Six-centred transition states in reactions involving halogen compounds

Harding, Charlie; Loudon, A. G.; Maccoll, Allan; Rodgers, P. G.; Ross, Robert A.; Wong, S. K.; Shapiro, J. S.; Swinbourne, E. S.; Stimson, VR; Thomas, Philipp

doi:10.1039/c19670001187

Cited by 3 publications

(3 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The experimental rate expression which we have found in this study is consistent with two quite different processes ; the molecular process suggested by Maccoll [7], involving a cyclic transition state, and a free radical hydrogen abstraction and restoration mechanism, given below.…”

Section: Discussionsupporting

confidence: 78%

“…The kinetics of the HBr-catalyzed isomerization of 1 -butene to 2-butene has been investigated by Maccoll and Ross [2], with the conclusion that the reaction is molecular and homogeneous in the gas phase. A similar molecular mechanism has been proposed for a variety of additional reactions [7]. O n the other hand, Benson and Bose [8] have suggested a hydrogen abstraction and replacement mechanism, in which iodine atoms abstract hydrogen from the allylic position of the olefin, and H I replaces a hydrogen at the other end of the allylic system, but they do not give a kinetic verification of the scheme.…”

Section: Introductionmentioning

confidence: 95%

See 1 more Smart Citation

Kinetics of the HBr‐catalyzed photoisomerization of the allyl halides

Abell

Adolf

1969

Int J of Chemical Kinetics

View full text Add to dashboard Cite

The kinetics of the gas phase bond isomerization of allyl fluoride, allyl chloride and allyl bromide, catalyzed by HBr and ultraviolet light, has been studied in the temperature range of 150-250' and at pressures of 3.5 to 50 mm. The reactions are very clean, first order in allyl halide and HBr, and have a light intensity exponent of unity. A quantum yield for allyl chloride of 3200 indicates a chain reaction. Dilution with inert gases is almost without effect, indicating that excited state intermediates are not involved. A small wall effect is observed. The evidence indicates a free radical reaction, involving hydrogen abstractions by bromine atoms, with replacement at the other end of the allylic radical.

show abstract

Section: Discussionsupporting

confidence: 78%

Section: Introductionmentioning

confidence: 95%

Kinetics of the HBr‐catalyzed photoisomerization of the allyl halides

Abell

Adolf

1969

Int J of Chemical Kinetics

View full text Add to dashboard Cite

show abstract

“…It was found that, under E2 conditions, the amount of hex-1-ene steadily decreased and the hex-2-enes increased as the leaving group was increased in size from fluoride to chloride, bromide, and iodide, the reverse of what might be expected from a consideration of steric factors.223 Moreover, the rates of formation of terminal olefin were accurately related to the rates of formation of cis-and tram-2-olefi11, separately suggesting that all three arise from the same transition state. These observations were interpreted by donsideration of a continuum of mechanistic types within the scope of the E2 mechanism, varying from the 'nearly El' (147) through an intermediate type (148) to a 'nearly carbanion' type (149) with some degree of carbanion character on the P-carbon. The orientation of elimination is governed by the nature of the transition state ; thus, since an a-alkyl group stabilises the intermediate and 'nearly El' transition states more than the 'nearly carbanion' one where the latter prevails, Hoffman elimination will be preferred.…”

Section: Phmentioning

confidence: 99%

Chapter 3. Part (ii) Reaction mechanisms

Isaacs¹

1968

Annu. Rep. Prog. Chem., Sect. B

View full text Add to dashboard Cite

Stable CarboniumIons.-Rates of reaction of the tris-(p-methoxypheny1)carbonium ion with water, hydroxide ion, and ammonia have been measured by a stopped-flow technique. The second-order rate constants are 0.28, 4-6 x lo3, and 2.1 x lo3 1. mole-1 set.-' respectively and a negative salt effect was observed. The spectroscopic properties of a large number of substituted and bridged trityl cations have been recorded2 and resolution of the first asymmetric carbonium ion (1) (by virtue of restricted rotation) has been achieved. The n.m.r. spectra of some diphenyl (2and 3-pyridinium)methyl dications (2) have shown that the heterocyclic ring protons are more deshielded than the phenyl proton^.^ Among new aromatic cations are the ditropylium dication (3) which exhibits a singlet n.m.r. spectrum at z 0.05 (cf: the tropylium ion at z 1-31) and is assumed to be twisted about the central bond.5 The tri-t-butylcyclopropenium ion appears to be less stable (pKR+ = 6.1) than the tri-isopropyl analogue, (pKR+ = 7.0) as measured by titration against sodium hydroxide in 50% aqueous acetonitrile.6 How much this is due to steric crowding and how much to a reduction in electron release by the t-butyl group is difficult to assess, but it may be relevant that the heptaphenyltropylium ion is less stable than '

show abstract

Evidence for α‐elimination from neopentyl chloride in the gas phase

Failes

Mollah

Shapiro

1979

Int J of Chemical Kinetics

View full text Add to dashboard Cite

Experimental evidence is presented for a unimolecular gas-phase Wagner-Meerwein shift in neopentyl chloride pyrolysis. In the decomposition of a,a-neopentyl chloride-dz at 445'C, maximally inhibited by cyclohexene, the initial products were isotopically pure 2-methyl-1-butene-dz and 2-methyl-2-butene-dI. Rearrangement, accompanied by loss of either aor y-hydrogen in the formation of hydrogen chloride, is consistent with an incipient ion-pair type of transition state. The cyclohexene maximally inhibited pyrolysis of neopentyl chloride was also examined over the temperature range 424478°C and Arrhenius parameters of E , 258.7 kJ/mole and logA/sec-', 13.78, were determined.

show abstract

Six-centred transition states in reactions involving halogen compounds

Cited by 3 publications

References 0 publications

Kinetics of the HBr‐catalyzed photoisomerization of the allyl halides

Kinetics of the HBr‐catalyzed photoisomerization of the allyl halides

Chapter 3. Part (ii) Reaction mechanisms

Evidence for α‐elimination from neopentyl chloride in the gas phase

Contact Info

Product

Resources

About