Effect of chain length on the rate of quaternization of alkyl amines

Abstract

Trialkyl amines from triethyl to tristearylamine were quaternized with benzyl chloride and compared for the effect of chain length on the rate of quaternization. Triethylamine reacted about twice as fast as tripropylamine with virtually no chain-length effect from propyl to stearyl. Results are compared to previous rate studies on steric hindrance and chain-length dependence.In the course of studying process improvements on alkylation of tertiary amines to quaternary ammonium salts, the reaction properties of trialkyl amines (Structure 1) were examined. R3N STRUCTURE1Industrially, amines from trimethylamine [R = Me) to tristearylamine (R --ClsH37) are readily available. Molecular models of the long-chain analogs (Structure 2) give the distinct impression of an aquatic organism. STRUCTURE 2 a 3-legged octopus.The flexibility and possible conformations are almost limitless. Depending on the conformations most favorable under a particular condition or solvent system, it is possible to theorize either equal quaternization rates for the short-and long-chain homologs of the tertiary amine series, or vastly differing rates. Three basic types of conformations seem possible in solution: (i) parallel (Structure 3): where the hydrophobic tails cling together with no solvent molecules separating them: N STRUCTURE3 (ii) separated (Structure 4): where the hydrocarbon chains are widely separated by solvent molecules: .j STRUCTURE4 (iii) ball (Structure 5): where the chains are coiled and twisted around the nitrogen: STRUCTURE 5Structures 3 and 4 should offer no particular steric hindrance to quaternization, while Structure 5 should be very slow to quaternize due to steric problems. It is known that quaternizations occur best in solvents with high dipole moments and dielectric constants {1). Acetonitrile, ethanol, or tetrahydroforan are much better reaction media than hexane or toluene. The coiled conformation should create a non-polar hydrocarbon environment around the reactive nitrogen, disfavoring formation of polar transition states typical in SN2 displacements. The effect could further slow the rate of reaction of Structure 5.While long-chain trialkyl amines do have appreciable solubility (at temperatures above their melting points) in polar organic solvents, it seems likely that they are not totally compatible on a molecular level. This conjecture is the basis for the three suggested conformation types. Rapid conversion between the types likely occurs with the important feature being the equilibrium ratio of the conformers. NMR evidence on the conformations of the amines in solution cannot be determined with certainty. The techniques are very difficult and not fully developed at this time (2).The literature shows that the effect of steric hindrance on rates of various quaternizations has been studied. Pyridine derivatives (3-11), aromatic systems (12-14), thiazoles (15,16), polymers (17), and miscellaneous nitrogen and phosphorous systems (18-22) all exhibit a rate change with general steric crowding.Berg et al. (23) discu...

Kinetic study of the reaction of dimethyl carbonate with trialkylamines

“…The energies of activation (also in Table II) calculated from the slopes for the three reactions were identical within the uncertainties of the measurements. This is consistent with Friedli's observation [16] that alkylation rates of tertiary amines are independent of carbon chain length when alkyl chains are longer than three carbons. The H ‡ and S ‡ values (Table III) for the same three reactions determined from the Eyring equation (Eq.…”

Closing One of the Last Gaps in Polyionene Compositions: Alkyloxyethylammonium Ionenes as Fast‐Acting Biocides

“…In contrast to this, the reactivity of those bis(2‐N,N‐dialkylamino)ethyl ether compounds was lower compared to bis[2‐(N,N)‐dimethylamino]ethyl ether in the conducted poly‐Menshutkin‐reactions with alkyl dibromides, since lower yields were obtained. This can be deduced to a deleterious steric effect of the alkyl substituents and a less efficient nucleophilic performance 36. Nevertheless, both monomers reacted at room temperature to form oligomers, as proven by mass spectrometry.…”

Degradation Kinetics and Solvent Effects of Various Long-Chain Quaternary Ammonium Salts

“…in 1990 19 . The described trends apply both at 120 and 200 • C. It is peculiar that the effect of chain length (TDMA-MC vs. TBMA-MC) seems to be reversed at 200 • C in DMC.…”