Hydrochloric Acid → 4-ethynylbenzoyl Chloride

Participants

Reaction Steps

An alternate and more direct approach to the preparation of the ethynylbenzoate-terminated PS involved the reaction of the hydroxy-terminated PS with 4-ethynylbenzoyl chloride as in Eq. 2. The 4-ethynylbenzoyl chloride was prepared as follows: 4-Bromobenzoic acid (11.0 g, 0.055 mole; source, Aldrich Chemical Company), dichlorobis(triphenylphoshine)palladium (0.3g; source, Strem Chemical, Inc.), pyridine (60 ml), triethylamine (40 ml), and trimethylsilylacetylene (8.3 g, 0.085 mole; source, Silar Laboratories Inc.) were stirred under nitrogen at 70°-74° C. for six hours. The yellow solution turned brown and a precipitate appeared (presumably triethylammonium bromide). The reaction mixture was poured into cold dilute aqueous hydrochloric acid (10° C.-20° C.) to yield 4-trimethylsilylethynylbenzoic acid as a crude tan solid (11.0 g), m.p. 175°-185° C. decomp. [lit. m.p. 181°-182.5° C.)]. The tan solid was dissolved in dilute aqueous sodium hydroxide solution, filtered, and the filtrate acidified to yield a light tan solid (9.6 g). Recrystallization twice from toluene provided 4-ethynylbenzoic acid as golden needles (4.3 g), m.p. 222°-223° C. decomp. (Sample introduced in a preheated melting point apparatus at 220° C.) [lit m.p. 218° C. decomp.] [IR, strong sharp absorption at 3300 cm-1 characteristic of H--C≡C stretch. Mass spectroscopy (MS), molecular ion (M+) at 146 [4-ethynybenzoic acid (mol. wt. 146)], major fragments m/e 129 (HC≡C--C6H4 --CO+) and 101 (HC≡C--C6H4 +)]. 4-Ethynylbenzoic acid (4.2 g) was stirred in thionyl chloride (30 ml) containing a few drops of N,N-dimethylformamide at ambient temperature overnight (8-20 hours). The light orange solution was concentrated to dryness at temperature <40° C. in vacuo to yield an orange crystalline solid which was thoroughly washed with cold n-hexane (5° C.-10° C.). The resultant yellow crystalline solid (3.1 g) was dissolved in warm n-hexane (30 ml) and the solution cooled to 5° C. to 10° C. Since no solid appeared, the solution was partially concentrated to yield 4-ethynylbenzoyl chloride as crude yellow needles (3.0 g), m.p. 32°-34° C., (IR, H--C≡C at 3300 cm-1, C=O doublet at 1800 and 1770 cm-1). Sublimination in nitrogen atmosphere under vacuum provided white crystalline needles, m.p. 69°-70° C.

Provenance & Evidence