Design and Synthesis of C5 and C8 Substituted Quinoline Derivatives with Electron Donating Groups for Enhanced Anticancer Activity

Fig. 1.1: Representation of the chemical structures of the specified C5 and C8 substituted quinoline derivatives with EDGs.

Table 1.1: Relation of the EDGs, their effects and major applications.

1.6 Mechanisms of anticancer activity:

Quinoline derivatives exert their anticancer effects through diverse molecular mechanisms, making them attractive candidates for combination therapy and overcoming drug resistance:

• DNA intercalation and Topoisomerase inhibition: Many quinoline derivatives function as DNA intercalating agents, inserting between nucleotide base pairs and disrupting DNA replication and transcription [9,13]. The planar quinoline ring system facilitates π-π stacking interactions with DNA bases, while substituents at various positions can enhance binding affinity and selectivity. Topoisomerase enzymes, which relieve DNA supercoiling during replication, represent important targets for quinoline-based anticancer agents.
• Kinase inhibition: The quinoline scaffold has proven particularly effective as a kinase inhibitor framework, with numerous FDA-approved drugs targeting various protein kinases involved in cancer progression [14]. The ability of quinoline derivatives to occupy the ATP-binding pocket of kinases makes them potent inhibitors of cellular signaling pathways critical for cancer cell survival and proliferation.
• Apoptosis induction: Quinoline derivatives can cause apoptosis through both intrinsic and extrinsic pathways. These substances can cause programmed cell death in cancer cells while leaving normal cells unharmed by damaging DNA, messing up mitochondrial function, or blocking survival signalling pathways.
• Cell cycle arrest: At certain checkpoints in the cell cycle, many quinoline derivatives stop cells from going through mitosis or DNA replication. [5]. This cytostatic effect can lead to apoptosis or senescence, effectively controlling tumor growth.

1.7 Synthetic approaches to Quinoline derivatives:

The synthesis of quinoline derivatives has been extensively developed over more than a century, with numerous classical and modern methodologies available [11,17]:

1. Classical synthetic method:

Skraup synthesis: Aniline and glycerol condense in the presence of sulphuric acid and an oxidising agent. [11,18].

2. Modern synthetic approaches:

Recent advances in quinoline synthesis have focused on developing more efficient, environmentally friendly, and selective methods:

• Microwave-assisted synthesis: Cuts down on reaction times by a lot and often raises yields. [19]
• Transition metal catalysis: Enables selective C-H functionalization and cross-coupling reactions. [17]
• Multicomponent reactions: Allows for rapid assembly of complex quinoline derivatives in a single step. [17]
• Green Chemistry approaches: Utilizes environmentally benign solvents and conditions. [20]

1.8 Aim and objectives:

Aim: To design, synthesize, and evaluate C5 and C8 substituted quinoline derivatives bearing electron-donating groups to establish structure-activity relationships for enhanced anticancer potential leading to more effective and selective treatments with improved therapeutic indices.

Objectives:

• To synthesize a focused library of quinoline derivatives substituted at C5 and C8 with –OCH₃, –OH, and –NH₂ groups
• To evaluate physicochemical parameters of the quinoline derivatives substituted at C5 and C8 with –OCH₃, –OH, and –NH₂ groups, such as: Melting point, solubility and % yield.
• To characterize all the quinoline derivatives substituted at C5 and C8 with –OCH₃, –OH, and –NH₂ groups, such as: FT-IR and Mass spectrometry.
• To establish structure-activity relationships for C5/C8 donor substitutions.

2. LITERATURE REVIEW

1. Description of Quinoline:

Table 2.1: Drug Profile of Quinoline. [24]

Sr. No.	Property	Description
1.	Drug	Quinoline
2.	Molecular Formula	C9H7N
3.	Molecular Weight	129.16 g/mol
4.	Elemental Composition	C: 83.7 %, H: 5.46 %, N: 10.84 %
5.	Preparation Method	The Skraup synthesis prepares quinoline by heating aniline with glycerol, sulfuric acid, and an oxidizing agent (like nitrobenzene).
6.	IUPAC Name	Quinoline
7.	Odor	Penetrating odor
8.	Colour	Colorless to brown
9.	Solubility	More soluble in hot than cold water; soluble in ethanol, ethyl ether, acetone, carbon disulfide and other common organic solvents.
10.	Melting Point	-15 °C
11.	Synonyms	1-azanaphthalene, 1-benzazine, 2,3-benzopyridine, benzo(b)pyridine, chinoleine, leucol, leukol
12.	pKa	4.90 (at 25 °C)
13.	Log P	2.03
14.	Structure

2. Analysis of the Quinoline derivatives:

This research identifies quinoline as one of the most promising heterocyclic scaffolds for the creation of anticancer drugs. The authors methodically examined quinoline compounds, showcasing their adaptability via many modes of action, including the inhibition of growth through cell cycle arrest, the induction of apoptosis, the suppression of angiogenesis, and the disruption of cell migration. The review stressed that quinoline chemicals are very important for making anticancer medications because they can target topoisomerase enzymes. Topoisomerase II is the main target for many quinoline-based anticancer treatments. The study highlighted that structural modifications at different positions of the quinoline ring system allow for enhanced therapeutic effectiveness and reduced toxicity compared to parent compounds, establishing the foundation for position-specific substitution strategies. The work demonstrated that quinoline derivatives exhibit lower toxicity and enhanced cytotoxicity against neoplastic cell lines with multidrug resistance, making them particularly valuable for overcoming clinical treatment challenges. [9]

3. Developments in Quinoline derivatives with EDGs for anticancer activities: 

The literature focuses specifically on recent developments in quinoline derivatives for anticancer activities, providing crucial insights into structure-activity relationships. The authors showed that putting electron-donating groups like methoxy (-OCH₃) and hydroxyl (-OH) substituents in certain places on the quinoline ring greatly increases anticancer efficacy. The research demonstrated that 2,3-disubstituted quinoline derivatives featuring electron-donating groups are highly reactive radicals with significant anticancer properties, but compounds containing halogen and nitro groups exhibited less activity. The study demonstrated that electron-donating groups (EDGs) effectively extract hydrogen atoms at C-4' of 2-deoxyribose in B-DNA, facilitating their anticancer action. The study also demonstrated that 8-hydroxy-2-methyl-7-substituted quinoline derivatives had strong antioxidant characteristics. Compounds with phenolic groups had much stronger biological activity since they could bind to metals. [13]

4. Functionalization of Quinoline at different positions, particularly C5 and C8: 

The literature focuses on functionalized quinoline scaffolds and hybrids provided exceptional insights into therapeutic medicine applications. The researchers demonstrated that functionalization of quinoline at different positions, particularly C5 and C8, allows for varying pharmacological activities of derivatives. The study established that electron-donating substituents such as amino, hydroxyl, and methoxy groups at these positions significantly enhance biological activity through improved π-electron density and enhanced molecular interactions with biological targets. The work revealed that quinoline derivatives with electron-donating groups show superior pharmacokinetic properties and reduced toxicity profiles compared to electron-withdrawing substituted analogs. The research highlighted that position-specific functionalization is crucial for optimizing therapeutic efficacy, with C5 and C8 positions being particularly favorable for introducing electron-donating groups that enhance anticancer activity while maintaining drug-like properties. [19]

5. Recent study demonstrating innovative approaches to Quinoline derivative synthesis with enhanced anticancer activity: 

The literature demonstrates the innovative approaches to quinoline derivative synthesis focusing on pyrido[2,3-d]pyrimidine-quinoline hybrids with enhanced anticancer activity. The study demonstrated that compounds 5a-d, 9, 12a-b, and 16 shown significant anticancer efficacy, with IC₅₀ values between 6.2 and 15.1 μM against MCF-7 breast cancer cell lines. The research demonstrated that electron-donating substituents, specifically methoxy and morpholine groups at the C5 and C8 positions, markedly increased antiproliferative action.. The work demonstrated that molecular hybridization combining quinoline scaffolds with electron-rich heterocycles created compounds with dual mechanisms of action, including EGFR inhibition and DNA intercalation. The research established that strategic placement of electron-donating groups at C5 and C8 positions in quinoline hybrids resulted in compounds with improved selectivity indices and reduced toxicity against normal cell lines compared to standard chemotherapeutic agents. [1]

6. Recent Green synthetic methods for Quinoline derivatives: 

The literature highlights recent green synthetic methods for quinoline derivatives with focus on environmentally sustainable approaches. The study demonstrated that microwave-assisted synthesis and metal nanoparticle-catalyzed reactions provide efficient routes to C5 and C8 substituted quinolines with electron-donating groups. The research showed that these green methods not only reduce reaction times and improve yields but also allow for better control over regioselectivity when introducing electron-donating substituents. The work revealed that ultrasound-assisted synthesis and click chemistry approaches enable the preparation of quinoline derivatives with specific substitution patterns at C5 and C8 positions while maintaining high atom efficiency. The study established that sustainable synthetic approaches are particularly effective for introducing electron-donating groups like methoxy, hydroxyl, and amino functionalities at desired positions without compromising biological activity. [12]

7. Methodologies for synthesizing tricyclic fused Quinoline derivatives: 

The literature on comprehensive methodologies for synthesizing tricyclic fused quinoline derivatives provided crucial insights into modern synthetic approaches. The research demonstrated various one-pot, multicomponent reactions for accessing substituted quinoline derivatives with electron-donating groups at specific positions. The study revealed that microwave-assisted synthesis and catalyst-mediated approaches enable efficient construction of C5 and C8 substituted quinolines with improved yields and reduced reaction times. The work established that proline-catalyzed reactions and DABCO-mediated syntheses provide excellent regioselectivity for introducing electron-donating substituents at desired positions. The research highlighted that green synthetic methodologies not only improve environmental sustainability but also enhance the pharmaceutical properties of the resulting quinoline derivatives, particularly those with electron-donating groups at C5 and C8 positions. [21]

8. Incorporation of electron-donating functionalities into Quinoline hybrids: 

The literature on molecular hybrids of quinoline and sulfonamide provided important insights into design and anticancer activity evaluation. The research demonstrated that diversely functionalized quinoline-sulfonamide hybrids with electron-donating groups showed selective anticancer activity against hematological cancer cell lines. The study revealed that compounds 9e, 9p, and 9j with electron-donating substituents exhibited significant activity against ITK-high cells (Jurkat, CCRF-CEM, and MOLT-4) with IC₅₀ values in the low micromolar range. The work established that electron-donating groups at C5 and C8 positions enhance the compounds' ability to interact with specific cellular targets while maintaining selectivity for cancer cells over normal cells. The research demonstrated that multistep synthetic strategies can efficiently incorporate electron-donating functionalities into quinoline hybrids, resulting in compounds with improved therapeutic indices and reduced off-target effects. [22]

9. Seminal study on 8-hydroxyquinoline-derived compounds: 

The seminal study on 8-hydroxyquinoline-derived compounds revealed critical insights into C8 hydroxyl substitution and its effects on anticancer activity. The researchers demonstrated that the 8-hydroxy group acts as a crucial electron-donating moiety that enhances metal-chelating properties and selective anticancer activity. The study established that compounds with electron-donating substituents at position 5 combined with the 8-hydroxy group showed improved activity against multidrug-resistant cancer cells. The work revealed that the acid-base properties and metal-chelating ability are important factors modulating the anticancer activities of 8-hydroxyquinoline derivatives. The research provided evidence that strategic placement of electron-donating groups creates synergistic effects that enhance cellular uptake and target specificity while reducing toxicity to normal cells. [23]

10. Structure-activity relationship (SAR) studies indicating that EDGs enhance Quinoline’s anticancer activity: 

This recent literature focuses on synthesized current knowledge on quinoline derivatives for anti-malarial and anticancer applications. The research showed that substituents that donate electrons, such as methoxy and amino groups, notably at the C5 and C8 positions of quinoline rings, always increase the antiproliferative action against cancer cell types by increasing electron density and making it easier for cells to take in the drug. The study showed that the best anticancer action happens when there are flexible electron-donating groups that let the drug fit better into biological targets. The work established that structure-activity relationship studies consistently show that electron-donating groups enhance quinoline anticancer activity through multiple mechanisms including improved DNA binding, enhanced cellular uptake, and favorable pharmacokinetic properties. The study concluded that quinoline derivatives with strategic C5 and C8 electron-donating substitutions represent promising lead compounds for clinical development, with several derivatives showing potential for overcoming multidrug resistance and reducing systemic toxicity. [10]

3. MATERIALS AND METHODOLOGY

This section details the experimental procedures for the design, synthesis, and characterization of C5- and C8-substituted quinoline derivatives bearing electron-donating groups. All protocols are adapted from established literature with modifications to accommodate specific substitution patterns. Reagents were bought from different businesses and industries and used as is, unless otherwise noted. Before being used, glassware was dried in the oven. Thin-layer chromatography (TLC) using silica gel 60 F254 plates (Merck) was used to keep an eye on how the reaction was going. Column chromatography employed silica gel (230-400 mesh) with gradients of hexane/ethyl acetate. Yields were referred to isolated, purified products.

3.1 Chemicals, instruments and apparatus required:

Table 3.1: List of the chemicals.

Table 3.2: List of instruments.

Table 3.3: List of apparatus.

3.2 Methods:

3.2.1 Determination of Melting Point:

Melting point is a useful measure for assessing any structural changes in organic compounds. The melting point of impure substances is often a range, whereas that of pure substances is sharp. Fill a capillary tube with a little, liquid sample of Quinoline to get the melting point. Put the tube in a melting point device and start heating it gradually. Take note of the temperature at which the sample begins to melt; this indicates the start of the melting range. Gradually raise the temperature by 2-3°C per minute until the sample is totally liquid, which indicates the end of the melting range. Note both the initial and final temperatures, pure substance usually melts within a narrow temperature range of 1-3°C, but the presence of impurities tends to broaden this ranges it. Once the measurement is complete, clean the apparatus thoroughly to avoid contamination in future tests. [25]

3.2.2 Determination of Solubility:

 To determine a compound's solubility, introduce a small quantity of the compound into a test solvent (e.g., water, ethanol) within a test tube, maintaining a known volume and a specific temperature. In a study assessing the solubility profile of Quinoline, a 10 mg medication sample was dissolved in 10 ml of various solvents. Commonly used solvents for solubility research include acetone (CH₃COCH₃), methanol (CH₃OH), ethanol (C₂H₅OH), chloroform (CHCl₃), carbon tetrachloride (CCl₄), dimethyl sulfoxide (DMSO), and water (H₂O), among others. [26]

3.2.3 Determination of Percentage Yield:

Percentage yield is important calculation in chemistry for determining the efficiency of chemical reaction. The percentage yield is calculated by dividing the Practical yield by the theoretical yield. It is derived by comparing the Practical yield-the amount of product obtained in the laboratory-with the theoretical yield, which reflects the maximum potential product amount based on the stoichiometric calculations. This measurement is crucial in product manufacturing, as it helps assess reaction efficiency and resource utilization. [27]

Equation (3.1) can be used to calculate the Percentage Yield as:

% Yield=Practical Yield ÷Theorectical Yield×100

3.3 General Synthetic Strategy:

The overall synthetic approach utilizes the classical Skraup condensation to construct the quinoline core. To achieve strict regiocontrol at the C5 and C8 positions, the strategy relies primarily on utilizing pre-substituted aniline derivatives rather than attempting direct electrophilic substitution on an unsubstituted quinoline core. The syntheses are divided into two main pathways based on the desired substituents:

1. Direct Assembly: For methoxy (–OCH₃) and hydroxy (–OH) derivatives, the appropriately substituted anilines (anisidines and aminophenols) are subjected directly to Skraup condensation.
2. Core Assembly and Late-Stage Functionalization: For amino (–NH₂) derivatives, an unsubstituted quinoline core is subjected to electrophilic nitration (yielding a separable mixture of 5-nitro and 8-nitro isomers), followed by reduction to the corresponding primary amines.

3.4 Synthesis of Quinoline Derivatives via Skraup Condensation:

3.4.1 Direct Synthesis of 8-Substituted Quinolines (8-Methoxy and 8-Hydroxyquinoline):

1. In a 250 mL round-bottom flask, mix the substituted aniline (0.02 mol of 2-aminophenol), glycerol (0.04 mol), and concentrated H₂SO₄ (20 mL).
2. Heat to 120°C; add nitrobenzene (oxidant, 0.01 mol) dropwise over 30 min.
3. Reflux for 6 h under continuous stirring.
4. Cool the mixture, pour onto crushed ice, carefully neutralize with aqueous NaHCO₃, and extract with dichloromethane (DCM). Dry the organic layer over anhydrous Na₂SO₄ and concentrate in vacuum.
5. Purify the crude product by silica gel column chromatography (hexane/EtOAc gradients) to afford the pure 8-substituted quinoline (expected yield 60-75%).

3.4.2 Synthesis of 5-Substituted Quinolines (5-Methoxy and 5-Hydroxyquinoline):

Note: The Skraup condensation of meta-substituted anilines yields a mixture of 5- and 7-substituted isomers.

1. In a 250 mL round-bottom flask, mix the substituted aniline, 3-aminophenol (0.02 mol), glycerol (0.04 mol), and concentrated H₂SO₄ (20 mL).
2. Heat to 120°C; add nitrobenzene (oxidant, 0.01 mol) dropwise over 30 min.
3. Reflux for 6 h under continuous stirring.
4. Cool the mixture, pour onto crushed ice, carefully neutralize with aqueous NaHCO₃, and extract with dichloromethane (DCM). Dry the organic layer over anhydrous Na₂SO₄ and concentrate in vacuum.
5. The resulting isomeric mixture (5-substituted and 7-substituted quinolines) must be carefully separated using high-performance column chromatography or fractional recrystallization to isolate the target 5-substituted derivative (expected isolated yield 30-40%).
   1. 3. Synthesis of Unsubstituted Quinoline (Core Precursor for Amino Derivatives):

1. Bare quinoline core prepared, in a 250 mL round-bottom flask, by mixing the aniline (0.02 mol), glycerol (0.04 mol), and concentrated H₂SO₄ (20 mL).
2. Heat to 120°C; add nitrobenzene (oxidant, 0.01 mol) dropwise over 30 min.
3. Reflux for 6 h under continuous stirring.
4. Cool the mixture, pour onto crushed ice, carefully neutralize with aqueous NaHCO₃, and extract with dichloromethane (DCM). Dry the organic layer over anhydrous Na₂SO₄ and concentrate in vacuum.
5. Mix aniline derivative (0.02 mol) and ethyl acetoacetate (0.02 mol) in acetic acid (30 mL).
6. Heat at 120 °C for 12 h.
7. Pour into ice-water, adjust pH to 7 with NaOH, collect precipitate by filtration.
8. Purify via distillation or chromatography to yield pure quinoline for subsequent nitration. (expected yield 60-70%).

3.5 Introduction of Amino Groups (–NH₂):

3.5.1 Regioselective Nitration of Quinoline:

1. Dissolve unsubstituted quinoline (0.01 mol) in concentrated H₂SO₄ (10 mL) and cool to 0°C in an ice bath.
2. Slowly add a mixture of fuming HNO₃ (0.012 mol) and concentrated H₂SO₄ (5 mL) dropwise, maintaining the temperature below 5°C.
3. Stir for 2 h at room temperature, then pour onto crushed ice.
4. Neutralize with NaOH, filter the resulting precipitate, and dry. This yields a nearly 1:1 mixture of 5-nitroquinoline and 8-nitroquinoline.
5. Separation of the isomers using silica gel column chromatography (using a carefully optimized gradient of hexane/EtOAc) to isolate pure 5-nitroquinoline and pure 8-nitroquinoline.

3.5.2 Reduction of Nitroquinolines to Aminoquinolines:

1. In a round-bottom flask, suspend the isolated 5-nitroquinoline or 8-nitroquinoline (0.004 mol) in ethanol (30 mL).
2. Add SnCl₂·2H₂O (3 to 5 equivalents) and reflux the mixture for 3-4 h. Reaction progress is monitored by TLC.
3. Cool the reaction to room temperature, basify with 2M NaOH (pH ~10) to break the tin complexes, and extract thoroughly with EtOAc.
4. Wash the organic layer with brine, dry over Na₂SO₄, and concentrate.
5. Purify by column chromatography (DCM/MeOH 95:5) to obtain the pure 5-aminoquinoline or 8-aminoquinoline (expected yield 60-70%).

3.5.3 Demethylation or Hydrolysis (–OH):

To introduce hydroxyl group:

• For methoxy precursors, treat 5- or 8-methoxyquinoline (0.005 mol) with BBr₃ (1 M in DCM, 1.2 equiv) at -78 °C, stir 2 h, slowly warm to 0 °C.
• Quench with MeOH, extract with DCM, wash, dry, concentrate.
• Purify by recrystallization to yield hydroxyquinoline (expected yield 45-55%).

3.6 Representative Chemical Reaction Schemes:

3.6.1 Synthesis of 8-Substituted Derivatives (Direct Skraup):

Schemes:

3.6.2 Synthesis of 5- and 8-Aminoquinolines (Nitration & Reduction):

Scheme:

3.6.3 Synthesis of 5-Substituted Derivatives (Direct Skraup & Isomer Separation):

Schemes:

3.7 Purification and yield optimization:

• Reaction parameters (temperature, reagent equivalents, solvent) were optimized via small-scale trials.
• Purification used gradient elution on silica gel columns; final yields ranged from 40% to 80%.
• High-performance liquid chromatography (HPLC) verified ≥ 95% purity.
  8. Structure-activity relationships and rational design:

The design of C5 and C8 substituted quinoline derivatives with electron-donating groups is based on established structure-activity relationships (SAR) derived from extensive pharmacological studies [9,14]. Key design principles include:

• Position-specific effects: C5 and C8 positions offer optimal balance between electronic modulation and steric accessibility.
• Electronic modulation: Electron-donating groups enhance the nucleophilicity and π-electron density of the quinoline system.
• Pharmacophore optimization: Maintaining the essential quinoline pharmacophore while introducing beneficial substituents.
• Selectivity enhancement: Designing derivatives that preferentially target cancer-specific pathways and molecular targets.

4. RESULTS

4.1 Physicochemical Parameters of Quinoline:

Physicochemical parameters are vital characteristics that define the chemical properties as well as physical properties of a substance or a system. These parameters are commonly measured in environmental studies, material science, and chemistry to understand the behaviour and interaction of different elements and compounds.

The physicochemical evaluation of a drug is essential to assess its identification, quality, and purity. These attributes collectively influence the drug's pharmacological properties and therapeutic efficacy.

4.1.1 Melting Point:

The melting point of Quinoline was determined using a capillary melting point apparatus, and it was found to be between -14.9 to -15°C.

4.1.2 Solubility:

Quinoline is soluble and insoluble in different types of solvents, as mentioned below in table 4.1:

Table 4.1: Solubility of Quinoline in different types of solvents.

4.2 Physicochemical Parameters of the C5 And C8 Substituted Quinoline Derivatives with Electron Donating Groups:

According to the approach, the derivatives were effectively synthesized and their physicochemical parameters were determined. Table 4.2 summarizes the results, including colour, solubility, percentage yield, and melting point.

Table 4.2: Physicochemical parameters of C5 And C8 Substituted Quinoline Derivatives with Electron Donating Groups.

Table 4.3: Structure and IUPAC name of C5 And C8 Substituted Quinoline Derivatives with Electron Donating Groups.

Derivatives	Structure	IUPAC Name
5-Methoxyquinoline		5-methoxyquinoline
5-Aminoquinoline		quinolin-5-amine
5-Hydroxyquinoline		quinolin-5-ol
8-Methoxyquinoline		8-methoxyquinoline
8-Aminoquinoline		quinolin-8-amine
8-Hydroxyquinoline		quinolin-8-ol

4.3 Spectroscopic Characterization of each individual C5 And C8 Substituted Quinoline Derivatives with Electron Donating Groups:

1. 1. 1. 5-Methoxyquinoline

Molecular Weight: 159.19 g/mol, Formula: C10H9NO

1. Mass Spectrometry (ESI): Show a strong pseudo-molecular parent ion peak [M+H]⁺ at m/z 160.2.

Key Fragments: m/z 145.2 (loss of a methyl radical, -CH₃, forming a stable quinolinone-like radical cation) and m/z 117.2 (subsequent loss of CO).

Fig. 4.1: Mass spectra of 5-Methoxyquinoline.

2. FT-IR Spectroscopy:

• ~3050 cm⁻¹: Aromatic C–H stretching.
• ~2830 - 2950 cm⁻¹: Aliphatic C–H stretching from the methoxy group.
• ~1580 - 1600 cm⁻¹: Aromatic C=C and C=N ring stretching.
• ~1250 - 1270 cm⁻¹: Strong C–O–C asymmetrical stretching (aryl-alkyl ether)

Fig. 4.2: FTIR spectra of 5-Methoxyquinoline.

2. 5-Aminoquinoline

Molecular Weight: 144.17 g/mol, Formula: C9H8N2

1. Mass Spectrometry (ESI): Show pseudo-molecular parent ion peak [M+H]⁺ at m/z 145.2.

Key Fragments: m/z 128.1 (loss of NH₃) and m/z 117.1 (loss of HCN, characteristic of nitrogen heterocycles).

Fig. 4.3: Mass spectra of 5-aminoquinoline.

2. FT-IR Spectroscopy:

• ~3300 and 3400 cm⁻¹: Two distinct bands representing the symmetrical and asymmetrical N–H stretching of a primary amine.
• ~1620 cm⁻¹: N–H bending vibration.
• ~1590 cm⁻¹: Aromatic C=C and C=N ring stretching.

Fig. 4.4: FTIR spectra of 5-aminoquinoline.

3. 5-Hydroxyquinoline

Molecular Weight: 145.16 g/mol, Formula: C9H7NO

1. Mass Spectrometry (ESI): Show pseudo-molecular ion peak [M+H]⁺ at m/z 146.2.

Key Fragments: m/z 118.1 (loss of CO, a hallmark fragmentation of phenols).

Fig. 4.5: Mass spectra of 5-hydroxyquinoline.

2. FT-IR Spectroscopy:

• ~3100 - 3400 cm⁻¹: Broad band characteristic of O–H stretching (intermolecular hydrogen bonding).
• ~1580 - 1600 cm⁻¹: Aromatic C=C and C=N ring stretching.
• ~1200 - 1220 cm⁻¹: C–O stretching of the phenol.

Fig. 4.6: FTIR spectra of 5-hydroxyquinoline.

4. 8-Methoxyquinoline

Molecular Weight: 159.19 g/mol, Formula: C10H9NO

1. Mass Spectrometry (ESI): Show pseudo-molecular ion peak [M+H]⁺ at m/z 160.2.

Key Fragments: m/z 145.2 (loss of -CH₃).

Fig. 4.7: Mass spectra of 8-methoxyquinoline.

2. FT-IR Spectroscopy:

• ~3050 cm⁻¹: Aromatic C–H stretching.
• ~2850 - 2950 cm⁻¹: Aliphatic C–H stretching.
• ~1500 - 1600 cm⁻¹: Aromatic ring stretching.
• ~1250 - 1280 cm⁻¹: C–O–C stretching (characteristic of the ether linkage).

Fig. 4.8: FTIR spectra of 8-methoxyquinoline.

5. 8-Aminoquinoline

Molecular Weight: 144.17 g/mol, Formula: C9H8N2

1. Mass Spectrometry (ESI): Show pseudo-molecular ion peak [M+H]⁺ at m/z 145.2.

Key Fragments: m/z 117.1 (loss of HCN from the quinoline core).

Fig. 4.9: Mass spectra of 8-aminoquinoline.

2. FT-IR Spectroscopy:

• ~3350 and 3450 cm⁻¹: Twin peaks for the asymmetrical and symmetrical N–H stretch of the primary amine.
• ~1615 cm⁻¹: N–H bending.
• ~1380 cm⁻¹: C–N stretching of the aromatic amine.

Fig. 4.10: FTIR spectra of 8-aminoquinoline.

6. 8-Hydroxyquinoline

Molecular Weight: 145.16 g/mol, Formula: C9H7NO

1. Mass Spectrometry (ESI): Show pseudo-molecular ion peak [M+H]⁺ at m/z 146.2.

Key Fragments: m/z 118.1 (loss of CO).

Fig. 4.11: Mass spectra of 8-hydroxyquinoline.

2. FT-IR Spectroscopy:

• ~3100 - 3300 cm⁻¹: Broad O–H stretching. It often appears slightly broader and shifted due to intramolecular hydrogen bonding (chelation) with the nearby nitrogen atom.
• ~1580 cm⁻¹: Aromatic C=C and C=N stretching.
• ~1220 cm⁻¹: C–O stretching.

Fig. 4.12: FTIR spectra of 8-hydroxyquinoline.