Further reactions

Another problem arises when reactions do not stop at the desired product  

Once again, modifying the reaction conditions may offer scope for improvements, one option being to settle for low conversion and recycle reactants a number of times, the approach used in the first stage of the adipic acid process mentioned above. The cost of separating reactants and recycling them will usually be an issue. 

Dow Chemical introduced a superior process, again starting from acrylonitrile and using a fixed bed copper catalyst, but further reaction can take place to acrylic acid, although the amount can be kept at low levels  

Byproducts arising from reaction of impurities in main reactants or auxiliary chemicals 

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When more than one reactant is used, it is often desirable to use an excess of one of the reactants. It is sometimes desirable to feed an inert material to the reactor or to separate the product partway through the reaction before carrying out further reaction. Sometimes it is desirable to recycle unwanted byproducts to the reactor. Let us now examine these cases. 

Intermediate separation followed by further reaction is clearly most appropriate when the intermediate separation is straightforward, as in the case of sulfuric acid production. 

Another way to keep the concentration of PRODUCT low is to remove the product as the reaction progresses, e.g., by intermediate separation followed by further reaction. For example, in a reaction system such as Eq. (2.18), intermediate separation of the PRODUCT followed by further reaction maintains a low concentration of PRODUCT as the reaction progresses. Such intermediate separation is most appropriate when separation of the product from the reactants is straightforward. 

Homogeneous catalysts. With a homogeneous catalyst, the reaction proceeds entirely in the vapor or liquid phase. The catalyst may modify the reaction mechanism by participation in the reaction but is regenerated in a subsequent step. The catalyst is then free to promote further reaction. An example of such a homogeneous catalytic reaction is the production of acetic anhydride. In the first stage of the process, acetic acid is pyrolyzed to ketene in the gas phase at TOO C ... 

Can the byproduct be subjected to further reaction and its value upgraded For example, most organic chlorination reactions produce hydrogen chloride as a byproduct. If this cannot be sold, it... 

If waste byproducts are formed, it may be possible to upgrade them by further reaction in a different reaction system. 

Separate the product partway through the reaction before carrying out further reaction and separation. 

Additional reaction and separation of waste streams. Sometimes it is possible to cany out further reaction as well as separation on waste streams. Some examples have already been discussed in Chap. 4. 

Once the reaction is accomplished, it is usually desirable to store the product, i.e. a further reaction... 

Further reaction then occurs between the disulphur dichloride and the carbon disulphide ... 

Normally the chloramine immediately undergoes further reaction, giving oft nitrogen ... 

Further reaction between hydrogen sulphide and the sulphite ion yields sulphur together with thionic acids) ... 

Uses of Picric Acid. The following further reactions of picric acid are used for analytical purposes in the laboratory. 

The sodium fusion and extraction, if performed strictly in accordance with the above directions, should be safe operations. In crowded laboratories, however, additional safety may be obtained by employing the follow ing modification. Suspend the hard-glass test-tube by the rim through a hole in a piece of stout copper sheet (Fig. 69). Place 1 -2 pellets of sodium in the tube, and heat gently until the sodium melts. Then drop the organic compound, in small quantities at a time, down — =. the tube, allowing the reaction to subside after each addition before the next is made. (If the compound is liquid, allow two or three small drops to fall at intervals from a fine dropping-tube directly on to the molten sodium.) Then heat the complete mixture as before until no further reaction occurs. 

Place about o-i g. of the following substances in a boiling tube and add about 5 ml. of 10% aqueous NaOH solution. Note any reaction in the cold. Then add a small piece of unglazed porcelain (to prevent bumping ), and boil gently for a few minutes. Note any further reaction. 

Reaction of cyclic ketones with diazoraethane leads to ring enlargement. Thus cyctohexanone (I) (1 mol) and diazomethane (1 mol) give /oheptanone (II) in about 60 per cent, yield together with a little epoxide (III) as by product and some cj/cioootanone (IV) resulting from further reaction of cyctoheptanone with diazomethane. ... 

The chemical recycling of carbon dioxide into usable fuels provides a renewable carbon base to supplement and eventually replace our diminishing natural hydrocarbon resources. Methanol (or dimethyl ether), as discussed, can be readily converted into ethylene or, by further reaction, into propylene. 

Beeause the product is at least as reactive as the starting material, and further reaction occurs ... 

Using a standard vacuum distillation the solvent is distilled off. This shouldn t take too long. The first thing to come over after the solvent was the safrole, which with my vacuum (2mm) started at around 9CfC. The safrole will be a clear liquid, slightly viscous and will smell of liquorice. With the above measurements one can expect a yield of around 85g. No further cleaning up is necessary, and the safrole can be used as is for any further reactions. ... 

Apparatue 500-ml three-necked round-bottomed flask, with a dropping funnel, a mechanical stirrer and a thermometer, combined with a vent, for the addition to the isopropenyl ether for the further reaction the dropping funnel was replaced with a thermometer and the combination of thermometer and vent with a reflux condenser. 

The transmetallation of various organometallic compounds (Hg, Tl, Sn, B, Si, etc.) with Pd(II) generates the reactive cr-aryl, alkenyl, and alkyl Pd compounds. These carbopalladation products can be used without isolation for further reactions. Pd(II) and Hg(II) salts have similar reactivity toward alkenes and aromatic compounds, but Hg(II) salts form stable mercuration products with alkenes and aromatic rings. The mercuration products are isolated and handled easily. On the other hand, the corresponding palladation products are too reactive to be isolated. The stable mercuration products can be used for various reactions based on facile transmetallation with Pd(II) salts to generate the very reactive palladation products 399 and 400 in rim[364,365]. 

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

The reaction of the 1- and (2-ethoxyvinyl)tributylstannanes (721) and (723) as masked carbonyls with aryl halides proceeds smoothly and the products 722 and 724 are used for further reactions[592,593]. 

The intramolecular insertion of a conjugated diene into 7r-allylpalladium, initially formed in 789, generates another rr-allyl complex 790, which is trapped with acetate anion to give a new allylic acetate 791. No further reaction of the allylic acetate with alkene takes place[489]. 

Another reaction occurs by the attack of a soft nucleophile at the central carbon to form the 7r-allylpalladium complex 7, which undergoes further reaction with the nucleophile typical of rr-allylpalladium complexes to form the alkene 8,... 

The allenyl moiety (2,3-aikadienyl system) in the carbonylation products is a reactive system and further reactions such as intramolecular Diels-Alder and ene reactions are possible by introducing another double bond at suitable positions of the starting 2-alkynyl carbonates. For example, the propargylic carbonate 33 which has l,8(or 1.9)-diene-3-yne system undergoes tandem carbonylation and intramolecular Diels-Alder reaction to afford the polycyclic compound 34 under mild conditions (60 C, 1 atm). The use of dppp as ligand is important. One of the double bonds of the allenyl ester behaves as part of the dieneflSj. 

The first step of the reaction is the oxypalladation of the triple bond with PdCl2 as shown by 228 to form the alkenylpalladium species 229, and the Pd is displaced with proton to regenerate Pd(TI) species and the lactone 224. The alkenylpalladium species 229 can be utilized for further reaction. When allyl chloride (230) is added, double bond insertion is followed by elimination of... 

Small amounts of salt-like addition products (85) formed by reaction on the ring nitrogen may be present in the medium. (Scheme 60) but. as the equilibrium is shifted by further reaction on the exocyclic nitrogen, the only observed products are exocyclic acylation products (87) (130. 243. 244). Challis (245) reviewed the general features of acylation reactions these are intervention of tetrahedral intermediates, general base catalysis, nucleophilic catalysis. Each of these features should operate in aminothiazoles reactivity. 

When malonic esters are the acvdating agents, further reaction may occur with an excess of 2-aminothiazole. yielding 118 (Scheme 79) (278). 

Formamidinoyl isothiocyanates (157) combine with 2-aminothiazoles the ring nitrogen attacks the spC part of the electrophilic reagent (312) further reaction then yields aza-condensed thiazolo-s-triazines (158) (Scheme 99) (313). Mesoionic S-alkvlthiazolo[3.2-fl]-i-tria2ine-5,7-diones (159) are obtained when 2-alkylaminothiazoles react with phenoxycar-bonyl isocyanate (304). 

The halogen in the 5-position of 2-aminothiazoles is usually reactive and is used for further reaction (see Chapter V). The reaction may take place in the same medium as thiocyanation (437-440), rhodanation (441). or reaction with NaNO (435). Similarly, a mixture of 2-amino-4-methylthiazole and thiourea in H2O yields 5,5 -thiobis(2-amino-4-methyDthiazole (202) after addition of iodine (Scheme 128) (442). 

These selenopyrazolones (Table X-7) can undergo further reaction, as described in Section III. 4 (34). 

Rearrangement to an open chain imine (165) provides an intermediate whose acidity toward lithiomethylthiazole (162) is rather pronounced. Proton abstraction by 162 gives the dilithio intermediate (166) and regenerates 2-methylthiazole for further reaction. During the final hydrolysis, 166 affords the dimer (167) that could be isolated by molecular distillation (433). A proof in favor of this mechanism is that when a large excess of butyllithium is added to (161) at -78°C and the solution is allowed to warm to room temperature, the deuterolysis affords only dideuterated thiazole (170), with no evidence of any dimeric product. Under these conditions almost complete dianion formation results (169), and the concentration of nonmetalated thiazole is nil. (Scheme 79). This dimerization bears some similitude with the formation of 2-methylthia-zolium anhydrobase dealt with in Chapter DC. Meyers could confirm the independence of the formation of the benzyl-type (172) and the aryl-type... 

Methanol is included to prevent the further reaction of py SO3 with water. The titration s end point is signaled when the solution changes from the yellow color of the products to the brown color of the Karl Fischer reagent. 

In practice, intermediate, Hquid resins, capable of further reaction are usually prepared. Polymerization is carried to an estabUshed end-point as determined by viscosity or other measurements. When the proper end-point has been reached, the reaction is terminated by adjusting the pH of the system to 5—8. Such hquid resins can be stored for six months or longer, then catalyzed and reacted further to obtain the final, desired product. 

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