Reaction yields

The amount of limiting reagent present at the start of a reaction determines the theoretical yield of the reaction, that is, the amount of product that would result if all the limiting reagent reacted. The theoretical yield, then, is the maximum obtainable yield, predicted by the balanced equation. In practice, the actual yield, or the amount of product actually obtained from a reaction, is almost always less than the theoretical yield. There are many reasons for the difference between actual and theoretical yields. For instance, many reactions are reversible, and so they do not proceed 100 percent from left to right. Even when a reaction is 100 percent complete, it may be difficult to recover all of the product from the reaction medium (say, from an aqueous solution). Some reactions are complex in the sense that the products formed may react further among themselves or with the reactants to form still other products. These additional reactions will reduce the yield of the first reaction. 

To determine how efficient a given reaction is, chemists often figure the percent yield, which describes the proportion of the actual yield to the theoretical yield. It is calculated as follows  

Percent yields may range from a fraction of 1 percent to 100 percent. Chemists sriive to maximize the percent yield in a reaction. Factors that can affect the percent yield include temperature and pressure. We will study these effects later. 

In Example 3.16 we will calculate the yield of an industrial process. 

Titanium is a strong, lightweight, corrosion-resistant metal that is used in rockets, aircraft, jet engines, and bicycle frames. It is prepared by the reaction of titanium(IV) chloride with molten magnesium between 950°C and 1150°C  

Percent yields may range from a tiny fraction to 100 percent. (They cannot exceed 100 percent.) Chemists try to maximize percent yield in a variety of ways. Factors that can affect percent yield, including temperature and pressure, are discussed in Chapter 15. Sample Problem 3.13 shows how to calculate the percent yield of a pharmaceutical manufacturing process. 

Aspirin, acetylsalicylic acid (C9H8O4), is the most commonly used pain reliever in the world. It is produced by the reaction of salicylic acid (C7H, 03) and acetic anhydride (C4H, 03) according to the following equation  

In a certain aspirin synthesis, 104.8 g of salicylic acid and 110.9 g of acetic anhydride are combined. Calculate the percent yield of the reaction if 105.6 g of aspirin are produced. 

Strategy Convert reactant grams to moles, and determine which is the limiting reactant. Use the balanced equation to determine the number of moles of aspirin that can be produced, and convert this number of moles to grams for the theoretical yield. Use the actual yield (given in the problem) and the calculated theoretical yield to calcrrlate the percent yield. 

Check Does your answer seem reasonable 18.71 moles of product are formed. What is the mass of 1 mole of (NH2)2CO  

Solution Starting with 18.71 moles of (1SIH2)2C0, we can determine the mass of CO2 that reacted using the mole ratio from the balanced equation and the molar mass of CO2. The conversion steps are 

The amount of CO2 remaining (in excess) is the difference between the initial amount (1142 g) and the amount reacted (823.4 g)  

Practice Exercise The reaction between aluminum and iron(in) oxide can generate 

In one process, 124 g of A1 are reacted with 601 g of Fc203. (a) Calculate the mass (in grams) of AI2O3 formed, (b) How much of the excess reagent is left at the end of the reaction  

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Here we consider uni- and bimolecular reactions yielding tln-ee different combinations. The resulting rate laws can all be integrated in closed fonn. 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Consequently, the reaction yield F in figure B2.5.15 is shown as a fiinction of the fluence, F. At the end of a laser-pulse sequence with a typical fluence F 3 J cm, practically 100% of the CF I is photolysed. As described in section B2.5.4.3. the product-level distribution of the iodine atoms fonned in this type of reaction can be detemiined... 

Further evidence that the nitronium ion was not the electrophile in the uncatalysed reaction, and yet became effective in the catalysed reaction, came from differences in the orientation of substitution. The nitration of chlorobenzene in the uncatalysed reaction yielded only 43 % of the para compound, whereas, when the catalysed reaction was made important by adding some nitric acid, the ratio of substitution was that usually observed in nitration involving the nitronium ion ( 5.3.4). In the case of the uncatalysed reaction however, the reaction was complicated by the formation of nitrophenols. 

The synthesis of five-, six-, and seven-membered cyclic esters or timides uses intramolecular condensations under the same reaction condifions as described for intermolecular reactions. Yields are generally excellent. An example from the colchicine synthesis of E.E. van Ta-melen (1961) is given below. The synthesis of macrocyclic lactones (macrolides) and lactams (n > 8), however, which are of considerable biochemical and pharmacological interest, poses additional problems because of competing intermolecular polymerization reactions (see p. 246ff.). Inconveniently high dilution, which would be necessary to circumvent this side-... 

Reaction takes place on nitrogen when the electrophilic center is an sp carbon, particularly if it is charged. Thus Mannich reaction yields the N-substituted compound (71 and 72) (Scheme 34) (54. 157-159). The same reaction is reported with piperidine, o-toluidine. and methylaniline (158). 

When unsubstituted, C-5 reacts with electrophilic reagents. Thus phosphorus pentachloride chlorinates the ring (36, 235). A hydroxy group in the 2-position activates the ring towards this reaction. 4-Methylthiazole does not react with bromine in chloroform (201, 236), whereas under the same conditions the 2-hydroxy analog reacts (55. 237-239. 557). Activation of C-5 works also for sulfonation (201. 236), nitration (201. 236. 237), Friede 1-Crafts reactions (201, 236, 237, 240-242), and acylation (243). However, iodination fails (201. 236). and the Gatterman or Reimer-Tieman reactions yield only small amounts of 4-methyl-5-carboxy-A-4-thiazoline-2-one. Recent kinetic investigations show that 2-thiazolones are nitrated via a free base mechanism. A 2-oxo substituent increases the rate of nitration at the 5-position by a factor of 9 log... 

Furfural can be oxidized to 2-furoic acid [88-14-2] reduced to 2-furanmethanol [98-00-0] referred to herein as furfuryl alcohol, or converted to furan by decarbonylation over selected catalysts. With concentrated sodium hydroxide, furfural undergoes the Cannizzaro reaction yielding both 2-furfuryl alcohol and sodium 2-furoate [57273-36-6]. 

The principal advantage to this method is that the heat evolved for each carbon—fluorine bond formed, 192.5 kj/mol (46 kcal/mol), is much less than that obtained in direct fluorination, 435.3 kJ/mol (104 kcal/mol). The reaction yields are therefore much higher and less carbon—carbon bond scisson occurs. Only two metal fluorides are of practical use, Agp2 and GoF. ... 

Nitio olefins can be made in some cases by dehydration of the aromatic nitrohydroxy derivatives. Subsequent reduction yields the aromatic amine. The following three-step reaction yielding 2-amino-l-phenylbutane illustrates the synthesis of this class of valuable pharmaceutical compounds. 

Modified oligonucleotides can be used to cross-link DNA sequences via a reactive group tethered to an oligonucleotide. When irradiated with uv light, psoralens (31) reacts with thymine bases, and the reaction yields a cross-link if the thymine residues are adjacent to each other on opposite strands. Psoralen linked to oligonucleotides have been shown to induce site-specific cross-links in vitro (51). 

A number of other valuable aroma chemicals can be isolated from essential oils, eg, eugenol from clove leaf oil, which can also, on treatment with strong caustic, be isomerked to isoeugenol, which on further chemical treatment can be converted to vanillin (qv). Sometimes the naturally occurring component does not requke prior isolation or concentration, as in the case of cinnamaldehyde in cassia oil which, on dkect treatment of the oil by a retro-aldol reaction, yields natural ben2aldehyde (qv). This product is purified by physical means. 

RM can be a traditional Grignard reagent or an organolithium, 2inc, aluminum, or mercury compound. The Grignard route is employed commercially for production of tertiary phosphines, even though these reactions are subject to side reactions. Yields are often low, eg, 40—50% for (C4H )2P prepared via a Grignard reaction (18). A phosphoms—carbon bond can form from the metathetical reaction of a phosphoms haUde and a pseudohaUde salt. 

The direct conversion of aniline into aminophenols may be achieved by hydrogen peroxide hydroxylation in SbE —HE at —20 to —40° C. The reaction yields all possible aminophenols via the action of H20" 2 on the anilinium ions the major product is 3-aminophenol (64% yield) (70,71). This isomer may also be made by the hydrolysis of 3-aminoaniline [108-45-2] in dilute acid at 190°C (72). Another method of limited importance, but useful in the synthesis of derivatives, is the dehydrogenation of aminocyclohexenones (73). 

Natural Products. Many natural products, eg, sugars, starches, and cellulose, contain hydroxyl groups that react with propylene oxide. Base-cataly2ed reactions yield propylene glycol monoethers and poly(propylene glycol) ethers (61—64). Reaction with fatty acids results ia a mixture of mono- and diesters (65). Cellulose fibers, eg, cotton (qv), have been treated with propylene oxide (66—68). 

Other important reactions yielding carbonyl sulfide foUow (33,34) ... 

The enhancement of the electrophilic properties of thaHium(III) ttifluoroacetate makes it a very important thaHation reagent. The products of thaHation, eg, arylthaHium bis(ttifluoracetate), undergo a variety of substitution reactions, yielding iodides, fluorides, nitriles, thiophenols, phenols, and biaryls. 

With acid anhydrides, the exothermic reaction yields similat products, subject to the limitation on n. For higher anhydride ratios, condensed acylates form. If n = 3, the result is (R 0Ti(00CR)2)20 if n = 4, then ((RC00)2Ti)20 forms. Phthahc anhydride does not give a cychc product ... 

Condensation of vinyl chloride with formaldehyde and HCl (Prins reaction) yields 3,3-dichloro-l-propanol [83682-72-8] and 2,3-dichloro-l-propanol [616-23-9]. The 1,1-addition of chloroform [67-66-3] as well as the addition of other polyhalogen compounds to vinyl chloride are cataly2ed by transition-metal complexes (58). In the presence of iron pentacarbonyl [13463-40-6] both bromoform [75-25-2] CHBr, and iodoform [75-47-8] CHl, add to vinyl chloride (59,60). Other useful products of vinyl chloride addition reactions include 2,2-di luoro-4-chloro-l,3-dioxolane [162970-83-4] (61), 2-chloro-l-propanol [78-89-7] (62), 2-chloropropionaldehyde [683-50-1] (63), 4-nitrophenyl-p,p-dichloroethyl ketone [31689-13-1] (64), and p,p-dichloroethyl phenyl sulfone [3123-10-2] (65). 

In a novel approach to vitamin K, Hoffmann-La Roche has exploited the potential acidity at C-3 as a means to attach the side chain of vitamin (36). Menadione was reacted with cyclopentadiene to yield the Diels-Alder adduct. The adduct is treated with base and alkylated at C-3 with phytyl chloride. A retro Diels-Alder reaction yields vitamin K. Process improvements in this basic methodology have been claimed by Japanese workers (37). 

Conversion of the C-2 amide to a biologically inactive nitrile, which can be further taken via a Ritter reaction (29) to the corresponding alkylated amide, has been accomphshed. When the 6-hydroxyl derivatives are used, dehydration occurs at this step to give the anhydro amide. Substituting an A/-hydroxymethylimide for isobutylene in the Ritter reaction yields the acylaminomethyl derivative (30). Hydrolysis affords an aminomethyl compound. Numerous examples (31—35) have been reported of the conversion of a C-2 amide to active Mannich adducts which are extremely labile and easily undergo hydrolysis to the parent tetracycline. This reverse reaction probably accounts for the antibacterial activity of these tetracyclines. 

With Alcohols, Ethers, and Esters. Carbon monoxide reacts with alcohols, ethers, and esters to give carboxyHc acids. The reaction yielding carboxyHc acids is general for alkyl (53) and aryl alcohols (54). It is cataly2ed by rhodium or cobalt in the presence of iodide and provides the basis for a commercial process to acetic acid. 

Ureas. Chlorination of aqueous urea yields unstable A/-chloro compounds. With excess C10 decomposition yields CO2, N2O, and NCl the latter decomposes further to NO (96). Only two soHd derivatives have been isolated A/-chlorourea [3135-74-8] mp 74—76°C, and N,lSf-dichlorourea [2959-01-5] which decomposes at its mp of 83°C with evolution of NCl. As an amide, urea also undergoes the Hofmann reaction yielding hydrazine. This route to hydrazine was once employed commercially. 

A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

Diaziridines (156) unsubstituted on both nitrogens decompose at 125 °C by a redox reaction yielding one mole of a diazirine (157) together with two moles of ammonia and one mole of ketone from two moles of (156). The reaction proceeds below 60 °C when copper salts are present (64AG(E)229). 

Oxidation of trimethylene sulfide catalyzed by tungstic acid is preferred to the uncatalyzed reaction Yields are better and the reaction time... 

Under the best conditions, sodium cyclopentadienide gives pale yellow or orange solutions. Traces of air lead to red or purple solutions, as does insufficiently purified solvent, without, however, lowering the reaction yield appreciably. If 1,2-di-methoxyethane is used, in which sodium cyclopentadienide is less soluble than in tetrahydrofuran, white crystals may be obtained at this point. 

Middleton, J. C., Pierce, F., and Lynch, P. M., Computations of Flow Fields and Complex Reaction Yield in Turbulent Stirred Reactors and Comparison with Experimental Data, Chem. Eng. Res. Des., Vol. 64, pp. 18-21, 1986. 

Recently, Okahara and his co-workers have investigated a variety of one-pot crown ether syntheses which are referred to in Sects. 3.4, 4.3, 5.4. During the course of these investigations, they examined the temperature dependence of the cyclization yield . Using either sodium hydroxide or potassium hydroxide and forming 15-crown-5, 18-crown-6 and 21-crown-7, an attempt was made to correlate yield and reaction temperature. For most of the reactions, yield was recorded over the range from 20 ° — 120 °C... 

The true, all-aromatic system (see 18, below) described by Kime and Norymberski is unusual in the sense that all of the ether linkages bridge aromatic carbons ". Synthesis of 18, therefore, required extensive use of copper mediated coupling reactions. As expected for such reactions, yields were generally low. The aromatics such as 18 were ineffective at binding either alkali metal or ammonium cations ". ... 

Shock-modified zirconia powder was reacted with lead oxide in controlled differential thermal analysis (DTA) experiments and compared to the unmodified material by Hankey and co-workers [82H01]. This reaction yields... 

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