Side reaction

No side reaction accompanied this block copolycondensation, except for the formation of a very small amount of 2-imidazolin-5-one. 

The structure of the chains was carefully characterized by NMR and SEC the separation of the phases was studied by DSC. 

The preparation of block copolymers by polycondensation requires very well defined oligomers. More particularly, it is important to control (i) their exact composition because many commercial oligomers are mixtures of homopolymers 

In many reports, it is mentioned that the telechelic oligomers were used as received . Unfortunately, the actual structure can be far from the composition given in the analytical card. 

Heatley [36] provided very valuable NMR spectroscopic information on polyethers. The NMR spectra of the copolyethers after trifluorination allowed the determination of the oxyethylene-to-oxypropylene unit ratio in the chain and the concentrations of primary (triplet at 4.42 ppm) and secondary (sextet at 5.16 ppm) hydroxyl groups. Their NMR spectra were compared to those of a,o -dihydroxy-polyaxyethylene and a,cn-dihydroxy-polyoxypropylene, which confirmed the NMR data and permitted the identification of each configuration in the chain. 

4 Side-reactions As soon as the cell current density surpasses the limiting current density of one reaction, the electrode potential rises until additionally another reaction takes place (in Fig. 1 

If there are no detrimental organic side reactions, a cell current density in excess of the limiting current density - and as result a loss of current efficiency - may be acceptable for laboratory scale experiments. For example, a hydrogen evolution parallel to an electroorganic cathodic reduction can even be advantageous as it improves the mass transfer by moving gas bubbles and thus enhances the organic cathodic reduction. 

But if reactants or products of the desired reaction are lost and/or if undesrred compounds are formed by side reactions (i.e. yield and selectivity will be reduced), it is necessary to avoid any overstepping of the limiting current density. Especially in case of changing conditions (batch operation) with a rising degree of conversion - here, the most significant parameter will be the decreasing reactant concentration - a continuous adjustment of the current density is indispensable. Then it will be better to work at a constant electrode potential than at constant cell current (see Sect. 23.23). 

In the example of Fig. 1, the potential differences are so large that clearly discriminated reactions can be realized. At a potential of 0.3 V, only reactant 1 is active, even at low concentrations for high degrees of conversion. 

There are also some reactions known, which need - contrary to the normal case - a high current density for a sufficient selectivity, for example, the Kolbe reaction (see Chapter 6). 

Several possible reactions may give rise to impurities in the preparation of phosphonic or phosphinic acid esters by the Michaelis-Arbuzov reaction, and it is possible that, in some cases, such reactions become preponderant. Triethyl phosphite, for instance, has been successfully used as a dehalogenating agent, in particular, for debrominations. Isomerization of allylic groups may occur through S T-type processes (reaction 5) or be induced thermally (reactions 6 and 7) 25,126 

The later stages of reactions involving a,co-dihaloalkanes and related compounds may be accompanied by cyclization, particularly at higher temperatures (equation 8) The two-stage reactions between trialkyl phosphites and 1,4-dibromo- or 1,5-dibromoalkanes give rise to 1,2-oxaphosph(V)orinanes (57) n = 1) or l,2-oxaphosph(V)epanes (57) n -2) accompanied by monodehydrobromination during the formation of a linear ester. 

Reactions between cyclic phosphonites (58) and alkyl halides have been employed to prepare linear (ring-opened) polymeric phosphonates which, when heated more strongly, undergo depolymerization and furnish l,2-oxaphosph(V)olanes, e.g. 59 when R = H, a second product has been shown to be the phosphinic anhydride (60)  

A further, and more important, difficulty occurs in attempted Michaelis-Arbuzov reactions involving certain halogenated carbonyl compounds. In these cases, a reaction in 

Many variations in the Michaelis-Arbuzov reaction have been observed they range from slight changes in the nature of the alkylating species to a recognition that certain reactions, of an apparently totally different type, are in essence of the same mechanistic type, and give rise to similar products. 

In the course of polycondensation various side reactions may occur. 

Partly aromatic copoly(amide)s prepared by conventional processes have triamine contents greater than 0.5%. This effects a deterioration in the product quality and to problems in preparation by a continuous method. For example, dihexamethylenetriamine, formed from HMD is used in the preparation. Copoly(amide)s with a low trianune content have the same solution viscosity but lower melt viscosities compared with products of the same composition which have a higher triamine content. The processability and the product properties are significantly improved by a low triamine content. The dimerization of the diamine can be suppressed by using special methods of polymerization. Short residence times in the high-temperature stage of the polycondensation process suppress the formation of triamines.  

PAs with 2-methylpentamethylenediamine are difficult to prepare by conventional melt condensation methods. Namely, 2-methylpentamethylenediamine can be easily cyclized to methylpiperidine under the liberation of ammonia. Thus one potential amide group is lost and the compound becomes monofunctional and acts as chain stopper.  

Ornithine formation. Occurs when protonation or acyl-based groups are used for protection of the guanidine side-chain 

5-Lactam formation. Results from attack of the N -atom on the activated carboxy group. Most problematic with Arg PSAs 

Cyanoalanine formation. Occurs during the activation of Asn when the side-chain carboxamide is not protected 

Deamidation. Can occur in aqueous media, particularly under basic conditions, leading to formation of a- and p-aspartyl peptides. Most problematic with peptides containing Asn-Gly and Asn-Ser 

Prevented through the use of sulphonyl-based protection, such as Pbf and Pmc 

The formation of deoxyosones, such as, for example, the 3-deoxyosones, appears to be the most important of the dehydration reactions which may take place during Lobry de Bruyn-Alberda van Ekenstein transformations. This type of reaction, which NeP first proposed in suggesting mechanisms for saccharinic acid formation, is difficult to study because the products are seldom stable in the reaction mixtures in which they are formed. Nevertheless, several different lines of evidence now indicate that reducing sugars undergo primary dehydrations of this kind, and that deoxyosones do indeed mediate in saccharinic acid formation in basic solutions, as well as in production of 2-furaldehyde and its derivatives in acidic media. 

A dehydration of this type has actually been observed as a side reaction of a Lobry de Bruyn-Alberda van Ekenstein transformation in a very simple system. Thus, in experiments with the DL-glycerose-l,3-dihy-droxy-2-propanone isomerization in acetate, formate, and trimethylacetate buffers, pyruvaldehyde appeared in the reaction mixtures. (The formation of pyruvaldehyde from l,3-dihydroxy-2-propanone- and dl-glycerose-mineral acid mixtures had been observed much earlier.) Since these experiments in acidic buffers established that this reaction is subject to general acid and base catalysis, pyruvaldehyde must be formed in alkaline mixtures also. The results of Wohl s and Evans and Hass s experiments with DL-glycerose in alkaline solutions containing phenyl-hydrazine, in which pyruvaldehyde phenylosazone was isolated, support this view. 

Two other dehydration reactions, reversion and anhydride formation, may possibly complicate the transformations in acidic solutions. However, they seem to be generally unimportant. 

The reactions leading to the saccharinic acids and to 2-furaldehyde and its derivatives do not seem at this point to warrant further consideration as side reactions, since they have already been mentioned in connection with primary dehydration and related transformations. 

Both aldolization and dealdolization become significant side-reactions in alkaline reaction mixtures because of the pronounced catalysis of the aldol condensation by hydroxide ion. (Except for ammonia and compounds possessing a primary or secondary amino group, which exert a special catalytic effect, other bases do not seem to catalyze the aldol condensation in aqueous solution. ) 

While all of these reactions will not affect the kinetic order, since they occur after the rate-determining protonation of the unsaturated system, they must be considered if the reactions are to be used preparatively, or if detailed speculation about the mechanism is to be made. 

Several products other than 2,2 -biaryls have been isolated following reaction of pyridines with metal catalysts. From the reaction of a-picoline with nickel-alumina, Willink and Wibaut isolated three dimethylbipyridines in addition to the 6,6 -dimethyl-2,2 -bipyridine but their structures have not been elucidated. From the reaction of quinaldine with palladium-on-carbon, Rapoport and his co-workers obtained a by-product which they regarded as l,2-di(2-quinolyl)-ethane. From the reactions of pyridines and quinolines with degassed Raney nickel several different types of by-product have been identified. The structures and modes of formation of these compounds are of interest as they lead to a better insight into the processes occurring when pyridines interact with metal catalysts. 

By-products from the Reaction of Pyridines with Degassed Raney Nickel 

The crude 2,2 -bipyridine obtained from the reaction of pyridine and degassed Raney nickel was found to contain 1.5% of 2 6, 2 -terpyridine, but no 2,2 2, 2 6 ,2 -quaterpyridine could be detected. Moreover, experiments with 2,2 -bipyridine and Raney nickel have failed to yield quaterpyridine, and the amount of terpyridine formed in experiments with mixtures of pyridine and 2,2 -bipyridine was found to be no higher than in the reaction with pyridine itself.  

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

From the dimensions of the lattice of W-6 Raney nickel, it seems that the formation of 2,2 6, 2 -terpyridine would be expected when one molecule of 2,2 -bipyridine and one molecule of pyridine are 

In virtually all cases, the alkylation of an isoparaffin with an olefin yields not only the products to be expected from the condensation of one molecule of the isoparaffin with one or more molecules of olefin but also paraffins of intermediate molecular weight. Thus, for example, pentanes, heptanes, and other alkanes containing an odd number of carbon atoms are obtained as by-products of the alkylation of isobutane with ethylene in the presence of aluminum chloride. Indeed, isopentane is usually formed in alkylation reactions involving isobutane regardless of the olefin or catalyst employed. 

The formation of these by-products is apparently due in part to further reaction of the primary products by dissociation into new paraffins and olefins and the subsequent reaction of these olefins with the original isoparaffin (as well as of the new paraffins with original olefin). Such reactions, which may be termed destructive alkylation, are similar to those which have been suggested to account for the products of the autodestructive alkylation of paraffins (Ipatieff and Grosse, 23). 

By-product formation in the alkylation of isobutane with propene, for example, may be indicated as follows  

It is apparent that the ratio of reacting isoparaffin to olefin may thus be greater than unity and the yield of alkylate based on the olefin may be higher than theoretical. Such high yields have been obtained in the alkylation of isopentane in the presence of sulfuric acid or hydrogen fluoride (see below). 

In some cases, as in the alkylation of isobutane with ethylene, the destructive alkylation appears to involve the transfer of methylene. 

Diffusion of gases other than oxygen from the surrounding environment into metal-air batteries also is detrimental to operational lifetimes. Ingress of carbon dioxide may form solid carbonates (e.g., K2CO3 in aqueous metal-air cells or Li2C03 in nonaqueous Li-air cells). In addition, diffused water will corrode unprotected metal anodes, particularly when metallic lithium is used as the anode. These side reactions also make rechargeable metal-air batteries more difficult to operate in ambient environments. 

Polymerization of pPL with a variety of initiators is a terminationless process. Lyudvig a.o. have mostly contributed to our present knowledge of the influence of reaction variables on the rates of polymerization 24 27). They obtained quantitative yields in the polymerization of lactones using bulky complex counterions, like SbClf1. Thus, the decomposition of these anions, leading to transfer and termination in the polymerization of cyclic ethers and acetals, does not lead to side reactions in the polymerization of lactones. This can be ascribed to reversible reactions between acyl halides plus Lewis acids and the acylium cation 28)  

The addition of water or alcohols does not influence the reaction rate 26) it is remarkable that water present in a concentration exceeding 30 times that of the initiator does not change the rate of polymerization although induction periods appear26). The addition of ketones leads to termination 27) and thus to limited yields. 

Chain transfer to polymer and macrocyclization, frequently observed in cationic ring-opening polymerization, has also been studied for lactones 4). In the polymerization of PPL cyclic oligomers were not observed, whereas in the polymerization of e-CL initially high-molecular-weight polymer is formed which then is slowly degraded to cyclic oligomers, mostly from dimer to pentamer. 

Degradation occurs because the monomer is much more basic and nucleophilic than the linear ester unit. Therefore, monomer will be consumed before chain transfer to polymer will occur. Typical GPC traces of the oligomers formed are shown in Fig. 9.1. 

To decrease the proportion of cycles the reaction should be carried out at high monomer concentration and the polymerization should be terminated before cycles 

Alcohols, possessing substituents able to stabilize carbocations at the (3 position, may suffer a carbon-carbon bond breakage as in Equation below (route b), competing with the normal transformation to ketones on Jones oxidation (route a).75 

This explains the following side products from oxidation of alcohols with Jones reagent  

A carbocation stabilized on a to a hydroxy group—that is a protonated ketone—is generated by cleavage of a carbon-carbon bond. This also leads to the formation of an aldehyde, which is oxidized in situ to a carboxylic acid. 

A naive look at the product suggests an oxidation to a ketone followed by a Baeyer-Villiger like reaction. The product is best explained by a fragmentation from an intermediate chromate ester, resulting on an aldehyde and a stabilized tertiary carbocation that is transformed into a tertiary alcohol by reaction with water. The hydroxyaldehyde so obtained may evolve to the final lactone either via a lactol or a hydroxyacid. 

As the oxidative carbon-carbon bond breakage of alcohols, leading to a stable carbocation, depends not only on the stability of the resulting carbocation but also on very exacting stereoelectronic factors, many cases are known in which alcohols are successfully oxidized to ketones, regardless of apparently easy oxidative carbon-carbon bond breakages. In fact, in synthetic experimental practice, it is recommended not to fail in trying a Jones oxidation because of fear of such side reactions. 

The kinetics of the esterification of 1-octanol with hexanoic acid on zeolite BE A was studied by Nijhuis et al. [29], For the acid, a first-order behavior was found, whereas the alcohol showed a negative reaction order of -1. From the data, an Eley-Rideal mechanism was concluded. The acid adsorbs onto the surface of the catalyst and reacts with an alcohol. The adsorption of water, alcohol, ester or ether inhibits the reaction. Hoek [30] found that the adsorption constant of water is more than one order of magnitude higher than those of the other compounds. The rate law given by Nijhuis et al. [29] also includes the equilibrium limitation  

As mentioned earlier, two side reactions are present Octene is formed to a low extent ( 2 %), and this might occur by dehydration of the 1-octanol or by splitting of an ester. Due to the small amounts, neither the mechanism of octene formation was further investigated, nor the principally possible isomerization of octene and subsequent formation of secondary esters or ethers. 

The etherification of two 1-octanol molecules to di-octyl-ether is more important. Nijhuis et al. [29] suggested a dual-site mechanism and a second-order reaction for the alcohol  

Under the experimental conditions chosen, no equilibrium limitation for this reaction was observed. The values for all kinetic parameters are presented in Tab. 8.2. 

The coating of wire gauze DX-packings is also described in Ref. [31], and in this way a catalyst hold-up of 17 g in 50 cm of packing was achieved. The catalyst bags of the katapak-S were filled with a sieve fraction (250-500 pm) of ground H-BEA extrudates, and this resulted in ca. 66-67 g catalyst in 50 cm of packing. 

The by-products which arose in the course of the Hilbert-Johnson reaction and were isolated either directly from the reaction mixture or after the usual work-up with alcoholic hydrogen chloride, hydrogen chloride in chloroform, or alcoholic ammonia might be divided into 

Prystas and F. Sorm, Collection Czech. Chem. Commun. 29, 121 (1964). 

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Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

A side reaction occurs in which dichlorodecane (DCD) is produced ... 

Which structure is most effective in suppressing the side reaction ... 

An arrangement is to be chosen to inhibit the side reaction, i.e., give low selectivity losses. The side reaction is suppressed by starving the reactor of either monochlorodecane or chlorine. Since the reactor is designed to produce monochlorodecane, the former option is not practical. However, it is practical to use an excess of decane. 

CH rCHCH NHCSNH. Colourless crystalline solid with a faint garlic-like odour m.p. 74 C. Manufactured by treating propenyl isothiocyanate with a solution of ammonia in alcohol. It has been given by injection in the treatment of conditions associated with the formation of excessive fibrous tissue. Toxic side reactions may occur. Propenyl thiourea is a chemical sensitizer for photographic silver halide emulsions. 

The successful preparation of polymers is achieved only if tire macromolecules are stable. Polymers are often prepared in solution where entropy destabilizes large molecular assemblies. Therefore, monomers have to be strongly bonded togetlier. These links are best realized by covalent bonds. Moreover, reaction kinetics favourable to polymeric materials must be fast, so tliat high-molecular-weight materials can be produced in a reasonable time. The polymerization reaction must also be fast compared to side reactions tliat often hinder or preclude tire fonnation of the desired product. 

The gas is passed through caustic soda solution to remove any sulphur dioxide or carbon dioxide produced in side reactions. Carbon monoxide is also obtained when an ethanedioate (oxalate) is heated with concentrated sulphuric acid ... 

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

Once a reaction has been performed, we have to establish whether the reaction took the desired course, and whether we obtained the desired structure. For our knowledge of chemical reactions is stiU too cursory there are so many factors influencing the course of a chemical reaction that we are not always able to predict which products will be obtained, whether we also shall obtain side reactions, or whether the reaction will take a completely different course than expected. Thus we have to establish the structure of the reaction product (Figure 1-4). A similar problem arises when the degradation of a xenobiotic in the environment, or in a living organism, has to be established. 

Alkyl and aryl iodides usually react with magnesium more rapidly than the corresponding bromides, and the bromides very much more rapidly than the chlorides. Aryl (as distinct from alkyl) chlorides have usually only a slow reaction with magnesium and are therefore very rarely used. With alkyl and aryl iodides in particular, however, a side reaction often occurs with the formation of a hydrocarbon and magnesium iodide ... 

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

The above reversible equation indicates that one mol of aluminium iso-propoxlde will reduce directly three mols of the carbonyl compound. It is generally desirable to use excess of the reductant except for aromatic aldehydes for the latter side reactions (e.g., 2RCHO-----> RCOOCH R Tischenko re-... 

Selenium. The substance is heated with a large excess of selenium at 280-350° for 36-48 hours. Better yields (and less side reactions) are usually obtained than with sulphur, but, owing to the higher temperature, rearrangements are more likely. Oxygen-containing groups are particularly prone to elimination. 

The derivative should be prepared preferably by a general reaction, which under the same experimental conditions would yield a definite derivative with the other individual possibilities. Rearrangements and side reactions should be avoided. 

Ketene formation is a common side reaction- scambling of chiral centers... 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

Which giyes formaldehyde as one of the starting materials. Base-catalysed reactions with this yery reacfiye aldehyde often giye poor yields because of polymerisation and other side reactions. The Marmich reaction is used instead ... 

In principal, synthesis route prediction can be done from scratch based on molecular calculations. However, this is a very difficult task since there are so many possible side reactions and no automated method for predicting all possible products for a given set of reactants. With a large amount of work by an experienced chemist, this can be done but the difficulty involved makes it seldom justified over more traditional noncomputational methods. Ideally, known reactions should be used before attempting to develop unknown reactions. Also, the ability to suggest reasonable protective groups will make the reaction scheme more feasible. 

More information has appeared concerning the nature of the side reactions, such as acetoxylation, which occur when certain methylated aromatic hydrocarbons are treated with mixtures prepared from nitric acid and acetic anhydride. Blackstock, Fischer, Richards, Vaughan and Wright have provided excellent evidence in support of a suggested ( 5.3.5) addition-elimination route towards 3,4-dimethylphenyl acetate in the reaction of o-xylene. Two intermediates were isolated, both of which gave rise to 3,4-dimethylphenyl acetate in aqueous acidic media and when subjected to vapour phase chromatography. One was positively identified, by ultraviolet, infra-red, n.m.r., and mass spectrometric studies, as the compound (l). The other was less stable and less well identified, but could be (ll). 

I ve found that unfortunately, there is a hyper oxydation of oleofin as side reaction, and gives organic acids, probably MDPhenylace-tic acid and may be a bit of piperonylic acid. It s easy to realise it. 

This is a way to do this procedure without having to use one of those crazy tube furnaces stuffed with thorium oxide or manganous oxide catalyst [21]. The key here is to use an excess of acetic anhydride. Using even more than the amount specified will insure that the reaction proceeds in the right direction and the bad side reaction formation of dibenzylketone will be minimalized (don t ask). 18g piperonylic acid or 13.6g phenylacetic acid, 50mL acetic anhydride and 50mU pyridine are refluxed for 6 hours and the solvent removed by vacuum distillation. The remaining residue is taken up in benzene or ether, washed with 10% NaOH solution (discard the water layer), and vacuum distilled to get 8g P2P (56%). 

Diethyl 3-oxoheptanedioate, for example, is clearly derived from giutaryl and acetic acid synthons (e.g. acetoacetic ester M. Guha, 1973 disconnection 1). Disconnection 2 leads to acrylic and acetoacetic esters as reagents. The dianion of acetoacetic ester could, in prin-ciple,be used as described for acetylacetone (p. 9f.), but the reaction with acrylic ester would inevitably yield by-products from aldol-type side-reactions. 

Then N-Boc-O-benzylserine is coupled to the free amino group with DCC. This concludes one cycle (N° -deprotection, neutralization, coupling) in solid-phase synthesis. All three steps can be driven to very high total yields (< 99.5%) since excesses of Boc-amino acids and DCC (about fourfold) in CHjClj can be used and since side-reactions which lead to soluble products do not lower the yield of condensation product. One side-reaction in DCC-promoted condensations leads to N-acylated ureas. These products will remain in solution and not reaa with the polymer-bound amine. At the end of the reaction time, the polymer is filtered off and washed. The times consumed for 99% completion of condensation vary from 5 min for small amino acids to several hours for a bulky amino acid, e.g. Boc-Ile, with other bulky amino acids on a resin. A new cycle can begin without any workup problems (R.B. Merrifield, 1969 B.W. Erickson, 1976 M. Bodanszky, 1976). 

The synthesis described met some difficulties. D-Valyl-L-prolyl resin was found to undergo intramolecular aminoiysis during the coupling step with DCC. 70< o of the dipeptide was cleaved from the polymer, and the diketopiperazine of D-valyl-L-proline was excreted into solution. The reaction was catalyzed by small amounts of acetic acid and inhibited by a higher concentration (protonation of amine). This side-reaction can be suppressed by adding the DCC prior to the carboxyl component. In this way, the carboxyl component is "consumed immediately to form the DCC adduct and cannot catalyze the cyclization. 

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