Document Detail


Methanolysis of 4-bromobenzenediazonium ions. Effects of acidity, [MeOH] and temperature on the formation and decomposition of diazo ethers that initiate homolytic dediazoniation.
MedLine Citation:
PMID:  18931809     Owner:  NLM     Status:  PubMed-not-MEDLINE    
Abstract/OtherAbstract:
We have investigated the effects of solvent composition, acidity and temperature on the dediazoniation of 4-bromobenzenediazonium (4BrBD) ions in MeOH-H(2)O mixtures by employing a combination of spectrometric and chromatographic techniques. The kinetic behaviour is quite complex; in the absence of MeOH, dediazoniations follow first-order kinetics with a half-life t(1/2) approximately 3000 min (T = 45 degrees C), but addition of small concentrations of MeOH lead to more rapid but non-first-order kinetics, suggestive of a radical mechanism, with t(1/2) approximately 125 min at 25% MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to first-order behaviour, and at percentages of MeOH higher than 90%, t(1/2) approximately 1080 min. Analyses of reaction mixtures by HPLC indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-bromophenol (ArOH), 4-bromoanisole (ArOMe), and bromobenzene (ArH). At acidities (defined as -log[HCl]) < 2, the main dediazoniation products are the substitution products ArOH and ArOMe but, upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOMe, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given MeOH content, the plot of k(obs) or t(1/2) values against acidity is S-shaped, the inflexion point depending upon the MeOH concentration and the temperature. Similar S-shaped variations are found when plotting the dediazoniation product distribution against the acidity. The acid-dependence of the switch between the homolytic and heterolytic mechanisms suggests the homolytic dediazoniation proceeds via transient diazo ethers. The complex kinetic behaviour can be rationalized by assuming two competitive mechanisms: (i) the spontaneous heterolytic dediazoniation of 4BrBD, and (ii) an O-coupling mechanism in which the MeOH molecules capture ArN(2)(+) to yield a highly unstable Z-adduct which undergoes homolytic fragmentation initiating a radical process. Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed, for the first time, estimation of relevant thermodynamic parameters for the formation of diazo ethers under acidic conditions.
Authors:
Alejandra Fernández-Alonso; Carlos Bravo-Díaz
Related Documents :
10960039 - Preparation, hydrolysis and intramolecular transesterification of 3'-deoxy-3'-thioinosi...
18563889 - Nonlinear phenomena in light-mediated bromate-hydroquinone-benzoquinone reactions.
7200059 - Inhibition of nitrosation of amines by thiols, alcohols and carbohydrates.
12018319 - Enzymatic synthesis of medium chain monoglycerides in a solvent-free system.
11924579 - The study of modified calcium hydroxides with surfactants for acid gas removal during i...
10862119 - Reaction competition in the fragmentation of protonated dipeptides
18350389 - Screening and immobilization burkholderia sp. gxu56 lipase for enantioselective resolut...
10399019 - From beta-lactams to alpha- and beta-amino acid derived peptides.
12147249 - Fatty acids potentiate interleukin-1beta toxicity in the beta-cell line ins-1e.
Publication Detail:
Type:  Journal Article     Date:  2008-09-05
Journal Detail:
Title:  Organic & biomolecular chemistry     Volume:  6     ISSN:  1477-0539     ISO Abbreviation:  Org. Biomol. Chem.     Publication Date:  2008 Nov 
Date Detail:
Created Date:  2008-10-20     Completed Date:  2008-12-12     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101154995     Medline TA:  Org Biomol Chem     Country:  England    
Other Details:
Languages:  eng     Pagination:  4004-11     Citation Subset:  -    
Affiliation:
Universidad de Vigo, Facultad de Química, Dpt. Química Física, 36200 Vigo, Spain.
Export Citation:
APA/MLA Format     Download EndNote     Download BibTex
MeSH Terms
Descriptor/Qualifier:

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine


Previous Document:  Transannular rearrangement of activated 2,5-diketopiperazines: a key route to original scaffolds.
Next Document:  Synthetic studies on the cornexistins: synthesis of (+/-)-5-epi-hydroxycornexistin.