Acétylène

Résumé

L’acétylène (appelé éthyne par la nomenclature IUPAC) est un hydrocarbure de la classe des alcynes de formule brute . Il a été découvert par Edmund Davy en Angleterre en 1836. C’est l’alcyne le plus simple, constitué de deux atomes de carbone et deux d’hydrogène. Les deux atomes de carbone sont reliés par une triple liaison qui contient la majorité de son énergie chimique. Description Sa structure est linéaire : . Il a été dit que Berthelot, en 1862, fut le premier à synthétiser l’acétylène, dans un appareil surnommé « œuf de Berthelot », par la production d'un arc voltaïque entre deux électrodes de graphite baignant dans une atmosphère d’hydrogène : :2 C + → . Toutefois, le professeur Morren, doyen de la faculté des sciences de Marseille, avait publié avant lui la découverte de la production de carbures dans un tel dispositif (et Berthelot se comporta d'ailleurs de façon honteuse vis-à-vis de ce collègue). L’acétylène est un gaz incolore, pratiquement inodore quand il

Source officielle

https://fr.wikipedia.org/wiki/Ac%C3%A9tyl%C3%A8ne

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This course aims to teach essential notions of the structure of matter, chemical equilibria and reactivity. Classes and exercises provide the means to analyze and solve, by reasoning and calculation, novel problems of geeneral chemistry.

Closely interfacing with bioengineering and medicine, this course provides foundational concepts in applying small-molecule chemical toolsets to probe the functions of living systems at the mechanistic and molecular level with emphasis placed on quantitative understanding and improving human health.

Ce cours permet l'acquisition des notions essentielles relatives à la structure de la matière, aux équilibres et à la réactivité chimique en liaison avec les propriétés mécaniques, thermiques, électriques et magnétiques des matériaux.

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The present work deals with two subjects. The interaction of NO2 and HONO with different types of soot are examined in the first part whereas in the second part an experimental set-up is presented which has been built in order to measure the kinetics of the degradation of organic compounds by OH radicals. Both soot particles as well as NO2 are mainly produced by fossil fuel and biomass burning. The two species are therefore ubiquitous in the atmospheric boundary layer where they may react with each other. It has been shown that one possible product of this heterogeneous interaction is nitrous acid (HONO). HONO is an important trace gas in atmospheric chemistry because it is easily photolysed resulting in OH radical and NO. Thus, the photolysis of HONO significantly enhances photooxidation processes early in the morning. In the present study two different types of decane soot have been produced. It has been shown that the air/fuel ratio of the diffusion flame is a key parameter influencing the reactivity of soot towards NO2. Whereas soot from a rich flame ("grey" decane soot) leads to HONO with yields up to 100% upon interaction with NO2, only small amounts of HONO, but significant amounts of NO are formed in the presence of soot originating from a lean flame ("black" decane soot). A reaction mechanism has been developed showing that NO2 is initially converted to HONO by a redox reaction. HONO produced in this way is subsequently either quantitatively desorbed into the gas phase on "grey" decane soot or it decomposes to a large extent in a disproportionation reaction on "black" decane soot. The products of disproportionation are NO, which is instantaneously desorbed into the gas phase and NO2, which undergoes secondary reactions. In addition to the HONO producing pathway there is a reaction channel that leads to adsorption of NO2 for both types of soot which is irreversible on the time scale of our standard experiments. Saturation effects that are occurring on the time scale of our experiments are primarily caused by saturation of adsorption sites and not by the depletion of the reducing agent. It seems that the fraction of NO2 that is irreversibly adsorbed is blocking part of the surface sites where NO2 is initially adsorbed. Uptake coefficients γ take initial values of up to 0.1 for both types of soot. However, with increasing uptake of NO2 γ is decreasing. After a NO2 consumption of approximately 8×1013 molecule cm-2, which corresponds to 13% of a formal monolayer, γ drops to values of 3×10-7 for "grey" decane soot and 6×10-7 for "black" decane soot, respectively. Furthermore, our results show that the rate of initial uptake is the rate limiting step of the NO2/soot interaction mechanism. Experiments where "black" decane soot has been exposed to a flow of HONO revealed that it readily decomposes into NO and NO2. For HONO concentrations above 3.3×1011 molecule cm-3 a total product yield of 50%, whereof 40% NO and 10% NO2, has been observed. The initial uptake coefficient ( γ0) for HONO on "black" decane soot did not show clear saturation effects but varied randomly in the concentration range from 1.2×1012 to 5.9×1012 molecule cm-3. The average value measured at these concentrations was equal to 0.027. The strong interaction of HONO with "black" decane soot shows that soot is not only a potential source, but also a potential sink of HONO. On the other hand, no significant uptake could be observed when "grey" decane soot was exposed to HONO. This is consistent with the observation that NO2 is converted to HONO at yields of up to 100%. Acetylene soot is an additional type of soot that has been examined. The results for the initial uptake coefficients were basically the same as for "grey" and "black" decane soot, that is maximum values of γ0 of 0.1 for NO2 uptake decreasing with increasing concentrations of NO2. This similarity holds only for the first few seconds of interaction because acetylene soot saturates much faster than "grey" and "black" decane soot with continuous exposure. The acetylene samples are generally almost completely saturated after only three minutes of exposure to NO2. Soxhlet extractions of "grey" decane soot which have been performed using tetrahydrofuran as a solvent also produced HONO upon interaction with NO2. This shows that the active reactants are not part of the graphitic soot backbone but are extractable, rather polar compounds of the organic fraction of soot. This assumption is supported by the observation that the corresponding benzene extraction is not reactive in relation to NO2 uptake and HONO formation. In the second part of the present work an experimental set-up is described which has been built in order to measure the rate constants of the reaction of organic compounds with the OH radical in the gas phase using a relative rate technique at atmospheric pressure. The system may be called a "mini smog chamber" as the reaction cell has a volume of only 480 cm3. The system was validated by measuring the relative rate constants of the reactant pair cyclohexane/toluene. These measurements have been performed for temperatures ranging from 300 to 360 K and led to results which are in excellent agreement with literature data. However, the experimental set-up also had some important limitations. Biphenyl which has a boiling point of 529 K was the compound with the lowest volatility that could be examined in our reaction cell. Compounds with lower vapor pressures or compounds with polar groups adsorbed to a large extent onto the glass walls of the reaction cell even at temperatures of up to 373 K. Additional problems occurred in the closed ion source of the mass spectrometer leading to non linear and/or unstable MS signals. These phenomena were most probably caused by secondary ion-molecule reactions in the closed ion source.

EPFL2000

Chargement

2009

Chargement

Development of Catalytic Umpolung

Jonathan Brand

The development of efficient synthetic methodologies is crucial to access complex molecules in an economic, eco-friendly and safe way. The reversal of the intrinsic reactivity of functional groups, called Umpolung, allows alternative synthetic strategies and can improve synthetic efficiency. In this thesis, two new Umpolung methods have been investigated. First a novel approach for the direct alkynylation of aromatic C-H bonds has been developed to access aromatic alkynes. Secondly, novel bifunctional (thio)urea carbene catalysts have been synthesized and their use in asymmetric intermolecular heterocouplings has been investigated. Aromatic alkynes are compounds of utmost prominence as synthetic intermediates and for application in materials science. Classical synthetic methods require a prefunctionalization of the aromatic ring in addition to the alkynylation step. An alternative emerging strategy to introduce triple bonds is based on the Umpolung of acetylenes by the use of electrophilic acetylene reagents. In addition, the use of metal catalysis has recently arisen to couple aromatic C-H bonds with electrophiles in a single step. However, direct alkynylation has been only scarcely investigated. In this thesis, alkynyl hypervalent iodines, highly electrophilic acetylenes, were used for the metal-catalyzed direct alkynylation of aromatic C-H bonds. Indoles were alkynylated in C3 position using AuCl as catalyst and TIPS-EBX (1- [(triisopropylsilyl)ethynyl]-1λ3,2-benziodoxol-3(1H)-one) as source of electrophilic acetylene. The reaction proceeded under mild open-flask conditions (room temperature under air, commercial solvents). Importantly, it was the first example showcasing the superiority of alkynyl benziodoxolones over alkynyl iodoniums in alkyne transfer reactions. On contrary to other alkynylation methods, which have either a limited scope or require indole protection, this reaction tolerated a large range of functional groups and worked for unprotected indoles. Furthermore in the presence of pyridine, pyrroles were alkynylated in C2 position in high yields. This development was important in light of the instability of unprotected halogeno- pyrroles, which makes their use in cross-coupling reactions difficult. A one-pot 2- alkynylaniline cyclization/direct alkynylation was next developed. The influence of substituents on the acetylene, the iodoxole ring structure and the benzene ring substitution on the alkynyl benziodoxolones were examined. This study revealed that bulky silyl groups were ideal alkyne substituents. Preliminary mechanistic investigations indicated that the reaction proceeded either via a π activation mechanism or via an oxidative AuI/AuIII mechanism. iii Unfortunately it has not yet been possible to distinguish between both mechanisms. The alkynylation of electron-rich arenes was next explored. N,N-dialkylanilines were alkynylated with high para regioselectivity using iPrOH as solvent. This transformation represented the first one-step synthesis of para-alkynylanilines and was then extended to 1,3,5- trimethoxybenzenes.

EPFL2012

Chargement

EPFL2000

Development of Catalytic Umpolung

Jonathan Brand

EPFL2012