The solvent-dependent reactivity of sodium hexamethyldisilazide (NaHMDS) toward carbon-centered electrophiles reveals reactions that are poorly represented or unrepresented in the literature including direct aminolysis of aromatic methyl esters to give carboxamides, nitriles, or amidines, depending on the choice of solvent. S$_N$Ar substitutions of aryl halides and terminal epoxides are also examined. A combination of $^1$H and $^{29}$Si NMR spectroscopic studies using [$^{15}$N]NaHMDS, kinetic studies, and computational studies reveal the complex mechanistic basis of the preference for simple aryl carboxamides in toluene and dimethylethylamine promote and arylnitriles or amidines in THF. A prevalence of dimer- and mixed dimer-based chemistry even from the observable NaHMDS monomer in THF solution is notable. Following extensive studies on sodium hexamethyldisilazide (NaHMDS), the solution structures and reactivity of sodium isopropyl(trimethylsilyl)amide (NaPTA), sodium (2-phenylethyl)amide (NaPETA), sodium tertbutyl(trimethylsilyl)amide (NaBTA), and their isotopomers [$^{15}$N]NaBTA have been investigated. Solution structural studies using a combination of $^{29}$Si NMR spectroscopy, the Method of Continuous Variations (MCV), and density functional theory (DFT) computations provided insights into aggregation and solvation in a range of solvents including toluene, $N,N$-dimethylethylamine, triethylamine, MTBE, THF, dimethoxyethane (DME), diglyme, $N,N,N’,N’$-tetramethylethylenediamine (TMEDA), $N,N,N’,N’$-tetramethylcyclohexanediamine (TMCDA), $N,N,N’,N’’,N’’$-pentamethyldiethylenetriamine (PMDTA), 12-crown-4, 15-crown-5, and 18-crown-6 revealed solvent- and substituent-dependent dimer-monomer mixtures with affiliated solvation numbers. Complexation of the three crown ethers documented both crown and substituent dependencies. Qualitative studies of reactivity showed a variety of reactions of NaPETA. Aminolysis of methyl benzoate with dialkylamines mediated by NaPTA afforded high yields of benzamides. Quantitative rate studies of aminolysis of methyl benzoate by NaPTA revealed a 47,000-old range of rates. Detailed rate studies in toluene and THF showed dimer-based mechanisms. The role of primary- and secondary-shell solvation by THF is discussed, including nuances of methods used to separate the two contributions. PMDTA-solvated NaPTA monomer reacts as a monomer whereas bis-diglyme solvated monomer reacts as a dimer. Rate studies exploring the structure-reactivity correlations of the three crown ethers show mono- and bis-crown-based pathways in which 15-crown-5—the crown ether often said to be of choice for sodium—was decidedly inferior as an accelerant.