3-Chloro-1-propanol: a multifaceted molecule from molecular conformation to environmental fate

Introduction

3-Chloro-1-propanol (Cl–CH?–CH?–CH?–OH) has a simple structure—the two ends of the straight chain of three carbons are connected to chlorine atoms and hydroxyl groups respectively. However, it is this simple structure that has rich conformational diversity due to three rotatable single bonds (C–C and C–O). Scientists have conducted in-depth research on this molecule from different angles: What is its most stable conformation in the gas phase? How do microorganisms in the environment degrade it? How to use it to build ordered organic layers on semiconductor surfaces? These seemingly independent questions together outline a complete scientific picture of 3-Chloro-1-propanol as a "multifaceted molecule".

The mystery of conformation

There are three rotatable dihedral angles (Cl–C–C–C, O–C–C–C and H–O–C–C) in the 3-Chloro-1-propanol molecule, which can theoretically produce a variety of conformational isomers. As early as the 1980s, Fuller, Wilson and Caminati conducted pioneering research on this molecule using microwave spectroscopy [1]. They observed the rotational spectra of the two conformations and determined that in both conformations, the oxygen atoms were in the gauche position relative to the C?–C? bond, and the hydroxyl hydrogen was in the trans position relative to the C?–C? bond. The difference lies in the orientation of the chlorine atoms: in one conformation the chlorine is trans (T conformation) and in the other it is gauche (G conformation). Through relative intensity measurements, they obtained that the ground state of the G conformation is about 0.7 kcal/mol more stable than the T conformation. What is even more surprising is that although it is possible to form a six-membered ring hydrogen bond (i.e., O–H···Cl) within the molecule, no hydrogen bond conformation has been observed experimentally. The author speculates that this may be because the chlorine atoms and oxygen atoms in the hydrogen bond conformation are too close, much smaller than the sum of their van der Waals radii, causing the repulsion to exceed the stabilizing effect of the hydrogen bond [1].

This conclusion was subsequently verified by high-level theoretical calculations. Badawi and F?rner used DFT?B3LYP, MP2, MP3, MP4 and other methods in 2008, combined with the 6?311+G basis set, to systematically scan the potential energy surface of 3-Chloro-1-propanol [2]. They located 12 energy minimum points on the potential energy surface, and all of them had no imaginary frequency. Calculations show that the Ggt conformation (gauche?gauche?trans) has the lowest energy, which is in perfect agreement with the microwave experimental results. Further by calculating the equilibrium constant, they predicted that at 298.15 K, the gas phase equilibrium mixture of 3-Chloro-1-propanol contains approximately 32% Ggt, 18% Ggg1, 13% Tgt, 8% Tgg and 8% Gtt conformations [2]. In order to verify the existence of high-energy conformation, they also designed a solvent experiment: using high dielectric constant formamide (ε = 109.5) as a solvent to compare the Raman spectra of pure samples and solutions. It was found that the relative intensities of the two pairs of spectral lines at 907/868 cm?1 and 724/656 cm?1 changed significantly, directly proving that multiple conformations do exist in the sample [2].

Application

As an important organic synthesis intermediate, 3-Chloro-1-propanol has a wide range of applications.

In the pharmaceutical industry, it is often used as an alkylating reagent and linker for the synthesis of a variety of drugs. For example, it is a key intermediate in the synthesis of anticancer drugs (cyclophosphamide analogs), cardiovascular drugs (propranolol, metoprolol), and the anti-AIDS drug nelfinavir. In addition, it is also used in the preparation of fine chemicals such as 3-chloropropyl chloroformate.

In the field of materials science, 3-Chloro-1-propanol exhibits unique surface chemistry value. Shao et al. used high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) to systematically study the adsorption and photochemical reaction of this molecule on the Si(100)-2×1 surface [3]. They found that 3-Chloro-1-propanol was chemically adsorbed on the silicon surface through thermal dissociation of the OH group, forming a Si–O–CH?CH?CH?–Cl structure. At the same time, Si–H bonds were generated on the Si surface (2110 cm?1 peak in HREELS), while the C–Cl bond was completely retained (654 cm?1 peak) [3]. Subsequently, 193 nm laser irradiation can selectively cut off the C–Cl bond, and the generated carbon radicals undergo intermolecular coupling to form a double-layer structure of Si–O–CH?CH?CH?–CH?CH?CH?–O–Si. Even more cleverly, if a layer of 3-Chloro-1-propanol molecules is physically adsorbed on top of the chemical adsorption layer, light-induced free radical coupling can also occur between the physical adsorption layer and the chemical adsorption layer to achieve the construction of a secondary organic layer and obtain a Si–O–CH?CH?CH?–CH?CH?CH?–OH structure with a hydroxyl group at the end [3]. This strategy opens up a new path for layer-by-layer organic functional modification of semiconductor surfaces.

In insect pheromone synthesis, 3-Chloro-1-propanol is also used as a "linker" (linchpin) to connect two synthetic modules, reflecting its versatility as a bifunctional molecule.

Environmental trends

Chlorinated hydrocarbons are often persistent pollutants in the environment, and 3-Chloro-1-propanol is no exception. Bosma and Janssen studied the co-metabolic conversion ability of the methanotroph Methylosinus trichosporium OB3b (expressing soluble methane monooxygenase) to a variety of chlorinated propanes, including 3-Chloro-1-propanol [4]. They found that this strain was able to convert 1-chloropropane, and the main product was 3-Chloro-1-propanol (3-Chloro-1-propanol could accumulate to 0.3 mM after using cyclopropane to inhibit alcohol dehydrogenase) [4]. Further research showed that 3-Chloro-1-propanol can be further oxidized and dechlorinated, releasing inorganic chloride ions. This study revealed the biotransformation pathway of 3-Chloro-1-propanol under aerobic conditions and pointed out that toxic intermediates will be produced during co-metabolic transformation, leading to inactivation of enzyme activity [4].

5. Synthesis method

The industrial synthesis of 3-Chloro-1-propanol mainly adopts the following route:

1. 1,3-propanediol chlorination method: react 1,3-propanediol with hydrogen chloride gas in the presence of a catalyst (such as benzenesulfonic acid or Lewis acid). The improved continuous process uses a fixed-bed reactor and is purified by molecular distillation to achieve high yields and high purity (≥99%).

2. Reaction method of ethylene, formaldehyde and hydrogen chloride (Prins reaction route): produced by the one-step reaction of ethylene, formaldehyde and hydrogen chloride.

3. Esterification-transesterification method of 1-bromo-3-chloropropane: The target product is obtained by reacting 1-bromo-3-chloropropane with acetate and then hydrolyzing it.

Industrial products usually use gas chromatography (GC) for content detection, and the purity requirement is ≥99%.

Detection method

Due to the potential genotoxicity of 3-Chloro-1-propanol, it needs to be strictly tested in pharmaceutical quality control. At present, the most important detection method is gas chromatography-mass spectrometry (GC-MS). In addition, nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy are also commonly used characterization tools for this compound. In particular, the C–Cl stretching vibration (about 656 cm?1) and OH stretching vibration (about 3300-3570 cm?1) in the Raman spectrum can be used as characteristic fingerprint peaks for conformational analysis and surface reaction monitoring [2][3].

Safety and precautions

3-Chloro-1-propanol is a flammable liquid that decomposes when heated to release toxic chloride gases (such as HCl and phosgene). It is irritating to the eyes, skin and respiratory tract. The oral LD?? in rats is about 110 mg/kg. It is classified as a Class 3 carcinogen by IARC (insufficient evidence of carcinogenicity for humans, but sufficient evidence for animals). The operation needs to be carried out in a fume hood, wearing chemical protective gloves, safety glasses and a protective mask. During storage, it should be isolated from strong oxidants, strong acids and strong alkali, and protected from light and high temperature. This substance belongs to Category 6.1 dangerous goods, UN number 2849, and transportation and disposal must comply with relevant regulations.

References

[1] FULLER M J, WILSON E B, CAMINATI W. Rotational Isomerism in 3-Chloro-1-Propanol from the Microwave Spectrum[J]. Journal of Molecular Spectroscopy, 1982, 96: 131-145. https://doi.org/10.1016/0022-2852(82)90158-4

[2] BADAWI H M, F?RNER W. Analysis of vibrational spectra of 3-halo-1-propanols CH?XCH?CH?OH (X is Cl and Br)[J]. Spectrochimica Acta Part A, 2008, 71(1): 1-7. https://doi.org/10.1016/j.saa.2008.03.002

[3] SHAO Y X, et al. Photoinduced Reactions of 3-Chloro-1-propanol on Si(100)-2×1[J]. The Journal of Physical Chemistry C, 2010, 114(40): 17157-17164. https://doi.org/10.1021/jp103945m

[4] BOSMA T, JANSSEN D B. Conversion of chlorinated propanes by Methylosinus trichosporium OB3b expressing soluble methane monooxygenase[J]. Applied Microbiology and Biotechnology, 1998, 50: 105-112. https://doi.org/10.1007/s002530051264