INTRODUCTION:

The relentless rise of antimicrobial resistance (AMR) represents one of the most severe threats to global public health, leading to increased mortality, prolonged hospital stays, and escalating healthcare costs^1,2. The World Health Organization (WHO) has declared AMR a top-tier priority, emphasizing the critical need for novel antibacterial agents with new mechanisms of action to tackle multidrug-resistant pathogens^3,4.

The pipeline for new antibiotics remains alarmingly sparse, highlighting a dire need for innovative research in this field^5,6.

In the quest for new chemotherapeutic agents, nitrogen-containing heterocyclic scaffolds have garnered significant attention. Among them, carbazole—a tricyclic structure comprising two benzene rings fused to a pyrrole ring—stands out as a "privileged scaffold" due to its presence in numerous natural products and pharmaceuticals^7,8. Carbazole derivatives exhibit a broad spectrum of biological activities, including anticancer, anti-inflammatory, antioxidant, and notably, antimicrobial properties^9,10. The planar, aromatic structure of carbazole allows for effective intercalation with biological macromolecules, while its synthetic versatility enables facile derivatization to fine-tune pharmacological profiles^11,12.

Functionalization at the N-9 position of the carbazole nucleus has been a successful strategy to enhance bioactivity¹³. Incorporating hydrazide functionalities (-CONHNH₂) is particularly attractive, as this group is known to contribute to antimicrobial potency by forming hydrogen bonds with active sites of microbial enzymes^14,
15. Furthermore, the introduction of halogen atoms, such as chlorine and bromine, is a classic bioisosteric strategy known to improve lipophilicity, membrane permeability, and overall drug-receptor interactions^16,17.

Inspired by these considerations and as part of our ongoing research into bioactive heterocycles, this work describes the design and synthesis of two novel N-9 substituted carbazole derivatives incorporating a hydrazide moiety and either a chlorine or bromine substituent. The primary objectives were: (i) to synthesize 2-(2-chloro-9H-carbazol-9-yl)acetohydrazide (IIIa) and 2-(3-bromo-9H-carbazol-9-yl)acetohydrazide (IIIb) via a straightforward and efficient route, (ii) to characterize their structures comprehensively using FTIR and ¹H NMR spectroscopy, and (iii) to evaluate their in-vitro antibacterial activity against representative Gram-positive and Gram-negative bacteria to establish a preliminary structure-activity relationship (SAR).

MATERIALS AND METHODS:

Planning and Reagents:

A retrosynthetic analysis was employed to design a high-yielding and reproducible synthetic pathway for the target molecules. The synthesis was planned to prioritize percent economy (yield) and feasibility with available laboratory resources over complex synthetic routes. All starting materials, including substituted carbazoles, ethyl chloroacetate, and hydrazine hydrate, as well as solvents (ethanol, DMSO), were procured from SD Fine Chemicals, Himedia, Spectrochem Ltd., and Aldrich Chemical Co. All commercial-grade reagents were used without further purification, while solvents were dried and distilled as per standard protocols.

General Experimental Procedures:

Reaction progress was monitored by thin-layer chromatography (TLC) on pre-coated silica gel G aluminum sheets, with spots visualized using iodine vapor. Melting points were determined in open capillary tubes using a Gallenkamp electrical melting point apparatus and are uncorrected. IR spectra were recorded on a PerkinElmer Spectrum 2 FTIR spectrophotometer. ¹H NMR spectra were obtained on a Bruker 400 MHz FT-NMR spectrometer using DMSO-d₆ as the solvent.

Synthetic Procedure:

Step 1: Synthesis of substituted ethyl 9H-carbazol-9-ylacetate (IIa-b)

To a stirred solution of substituted carbazole(2-chloro-9H-carbazole for IIa or 3-bromo-9H-carbazole for IIb) (0.02mol) in absolute ethanol (25mL), ethyl chloroacetate (0.02mol) was added dropwise. The reaction mixture was stirred at room temperature for 12 hours (overnight). After completion (monitored by TLC), the mixture was poured into ice-cold water (100 mL). The resulting solid was filtered, washed thoroughly with water, and recrystallized from ethanol to yield the intermediate esters IIa and IIb as colorless needles.

Step 2: Synthesis of substituted 2-(9H-carbazol-9-yl)acetohydrazide (IIIa-b)

A solution of the respective esterIIa or IIb (0.01mol) in absolute ethanol (25mL) was treated with hydrazine hydrate (98%, 0.021mol). The reaction mixture was heated under reflux for 4hours and then left stirring at room temperature overnight. The solid product that separated was filtered, dried, and recrystallized from a mixture of ethanol and water (2:2) to afford the final hydrazide products IIIa and IIIb as crystalline solids.

Scheme 1. Synthetic pathway for the target carbazole hydrazides (IIIa-b).

Scheme 1. Scheme illustrating the synthetic strategy for substituted carbazole derivatives.

RESULTS AND DISCUSSION:

Chemistry and Physical Characterization:

The target compounds IIIa and IIIb were successfully synthesized in two steps with good to excellent yields (IIIa: 76%, IIIb: 68%), as summarized in Table 2. The physical appearance of the compounds differed, with IIIa forming pale yellow crystals and IIIb forming reddish-brown crystals (Table 2). The sharp melting points (Table 3) and a single spot on TLC for both compounds indicated their high purity, which was essential for reliable spectroscopic and biological evaluation.

Table 2: Percentage yield and physical appearance of synthesized compounds.

Sr. No	Compound	Yield (%)	Appearance
1	IIIa	76	Pale yellow crystals
2	IIIb	68	Reddish brown crystals

Table 3: Melting point of synthesized compounds (Mean ± SD; n = 3).

Sr No	Compound	Melting Point (°C)
1	IIIa	198.3±0.05
2	IIIb	199.6±0.03

Spectroscopic Characterization:

FTIR Analysis:

The FTIR spectra provided crucial evidence for the formation of the target hydrazides.

Compound IIIa: The spectrum showed key absorption bands at: 3300 cm⁻¹ (m, N-H stretch of hydrazide), 3050 cm⁻¹ (s, aromatic C-H stretch), 1710 cm⁻¹ (s, C=O stretch of amide), 1600 cm⁻¹ (m, N-H bend), 1580 cm⁻¹ (m, C=C aromatic stretch), 1250 cm⁻¹ (s, C-N stretch), and 750 cm⁻¹ (s, C-Cl stretch).

Compound IIIb: The spectrum was similar, with characteristic bands for the hydrazide and aromatic moieties. The key difference was the appearance of a C-Br stretch band at 650 cm⁻¹ (s), confirming the presence of the bromo substituent.

The disappearance of the ester C=O stretch (~1735 cm⁻¹) and the appearance of a slightly lower frequency amide C=O stretch, along with N-H stretches, confirmed the successful conversion of the ester intermediate to the desired hydrazide.

¹H NMR Analysis:

The ¹H NMR spectra in DMSO-d₆ further confirmed the proposed structures.

Compound IIIa: δ 8.17 (t, J = 3.5 Hz, 1H), 8.09 – 8.04 (m, 1H), 8.00 (d, J = 7.8 Hz, 1H), 7.51 (dd, J = 6.3, 1.6 Hz, 1H), 7.38 – 7.27 (m, 3H), 4.56 (s, 2H, -N-CH₂-C=O), 3.95 (d, J = 3.5 Hz, 2H, -NH-NH₂, exchangeable with D₂O).

Compound IIIb: δ 8.33 (d, J = 2.4 Hz, 1H), 8.12 – 8.07 (m, 1H), 7.54 – 7.37 (m, 3H), 7.34 – 7.27 (m, 1H), 5.06 (q, J = 4.9 Hz, 1H), 4.09 (d, J = 3.3 Hz, 1H), 1.38 (s, 1H).

The spectra displayed complex multiplet signals in the aromatic region (δ 7.0-8.5 ppm) corresponding to the eight protons of the carbazole ring system. The distinctive singlet around δ 4.5 ppm for IIIa is assigned to the methylene group (-N-CH₂-C=O), a hallmark of N-alkylation. The broad signals for the -NH-NH₂ protons were also observed, confirming the hydrazide functionality.

In-Vitro Antibacterial Activity:

The synthesized compounds were evaluated for their antibacterial activity against S. aureus and P. aeruginosa, with ofloxacin as the standard drug. The results, expressed as Minimum Inhibitory Concentration (MIC in µg/mL), are presented in Table 4 and Figure 1.

Table 4: Antibacterial activity (MIC in µg/mL) of synthesized compounds.

Sr No.	Compound	S. aureus (Gram-positive)	P. aeruginosa (Gram-negative)
1	IIIa	12	9
2	IIIb	18	16
3	Ofloxacin	4	6

Figure 1. Graphical representation of MIC values.

Both compounds demonstrated notable antibacterial activity, though less potent than the standard drug ofloxacin. A clear structure-activity relationship (SAR) was observed. Compound IIIa, bearing a chlorine atom at the 2-position of the carbazole ring, exhibited superior activity compared to the bromo-substituted analogue IIIb. This suggests that the smaller, more electronegative chlorine atom might be more favorable for interaction with the bacterial target(s) or for cellular penetration. Interestingly, both compounds were more active against the Gram-negative P. aeruginosa than the Gram-positive S. aureus, which is an intriguing finding worthy of further investigation. The presence of the hydrazide moiety, capable of hydrogen bonding, and the planar carbazole ring, which may facilitate intercalation or membrane disruption, are likely key contributors to the observed bioactivity^18,19.

CONCLUSION:

In this study, two novel hydrazide-functionalized carbazole derivatives (IIIa and IIIb) were successfully synthesized and characterized. The synthetic route was efficient, yielding pure compounds as confirmed by melting point, TLC, and spectroscopic analyses (FTIR and ¹H NMR). The in-vitro antibacterial screening revealed that both compounds possess significant activity against Gram-positive and Gram-negative bacteria. The chloro-substituted derivative IIIa was found to be more potent than its bromo-substituted counterpart IIIb, establishing a preliminary halogen-dependent SAR. These findings underscore the potential of the carbazole-hydrazide hybrid scaffold as a promising template for developing new antimicrobial agents. Future work will focus on synthesizing a more extensive library of analogues, exploring their mechanism of action, and evaluating their in-vivo efficacy and pharmacokinetic properties.

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Received on 17.11.2025 Revised on 04.12.2025

Accepted on 20.12.2025 Published on 10.04.2026

Available online from April 13, 2026

Asian J. Research Chem.2026; 19(2):97-100.

DOI: 10.52711/0974-4150.2026.00016

This work is licensed under a Creative Commons Attribution-Non Commercial-Share Alike 4.0 International License. Creative Commons License.

Sr. No	Compound code	R	IUPAC name	Molecular Formula
1	IIIa	2-Cl	2-(2-chloro-9H-carbazol-9-yl)acetohydrazide	C₁₄H₁₂ClN₃O
2	IIIb	3-Br	2-(3-bromo-9H-carbazol-9-yl)acetohydrazide	C₁₄H₁₂BrN₃O