4.3.2. Synthesis of Ionic Liquids

1-alkylpyridinium ionic liquids were synthesized according to Scheme 1.

1-dodecylpyridinium chloride (PyrC₁₂-Cl) (3)

The mixture of pyridine (10 g, 0.12 mol) and 1-chlorododecane (31 g, 0.15 mol) was stirred at 140 °C for 20 h. The obtained solid product was purified by recrystallization from ethyl acetate-hexane mixture (1:3 v/v). Yield: 65% (22 g), white solid, mp 92–94 °C.

¹H NMR (400 MHz, DMSO-D₆): δ = 0.84 (t, 3H, CH₃), 1.24 (m, 18H, CH₃(CH₂)₉), 1.9 (m, 2H, NCH₂CH₂), 4.6 (t, 2H, NCH₂), 8.2 (t, 2H, C₃-H, C₅-H), 8.6 (t, 1H, C₄-H), 9.2 (d, 2H, C₂-H, C₆-H).

1-tetradecylpyridinium bromide (PyrC14-Br) (4)

The mixture of pyridine (10 g, 0.12 mol) and 1-bromotetradecane (36 g, 0.13 mol) was stirred at 120 °C for 2 h. The obtained solid product was purified by double recrystallization from ethyl acetate-hexane mixture (1:1 v/v). Yield: 76% (32 g), white solid, mp 64‒66 °C.

¹H NMR (400 MHz, DMSO-D₆): δ = 0.83 (t, 3H, CH₃), 1.21 (m, 22H, CH₃(CH₂)₁₁), 2.0 (m, 2H, NCH₂CH₂), 4.9 (t, 2H, NCH₂), 8.14 (t, 2H, C₃-H, C₅-H), 8.51 (t, 1H, C₄-H), 9.44 (d, 2H, C₂-H, C₆-H).

Long-chain imidazolium IL containing polar ester group in the alkyl radical was synthesized according to Scheme 2.

1-(dodecyloxycarbonylmethyl)-3-methylimidazolium chloride (IMC1CH2COOC12-Cl) (13)

Chloroacetyl chloride (13.5 g, 0.12 mol) was added dropwise to the stirred mixture of 1-dodecanol (20 g, 0.1 mol) and potassium carbonate (14 g, 0.1 mol) in dry benzene (200 mL). The reaction was carried out for 6 h at room temperature. After completion of the reaction, the reactionary mixture was washed with water until the pH became neutral. The organic layer was separated and dried overnight over calcium chloride. Benzene was distilled, residual solvent was removed in vacuum 10 mbar at 50 °C. The prepared crude dodecyl chloroacetate was further used for the synthesis of the IL.

The mixture of 1-methylimidazole (5 g, 0.06 mol) and dodecyl chloroacetate (18 g, 0.07 mol) was stirred at 120 °C for 2 h. After cooling, the solid product was purified by double recrystallization from ethyl acetate. Yield: 68% (14 g), white solid, mp 58–60 °C.

¹H NMR (400 MHz, CDCl₃): δ = 0.86 (t, 3H, CH₃), 1.24 (m, 18H, (CH₂)₉), 1.63 (m, 2H, COOCH₂CH₂), 4.06 (s, 3H, NCH₃), 4.16 (t, 2H, COOCH₂), 5.45 (s, 2H, NCH₂CO), 7.47 (d, 1H, C₄-H), 7.54 (d, 1H, C₅-H), 10.24 (s, 1H, C₂-H).

Long-chain imidazolium ILs comprising polar 2-hydroxyethyl groups were synthesized according to Scheme 3.

1-(2-hydroxyethyl)-3-dodecylimidazolium chloride (IMC₂OHC₁₂-Cl) (16)

Potassium carbonate (38 g, 0.28 mol) was added to the solution of imidazole (12 g, 0.16 mol) and 1-chlorododecane (24 g, 0.12 mol) in 100 mL DMF. The mixture was stirred at 60 °C for 20 h after that it was poured into water (300 mL). The top layer of water immiscible oil product was separated, dissolved in methylene chloride (200 mL) and washed again with water (2 × 300 mL). The solution was dried over sodium sulfate. Methylene chloride was distilled and residual solvent was removed in vacuum 15 mbar at 60 °C. 1-dodecylimidazole was obtained as light yellow liquid.

¹H NMR (400 MHz, CDCl₃): δ = 0.87 (t, 3H, CH₃), 1.24 (m, 18H, (CH₂)₉, 1.75 (m, 2H, NCH₂CH₂), 3.9 (t, 2H, NCH₂), 6.89 (d, 1H, C₄-H), 7.04 (d, 1H, C₅-H), 7.44 (s, 1H, C₂-H).

The mixture of 1-dodecylimidazole (15 g, 0.06 mol) and 2-chloroethanol (7.6 g, 0.095 mol) was stirred at 120–130 °C for 24 h. The residual reagent was removed in vacuum 5 mbar at 80 °C. The prepared semi-solid product of light brown color was purified by washing with hexane-ethyl acetate mixture (1:3 (v/v). Residual solvents were removed in vacuum 15 mbar at 60 °C. Yield: 45% (8.5 g), white semi-solid.

¹H NMR (400 MHz, DMSO-D₆): δ = 0.85 (t, 3H, CH₃), 1.24 (m, 18H, CH₃(CH₂)₉), 1.79 (m, 2H, NCH₂CH₂), 3.72 (t, 2H, NCH₂CH₂OH), 4.16 (t, 2H, NCH₂), 4.22 (t, 2H, NCH₂CH₂OH), 5.35 (m, 1H, OH), 7.78 (br s, 1H, C₄-H), 7.81 (br s, 1H, C₅-H), 9.27 (s, 1H, C₂-H).

1-(2-hydroxyethyl)-3-tetradecylimidazolium bromide (IMC₂OHC₁₄-Br) (17)

Potassium carbonate (38 g, 0.28 mol) was added to the solution of imidazole (10 g, 0.14 mol) and 2-chloroethanol (11 g, 0.14 mol) in 120 mL acetonitrile. The mixture was stirred at 60 °C for 20 h. The residue of inorganic salts was filtered off. Acetonitrile was distilled from reactionary mixture. Residual 2-chloroethanol was removed in vacuum 65 mbar at 80 °C. The crude product, 1-(2-hydroxyethyl)imidazole (Scheme 4) was prepared as viscous oil and used in the next stage without purification.

The mixture of crude 1-(2-hydroxyethyl)imidazole and 1-bromotetradecane (38 g, 0.14 mol) was stirred at 120 °C for 2 h. The obtained semi-solid product of light brown color was purified by washing with ethyl acetate (3 × 100 mL). Yield: 52% (28 g), white semi-solid.

¹H NMR (400 MHz, DMSO-D₆): δ = 0.85 (t, 3H, CH₃), 1.23 (m, 22H, CH₃(CH₂)₁₁), 1.78 (m, 2H, NCH₂CH₂), 3.73 (t, 2H, NCH₂CH₂OH), 4.18 (t, 2H, NCH₂), 4.21 (t, 2H, NCH₂CH₂OH), 5.16 (m, 1H, OH), 7.77 (br s, 1H, C₄-H), 7.81 (br s, 1H, C₅-H), 9.2 (s, 1H, C₂-H).

Guanidinium based long-chain ILs were synthesized according to Scheme 4.

2-dodecylaminoimidazoline-2 hydrochloride (C₁₂AIM-Cl) (20)

2-methylmercaptoimidazoline-2 chlorohydrate was synthesized using the method described in [40]. 40 g (0.39 mol) of 2-imidazolidinethione were suspended in the mixture of methanol (50 mL) and concentrated hydrochloric acid (50 mL) with magnetic stirrer. The mixture was heated to reflux for 12 h. Residual methanol was distilled and water solution was evaporated. The solid residue was then purified by recrystallization from isopropanol. Yield: 70% (41.6 g), white solid, mp 160 °C.

¹H NMR (300 MHz, DMSO-D₆): δ = 2.71 (t, 3H, CH₃), 3.84 (s, 4H, CH₂), 10.64 (s, 2H, NH).

2-methylmercaptoimidazoline-2 chlorohydrate (15 g, 0.1 mol) was added to the stirred solution of dodecylamine (18.5 g, 0.1 mol) in 150 mL of isopropanol. The mixture was heated to boiling for 6 h. Methyl mercaptan released during the reaction was captured with 20% aqueous potassium hydroxide solution. Isopropanol was removed from the reactionary mixture at reduced pressure. The solid residue was purified by recrystallization from ethyl acetate-hexane (1:3 v/v) mixture. Yield: 74% (22 g), white solid, mp 54–56 °C.

¹H NMR (300 MHz, DMSO-D₆): δ = 0.85 (t, 3H, CH₃), 1.25 (m, 18H, CH₃(CH₂)₉), 1.46 (m, 2H, NCH₂CH₂), 3.18 (t, 2H, NCH₂), 3.4 (s, 2H, 4-CH₂), 3.56 (s, 2H, 5-CH₂), 8.1–8.9 (br s, 3H, NH).

N-dodecylguanidine hydrochloride (C12G-Cl) (22)

S-methylisothiuronium chloride (MIT-Cl) was synthesized by the method similar to that described for the synthesis of 2-methylmercaptoimidazoline-2 chlorohydrate, using thiourea instead of ethylenethiourea. Yield: 65% (32 g), white solid, mp 116 °C. The ionic liquid C₁₂G-Cl was prepared using the method similar to that described for the synthesis of C₁₂AIM-Cl, using S-methylisothiuronium chloride instead of 2-methylmercaptoimidazoline-2 chlorohydrate (Scheme 4). After recrystallization from ethyl acetate-hexane (1:2 v/v) mixture, white solid residue was obtained. Yield: 65% (17 g), white solid, mp 116 °C.

¹H NMR (400 MHz, DMSO-D₆): δ = 0.86 (t, 3H, CH₃), 1.25 (m, 18H, CH₃(CH₂)₉), 1.46 (m, 2H, NCH₂CH₂), 3.11 (t, 2H, NCH₂), 6.8–7.8 (br s, 4H, NH), 7.86 (s, 1H, NH).