Question

Name or write the condensed structural formula for the following compounds: \begin{equation}\begin{array}{l}{\text { (a) } 4 \text { -methyl-2-pentene }} \\\ {\text { (b) } \text {cis}-2,5 \text { -dimethyl-3-hexene }} \\ {\text { (c) ortho-dimethylbenzene }}\\\\{\text { (d) } \mathrm{HC}=\mathrm{CCH}_{2} \mathrm{CH}_{3}} \\ {\text { (e) } \text {trans}-\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}}\end{array}\end{equation}

Short answer

Short Answer: (a) \(CH_3CH=C(CH_3)CH_2CH_3\) (b) \(CH_3CH(CH_3)CH=C(CH_3)CH_2CH_3\) (c) 1,2-dimethylbenzene (d) 1-butyne (e) trans-2-heptene

Step-by-step solution

(a) 4-methyl-2-pentene Condensed Structural Formula

:\ For 4-methyl-2-pentene, we can visualize a pentene compound with a methyl group attached to the 4th carbon and a double bond between carbons 2 and 3. The condensed structural formula is given by \(CH_3CH=C(CH_3)CH_2CH_3\).

(b) cis-2,5-dimethyl-3-hexene Condensed Structural Formula

:\ For cis-2,5-dimethyl-3-hexene, we visualize a hexene compound with a double bond between carbons 3 and 4 and methyl groups attached to carbons 2 and 5 in a cis configuration. The condensed structural formula is given by \(CH_3CH(CH_3)CH=C(CH_3)CH_2CH_3\).

(c) ortho-dimethylbenzene IUPAC Name

:\ The compound ortho-dimethylbenzene refers to a benzene ring with two methyl groups attached to adjacent carbon atoms within the ring. In IUPAC nomenclature, this compound is known as 1,2-dimethylbenzene.

(d) HC= CCH₂CH₃ IUPAC Name

:\ For the given condensed formula \(HC= CCH_2CH_3\), we see a compound with a triple bond starting at the first carbon and extending to the second carbon. Counting the carbons, we have a total of 4. Thus, the IUPAC name for the compound is 1-butyne.

(e) trans-CH₃CH=CHCH₂CH₂CH₂CH₃ IUPAC Name

:\ For the given condensed formula \(\text{trans}-CH_3CH=CHCH_2CH_2CH_2CH_3\), we can visualize a compound with seven carbon atoms in the chain and a double bond between carbons 2 and 3 in the trans configuration. The IUPAC name for this compound is trans-2-heptene.

Key concepts

Understanding Structural Formulas

In the realm of organic chemistry, the structural formula of a compound is a graphical representation that conveys the arrangement of the atoms within the molecule. Unlike molecular formulas which merely indicate the number and types of atoms, structural formulas provide insight into the specific connectivity between atoms.

Structural formulas can be written in several ways, including Lewis structures, condensed structures, and skeletal formulas. The condensed structural formula simplifies the molecular structure by showing the arrangement of atoms in a more compact form, without explicitly drawing all bonds. For example, in the compound 4-methyl-2-pentene, the condensed structural formula is written as \(CH_3CH=C(CH_3)CH_2CH_3\), showing how groups of atoms like methyl (\(CH_3\)) are connected.

An understanding of structural formulas is essential for students as it helps visualize and predict the physical and chemical properties of organic compounds. It also lays the groundwork for more complex topics such as isomerism and reaction mechanisms.

Alkene Naming Basics

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The naming of alkenes is governed by rules set by the International Union of Pure and Applied Chemistry (IUPAC). To correctly name an alkene:

• Determine the longest carbon chain that includes the double bond and name it using the corresponding alkane name, but replace the -ane ending with -ene to indicate the double bond.
• Number the carbon chain starting from the end nearest the double bond.
• If there are substituents, such as methyl groups, indicate their position using the number of the carbon they are attached to.

For example, '4-methyl-2-pentene' means the compound has a methyl group at the fourth carbon and a double bond starting at the second carbon. Remember to consider any cis-trans isomerism, which can affect the properties of the compound.

IUPAC Nomenclature Simplified

IUPAC nomenclature is the standardized system for naming chemical compounds. It's designed to provide a unique and unambiguous name for each compound. The steps involved in determining an IUPAC name include:

• Firstly, select the longest continuous carbon chain as the parent hydrocarbon.
• Number the carbon atoms in the chain starting from the end nearest to a substituent or functional group.
• Identify and name the substituents.
• Assign a number to each substituent according to its position on the carbon chain.
• Combine the substituent names and numbers with the parent hydrocarbon name.

Rings and multiple bonds are also accounted for in the naming process. For instance, 'ortho-dimethylbenzene' is formally named '1,2-dimethylbenzene', indicating two methyl groups on adjacent carbons of the benzene ring. Mastery of IUPAC nomenclature allows students to systematically name an endless variety of organic molecules.

Geometric Isomers: cis-trans Distinctions

Geometric isomers are compounds that have the same molecular formula but differ in the spatial arrangement of their atoms. The most common type of geometric isomerism arises in alkenes due to the presence of a carbon-carbon double bond, which restricts rotation and locks the substituents in place.

There are two primary geometric isomers: 'cis' and 'trans'. In the 'cis' configuration, similar or identical substituents on a double bond are on the same side, while the 'trans' configuration has them on opposite sides. Take, for instance, 'cis-2,5-dimethyl-3-hexene' versus 'trans-2-heptene' from the exercise; the 'cis' and 'trans' prefixes define the relative positions of the methyl groups and other substituents.

Understanding these concepts helps explain the distinct physical and chemical properties of geometric isomers and is crucial for topics such as stereochemistry and the study of molecular interactions.