Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review

Natural waters, water droplets in the air at coastal regions and wastewaters usually contain chloride ions (Cl-) in relatively high concentrations in the milimolar range. In the reactions of highly oxidizing radicals (e.g., •OH, •NO3, or SO4•-) in the nature or during wastewater treatment in advanced oxidation processes the chloride ions easily transform to chlorine containing radicals, such as Cl•, Cl2•-, and ClO•. This transformation basically affects the degradation of organic molecules. In this review about 400 rate constants of the dichloride radical anion (Cl2•-) with about 300 organic molecules is discussed together with the reaction mechanisms. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation state), and molecules with conjugated electron systems are suggested to take place with electron transfer mechanism. The rate constant is high (107–109 M-1 s-1) when the reduction potential the one-electron oxidized species/molecule couple is well below that of the Cl2•-/2Cl- couple. Supplementary Information The online version contains supplementary material available at 10.1007/s11356-021-14453-w.


Introduction
The presence of chloride ions in wastewaters is widespread. Therefore, the effect of chloride ions on Advanced Oxidation Processes (AOP), emerging technology for wastewater purification, is of outermost importance since Clefficiently scavenges the reacting radicals (hydroxyl radical ( • OH), sulfate radical anion (SO 4 •-)) transforming them to chlorine containing radicals, e.g., chloride atom and dichloride radical anion ( • Cl and Cl 2

•-
). However, the latter radicals also react with organic molecules, with altered rate constants and selectivity (Caregnato et al. 2013). In surface waters and treated wastewaters rich in Clions, including oceans, estuaries, and brines chloride radical anions can occur at concentrations that are orders of magnitude higher than the concentrations of other radicals such as • OH. These radicals may also form during disinfection by chlorine or during chlorine photolysis (Zhang and Parker 2018;Liu et al. 2019). Therefore, determination of the reactivity of chloride radicals, among them also that of Cl 2 •toward organic molecules is of environmental importance.
The standard reduction potential of the Cl 2 •-/2Clcouple has been reported to be between 2.1 and 2.3 V vs. NHE (Wardman 1989;Armstrong et al. 2015), here we use the frequently referred value: E o (Cl 2 •-/2Cl -) = 2.1 V. We often compare the rate constants measured in Cl 2 •reactions with values measured in reactions of other one-electron oxidants. Comparison of the reduction potentials of the one-electron oxidants and those of the semioxidized and nonoxidized forms of organic molecule couples often gives possibilities for explaining the rate constant ranges of radical reactions. Table 1S in the Supplementary material lists several oneelectron oxidants with reduction potentials ranging from 2.43 to 0.934 V.
Determination of the Cl 2 •reaction rate constants contrast to other oxidizing radicals (e.g., • OH, SO 4 •-, and CO 3 •-), competitive techniques were rarely used in rate constant determinations . This is probably due to the rather complex reaction system involved in Cl 2 •reactions (see below). A reference compound in the solution would make further complications. In pulse radiolysis Cl 2 •reactions are generally investigated in N 2 O-saturated solutions Buxton et al. (1988b) at high Clconcentration relative to the compound of interest (S), where most of • OH are scavenged by Cl -, transforming the hydroxyl radical to Cl • and then to Cl 2 •- (Hasegawa and Neta 1978). The pH is adjusted to the acidic range (pH 1-4) to ensure the efficient formation of Cl 2
Cl 2 •has a wide transient absorption band with λ max = 340 nm and ε 340 nm = 9600 M -1 cm -1 , at 340 nm the absorbance of Cl • is negligible compared to that of Cl 2 •- (Guha et al. 1992;Caregnato et al. 2013). In transient experiments mostly this band is used for rate constant determination. However, many organic radical intermediates formed in Cl 2 •reaction, e.g., cyclohexadienyl or phenoxyl radicals, have absorption bands around or slightly above 340 nm. This coincidence complicates the investigation of organic radical intermediates; the mechanistic suggestions are usually based on indirect information. There are exceptions, e.g., certain dye cations (methylene blue, toluidine blue, safranine T, Kishore et al. 1989;Mahadevan et al. 1990;Guha et al. 1992) have strong absorbances out of this range which allow direct observation of product build-up. Transient products were identified only in few cases (e.g., Dwibedy et al. 2005;Osiewala et al. 2013;Caregnato et al. 2013). In mechanistic studies, instead of Cl 2 •-, sometimes azide radical (N 3 • ) is used, this radical does not have light absorption in the 300-500 nm range (Buxton and Janovský 1976;Hug 1981) .
When Cl • is produced in the SO 4 •-+ Clreaction, the ab-  (Yu and Barker 2003). However, at sufficiently high Clconcentration the intense absorbance of Cl 2 •builds-up practically during the pulse. Fitting to the decay curve of the 340 nm absorbance supplies the pseudo-first-order rate constants (k obs ). Under suitable conditions the slope of the pseudo-first-order rate constants vs. solute concentration plot gives the second order rate constants of the Cl 2 •-+ S reaction (k Cl2•-).
Cl • , and through the chlorine atom, Cl 2 •may also be produced in VUV photolysis of Clcontaining solutions (Takahashi et al. 1985). In steady-state experiments sometimes k Cl2•-is determined by fitting to complex kinetic models without applying real competitor. Generally, photon intensities, quantum yields, rate constants of some basic reactions of intermediates and the time dependence of degradation are used for k Cl2•-calculation. In some works, very large set of reactions was considered, and modeling software was used for obtaining rate constants. In this review we evaluate the rate constants and reaction mechanisms of Cl 2 •reactions with about 300 organic molecules, most of which have some environmental implications either as water pollutant, or as an atmospheric contaminant in water droplets. Although in several publications larger number of rate constants were published (Hasegawa and Neta 1978;Cornelius 1998;Jacobi et al. 1999;Jasper et al. 2016;Lei et al. 2019), only a few works discussed structure effects and compared the reactions induced by several one-electron oxidants, including also Cl 2

•-
. In the tables we collected the rate constants measured around room temperature, only a few termperature dependence studies are published in the literature. The tables show also the pK a values collected from a number of publications, e.g., Babic et al. (2007), Shalaeva et al. (2008). The error bounds represent the σ-level uncertainty published in the original works. The methods of k Cl2•determinations are indicated by the following abbreviations: PR pulse radiolysis, FP flash photolysis, LFP laser flash photolysis, Comp. competitive method, and Complex calculations involving complicated reaction sequence in steadystate experiments.

Molecules with interest from tropospheric point of view
In tropospheric liquid phase (e.g., in droplets), chloride is an abundant species. In Clcontaining aerosols and smaller cloud droplets Cl 2 •is expected to be formed in reactions of other highly reactive radicals such as • OH, • NO 3 , or SO 4 •- (Jacobi et al. 1999). Soluble oxygenated compounds, alcohols, aldehydes, terpenes, sulfoxides, etc., represent important classes of tropospheric species (Table 1). They may originate either from the gas-phase oxidation of volatile organic compounds or from direct emission (Herrmann et al. 2000(Herrmann et al. , 2015. Most rate constants of Cl 2 •reactions with small oxygen containing molecules (Scheme 1S, Supplementary material) were measured by Hasegawa and Neta (1978) and by Jacobi et al. (1999). Eight of the reactions in Table 1 were investigated by both groups. All values are very small, they are in the in the 10 3 -10 6 M -1 s -1 range. In this range the uncertainty in the transient measurements is rather large. However, in some cases (e.g., 2-propanol, acetone) the results of the two groups agree excellently. In other cases the k Cl2•-values agree within one order of magnitude. The experiments of Hasegawa and Neta were carried out in the presence of 1 M NaCl, while Jacobi et al. applied one order of magnitude smaller NaCl concentration. The differences between the values determined by the two groups may also be attributed to the different experimental conditions. We suggest accepting the values of the latter authors due to the smaller ionic strength effect. The k Cl2•values show some correlation with the energy of the weakest C-H bonds suggesting that the rate is controlled by the energy of the C-H bond being ruptured. For a HROH molecule Hasegawa and Neta (1978) supposed the reaction in the following way: Jacobi et al. (1999) in cases of compounds with high bond dissociation energy (BDE ≈ 410 kJ mol -1 ) speculated about an addition/elimination mechanism: α-Pinene may serve as a representative of terpenes. Terpenes are emitted in the atmosphere in large quantities by both anthropic and natural sources. Radical reactions are
Unsaturated alcohols and carboxylic acids Hasegawa and Neta (1978), Padmaja et al. (1992) and Alfassi et al. (1993) published several rate constants on Cl 2 •reactions with unsaturated alcohols (Scheme 2S). The values increase with the increasing alkyl substitution at the double bond from 5 × 10 7 to 7 × 10 8 M -1 s -1 indicating an electrophile addition mechanism. Cl-adducts were observed by ESR spectroscopy in Cl 2 •reaction with several compounds.
Due to the limited pH range in Cl 2 •reaction investigations few pH dependence studies were published in the literature. In the practically used pH range Cl 2 •is a single species (no protonation). Therefore, the changes in the reactivity with the pH must be attributed to the substrate molecules themselves. This is well exemplified by the reactions of maleic and fumaric acids (Hasegawa and Neta 1978;Wojnárovits et al. 2008). These acids undergo protolytic dissociations with pK a1 = 3.02, pK a2 = 4.39, and pK a1 = 1.92, pK a2 = 6.23, respectively. Therefore, in the 1-8 pH range the mole fractions of the different forms (protonated, monoanion, dianion) change continuously with pH. The k Cl2•-of fumaric acid shows the tendency protonated form<monoanion<dianion (1.2±1.0 × 10 5 , 2.0±0.3 × 10 6 and 4.5±0.3 × 10 6 M -1 s -1 ) in agreement with the electrophilic character of reaction. Neutral maleic acid has somewhat higher reactivity (1.7±0.2 × 10 6 M -1 s -1 ) as the monoanion (1.25±0.1 × 10 6 M -1 s -1 ). This is attributed to a prevalence of steric or polar effects for the monoanion. Acrylic acid, n-butyl acrylate, 3-sulfo propylmethacrylate, acrylamide, N-isopropyl acrylamide, and acrylonitrile (monomers used in polymerization reactions) have rate constants in the 10 6 -10 7 M -1 s -1 range. Muconic acid, a metabolite of benzene, is an important intermediate of chemical industry, sorbic acid is natural food preservative. These two compounds, due to the two conjugated double bonds in their structures react with rate constants in the 10 8 M -1 s -1 range (Hasegawa and Neta 1978). 3-Hexenedioic acid (used e.g., in nylon production), reacts with k Cl2•-in the same order of magnitude as the previous compounds, although it has only one double bond.

Small organic molecules containing halogen atoms
Organic acids dissolve in water to form anions which can transfer an electron to oxidizing radicals (e.g., Cl 2
Scheme 2 Three-electron bonded systems in one-electron oxidation of dialkylsulfoxides, diethylthiourea and 2,5-dimercaptothiadiazole (Kishore and Asmus 1991;Kishore et al. 1995;Dey et al. 1994c)  (Hasegawa and Neta 1978;Mártire et al. 2001). In the latter two compounds the electron withdrawing group on the ring is expected to decrease the reactivity. Hasegawa and Neta attempted to find a correlation between the k Cl2•-values and the Hammett parameters including in the investigations also compounds with electron donating groups (anisole, phenol, aniline). This attempt failed because two different mechanisms appear to be involved in the reactions with Cl 2 •-, i.e., addition to the aromatic ring and direct oxidation by electron transfer. The I-atom in iodobenzene has weak electron withdrawing properties: Mohan and Moorthy (1989) published much higher rate constant (3.5 × 10 8 M -1 s -1 ) as the k Cl2•-values of other simple aromatics. Iodobenzene reacts with Cl 2 •by oneelectron oxidation: Similar oxidation was not observed in reaction with Br 2 •-: the reduction potentials of Cl 2 •and Br 2 •are published to be 2.1 and 1.60, respectively, vs. NHE (Table 1S). The reduction potential of C 6 H 5 I •+ /C 6 H 5 I couple is suggested as >2.0 V. The reaction of water insoluble Co 60 (Buckminsterfullerene) was investigated as a soluble γ-cyclodextrine/Co 60 complex, the fullerene is suggested to be enclosed by two γ-cyclodextrine molecules. The reaction produces a transient species that was assigned as • Co 60 Cl radical adduct (Priyadarsini et al. 1994), rate constant: 3.8 × 10 9 M -1 s -1 .

Anisoles
The k Cl2•-of anisole and, in general of aromatic molecules with side chains connected to the ring through an O-atom (Scheme 5S) are higher than those of the simple aromatic molecules. The k Cl2•-of anisole (1.62±0.09 × 10 8 M -1 s -1 ) and 4-nitroanisole (2.5±0.3 × 10 7 M -1 s -1 ) nicely exemplify the effect of electron density on the ring. The k Cl2•-of 4nitroanisole with electron withdrawing -NO 2 group is one order of magnitude smaller than that of the molecule without this substituent. The three methoxy groups in 1,3,5trimethoxybenzene by enhancing the electron density on the ring increase the rate constant of electrophile reactions to a high value, k Cl2•-= 2.87±0.26 × 10 9 M -1 s -1 . As it will be discussed later Cl 2 •has high reactivity with sulfur atoms in lower oxidation state in organic molecules. Thioanisole and 2-(phenylthio) ethanol react with Cl 2 •with rate constants of 4.8 × 10 9 and 3.5 × 10 9 M -1 s -1 , respectively (Mohan and Mittal 1997;Gawandi et al. 1999a). The blood pressure regulators (β-blockers) atenolol, metoprolol and propranolol have common structure of R-O-CH 2 -CH(OH)-CH 2 -NH-CH(CH 3 ) 2 with optical centers at the OH group on the alkyl chain (R is aromatic). At neutral pH they have positive charge on the N-atom (pK a ≈ 9.5). The rate constants of reactions with Cl 2 •are in the 10 8 -10 9 M -1 s -1 range (Jasper et al. 2016;Lei et al. 2019;Pan et al. 2019). These values in reaction with SO 4 •are around 1 × 10 10 M -1 s -1 , while those of reactions with CO 3 •are in the 2 × 10 6 -5 ×10 7 M -1 s -1 range (Wojnárovits and Takács 2019; Wojnárovits et al. 2020). The rate constants increasing in the CO 3 •-<Cl 2 •-

<SO 4
•order are in agreement with the increasing reduction potential of oxidant (Table 1S). The similar rate constants of the three molecules in all three one-electron oxidations reflect reactions occurring on the same center, mainly on the aromatic rings. Napropamide (Nap, herbicide), reacts with Cl 2 •and Br 2

Phenols and benzoic acids
The rate constant of Cl 2 •reaction with phenol has been reported in four laboratories (Table 3, Scheme 7S), the values are close to each-other with average of 3.8±1.0 × 10 8 M -1 s -1 (Willson 1973;Hasegawa and Neta 1978;Alfassi et al. 1990;Lei et al. 2019). In the reaction chlorinated phenols were observed among the final products; these highly poisonous compounds underline the importance of studying Cl 2 •reactions.
In 4-hydroxybenzyl alcohol a CH 2 -OH, in 4hydroxycinnamic acid a CH=CH-COOH group in para-position on the ring practically does not influence the rate constant. The values, 2.3 × 10 8 and 2.9 × 10 8 M -1 s -1 , respectively (Bobrowski 1984;Dhiman and Naik 2010), are close to the k Cl2•-of phenol. The 4-hydroxybenzyl alcohol + Cl 2 •reaction gives phenoxyl radical. Build-up of this radical was used in rate constant determination (Dhiman and Naik 2010). The mechanisms of reactions were suggested as simple electron transfer. The rate constants of the sulfur compounds, 4methylthiophenol and 4,4'-thiodiphenol, 4.7 × 10 9 and 6.9 × 10 9 M -1 s -1 , respectively, are much higher than those of phenols Mohan and Mittal 1999). Benzoic acid at the pH in the measurements of Mártire et al. (2001) was mainly in the neutral, while in the investigations of Hasegawa and Neta (1978) and Lei et al. (2019) in the anionic form (Scheme 8S). The k Cl2•-of the anionic form (2 × 10 6 M -1 s -1 ) is higher than that of the neutral molecule (<10 6 M -1 s -1 ). Zhou et al. (2019) using a complex reaction system and a multicomponent fitting procedure for 3-methyl-, 4-fluoro-, The rate constants of 4-hydroxybenzoic and 4phenoxybenzoic acids are two orders magnitude higher than those of the previously mentioned benzoic acid derivatives due to the increased electron density on the rings. As the measurements of Hasegawa and Neta (1978) with 4hydroxybenzoic acid show that k Cl2•-is increasing with increasing pH, the highest value was measured at pH 9.5, 1.5 ±0.1 × 10 9 M -1 s -1 .
Parabens, p-hydroxybenzoic acid derivatives, are widely used as preservatives in cosmetic and pharmaceutical products because of their bactericidal and fungicidal properties. For the reaction of methyl paraben, Lei et al.
Dithiothreitol (DTT) is a small-molecule redox reagent with a very low reduction potential of 0.33 V vs. NHE at pH 7 (Redpath 1973). It is often used in racemic form in the acidic pH range. DTT reacts with Cl 2 •with a rate constant of 3.0 ±0.3 × 10 9 M -1 s -1 . Cimitidine, famotidine and ranitidine are used to control stomach acid overproduction. All contain sulfur atom in the alkyl chain. Based on analogous reactions, this S bridge is attacked in one-electron oxidation reaction giving explanation for the unusually high rate constants: 1.65 × 10 9 -4.5 × 10 9 M -1 s -1 (Jasper et al. 2016;Lei et al. 2019). The use of methidation and dimethoate organophosphate insecticides are banned in several countries. In these molecules, the Satom is connected to a thiophoshate group. The k Cl2•-values were determined as 1.3±0.4 × 10 8 and 1.1±0.4 × 10 8 M −1 s −1 , respectively. A mechanism involving charge transfer from the sulfide groups is proposed and supported by the identified intermediates and reaction products (Caregnato et al. 2013). In the reaction P(OCH 3 )SCl fragment forms. In phenyl trifluoromethyl sulfide (PTS) the CF 3 group decreases the reactivity (Shirdhonkar et al. 2008).
Dimethyl sulfide oxidation in marine atmosphere may play an important role in modifying the global climate since several of its free radical induced oxidation products are water soluble (Zhu et al. 2005) contributing to atmospheric aerosols formation. The less oxidized sulfur compounds, dimethyl-, diethyl-, and di-n-propyl sulfoxides react with moderate k Cl2•-of 1.2 × 10 7 -3.9 × 10 7 M -1 s -1 (Kishore and Asmus 1991;Zhu et al. 2005). In reactions with alkylsulfoxides also three-electron bonded Cl-adducts were observed as intermediates (R 1 R 2 S(O)∴Cl, Scheme 2). These intermediates may form in one step (20) or two steps (21) reactions.
The rate constants of reactions with the higher oxidized sulfur compounds, dimethyl sulfone and methanesulfonate are very small, they are 8.2±5.5 × 10 3 and 3.9±0.7 × 10 3 M -1 s -1 , respectively (Zhu et al. 2005). However, a higher k Cl2•-value, 8.0±1.0 × 10 8 M -1 s -1 , was published for methanesulfinate. As the authors mentioned, in their experiments the reactions of Cl • might disturbe the determination of the rate constant.
Thiazole is the structural unit of several drugs, e.g., sulfathiazole, it reacts with moderately high rate constant of 3.9 ±0.2 × 10 8 M -1 s -1 . Dey et al. (1994b, c) published high k Cl2•-for the reaction of phenylthiourea and diethythiourea: 4.0 × 10 9 M -1 s -1 . High value was also found for n-allylthiourea (4.6 × 10 9 M -1 s -1 , Naik and Mukherjee 2006). Electron transfer, followed by deprotonation of the intermediate radical cation is suggested as a possible mechanism. It is proposed that an intramolecular 3-electron bond is formed between sulfur and nitrogen after the deprotonation. Such sulfur-nitrogen three-electron bond was also reported, e.g., in the radical reactions of methionine (Asmus 1990). A similar mechanism is suggested for Cl 2 •reaction with 2mercaptobenzimidazole, k Cl2•-is at the diffusion controlled limit: 8.8 × 10 9 M -1 s -1 . High value was also published for selenourea (Mishra et al. 2004). The radical chemistry of thioacetamide is similar to that of thiourea, rather than that of acetamide (2.5 × 10 9 M -1 s -1 , Kishore et al. 1998).

Antibiotics and model compounds β-Lactams
In these antibiotics the bicyclic system includes a thioether moiety, an especially susceptible part of the molecules in radical reactions (Szabó et al. 2016). 6-Aminopenicillanic acid and 4-hydroxy-D-phenyl glycine ( Table 5, Scheme 10S) are regarded as model compounds of the more complex β-lactam antibiotics, e.g., amoxicillin (Song et al. 2008;Rickman and Mezyk 2010;Szabó et al. 2016). The rate constant of Cl 2 •reaction, measured for 6-aminopenicillanic acid at pH 2 is an order of magnitude higher (1.3 × 10 9 M -1 s -1 ) than that determined for 4-hydroxy-D-phenyl glycine (1.8 × 10 8 M -1 s -1 ). The attack on the former compound is suggested to take place on the sulfur atom giving rise to formation of threeelectron bonded complexes. The reaction with 4hydroxy-D-phenyl glycine is suggested to proceed by one-electron oxidation at the ring yielding a radical cation: the intermediate by deprotonation rearranges to phenoxyl radical (Szabó et al. 2016). It is evident that at pH 2 the reaction with amoxicillin occurs principally with the β-lactam part of the molecule (1.6 × 10 9 M -1 s -1 ). Lei et al. (2019) measured much smaller value for amoxicillin reaction at pH 7 (4.20±0.11 × 10 8 M -1 s -1 ). 7-Aminocephalosporanic acid may be regarded as the core part of the cephalosporin β-lactam antibiotics . This model compound reacts with k Cl2•-= 2.29±0.11 × 10 8 M -1 s -1 . The rate constants of the cephalosporin antibiotics in the table are just slightly higher than this value sugg e s t i n g r e a c t i o n p r e d o m i n a n t l y w i t h t h e 7aminocephalosporanic acid part. All values measured for βlactams are in the same order of magnitude as measured for dimethyl-and diethyl sulfide (3.0 × 10 8 and 4.7 × 10 8 M -1 s -1 , respectively, Bonifacic and Asmus 1980).

Benzenesulfonates and trimethoprim
In benzenesulfonates a -SO 2 R group with highly oxidized sulfur atom is attached to a benzene ring. These molecules have good water solubility. They are used for numerous purposes including solubilisation or serve as starting molecules for synthesis of many drugs. Benzene sulfonic acid (Scheme 11S) is the basic molecule of the so-called sulfa drugs, sulfacetamide is the simplest antibiotic of the group, pcumenesulfonate may serve as a model for sulfonate type surfactants, while p-styrene sulfonate is used to produce ionic polymers. Benzenesulfonic acid practically does not react with Cl 2 •due to the presence of the strong electron withdrawing sulfonate group (Hasegawa and Neta 1978). Osiewala et al. (2013) published an unexpectedly high value for the molecule which has a cumene group in para position (pcumenesulfonate, 9.4 × 10 9 M -1 s -1 ). Cl 2 •is suggested to react by electron transfer from aromatic ring and also by abstracting the tertiary H-atom of the cumene group. In the latter reaction benzyl type radical forms. The intermediate in reaction with a water molecule transforms to hydroxycyclohexadienyl radical.
Most of sulfa drugs have two pK a values, the first acid-base dissociation occurs at pH 2-3 at the NH 2 group attached to the benzene ring. The second one is at the -SO 2 -NH-R unit (-NH--N --+ H + ) (Babic et al. 2007). In acidic solutions of sulfacetamide the transient formed in Cl 2 •reaction (and also in reactions of other one-electron oxidants) was inferred to be the radical cation, k Cl2•-is 8.0 × 10 8 M -1 s -1 (Sabharwal et al. 1994). Lei et al. (2019) published k Cl2•-≈ 5 × 10 8 M -1 s -1 for several sulfonamide antibiotics. At the pH of their measurements the molecules were in the neutral/anionic form. It is assumed that there is a common reaction center in these molecules. It should be noted that the rate constants of individual sulfonamides were very close to each-other also in CO 3 •reactions (~10 8 M -1 s -1 , Jasper and Sedlak 2013; Wols et al. 2014;Zhang et al. 2016). It is surprising that Liu et al. (2020) published lower value for the Cl 2 •-+ sulfapyridine reaction, 3.82±0.68 × 10 6 M -1 s -1 , as measured for other sulfonamide antibiotics. Trimethoprim as antibiotic often used in combination with sulfa drugs, mainly with sulfamethoxazole. Jasper et al. (2016) and Lei et al. (2019) published similar high rate constant values with an average of 2.1 × 10 9 M -1 s -1 . In reaction with SO 4 •a rate constant of 7.7 × 10 9 M -1 s -1 was measured while the reaction with CO 3 •proceeded with rate constant of 3.4 × 10 7 M -1 s -1 (Zhang et al. 2015).

Fluoroquinolones
Fluoroquinolones (Scheme 12S), ciprofloxacin, enrofloxaxin, flumequine and ofloxacin are used to treat both human and veterinary diseases caused by both Gram positive and Gram negative bacteria. These molecules contain a central fluoroquinolone unit and, in ciprorofloxacin, enrofloxaxin and ofloxacin a piperazine ring is attached to the benzene ring. Flumenique has three condensed rings. Ciprofloxacin, enrofloxacin, and ofloxacin at pH 7 are zwitterions with positive charge on the piperazine ring and negative charge on the carboxyl group. In ciprofloxacin the carboxyl group deprotonates with pK a 6.1 and the deprotonation of the secondary nitrogen atom occurs with pK a 8.7 (Jiang et al. 2016). In

Tetracycline antibiotics and tylosin
The four fused ring tetracycline antibiotics (Scheme 13S) exhibit wide range of activity against Gram positive and Gram negative bacteria and several classes of parasites. At pH 7 they are neutral molecules or zwitterions (Babic et al. 2007). Their k Cl2•-values fall in narrow range with an average of 1.03±0.16 × 10 9 M -1 s -1 . The macrolide antibiotic tylosin inhibits bacteria by binding to the 50S ribosome and inhibiting protein synthesis. The acivity spectrum is limited primarily to Gram positive aerobic bacteria. Tylosin reacts with k Cl2•-of 4.6±0.3 × 10 7 M -1 s -1 ).

Molecules with nitrogen atom(s) in the ring
3-Hydroxy pyridine at pH 2 is protonated, and as such has low reactivity toward Cl 2 •-(<1.6 × 10 6 M -1 s -1 , Table 6, Scheme 14S). At pH 6.8, it is neutral (or a zwitterion with negative charge on oxygen (O -) and positive charge on nitrogen (NH + )). In the reaction, the semioxidized form is produced with k Cl2•-= 1.44 × 10 8 M -1 s -1 (Naik and Moorthy 1991b). 2-Hydroxy pyridine undergoes tautomerism to give 2-pyridone (a carbonyl compound). Pyridones are still aromatics as the lone pair of electrons on nitrogen can be delocalized into the ring. The k Cl2•-of 2-hydroxy pyridine, 1.31 × 10 8 M -1 s -1 , is close to the value measured for 3-hydroxy pyridine (Naik and Moorthy 1991a). The rate constants of Cl 2 •reactions with 2-and 4-mercaptopyridines are in the 10 9 M -1 s -1 range Kishore et al. 2002). Imidazole, a diazole type aromatic heterocycle, shows high reactivity in one-electron oxidation: the rate constants with SO 4 •and Cl 2 •are 5.3 × 10 9 M -1 s -1 and 1.82±0.17 × 10 8 M -1 s -1 , respectively (Steenken 1989;Lei et al. 2019). The nitroimidazole type of antibiotics, dimetridazole, metridazole, ornidazole and ronidazole, have central nitroimidazole units, and substituents are added to one of the N-and C-atoms. They react with average k Cl2•-of 1.14±0.32 × 10 8 M -1 s -1 . This value is between the rate constants determined in CO 3 •and SO 4 •reactions: 3.34±0.47 × 10 7 and 2.72±0.41 × 10 9 M -1 s -1 , respectively (Wojnárovits and Takács 2019;Wojnárovits et al. 2020). The trend of the average values reflects the trend of reduction potentials of the oxidants. The similar values within the group suggest that these radicals attack the same parts of the molecules, most probably the imidazole ring. The k Cl2•-values measured for imidazole and the nitroimidazoles practically coincide.
In reaction with one-electron oxidants, among them also with Cl 2 •-, pyrimidine shows low reactivity (Steenken 1989;Lei et al. 2019). The reactivity is increasing when two electron donating methoxy groups are attached to the ring in 2,4dimethoxypyrimidine. The basic structure of the bicyclic compounds in Table 7 is purine. Purine is a heterocyclic aromatic compound with a pyrimidine ring fused to an imidazole ring. Adenine and guanine nucleobases react with Cl 2 •with low rate constants of <5 × 10 6 and 8.1 × 10 7 M -1 s -1 , respectively (Ward and Kuo 1968): the pyrimidine ring deactivates the imidazole ring against radical attack. In xanthine, caffeine, and theophylline (stimulants in coffee, tea, and cola) the pyrimidine ring has no aromatic character, the k Cl2•-values are in the 10 8 M -1 s -1 range . Carbamazepine is used in cases of neuropathic disorders. Jasper et al. (2016) suggested a k Cl2•-of <5 × 10 7 M -1 s -1 , while the value of Lei et al. (2019) is 4.3±0.3 × 10 7 M -1 s -1 . In SO 4 •reaction a rate constant of 1.92 × 10 9 M -1 s -1 was determined (Matta et al. 2011), whereas in CO 3 •reaction 3.3 ±1 × 10 6 M -1 s -1 (average of 4 values) was found (Wojnárovits In 2-thiouracil and 4-thiouracil the S-atom essentially determines their reactivity with Cl 2

Miscellaneous compounds
T e t r a m e t h y l -, t e t r a e t h y l -, t e t r a p r o p y l , a n d tetrabutylammonium ions have low reactivity with Cl 2 •-(Scheme 15S, Bobrowski 1980). The reactivity is increasing with the increasing number of C-H bonds in the molecule. The values (10 3 -10 4 M -1 s -1 ) are in good agreement with those of the other aliphatic compounds which undergo H-abstraction. Cetyltrimethylammonium chloride (CTACl) is used as a cationic surfactant ( Table 7). The molecule has 14 -CH 2 -units, therefore, many possibilities for H-abstraction. The rate constant, 1.2 × 10 7 M -1 s -1 , is much higher than that of the previously mentioned trialkylammonium ions (Patterson et al. 1972). In vinyltrimethylammonium chloride, the reaction takes place on the styrene part of the molecule forming radical cation (2.3 × 10 8 M -1 s -1 , Kumar et al. 2003). Table 7 lists also the rate constant of the cationic surfactant sodium dodecyl sulfate (NaLS) and the neutral nonylphenol ethoxylate (Igepal CO-730) surfactant (3.9 × 10 6 and 2.1 × 10 8 M -1 s -1 , respectively, Patterson et al. 1972). The k Cl2•-for the latter is much higher due to the presence of the -O-CHmoiety, the rate constant is similar to that of anisole, 1.62 ±0.09 × 10 8 M -1 s -1 . When the concentrations of surfactants were above critical micelle concentrations (CMC) the rate constants decreased considerably due to aggregation.
Clenbuterol, mabuterol, salbutamol, and terbutaline are used by patients with breathing disorders. These compounds have the same -CH(OH)-CH 2 -NH-C(CH 3 ) 3 side on a benzene ring. Similarly, to the previously discussed blood pressure regulators, they also have a secondary amine in the chain. In clenbuterol and mabuterol the ring is deactivated by chlorine or trifluoromethyl groups. In salbutamol and terbutaline activating -OH or -CH 2 -OH groups are attached to the rings. Despite the activating or inactivating groups the k Cl2•-values are close to each-other, they are in the 3 × 10 8 -1.2 × 10 9 M -1 s -1 range . Cl 2 •probably attacks the amine containing side-chain, which is common in all of them. The k Cl2•-values are one order of magnitude higher as measured  (1973) 2,3-Dihydroxy-2-propenal 1 × 10 9 Horii et al. (1985) Microcystin-LR 5.58±0.42 × 10 7 LFP, 3.9 Zhang et al. (2019) for CO 3

•-
, but the reaction with I 2 •is very slow. The reduction potential of the semioxidized ACR • /ACR couple is expected to be around 1 eV vs. NHE. One-electron oxidants readily oxidize acridine-1,8-dione which has gained importance as a laser dye. They remove the electron from the N-atom (k Cl2•-= 4 × 10 9 M -1 s -1 , Mohan et al. 1996). The Cl 2 •reaction with riboflavine (Rf) by removing an electron from the extended conjugated system is an equilibrium process. Using the equilibrium constant the reduction potential of Rf + /Rf couple has been evaluated to be 2.28 V vs. NHE . k Cl2•-was reported to be 2.1 × 10 10 M -1 s -1 . This value is unrealistically high, higher than the diffusion controlled limit (c.a. 7.0 × 10 9 M -1 s -1 , Wojnárovits and Takács 2016). Neutral red, a phenazine type dye, is used for staining in histology (staining lysosomes red) and as a pH indicator. Guha et al. (1993) published high rate constants for its reaction with Cl 2 •-: 7.5 × 10 9 M -1 s -1 . High value (5.5 × 10 9 M -1 s -1 ) was published also for the reaction of safranine T cation (Guha et al. 1992). Safranines are azonium compounds of the symmetrical 2,8-dimethyl-3,7diaminophenazine. The reduction potential in the one-electron oxidation is 1.15 V vs. NHE. Safranine T is as a biological stain used in histology and cytology.

Mechanism of electron transfer
It is apparent from the discussion of Cl 2 •reactions with individual compounds that the authors mostly suggested electron transfer as a probable mechanism. Previously in connection with the possibility of reaction we often referred to the differences between the reduction potentials of the Cl 2 •-/2Clcouple and those of the one-electron oxidized/nonoxidized organic molecule couples. The higher, or at least similar reduction potential value of the former couple to that of the latter one gives only a possibility for the transfer. In the Supplementary Material, using the Marcus theory, we discuss the relation between rate constant values and the reduction potential differences for the reactions of phenols and anilines with Cl 2

•-
, and SO 4 •one-electron oxidants. The analysis shows that there is a correlation between the two quantities. However, other effects, e.g., solvent reorganization around the charged species and steric arrangement of the reactive parts of molecules, also strongly influence the rate constant values.

Concluding remarks
1. About 400 rate constants for~300 compounds were collected from the literature, for most compounds only one value was published. Therefore, we had little possibilities to compare the values measured for the same compound, eventually by different techniques. When there were possibilities for comparison the k Cl2•-values often differed considerably. This may be due to the rather complex reaction systems; it is difficult to find optimal conditions to measure k Cl2•-. The large differences, in some cases are due to improper handling the transient data. 2. Practically all rate constants were measured by transient techniques, by (laser) flash photolysis or pulse radiolysis following the decay of Cl 2 •-. Rate constant determination by competitive technique was reported in few publications. 3. The measured k Cl2•-values span over at least 6 orders of magnitude. The highest values are around the diffusion controlled limit (~7.0 × 10 9 M -1 s -1 ). 4. Most of the information about the reaction mechanism is coming from indirect sources. Actually, the authors rarely analysed the absorption spectra in transient measurements.

Cl 2
•can abstract an H-atom from aliphatic compounds (e.g., methanol, ethanol, 2-propanol) with rate constants between 10 3 and 10 5 M -1 s -1 . The k Cl2•-values were shown to relate to the energy of the bond broken during H-abstraction. 6. Olefin compounds react with Cl 2 •by three orders of magnitude more rapidly (10 5 -10 8 M -1 s -1 ) than the saturated analogs: the reactions with olefins occur with Cl-addition. Addition to the aromatic ring may also take place with k Cl2•-of <10 7 M -1 s -1 , but direct oxidation of the ring by electron transfer seems to be the predominant pathway. 7. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation stste), molecules with conjugated electron systems are suggested to take place with electron transfer (10 7 -10 9 M -1 s -1 ). However, it is noted in some works that electron transfer and Cl 2 •addition to a double with eliminations of two Clmay give the same result. The rate constant is high when the reduction potential of the one-electron oxidized species/molecule couple is well below that of the Cl 2 •-/2Clcouple. However, other effects, e.g., solvent reorganization around the charged species and steric arrangement of the reactive parts of molecules, also strongly influence the values. 8. The rate constant values measured for CO 3

•-
, and SO 4 •one-electron oxidants increase in this order; it is the order of increasing one-electron reduction potentials.
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Declarations
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Competing interests The authors declare that they have no competing interests.
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