with a clear and systematic way to distinguish between isomers.