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The hydrolysis of geminal ethers: a kinetic appraisal of orthoesters and ketals

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ABSTRACT

A novel approach to protecting jet fuel against the effects of water contamination is predicated upon the coupling of the rapid hydrolysis reactions of lipophilic cyclic geminal ethers, with the concomitant production of a hydrophilic acyclic hydroxyester with de-icing properties (Fuel Dehydrating Icing Inhibitors - FDII). To this end, a kinetic appraisal of the hydrolysis reactions of representative geminal ethers was undertaken using a convenient surrogate for the fuel–water interface (D2O/CD3CN 1:4). We present here a library of acyclic and five/six-membered cyclic geminal ethers arranged according to their hydroxonium catalytic coefficients for hydrolysis, providing for the first time a framework for the development of FDII. A combination of 1H NMR, labelling and computational studies was used to assess the effects that may govern the observed relative rates of hydrolyses.

No MeSH data available.


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Newman projections of 9, 12 and 16 (viewed along Cβ→Cα).
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Figure 4: Newman projections of 9, 12 and 16 (viewed along Cβ→Cα).

Mentions: McClelland et al. [23] has determined kH+ for 5, 7, 9, 12 and 16 in a water/phosphate buffer system at pH 6–7; we prepared 5 and 16 to calibrate their data with the conditions employed here (i.e., D2O/CD3CN/HCl), whilst assuming that the relative magnitudes of the hydroxonium catalytic coefficients remain consistent throughout (Table 2, Experimental section). The rates of hydrolysis for 5 and 7 are essentially the same, indicating that a tert-butyl group at Cα exerts little or no transannular steric demand which might manifest itself in the rate determining step. In the case of Cβ substituted derivatives, a gradual increase in the hydroxonium catalytic coefficient is observed with respect to 5, with a dramatic acceleration noted for the case of 16 (i.e., kH+ 5 = 6.5; 9 ≈ 11; 12 ≈ 14; 16 = 75.9 M−1 s−1; Fig. 1). Inspection of Newman projections (Fig. 4) of 9 and 12 reveal incremental 1,3-transannular steric demand associated with Me–Cβ and C(4/5)–H atoms; consistent with the gradual increase of kH+. In the case of neopentyl 16 however, two Me–Cβ groups are oriented toward transannular C(4/5)–H atoms at all times, suggesting a means by which this substituent affects a dramatic (>400%) rate increase for this substrate – this is examined further.


The hydrolysis of geminal ethers: a kinetic appraisal of orthoesters and ketals
Newman projections of 9, 12 and 16 (viewed along Cβ→Cα).
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979634&req=5

Figure 4: Newman projections of 9, 12 and 16 (viewed along Cβ→Cα).
Mentions: McClelland et al. [23] has determined kH+ for 5, 7, 9, 12 and 16 in a water/phosphate buffer system at pH 6–7; we prepared 5 and 16 to calibrate their data with the conditions employed here (i.e., D2O/CD3CN/HCl), whilst assuming that the relative magnitudes of the hydroxonium catalytic coefficients remain consistent throughout (Table 2, Experimental section). The rates of hydrolysis for 5 and 7 are essentially the same, indicating that a tert-butyl group at Cα exerts little or no transannular steric demand which might manifest itself in the rate determining step. In the case of Cβ substituted derivatives, a gradual increase in the hydroxonium catalytic coefficient is observed with respect to 5, with a dramatic acceleration noted for the case of 16 (i.e., kH+ 5 = 6.5; 9 ≈ 11; 12 ≈ 14; 16 = 75.9 M−1 s−1; Fig. 1). Inspection of Newman projections (Fig. 4) of 9 and 12 reveal incremental 1,3-transannular steric demand associated with Me–Cβ and C(4/5)–H atoms; consistent with the gradual increase of kH+. In the case of neopentyl 16 however, two Me–Cβ groups are oriented toward transannular C(4/5)–H atoms at all times, suggesting a means by which this substituent affects a dramatic (>400%) rate increase for this substrate – this is examined further.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

A novel approach to protecting jet fuel against the effects of water contamination is predicated upon the coupling of the rapid hydrolysis reactions of lipophilic cyclic geminal ethers, with the concomitant production of a hydrophilic acyclic hydroxyester with de-icing properties (Fuel Dehydrating Icing Inhibitors - FDII). To this end, a kinetic appraisal of the hydrolysis reactions of representative geminal ethers was undertaken using a convenient surrogate for the fuel–water interface (D2O/CD3CN 1:4). We present here a library of acyclic and five/six-membered cyclic geminal ethers arranged according to their hydroxonium catalytic coefficients for hydrolysis, providing for the first time a framework for the development of FDII. A combination of 1H NMR, labelling and computational studies was used to assess the effects that may govern the observed relative rates of hydrolyses.

No MeSH data available.


Related in: MedlinePlus