Foad M.S. Mahmoud

Simultaneous Spectrophotometric Determination of Iron(II) and Iron(III) in Mixtures Using Di-2-pyridyl Ketone Benzoylhydrazone

Abstract A method for simultaneous spectrophotometric determination of total iron, iron(II) and iron(III) in mixtures containing other metal ions has been described. The method is based on the complexation of iron with di-2-pyridyl ketone benzoylhydrazone (DPKBH) in 50% (v/v) ethanolic solution. Iron(II) complex with DPKBH exhibits two absorption maxima at 360 and 650 nm. meanwhile iron(II1) complex with DPKBH exhibits only one maximum at 360nm. Iron(II) and iron(III) complexes with DPKBH have similar behaviour at 360nm. Iron forms 1:2 complexes with the reagent. Beers laws are obeyed over the ranges 0.1–2 μgml-1 and 0.4–5 μgrn1-l for iron(II) complexes at 360 and 650nm respectively. Iron(III) showed results similar to those obtained for iron(II) at 360nm. The effect of pH, effect of excess reagent. the stability of complexes. and the tolerance limit of many metal ions have been reported. The method is applied to the determination of total iron, iron(I1). and iron(II1) in synthetic solutions.

The SN1 methanolysis of a tertiary organosilicon iodide (Me3Si)3CSiMe2I

The highly sterically-hindered organosilicon iodide (Me3Si)3CSiMe2I undergoes solvolysis in MeOH by an SN1 process; the half-life is approximately 13 days at 50°C, and the reaction is little accelerated by the presence of NaOMe, which does, however, induce a competing fragmentation.

A unimolecular solvolysis of an organosilicon perchlorate

Abstract Kinetic studies have been carried out on the reaction of the perchlorate (Me 3 Si) 3 CSiMe 2 OClO 3 with anhydrous methanol. The effects of added NaOMe, LiCl, LiNO 3 , and NaClO, are consistent with rate-determining ionization of the perchlorate.

Base cleavage of the benzyl−silicon bond in PhCH2SiMe2(CH2)nOH (n = 2 or 3) compounds

Abstract The compounds PhCH 2 SiMe 2 (CH 2 ) n OH with n = 2 or 3 arecleaved 0.75 and 95135 times, respectively, as readily as PhCH 2 SiMe 3 by NaOMe/MeOH at 50 0 C. The high reactivity of the compound with n = 3 may arise from intramolecular attack of the alkoxide centre on silicon in the anion PhCH 2 SiMe 2 (CH 2 ) 3 O - .

The mechanism of the gas-phase pyrolysis of esters. Part 13. The very strong activating effects of β-trialkylmetal groups

The rates of gas-phase pyrolysis of β-substituted ethyl acetates AcOCH2CH2X where X = SiMe3, SiEt3, GeEt3, and SiMe2Ph, and of 1-aryl-2-trimethylsilylethyl acetates have each been measured over a minimum of 50 °C, between 282 and 397 °C; the rates of pyrolysis of 2-aryldimethylsilylethyl acetates have been measured at 396.9 and 378.2 °C. The β-organometallic substituents are not themselves eliminated (except at considerably higher temperatures) but strongly accelerate the normal elimination of acetic acid, the relative rates per β-hydrogen at 327 °C for X being: H, 1.0; SiMe3, 125; SiEt3, 179; GeEt3, 108; SiMe2Ph, 144. These groups appear to activate by a combination of increasing the acidity of the β-hydrogen via stabilisation of the forming β-carbanion through (p→d)π bonding, stabilisation of the incipient α-carbocation by C–X hyperconjugation, and steric acceleration. The effect of substituents in the aryl ring of 1-aryl-2-trimethylsilylethyl acetates gave an excellent correlation with σ+ values with ρ–0.52 at 327 °C. The lower ρ-factor for this reaction compared to that for 1-arylethyl acetates (–0.66) is consistent with either conjugative stabilisation of the α-carbocation or increased β-hydrogen acidity. Substituents in the aryl ring of 2-aryldimethylsilylethyl acetates gave a very small positive ρ-factor indicating that overall their effect on β-hydrogen acidity is larger than that on the forming carbocation. The product of pyrolysis of AcO·CH2·CH2·GeEt3viz. CH2CHGeEt3 underwent increasingly rapid elimination of successive moieties, believed to be ethylene, in a reaction of stoicheiometry 4.0, which did not occur with the silicon analogue.

Kinetic studies of the hydrolysis of furfurylidene benzoylhydrazone

SummaryThe hydrolysis of furfurylidene benzoylhydrazone,FBH, in hydrochloric acid medium has been found to follow specific acid catalysis, whereas the hydrolysis in universal buffer medium has been found to follow general acid-base catalysis. Mechanisms for the hydrolysis in different acidic media have been postulated. The observed rate constants, secondary rate constants, activation and thermodynamic parameters for the hydrolysis of furfurylidene benzoylhydrazone have been reported. The effect of change in percentage of ethanol in the ethanol-buffer mixture on the rate of hydrolysis has been discussed.ZusammenfassungEs wurde festgestellt, daß die Hydrolyse von Furfurylidenbenzoylhydrazon (FBH) in HCl-Medium einer speziellen Säurekatalyse gehorcht, wohingegen die Hydrolyse in universellen Puffer-Medien nach einer generellen Säure-Base Katalyse abläuft. Es werden Mechanismen für die verschiedenen sauren Medien postuliert. Die beobachteten Geschwindigkeitskonstanten, die sekundären Geschwindigkeitskonstanten, Aktivierungs- und sonstige thermodynamische Parameter für die Hydrolyse von Furfurylidenbenzoylhydrazon werden berichtet. Der Effekt wechselnder Ethanolkonzentration auf die Hydrolysengeschwindigkeitskonstante in Ethanol-Puffer-Mischungen wird diskutiert.

Effects of varying the media for base cleavage of some Benzyl-silicon and -tin compounds in alcohols

Abstract The effects on the rates of cleavage of 3-ClC 6 H 4 CH 2 MMe 3 and 3,5-Cl 2 C 6 H 3 CH 2 MMe 3 compounds with M = Si and Sn have been examined for (a) various concentrations of NaOMe-MeOH; (b) various concentrations of H 2 O in NaOHH 2 O-eOH; and (c) variation of R from Me to Et, i-Pr and t-Bu for metal alkoxide-ROH mixtures. Rates have also been measured for cleavages in NaOCH 2 CF 3 HOCH 2 CF 3 -Me 2 SO. In all cases there are differences in behaviour between the silicon and tin compounds which are consistent with the postulated difference in the mechanisms of their reactions.

The mechanism of cleavage of SiGe bond by base

Abstract The compounds R 3 SiGePh 3 with R = Me or Et have been shown to undergo cleavage in NaOMe-MeOH to give Ph 3 GeH. From rate measurements and solvent isotope effect studies it is concluded that the Ph 3 Ge − anion separates in the rate-determining transition state. There is an unusually large steric effect, Me 3 SiGePh 3 being roughly 1300 times as reactive as Et 3 SiGePh 3 .

Some properties of the sterically-hindered aminosilane (Me3Si)3CSiMe2NH2

The amine (Me3Si)3CSiMe2NH2 has been made by treatment of (Me3Si)3CSiMe2OClO3 with liquid ammonia. It is recovered unchanged from its solutions in methanol containing acid or base or in trifluoroacetic acid, and is unaffected by various diazotizing agents. The hydrochloride (Me3Si)3CSiMe2NH3Cl has a pKa of 4.1 in ca. 42/58 v/v H2O/THF at 20°C.

Unimolecular solvolysis of some organosilicon perchlorates and iodides

Kinetic studies have been made of the solvolysis (mainly the methanolysis) of the highly sterically hindered compounds TsiSiMe2OClO3[Tsi =(Me3Si)3C], (Me3Si)2(Ph2MeSi)CSiMe2OClO3, TsiSiHPhX (X = I, Br, ONO2), and TsiSiHMel. The rate of methanolysis of TsiSiMe2OClO3 is increased by only ca. 20% upon addition of 0.1MNaOMe, and further additions of base have smaller effects. Additions of LiCl or LiNO3 cause even smaller rate increases, but in the presence of LiNO3 substantial amounts of TsiSiMe2ONO2 are formed, the amounts being greater than would correspond to the rate increases. Water has a very large accelerating effect, and even with only 1 vol % of water present the product is very predominantly TsiSiMe2OH. The solvolysis is slower in EtOH and PriOH, and occurs only very slowly, if at all, in CF3CH2OH. The methanolysis of (Me3Si)2(Ph2MeSi)CSiMe2OClO3 is slower than that of TsiSMe2OClO3. The methanolysis of TsiSiHPhl and TsiSiHMel are also accelerated only to a small extent by NaOMe. The above results are interpreted in terms of an SN1 mechanism involving anchimerically assisted ionization of the Si–OClO3 or Si–I bond to give a methyl-bridged cation with unusual solvation requirements. In contrast, the methanolyses of TsiSiHPhX (X = Br or ONO2) are markedly accelerated by base, and it seems that, at least in the presence of base, attack by the nucleophile is involved in the rate-determining step in these cases.