Kinetics and mechanism of the reaction between phenylacetylene and triethylaluminium + tertiary amine complexes in hydrocarbon solution

Abstract

The reaction of phenylacetylene with triethylaluminium in solutions containing tertiary amines has been investigated using n.m.r. and kinetic methods over the temperatures 333–378 K. A binary 1 : 1 complex between triethylaluminium and the amine ½Et₆Al₂+ NR₃⇌ Et₃Al â†� NR₃(I) reacts according to the stoichiometric equation Et₃Al â†� NR₃+ HCCPh = Et₂(PhCC)Al â†� NR₃+ C₂H₆.

The apparent order of reaction of this complex, in hydrocarbon solution, is non-integral. A phenomenological rate equation: v₀=(k₁[Et₃Al â†� NR₃]₀+k₂[Et₃Al â†� NR₃]²₀)[C₈H₆]₀(where R₃NEt₃N, n-Bu₃N or Et₂NPh) is established.

Values of log₁₀(k₁/dm³ mol^–1 s^–1) are 5.0–(62 000 J mol^–1/RT) for the triethylamine complex and 11.1–(102 000 J mol^–1/RT) for the tri-n-butylamine complex. The activation parameters are compared with those for proton-transfer reactions in non-aqueous solutions. The third order term is believed to derive from a complex parallel reaction involving a ternary complex. Log₁₀(k₂/dm⁶ mol^–2 s^–1) is 10.9–(97 000/RT)(for Et₃N) and 7.1–(71 000/RT)(for n-Bu₃N).

Et₃Al-pyridine and Et₃Al-tetrahydrofuran complexes did not react with phenylacetylene under these conditions.

N.m.r. spectrometry and ebulliometry were used to investigate whether any complexes other than (I) were present in the reaction mixture in significant concentrations. An ebulliometer was constructed which was capable of operating in an inert atmosphere of nitrogen at pressures 0.5–1.5 bar. The vapour pressure of Et₃Al â†� NEt₃ was measured over the range 294–410 K.