Halogenation rotational barriers

Thiirane, 2-phenyl-conformation rotational barriers, 7, 138 polymerization, 7, 144 Thiirane, tetraaryl-synthesis, 7, 175 Thiirane, tetrafluoro-halogenation, 7, 148 polymerization, 7, 144 reactions... 

The haloethanes all have similar rotational barriers of 3.2-3.7 kcal/mol. The increase in the barrier height relative to ethane is probably due to a van der Waals rqjulsive efiect. The heavier halogens have larger van der Waals radii, but this is ofiset by the longer bond lengths, so that the net efiect is a relatively constant rotational barrier for each of the ethyl halides. 

The issue of the role of bridged radicals in the stereochemistry of halogenation has recently been examined computationally and a new interpretation offered. The structure, rotational barriers, and for halogen atom abstraction for P-haloethyl radicals were studied. For the reactions where X=C1 or Br, the halogen atom abstraction reaction shows a preference for a trans TS. 

Racemization of sulphinamides in solution in an inert solvent involves a radical-chain mechanism. Rotation barriers concerning the S—bond in halogenosulphinamides and rapid halogen exchange in the case of the Cl and Br compounds have been estimated by variable-temperature n.m.r. methods. ... 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

Haloethanes CH3CH2X. X = Cl, Br, I) have similar barriers to rotation (3.2-3.7 kcal/mol) despite the fact that the size of the halogen increases. Cl Br I. Offer an explanation. 

The synthesis of a series of l//-pyrazolo[3,4-3]quinoxalines (flavazoles) 55 by acylation, alkylation, halogenation, and aminomethylation of the parent compound was reported and their structures were investigated by H, and N NMR spectroscopy <2005T2373>. Restricted rotation about the partial C-N double bond of the A -acyl derivatives was studied by dynamic NMR spectroscopy and the barriers to rotation were determined. N NMR data of a series of 3-alkyl [aryl]-substituted 5-trichloromethyl-l,2-dimethyl-l//-pyrazolium chlorides 56 (where the 3-substituents are H, Me, Et, -Pr, -Bu, -Pent, -Hex, (CH2)sC02Et, CH2Br, Ph, and 4-Br-C6H4) were reported <2002MRC182>. The N substituent chemical shift (SCS) parameters were determined and these data were compared with the SCS values and data obtained by molecular orbital (MO) calculations. 

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