Houben-Hoesch reaction


Houben-Hoesch reaction  [c.483]

An analogous reaction is the Houben-Hoesch reaction,(sometimes called the Hoesch reaction) using nitriles 7 to give aryl ketones 8. This reaction also is catalyzed by Lewis acids often zinc chloride or aluminum chloride is used. The Houben-Hoesch reaction is limited to phenols—e.g. resorcinol 6—phenolic ethers and certain electron-rich aromatic heterocycles  [c.134]

The synthetic importance of the Houben-Hoesch reaction is even more limited by the fact that aryl ketones are also available by application of the Friedel-Crafts acylation reaction.  [c.134]

Haworth reaction Hell-Volhard-Zelinsky reaction Hoesch reaction Hofmaim reaction.  [c.1210]

In contrast to the other methods discussed, and in spite of the high yields generally obtained, only limited use has been made of the Hofmann-Loffler Reaction in the field of steroids. All the starting amines belong to the one group of 20a- and 20) -methylamino steroids, containing free or acetylated hydroxyl groups, a dimethylamino or an a,j5-unsaturated ketone function. With a saturated 3-ketone, a-chlorination rather than angular substitution at C-18 is observed.  [c.258]

Reaction rates often may be improved by using more extreme operating conditions. More extreme conditions may reduce inventory appreciably. However, more extreme conditions bring their own problems, as we shall discuss later. A very small reactor operating at a high temperature and pressure may be inherently safer than one operating at less extreme conditions because it contains a much lower inventory. A large reactor operating close to atmospheric temperature and pressure may be safe for different reasons. Leaks are less likely, and if they do happen, the leak will be small because of the low pressure. Also, little vapor is produced from the leaking liquid because of the low temperature. A compromise solution employing moderate pressure and temperature and medium inventory may combine the worst features of the extremes. The compromise solution may be such that the inventory is large enough for a serious explosion or serious toxic release if a leak occurs, the pressure will ensure that the leak is large, and the high temperature results in the evaporation of a large proportion of the leaking liquid.  [c.263]

The way the substituents affect the rate of the reaction can be rationalised with the aid of the Frontier Molecular Orbital (FMO) theory. This theory was developed during a study of the role of orbital symmetry in pericyclic reactions by Woodward and Hoffinann and, independently, by Fukui Later, Houk contributed significantly to the understanding of the reactivity and selectivity of these processes.  [c.4]

Hoffman Degradation. Polyacrylamide reacts with alkaline sodium hypochlorite [7681-52-9], NaOCl, or calcium hypochlorite [7778-54-3], Ca(OCl)2, to form a polymer with primary amine groups (58). Optimum conditions for the reaction include a slight molar excess of sodium hypochlorite, a large excess of sodium hydroxide, and low temperature (59). Cross-linking sometimes occurs if the polymer concentration is high. High temperatures can result in chain scission.  [c.141]

Hafnium is a highly reactive metal. The reaction with air at room temperature is self-limited by the adherent, highly impervious oxide film which is formed. This film provides oxidation stabiUty at room temperature and resistance to corrosion by aqueous solutions of mineral acids, salts, or caustics. Thicker oxide films are formed at higher temperature, but slowly enough that forging or hot rolling of hafnium ingots is conducted in air at a temperature between 900 and 1000°C, with subsequent removal of surface scale by sandblasting and then a nitric—hydrofluoric acid pickling. High surface area hafnium powder or porous sponge metal ignites quite easily in air. Clean hafnium metal ignites spontaneously in oxygen of about 2 MPa (300 psi).  [c.440]

Another approach for speeding PF resin cure utilizes faster phenolics as accelerators. The most popular of these has been resorcinol. Many different approaches have been tried. Resorcinol has often been cooked into the PF resin. This is probably the least effective approach available. Depending on the amount of resorcinol used and other reaction conditions, different results are obtained. If enough resorcinol is added to the beginning of the cook, a resorcinol-formaldehyde polymer will rapidly form. This polymer will contain little or no phenol. Thus, in a viscosity-limited system, the resin may hit the viscosity endpoint while still containing large quantities of raw phenol and formaldehyde. Meanwhile, the resorcinol-formaldehyde polymer will have exhausted all of the resorcinol functionality during its formation. Such a polymer will not show the rapid curing characteristics hoped for and will have high VOC emissions. If only a small amount of resorcinol is used, it will become thoroughly incorporated into the phenolic polymer and will show no special reactivity in cure. Sometimes, the resorcinol polymer made in the first case will break down in the presence of the excess phenol to give the same result as the second case. This will only happen if the reaction time and conditions are sufficient to allow it.  [c.918]

The reason for this is simple. If the reaction chemistry is not "clean" (meaning selective), then the desired species must be separated from the matrix of products that are formed and that is costly. In fact the major cost in most chemical operations is the cost of separating the raw product mixture in a way that provides the desired product at requisite purity. The cost of this step scales with the complexity of the "un-mixing" process and the amount of energy that must be added to make this happen. For example, the heating and cooling costs that go with distillation are high and are to be minimized wherever possible. The complexity of the separation is a function of the number and type of species in the product stream, which is a direct result of what happened within the reactor. Thus the separations are costly and they depend upon the reaction chemistry and how it proceeds in the reactor. All of the complexity is summarized in the kinetics.  [c.297]

The effect of the metals used was then examined (Table 5.4). When the group 4 metals, titanium, zirconium, and hafnium, were screened it was found that a chiral hafnium catalyst gave high yields and enantioselectivity in the model reaction of aldimine lb with 7a, while lower yields and enantiomeric excesses were obtained using a chiral titanium catalyst [17].  [c.192]

High pressure reactors are frequently called bombs, an unfortunate term indeed. A major aim of any investigation in this area is to make certain that the phrase does not become an apt description. Serious accidents are most likely to happen not as part of the hydrogenation process but as a result of chemistry extrinsic to the hydrogenation, that is, confinement of an unstable material under elevated temperatures. In this regard there is some danger in trying to force a hydrogenation to completion by increasing the reaction temperature excessively. A large-scale destructive, runaway reaction occurred for this reason during hydrogenation of impure 3,4-dichloronitrobenzene, caused by  [c.20]

Friedel-Crafts acylation using nittiles (other than HCN) and HCI is an extension of the Gattermann reaction, and is called the Houben-Hoesch reaction (120—122). These reactions give ketones and are usually appHcable to only activated aromatics, such as phenols and phenoHc ethers. The protonated nittile, ie, the nitrilium ion, acts as the electrophilic species in these reactions. Nonactivated ben2ene can also be acylated with the nittiles under superacidic conditions 95% trifluoromethanesulfonic acid containing 5% SbF (Hg > —18) (119). A dicationic diprotonated nittile intermediate was suggested for these reactions, based on the fact that the reactions do not proceed under less acidic conditions. The significance of dicationic superelectrophiles in Friedel-Crafts reactions has been discussed (123,124).  [c.559]

The Marcus equation provides a nice conceptual tool for understanding trends in reactivity. Consider for example the degenerate Cope rearrangement of 1,5-hexadiene and the ring-opening of Dewar benzene (bicyclo-[2,2,0]hexa-2,5-diene) to benzene, Figure 15.29. The experimentally observed activation energies are 34 kcal/mol and 23 kcal/mol, respectively. The Cope reaction is an example of a Woodward-Hoffmann allowed reaction ([3,3]-sigmatropic shift) while the ring-opening of Dewar benzene is a Woodward-Hoffmann forbidden reaction (the cyclobutene ring-opening must necessarily be disrotatory, otherwise the benzene product ends up with a trans double bond). How come a forbidden reaction has a lower activation energy than an allowed reaction This is readily explained by the Marcus equation. The Cope reaction is thermoneutral (reactant and product are identical) and the activation energy is purely intrinsic, while the ring-opening is exothermic by 71 kcal/mol, and therefore has an intrinsic barrier of 52 kcal/mol. The forbidden reaction occurs only because it has a huge driving force in terms of a much more stable product, while the allowed reaction occurs even without a net energy gain.  [c.367]

The Woodward-Hoffmann rules state what the outcome of a pericyclic reaction will be, but they do not define the mechanism by which the reaction occurs. Many theoretical techniques have been applied to the study of these problems over the years [Houk et al. 1992] and a passionate debate has ensued on the nature of the transition structures involved in these reactions. The debate has been fuelled by fhe fact that different theoretical treatments (especially semi-empirical methods) give different results. For example, at one extreme the Diels-Alder reaction between butadiene and ethene would proceed via a two-step mechanism involving a brradial transition structure. At the other extreme the reaction would involve a symmetrical transition state formed in a concerted, synchronous reaction. Ab initio calculations at various levels of theory suggest the concerted transition structure. The geometry obtained for the prototypical Diels-Alder reaction between butadiene/ ethene using a CASSCF calculation and a 6-31G basis set is shown in Figure 5.35 [Houk et al. 1995]. The alternative biradial structure is also shown in Figure 5.35 this is predicted to be 6kcal/mol higher in energy than the symmetrical transition structure.  [c.309]

An RHF wave function is also inappropriate for a reaction where reactants and products share different occupied orbitals, as in a Woodward-Hoffmann orbital symmetry forbidden thermal reaction. For example, in the absence of symmetry, the disrotatory ring opening of cyclobutene to butadiene must involve a change in the orbital occupancy between reactants and products. The lowest virtual orbital in reactants becomes the highest occupied in the product. This gives rise to an artificial and undesirable cusp in the potential energy surface along the reaction path, plus an unrealistically high activation energy. You can prevent this problem and the breaking of a covalent bond by using a UHF wave function for the singlet state. Aminor disadvantage of the UHFmethod is that the space and spin symmetry of the molecular orbitals are broken. For example, the UHF wave function is a mixture of singlet and triplet states. To preserve spin symmetry, use RHF plus the Configuration Interaction option instead of UHF.  [c.46]

Even though form amide was synthesized as early as 1863 by W. A. Hoffmann from ethyl formate [109-94-4] and ammonia, it only became accessible on a large scale, and thus iadustrially important, after development of high pressure production technology. In the 1990s, form amide is mainly manufactured either by direct synthesis from carbon monoxide and ammonia, or more importandy ia a two-stage process by reaction of methyl formate (from carbon monoxide and methanol) with ammonia.  [c.507]

Metallocene Catalysts. Metallocene catalyst systems (Kaminsky catalysts) contain two components (35). The first is a metallocene complex of zirconium, titanium, or hafnium, which usually contains two cyclopentadienyl rings. The second component is either an organoaluminum compound, methylalurninoxane, or a perfiuorinated boron-aromatic compound. The most attractive feature of Kaminsky catalysts is that they are able to produce ethylene copolymers with high compositional uniformity. Although this feature is important for LLDPE resins, it is relatively insignificant in the case of copolymers with a low comonomer content, such as slightly branched HDPE. Another distinguishing feature of Kaminsky catalysts is high sensitivity of the polymer molecular weight to the presence of hydrogen in the reaction medium this is a useful feature in the manufacture of HDPE resins for injection molding.  [c.383]

In addition to the nitrile and alcohol routes just described, many other methods of preparation of fatty amines are available. Some commercially available tertiary fatty amines are prepared via a petrochemical route (13). The amines have been prepared by reaction of an olefin with ammonia or a primary or secondary amine in the presence of a catalyst prepared from a Group 8—10 metal or an ammonium haUde (14—16). Nitration of paraffins having from 6 to 30 carbon atoms with nitrogen dioxide at elevated temperatures followed by hydrogenation in the presence of a nickel or palladium catalyst produces secondary alkyl primary amines (17—20). Long-chain, unbranched, aUphatic tertiary amines can be prepared by reaction of an alkyl chloride with an alkyl secondary amine at 100—250°C (21,22). Other methods of producing amines include reaction of a carboxyHc acid ester with a secondary amine in the presence of hydrogen at high pressure using a metal oxide catalyst, 2inc oxide—chromium oxide or 2inc oxide—aluminum oxide (23), or by catalytic hydroammonolysis of carboxyHc acids at high pressure and temperature in the presence of a mixture of sulfides of metals of Groups 6 and 8 —10 (24). The Hofmann rearrangement, preparation of an amine from an unsubstituted amide using solutions of chlorine in sodium hydroxide, has been useful in preparing long-chain primary amines (25). Amines prepared using the Hofmann rearrangement contain one less carbon atom than the amide. Thus, odd-chain-length fatty amines, not available from natural sources, can be prepared. The Ritter reaction of olefins with hydrogen cyanide in the presence of a strong acid, after hydrolysis, produces tertiary-alkyl primary amines. Rohm and Haas Primene amines are prepared using the Ritter reaction.  [c.218]

A complete mechanistic description of these reactions must explain not only their high degree of stereospecificity, but also why four-ir-electron systems undergo conrotatory reactions whereas six-Ji-electron systems undergo disrotatory reactions. Woodward and Hoifinann proposed that the stereochemistry of the reactions is controlled by the symmetry properties of the HOMO of the reacting system. The idea that the HOMO should control the course of the reaction is an example of frontier orbital theory, which holds that it is the electrons of highest energy, i.e., those in the HOMO, that are of prime importance. The symmetry characteristics of the occupied orbitals of 1,3-butadiene are shown in Fig. 11.1.  [c.608]

Bond density surfaces are also superior to conventional models when it comes te describing chemical reactions. Chemical reactions can involve many changes in chemica bonding, and conventional formulas are not sufficiently flexible to describe what happen (conventional plastic models are even worse). For example, heating ethyl fonnate t( high temperatures causes this molecule to fragment into two new molecules, foraii( acid and ethene. A conventional formula can show which bonds are affected by ths reaction, but it cannot tell us if these changes occur all at once, sequentially, or in soms other fashion.  [c.26]


See pages that mention the term Houben-Hoesch reaction : [c.814]    [c.92]    [c.29]    [c.38]    [c.106]   
A life of magic chemistry (2001) -- [ c.198 ]

Named organic reactions 2nd edition (2005) -- [ c.134 ]