Nucleophilic Reactivity of Calcium Carbide: Its Catalytic Activation and Reaction with Acetone to Synthesize Non-Ionic Defoamers

Abstract

Methylbutynol (MB) is a typical propargylic alcohol with both alkynyl and hydroxyl groups, featuring excellent modifiability and broad applications. Currently, it is produced through the reaction of alkaline metallic acetylides and acetone, requiring expensive raw material and harsh reaction conditions. Herein, a novel method was proposed by replacing the metallic acetylide with calcium carbide (CaC₂) as a low-cost industrial acetylide reagent. The effects of solvent, activator, and proton donor on the ball mill reaction, and the defoaming performance of the resultant MB and its oxidative coupling product (2,7-dimethyl-3,5-octadiyn-2,7-diol), were studied. Nucleophilic reactivity of CaC₂ with acetone can be regulated by the activating effect of the ball mill, an appropriate activator, and a proton donor. High yield of MB (~94%) was obtained under synergistic action of TBAF·3H₂O and acetylene, which represents a facile synthesis process of MB under mild conditions. MB exhibits good defoaming performance, and 2,7-dimethyl-3,5-octadiyn-2,7-diol is more promising, being an excellent non-ionic defoamer. The result is of great significance for exploring new chemical reactions of CaC₂ and its high-value utilizations.

1. Introduction

Propargylic alcohols are typical multifunctional chemicals with terminal alkynyl and hydroxyl groups, showing good reactivity, chemical modifiability, and broad applications for the synthesis of pharmaceuticals and other functional materials. They are excellent non-ionic surfactant and steel corrosion inhibitor due to their amphiphilicity, good wetting, and defoaming properties [1,2]. Methylbutynol (MB) is an intermediate for the synthesis of isoprene, a monomer of synthetic rubber. Propargylic alcohols are currently produced via alkynylation of carbonyl compounds by metal acetylides, Grignard reagents or acetylene under basic conditions of liquid ammonia or KOH and low temperatures [3], which requires a higher raw material cost and is not suitable for large-scale production. The nucleophilic reaction of calcium carbide (CaC₂) with aldehydes/ketones has ever been explored [4]. As early as 1952, Ray et al. [5] reported the reaction of CaC₂ with acetone in KOH-diethyl ether solvent, yielding 33% of 2,5-dimethyl-3-hexyn-2,5-diol after a 33 h reaction at low temperatures (0–10 °C). Abolfazl et al. [6] used tetrabutylammonium fluoride (TBAF) to catalyze the reaction of CaC₂ with some aldehydes/ketones, except acetone, in DMSO to synthesize propargylic alcohols (Scheme 1a). For ketones without active hydrogens, the aldol condensation can be suppressed. For example, when CaC₂ and KOH were ball milled 30 min for activation, and then co-milled with benzophenone for 3 h, a yield of 41% propargyl diol and 25% propargylic alcohol was obtained (Scheme 1b) [7].

CaC₂ is an important coal chemical product with an annual production of ca. 30 million tons. Furthermore, its manufacture from bio-coke is a greener process [8,9], which makes it a potential low-carbon chemical. Currently, CaC₂ is primarily used as a source of acetylene for the production of various chemical products, such as vinyl chloride [10], acrylonitrile, acetylene black, and vinyl ethers [11,12,13]. The reactivity of its carbide anion (C₂²⁻) has received little attention and is worthy of further exploration. Therefore, developing new applications of CaC₂ in organic synthesis and manufacturing its high-value derivatives is of great significance. Theoretically, CaC₂ is an important industrial source of carbon anions and the cheapest metallic acetylide reagent, and its reaction potential with various reactants can be expected from a thermodynamic perspective [14]. Production of methylbutynol using CaC₂ is more viable from techno-economic points of view, compared to that using expensive sodium acetylide or acetylene with minimal reactivity. However, the strong lattice energy of CaC₂ makes it insoluble in all organic solvents, which greatly limits its availability and reactivity. Meanwhile, as a conjugated base of acetylene, its carbanion is of super-strong Lewis basicity, which may suppress its nucleophilic reactivity and give rise to an elimination reaction of the electrophiles. For instance, acetone condensation occurs easily at room temperature in the presence of CaC₂, yielding various condensation products like isophorone and methyl-substituted aromatics, where CaC₂ is functioned as a strong base and dehydrating agent rather than a nucleophilic reagent [15,16,17]. The aim of the present work is to investigate the nucleophilic reaction of CaC₂ with acetone to produce MB, focused on its basicity regulation, solvent effects, and process intensification of the heterogeneous reaction. This work not only provides a facile synthesis method of MB but also provides insights for the development of new reactions of CaC₂ toward its high-value applications.

2. Results

The reactivity of CaC₂ is greatly limited by its insolubility, while mechanical grinding helps to destroy its crystallinity, and increase its chemical potential, interface mass transfer, availability of the acetylide anion and the reaction rate [18,19]. Its mechanochemical reaction has been used to prepare various CaC₂-derived carbon materials [20,21,22,23]. Suitable solvents and additives can facilitate the dissolution of CaC₂ and regulate its basicity and nucleophilic reactivity; thus, they are studied separately in the following sections.

2.1. Solvent Effect of the Reaction

Solvent has a profound influence on a reaction due to its regulation on the acidity/basicity of the reactants and the activation energy of the reaction via specific intermolecular interaction. And a suitable solvent is crucial for the nucleophilic reactivity of CaC₂. Due to its super-strong basicity, CaC₂ is apt to be protonated by protic solvents like water, methanol, and glycerol, forming acetylene in situ with little reactivity [20]. In contrast, polar aprotic solvents are suitable for nucleophilic reactions due to their strong solvation ability for CaC₂ and chemical stability. Thus, aprotic polar solvents were studied here, i.e., dimethyl sulfoxide (DMSO), acetonitrile (CH₃CN), N,N-dimethylformamide (DMF), and so on. As shown in

Table 1. Effect of solvent on the reaction yield ^a.

Solvent	Yield
DMSO	31.8%
DMF	16.2%
DMAC	25.9%
THF	22.4%
CH3CN	19.0%

^a Reaction conditions: CaC₂ 0.02 mol, TBAF·3H₂O 0.02 mol, DMSO 30 mL, ball mill 4 h at 270 r/min.

, a nucleophilic reaction occurred in all solvents, and the highest yield was obtained in DMSO (31.8%), followed by Dimethylacetamide (DMAC) (25.9%), THF (Tetrahydrofuran) (22.4%), acetonitrile (19.0%), and DMF (16.2%). Thus, DMSO is chosen as a appropriate solvent for the reaction.

The solvent effect arises from its interaction with reactants, products, catalysts, and transition states via H-bonding, complexation, electron transfer, and so on, affecting the reaction mechanism, activation energy, reaction rate, and selectivity. For the polar aprotic solvents, their positive charge is shielded inside the molecules, which decreased their electrostatic interaction with the carbanion and enhanced its nucleophilic reactivity. The high polarity helps to stabilize the charged transition states, lower the activation energy, and increase the reaction rate. Chen et al. studied the interaction of CaC₂ with various solvents and its solvation energies using a SMD implicit solvent model. It was found that polar aprotic solvents show a stronger solvation effect and a weaker deprotonation ability, and DMSO has the greatest impact on the negative electrostatic potential of C₂²⁻ [24]. Thus, DMSO can better modulate the basicity of the acetylide anion, stabilize the transition state, and lower the activation energy, which is consistent with the experimental observation.

2.2. Activation and Basicity Regulation of CaC₂

Generally, CaC₂ hardly reacts with organic compounds due to its high lattice energy and insolubility. Additionally, as a conjugated base of acetylene, C₂²⁻ is of super basicity, which severely inhibits its nucleophilic reactivity. Benzophenone does not react with CaC₂, but a high yield of propargylic alcohols can be obtained when they are ball milled with solid KOH [22]. This may be attributed to the partial protonation of CaC₂ by KOH, resulting in a disrupted lattice structure and formation of monovalent anion of HC≡C⁻, which has lower basicity and better nucleophilic reactivity than the bivalent anion of C₂²⁻.

To adjust the reactivity of CaC₂, the effect of some activators was studied, including those with active hydrogen (acetylene, water, TBAF·3H₂O), Lewis acidity (MgCl₂), and inorganic fluorides/carbonates (TBAF·3H₂O, CsF, Et₃N·HF, KF, Cs₂CO₃). As such, the basicity of CaC₂ may be reduced by its partial protonation or Lewis acid–base pairing, and its nucleophilicity may be liberated indirectly via the strong affinity between calcium and the F⁻/CO₃²⁻ ions. As shown in

Table 2. Effect of activators on the reaction yield.

Entry	Activator	Acetone Conversion/% a	Yield of Monosubstituted Product/% b	Yield of Bi-Substituted Product/% c
1	—	57.4	0	0
2	TBAF·3H2O	62.3	37.8	0
3	BMIMPF6	71.2	14.7	3.3
4	CsF	37.4	5.1	0
5	(C2H5)3N(HF)	100	0	0
6	Cs2CO3	100	0	0
7	KF	100	0	0
8	MgCl2	58.2	9	0
9	H2O	52.3	3.2	0
10 d	C2H2	40.5	18.5	0
11 e	C2H2	70.4	5.8	0

^a Reaction conditions: CaC₂ 20 mmol, acetone 10 mmol, activator 20 mmol, DMSO 30 mL, 270 r/min, 4 h. ^b yield of 2-methyl-3-butyne-2-ol. ^c yield of 2, 5-dimethyl-3-hexyne-2,5-diol. ^d The milling jar was charged with C₂H₂. ^e C₂H₂ dissolved in DMSO.

, both water and acetylene can make CaC₂ protonated, forming corresponding propargylic alcohol. Water has a poor activation effect with only 3.2% of product yield. Acetylene, as a weaker proton donor, showed a better activation effect, and higher product yield (5.8%) was obtained when the reactants were ball milled for 4 h in acetylene-saturated DMSO solution. This is because the protonation of C₂²⁻ by acetylene can be exactly controlled to C₂H⁻ via proton transfer reaction, i.e., CaC₂ + C₂H₂ → Ca(C₂H)₂, without worry of over-protonation in water and some alcohols. Anhydrous MgCl₂ can activate CaC₂, yielding 9% propargylic alcohol. This may be ascribed to their Lewis acid–base interaction, forming a Grignard-like reagent via Schlenk Equilibrium [23,25], CaC₂ + 2MgCl₂→ClMgC≡CMgCl + CaCl₂, thereby lowering the basicity of the acetylide anion and enhancing its nucleophilicity. Among the inorganic salts (CsF, KF, Cs₂CO₃) tested, only CsF showed a slight activation effect with 5.1% of product yield, which may be ascribed to its higher solubility in DMSO and a specific complexing effect with the carbonyl group of acetone [24]. The complexation of the cesium with the carbonyl oxygen increases the positive charge of the carbonyl carbon and its electrophilic reactivity. KF and Cs₂CO₃ showed little activation effect, likely due to their minimal solubility in DMSO and metathesis with CaC₂ to form alkali metal carbides and corresponding CaF₂ and CaCO₃. The driving force of the metathesis reactions is the strong electrostatic affinity between calcium and fluoride/carbonate ions and the high formation energy of CaF₂ and CaCO₃. Among the activators studied, TBAF·3H₂O is the best one, yielding 37.8% of propargylic alcohol, which may be attributed to its high solubility and the synergistic activation effects of the fluoride ion and crystal water. Quantum chemical calculations indicate that trace water can react with calcium carbide on its interface, forming ethynyl calcium hydroxide HOCaC≡CH, which significantly increases the solubility of the acetylide ion in DMSO and enhances its reactivity [26,27]. However, excessive water may cause rapid hydrolysis of calcium carbide, generating acetylene gas with little nucleophilicity.

2.3. Effect of Reaction Time and Rotation Speed

The effect of reaction time on the product yield was explored under different conditions. As shown in

Table 3. Reactivity of CaC₂ with acetone under different activators and reaction time.

Entry	Additive	Reaction Time/h	Acetone Conversion/% a	Yield of Monosubstituted Product/% b	Yield of Bi-Substituted Product/% c
1	CsF	3	37.4	5.1	0
2	CsF	6	60.8	35.4	2.8
3	CsF	9	80.5	45.8	7.3
4	C2H2	1	31.9	4.55	0
5	C2H2	2	40.4	4.87	0
6	C2H2	4	43.8	18.5	0
7 d	C2H2	4	75.6	13.8	0
8	C2H2	9	81.4	18.8	0
9	TBAF·3H2O	3	59.9	31.8	0
10	TBAF·3H2O	4	62.3	37.8	0
11	TBAF·3H2O	6	75.2	56.0	3.3
12	TBAF·3H2O	9	79.2	56.1	3.6
13	C2H2-TBAF·3H2O	1	33.0	7.2	0.7
14	C2H2-TBAF·3H2O	3	94.5	80.1	1.5
15	C2H2-TBAF·3H2O	5	99.1	82.4	2.0

^a Reaction conditions: CaC₂ 20 mmol, acetone 10 mmol, DMSO 30 mL, 270 rpm. ^b yield of 2-methyl-3-butyne-2-ol. ^c yield of 2, 5-dimethyl-3-hexyne-2, 5-diol. ^d The milling jar was charged with C₂H₂.

, extending reaction time is conducive to the product yield. For example, the reaction with neat KF was very slow at the beginning with only 5.1% of MB yield in 3 h, but the yield rapidly increased to 35.4% at 6 h. This indicates that the reaction is controlled by mass transfer, and there exists an induction period. The slower initial rate is related to the larger particle size of CaC₂ and the limited reactive interface and activation degree. TBAF·3H₂O, with the dual effect of water and fluoride ions, showed a faster reaction rate, and the propargylic alcohol yield was increased from 31.8% to 56.0% as the reaction time prolonged from 3 h to 6 h and then levelled off. This is because, in the late stage, the concentration of CaC₂ is low, and its surface was covered by the insoluble calcium alkoxide that further hinders its reaction. The product yield was 18.8% after a 9 h reaction in the presence of C₂H₂. When acetylene and TBAF·3H₂O were used together, the effect was more significant, indicating their synergistic activation on CaC₂. After a 3 h reaction in acetylene-saturated DMSO, the product yield was over 80.1%, which is much higher than the yields from using either TBAF·3H₂O (31.8%) or acetylene (13.8%) alone. The low activity of the water in TBAF·3H₂O helps to control the protonation degree of CaC₂, along with the activation effect of its fluoride ions [6]. Additionally, CaC₂ can be partially protonated by acetylene, forming calcium acetylide (CaC≡CH) with lower basicity and higher nucleophilicity. The compositions of the solution before and after the reaction in the presence of acetylene and TBAF·3H₂O are compared in Figure 1. Obviously, acetone was converted almost completely after a 9 h reaction, and MB was formed with high selectivity (~100%). The influence of rotation speed on the reaction was also investigated under fixed other conditions (CaC₂ 20 mmol, acetone 10 mmol, C₂H₂-saturated DMSO 30 mL, TBAF·3H₂O 20 mmol, ball mill 3 h). The MB yield at 150, 270, and 400 rpm was 74%, 80%, and 75%, respectively. Thus, the rotation speed has little influence on the reaction within the range of 150–400 rpm, and 270 rpm was used hereinafter.

2.4. Reaction Mechanism of CaC₂ with Acetone

Acetylene (C₂H₂) is not only a H-donator, its nucleophilic addition with acetone is also possible under basic conditions to form methylbutynol. Herein, CaC₂ and the companying CaO may be functioned as a strong base for deprotonation of acetylene, forming monovalent acetylide anion and then reacting with acetone. To explore this possibility, the reactions of acetylene with acetone in DMSO catalyzed by TBAF, CsF, CsOH, and CaO were investigated. As seen from

Table 4. Effect of different additives on the reaction of acetylene with acetone and product yield ^a.

Entry	Additive	Acetone Conversion/%	Yield of Monosubstituted Product/% b	Yield of Bi-Substituted Product/% c
1	-	46.7	0	0
2	CsF	46.7	0	0
3	TBAF·3H2O	83.0	44.4	0
4	CaO	58.7	40.1	0.4
5	CaO-BAF·3H2O	91.9	55.2	0
6	CsOH	83.0	50.0	1.6

^a Reaction conditions: acetone 0.01 mol, DMSO 30 mL, reacted for 9 h in a planetary ball mill at 270 rpm. ^b yield of 2-methyl-3-butyne-2-ol. ^c yield of 2, 5-dimethyl-3-hexyne-2,5-diol.

, The reaction does not occur in the absence of a catalyst or in the presence of CsF only [28]. When TBAF or CaO were used alone, the product yield was 44.4% and 40.1%, respectively; when they were used together, the yield was enhanced to 55.2%. When industrial-grade calcium carbide (75% CaC₂–25% CaO) and TBAF were used together, the yield was increased drastically to 84.4% (see Table 3 entry 15) after a 5 h reaction, which may be ascribed to the stronger basicity and deprotonation capacity of CaC₂ than CaO for acetylene. Additionally, CsOH also showed a good catalytic effect, yielding 51.6% of MB products.

Based on the above analysis, the reaction mechanism of CaC₂ with acetone was proposed, as shown in Scheme 2. Herein, CaC₂ was protonated partially by C₂H₂ and/or H₂O to form [CaC₂H]⁺, which was further activated by TBAF·3H₂O, forming TBA⁺[HOCaFC≡CH]⁻ (Com 1) [29,30,31]. These processes significantly reduced the surface basicity of CaC₂ (from C₂²⁻ to C₂H⁻), suppressed the condensation of acetone, and enhanced the nucleophilic reactivity of the acetylide ions. The acetylide ion of TBA⁺[HOCaF(C≡CH)]⁻ reacted with acetone to yield Com 2, a calcium propargyl alcoholate complex. Upon rearrangement of Com 2, calcium propargyl alcoholate (Com 3) was formed along with the release of TBAF [6]. Calcium propargyl alcoholate was finally converted to MB via hydrolysis.

2.5. Scale-Up Experiment

To examine the reproducibility of the reaction results, scale-up experiments were conducted in a stirred ball mill with larger amounts of reactants and solvent. To a 2 L stirred ball mill jar, 6 g of calcium carbide, 15 mL of acetone, 150 mL of acetylene-saturated DMSO, and 18 g of TBAF·3H₂O were added sequentially and milled at 270 rpm for 4 h. The resulting slurry was treated with centrifugation, the supernatant was analyzed by GC, and the yield of MB was 94.3%. This indicates the reproducibility of the ball mill reaction and its viability for massive synthesis. When the ball milled reaction was conducted at 270 rpm for 9 h under the same other conditions, the acetone conversion and MB yield was 97.2% and 91.9%, respectively. The GC result for the solution sample after a 9 h reaction under the synergistic action of TBAF·3H₂O and acetylene is presented in Figure 2. Obviously, acetone was virtually completely converted to the main products of MB and disubstituted propargylic alcohol, along with minor byproducts of acetone condensation.

2.6. Defoaming Performance of Propargylic Alcohols

Alkynediols are widely used non-ionic defoamer, while the main product here is a mono-substituted methylbutynol, which was further converted to 2,7-dimethyl-3,5-octadiyn-2,7-diol via its oxidative coupling. For this, 4 mmol of methylbutynol, 60 mg of cuprous oxide, and 10 mL of DMSO were added to a stirred glass vial and reacted at 80 °C for 8 h under oxygen pressure of 0.03 MPa. The resulting solution was analyzed by GC. As shown in Figure 3, most of methylbutynol (1.83 min) was converted to 2,7-dimethyl-3,5-octadiyn-2,7-diol (11.9 min).

The defoaming rate of MB decreases significantly with its decreased usage from 10% to 3 wt%, as shown in

Table 5. Defoaming rate of methylbutynol and its oxidative coupling product at different usages.

Entry	Mass Content of Defoamer
	10%	8%	5%	3%
1 a	94%	81%	37%	15%
2 b	100%	100%	100%	100%

^a methylbutynol; ^b oxidative coupling product of methylbutynol.

. The defoaming performance of 2,7-dimethyl-3,5-octadiyn-2,7-diol is much better than that of MB, and complete deforming can be realized at 3% usage, which may be ascribed to its superhigh surface activity. For illustration, the surface tension of 0.5 wt% aqueous solution of sodium dodecylbenzenesulfonate (C₁₂H₂₅SO₃Na) with varying content of MB or its oxidized product (0~0.1 g·L⁻¹) was determined. As shown in Figure 4, MB can slightly reduce the surface tension of the C₁₂H₂₅SO₃Na solution. In contrast, the surface activity of its oxidative coupling product is enhanced greatly, as shown from the drastically decreased surface tension from 36 to 27 mN·m⁻¹ as its usage increased to 10%. This may be ascribed to the doubled length of the carbon chain and the number of alkynyl and hydroxyl groups of the oxidative coupling product of MB. The strong hydrophobicity and rigidity of the two alkynyl bonds impart a stronger hydrophobic–hydrophilic balance to the surface active molecule, making it more easily adsorbed at the gas–liquid interface, forming a closely packed monolayer. The synergistic effect of the longer carbon chain and multiple alkyne bonds further enhances its hydrophobicity.

3. Materials and Methods

The main instruments used in the experiments include planetary ball mill (QXQM-2, Tianchuang, Changsha, China), gas chromatograph (GC-2010, Shimadzu, Kyoto, Japan), gas chromatograph-mass spectrometer (GC-MS, Agilent 7890A-5975C, Agilent Technologies, Santa Clara, CA, USA), precision electronic balance (ME204E, METTLER TOLEDO, Zurich, Switzerland), crusher (ZN-04BL, Zknc, Beijing, China), and nuclear magnetic resonance spectrometer (NMR, AV600, Bruker, Rheinstetten, Germany).

The main chemicals used include calcium carbide (CaC₂, 75%), cesium fluoride (CsF, 99%), tetrabutylammonium fluoride (TBAF, 99%), triethylamine trihydrofluoride (TEA-3HF, 97%), and anhydrous magnesium chloride (MgCl₂, 99.9%), purchased from Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China, and acetylene (99.9999%) from Beijing Haipu Gas (Beijing, China). Dimethyl sulfoxide (DMSO, 99.8%, water content 0.03%) and methylbutynol (98%) were purchased from Aladdin (Shanghai, China) and used as received.

For the synthesis of MB, a certain amount of 400-mesh CaC₂ powder, TBAF, acetone, and DMSO saturated with acetylene were added to the ball mill jar. The admixture was ball milled at room temperature and 270 rpm for a period of time and acidified to neutral by formic acid. The as-treated reaction slurry was separated via centrifugation, and the supernatant was sampled using a syringe with a microfiltration head and analyzed by GC. The GC analysis was made using an FFAP (30 × 0.53 × 1.0) column under the following conditions: high-purity nitrogen as carrier gas at 27.7 kPa with flow rate of 100 mL/min, split injection mode with split ratio 16.8:1, injection temperature 300 °C, oven initial temperature 60 °C, temperature held at 60 °C for 0.5 min and ramped at 20 °C/min to 240 °C, and FID detector at 300 °C. When the set temperature was reached and stabilized, 1 μL of the sample was injected and analyzed via internal standard method. The analysis was repeated three times, and the average value was taken as the experimental result.

4. Conclusions

The activation effect of solvent, activator, and ball milling on the reactivity of calcium carbide with acetone was studied for the preparation of propargylic alcohols, a non-ionic surfactant and deformer. Additives like CsF, TBAF, MgCl₂, CsOH, water, and acetylene show certain activation effects on calcium carbide, but the propargylic alcohol yield is relatively low when they are used separately. However, 94.0% of product yield can be obtained when TBAF·3H₂O and acetylene are used together. Their synergistic activation effect may be attributed to the protonation of calcium carbide by acetylene and crystal water of TBAF·3H₂O, as well as the strong affinity between calcium and the fluoride ion of TBAF, forming fluoride-activated terminal acetylide intermediates with reduced basicity and enhanced nucleophilicity. Propargylic alcohol is a good defoamer, and its oxidative coupling product is better. This study is of great significance for the development of new reactions of calcium carbide and its high-value utilization.