Question

Give structural formulas for the following compounds: (a) 3-Methylindole (b) 3-Bromoquinoline (c) Nicotinic acid (d) 4-Methylisoquinoline (e) Histidine (f) 2-Methylimidazole

Short answer

The structural formulas for the given compounds are: (a) 3-Methylindole: \[ \chemfig{*6(=-=(-*5(-=-=-N(-[::+60]CH_3))-))=-=)} \] (b) 3-Bromoquinoline: \[ \chemfig{*6(=-=(-*5(-=N(-[::+60]Br)---))-))=-=)} \] (c) Nicotinic acid: \[ \chemfig{*6(=-=(-N-*6(-=-=-(=[::+60]O)-[::-60]OH))-))=-=)} \] (d) 4-Methylisoquinoline: \[ \chemfig{*6(=-=(-*5(-=N(-(-[::+60]CH_3))---))-))=-=)} \] (e) Histidine: \[ \chemfig{H_2N-CH([:270]-COOH)-C(=[:+60]N(-H)-*5(-N=-(-[::+60]H)--))R} \] (f) 2-Methylimidazole: \[ \chemfig{*5(=-N-(-N*5(-=-(=[::+60]CH_3)--))-))} \]

Step-by-step solution

Identify the parent structure

Indole is an aromatic heterocyclic organic compound with a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. The molecular formula for indole is C8H7N.

Add the substituent

3-Methylindole is derived from indole with a methyl group at the 3rd position. So, attach a methyl group on the 3rd carbon of the indole structure.

Draw the structural formula

The structural formula of 3-Methylindole is: \[ \chemfig{*6(=-=(-*5(-=-=-N(-[::+60]CH_3))-))=-=)} \] (b) 3-Bromoquinoline

Identify the parent structure

Quinoline is an aromatic heterocyclic organic compound consisting of a fused benzene and pyridine ring. The molecular formula for quinoline is C9H7N.

Add the substituent

3-Bromoquinoline is derived from quinoline with a bromine atom at the 3rd position. So, attach a bromine atom on the 3rd carbon of the quinoline structure.

Draw the structural formula

The structural formula of 3-Bromoquinoline is: \[ \chemfig{*6(=-=(-*5(-=N(-[::+60]Br)---))-))=-=)} \] (c) Nicotinic acid

Identify the parent structure

Nicotinic acid is also known as pyridine-3-carboxylic acid. Pyridine is an aromatic heterocyclic organic compound with a six-membered ring containing one nitrogen atom. The molecular formula for pyridine is C5H5N.

Add the functional group

The carboxylic acid group (-COOH) needs to be added to the 3rd carbon of the pyridine structure.

Draw the structural formula

The structural formula of nicotinic acid is: \[ \chemfig{*6(=-=(-N-*6(-=-=-(=[::+60]O)-[::-60]OH))-))=-=)} \] (d) 4-Methylisoquinoline

Identify the parent structure

Isoquinoline is an aromatic heterocyclic organic compound, structurally similar to quinoline but with a different arrangement of the nitrogen atom. The molecular formula for isoquinoline is C9H7N.

Add the substituent

4-Methylisoquinoline is derived from isoquinoline with a methyl group at the 4th position. So, attach a methyl group on the 4th carbon of the isoquinoline structure.

Draw the structural formula

The structural formula of 4-Methylisoquinoline is: \[ \chemfig{*6(=-=(-*5(-=N(-(-[::+60]CH_3))---))-))=-=)} \] (e) Histidine

Identify the parent structure

Histidine is an α-amino acid, commonly found in proteins and enzymes. It is composed of a central carbon atom, an amino group (-NH2), a carboxyl group (-COOH), and an imidazole side chain.

Draw the structural formula

The structural formula of histidine is: \[ \chemfig{H_2N-CH([:270]-COOH)-C(=[:+60]N(-H)-*5(-N=-(-[::+60]H)--))R} \] (f) 2-Methylimidazole

Identify the parent structure

Imidazole is an aromatic heterocyclic organic compound, consisting of a five-membered ring with two nitrogen atoms at non-adjacent positions. The molecular formula for imidazole is C3H4N2.

Add the substituent

2-Methylimidazole is derived from imidazole with a methyl group at the 2nd position. So, attach a methyl group on the 2nd carbon of the imidazole structure.

Draw the structural formula

The structural formula of 2-Methylimidazole is: \[ \chemfig{*5(=-N-(-N*5(-=-(=[::+60]CH_3)--))-))} \]

Key concepts

Aromatic Heterocyclic Compounds

Aromatic heterocyclic compounds are organic compounds that contain a ring structure composed of atoms from at least two different elements, typically carbon and nitrogen. These compounds are termed 'aromatic' due to their stable ring systems and unique electronic configuration. The electrons in these systems are delocalized, creating a resonance, which imparts stability and distinct chemical properties.
Examples of aromatic heterocyclic compounds include indole, quinoline, pyridine, isoquinoline, and imidazole. These structures are foundational in many biological and chemical processes. For instance:

• Indole: Found in many natural compounds like serotonin, indole is a bicyclic structure combining a benzene ring with a pyrrole ring.
• Quinoline: Known for its anti-malarial properties.
• Imidazole: A part of histidine, which plays a crucial role in enzyme activity.

Understanding these structures can help elucidate the behavior of these compounds in various contexts, from pharmaceuticals to everyday chemical reactions.

Amino Acids

Amino acids are the building blocks of proteins, which are crucial for life. They contain an amino group \((NH_2)\), a carboxyl group \((-COOH)\), and a side chain (R group) attached to a central carbon atom (Cα). This general structure allows them to link together, forming polypeptides and ultimately proteins.
Histidine, for example, is an amino acid with a side chain that includes an imidazole ring, making it basic and hydrophilic. This property allows it to participate in enzyme-mediated catalysis and stabilize or deinhibit proteins. Importantly:

• Histidine's Imidazole: The imidazole side chain is a versatile structural motif found in many biological systems.
• Biological Significance: Amino acids, including histidine, play critical roles in metabolism, cell signaling, and maintaining the structure of cells.

By examining amino acids at a structural level, we grasp why they are so vital in numerous biochemical pathways.

Molecular Structure Analysis

Molecular structure analysis is a method of determining the arrangement of atoms within a molecule. This knowledge helps us predict the molecule's reactivity, polarity, phase of matter, color, magnetism, biological activity, and more.
Key to this analysis are structural formulas, which depict the molecular structure and arrangement of atoms. For compounds like 3-methylindole or 2-methylimidazole, structural formulas help identify the position of substituents such as methyl groups or bromine atoms. This kind of analysis is significant because:

• Identifying Functional Groups: Locating functional groups within molecules helps predict reactivity.
• Understanding Compounds: As seen with 3-bromoquinoline, knowing the exact structure helps anticipate its chemical and physical properties.

Molecular structure analysis thus serves as the backbone in understanding and predicting the behavior of chemical compounds.

Functional Groups in Organic Chemistry

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing and understanding functional groups is crucial in organic chemistry as they dictate the chemical reactivity and properties of the compound.
Several functional groups were mentioned in the original exercise, such as:

• Methyl Group (-CH₃): Found in diverse molecules, adding a methyl group affects the molecule's polarity and solubility.
• Bromine (Br): Present in 3-bromoquinoline, bromine is an electronegative atom that affects the compound's reactivity.
• Carboxyl Group (-COOH): In nicotinic acid, this acidic group is vital in biochemical reactions and can donate a proton.

Functional groups determine how molecules interact with each other, making them foundational for constructing diverse chemical reactions that are central to both synthetic chemistry and real-world biochemical processes.