Question

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

Short answer

(a) \(CH_3CH=C(CH_3)CH_2CH_3\) (b) \(CH_3CH(CH_3)CH=C(CH_3)CH_2CH_3\) (c) \(\mathrm{C_6H_4}(CH_3)_2\) (d) 2-propyn-1-ol (e) trans-2-hexene

Step-by-step solution

(a) 4-methyl-2-pentene

First, understand the name of the compound: - "Pentene" means there's a five-carbon chain with a double bond (alkene). - The double bond is at the second carbon (2-pentene). - There's a methyl group (an extra carbon) attached to the fourth carbon (4-methyl). Now we can write the condensed structural formula: \(CH_3CH=C(CH_3)CH_2CH_3\)

(b) cis-2,5-dimethyl-3-hexene

First, understand the name of the compound: - "Hexene" means there's a six-carbon chain with a double bond (alkene). - The double bond is at the third carbon (3-hexene). - There are methyl groups (extra carbons) attached to the second and fifth carbons (2,5-dimethyl). - "cis" means the methyl groups are on the same side of the double bond. Now we can write the condensed structural formula: \(CH_3CH(CH_3)CH=C(CH_3)CH_2CH_3\)

(c) ortho-dimethylbenzene

First, understand the name of the compound: - "Benzene" means there's a six-carbon ring with alternating double bonds. - "ortho" means the two methyl groups are adjacent on the benzene ring. - "dimethyl" means there are two methyl groups attached to the benzene ring. Now we can write the condensed structural formula: \(\mathrm{C_6H_4}(CH_3)_2\)

(d) \(\mathrm{HC} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\)

For this compound, we already have the condensed structural formula. We just need to name it accordingly. First, understand the structure: - There is a triple bond between the first and second carbons (alkyne). - There's a three-carbon chain (propyl) after the triple bond. Now we can write the name of the compound: 2-propyn-1-ol

(e) trans-CH \(_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{2}\mathrm{CH}_{2} \mathrm{CH}_{3}\)

For this compound, we have the condensed structural formula. We need to name it accordingly. First, understand the structure: - There is a double bond between the second and third carbons (alkene). - There's a six-carbon chain in total (hexene). - "trans" means the two hydrogens attached to the carbon with a double bond are on opposite sides of the double bond. Now we can write the name of the compound: trans-2-hexene

Key concepts

Condensed Structures

In organic chemistry, understanding condensed structures is essential for depicting molecules. A condensed structure is a simplified way to represent molecular formulas without drawing all the bonds individually. For example, instead of writing out each single or double bond between atoms in a hydrocarbon chain, you group atoms together. You'd use parentheses to show branches or multiple identical groups. Take a compound like 4-methyl-2-pentene; its condensed structural formula, CH_3CH=C(CH_3)CH_2CH_3, quickly conveys the arrangement of different atoms, including the position of functional groups like double bonds and side chains. This makes condensed structures a compact and clear way to illustrate complex molecules.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) nomenclature system is a standardized method of naming chemical compounds. It ensures that each compound has a unique name that reflects its structure and key features. For example, in 4-methyl-2-pentene, each part of the name provides specific information:

• "4-methyl" indicates a methyl group attached to the fourth carbon.
• "2-pentene" suggests a five-carbon chain with a double bond starting at the second carbon.

This systematic approach is crucial for clear communication in the scientific community, making it easier to identify compounds. Another example is trans-2-hexene, where "trans" indicates the spatial arrangement around the double bond.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. They are unsaturated, meaning they have fewer hydrogen atoms than alkanes (saturated hydrocarbons). This double bond is significant because it impacts the chemistry of the molecule. For instance, in cis-2,5-dimethyl-3-hexene, the term "cis" describes the geometry of the molecule — where similar substituents are on the same side of the double bond. The double bond is also a site of reactivity, making alkenes versatile in chemical reactions like polymerization and addition reactions. Thus, knowing the position and geometry of double bonds is crucial for predicting the behavior of alkenes.

Alkynes

Alkynes are hydrocarbons characterized by at least one carbon-carbon triple bond. Like alkenes, they are unsaturated but have an even greater degree of unsaturation due to the triple bond, which reduces the number of hydrogen atoms further. For example, in the compound formula HC≡CCH_2CH_3, the triple bond between the first two carbons marks it as an alkyne. This property gives alkynes unique characteristics, like excellent reactivity in various chemical reactions, including hydrohalogenation and hydrogenation. Understanding the position of the triple bond is key to predicting alkyne behavior.

Aromatic Compounds

Aromatic compounds are a special class of cyclic hydrocarbons, typically containing a benzene ring — a six-carbon ring with alternating double and single bonds, known as resonance. Aromaticity imparts unique stability to these compounds beyond typical double-bonded structures. An example is ortho-dimethylbenzene, also known as o-xylene. In this compound, the term "ortho" indicates that two methyl groups are adjacent on the benzene ring. Aromatic compounds like this are significant in both chemical synthesis and various industrial applications, due to their stable and structured nature. Understanding how substituents are arranged on the aromatic ring is critical for grasping their chemical properties and behaviors.