Acetates a-functionalization

Since water adds to (at least some) cafbonyl compounds, it should come as no surprise that alcohols do too. The product of the reaction is known as a hemiacetal, because it is halfway to an acetal, a functional group, which you met in Chapter 2 (p. 35) and which will be discussed in detail in Chapter 14. The mechanism follows in the footsteps of hydrate formation just use ROll instead of HOH. 

Figure C2.1.9. Specific voiume of poiy (vinyi acetate) as a function of tiie temperature measured during heating two sampies which were preiiminary quenched from tiie meit to -20 °C. One sampie was stored for i min and tiie otiier for iOO h at -20 °C before heating. (Figure from [77], reprinted by pennission of Joim Wiiey and Sons Inc).

Acetate and triacetate exhibit moderate changes in mechanical properties as a function of temperature. As the temperature is raised, the tensile modulus of acetate and triacetate fibers is reduced, and the fibers extend more readily under stress (see Fig. 4). Acetate and triacetate are weakened by prolonged exposure to elevated temperatures in ah (see Fig. 5). 

Carbon Cha.in Backbone Polymers. These polymers may be represented by (4) and considered derivatives of polyethylene, where n is the degree of polymeriza tion and R is (an alkyl group or) a functional group hydrogen (polyethylene), methyl (polypropylene), carboxyl (poly(acryhc acid)), chlorine (poly(vinyl chloride)), phenyl (polystyrene) hydroxyl (poly(vinyl alcohol)), ester (poly(vinyl acetate)), nitrile (polyacrylonitrile), vinyl (polybutadiene), etc. The functional groups and the molecular weight of the polymers, control thek properties which vary in hydrophobicity, solubiUty characteristics, glass-transition temperature, and crystallinity. 

Membrane stmcture is a function of the materials used (polymer composition, molecular weight distribution, solvent system, etc) and the mode of preparation (solution viscosity, evaporation time, humidity, etc). Commonly used polymers include cellulose acetates, polyamides, polysulfones, dynels (vinyl chloride-acrylonitrile copolymers) and poly(vinyhdene fluoride). 

Fig. 5. Composition of cellulose acetate butyrate (propionate) as a function of butyryl (propionyl) content of esterification bath.

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatihty is largely determined by the solvent concentration and hence the S/F ratio. The relative volatility tends to increase as the S/F ratio increases. Thus, a given separation can be accomplished in fewer equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

FIG. 22-76 Constant -cost lines as a function of permeability and selectivity for CO2/CH4, Cellulose-acetate membrane mscf is one thousand standard cubic feet, CouHesy VP R. Grace.)... 

Furthermore, the stereochemistry of the product 1 changes as the solvent is changed. In aqueous dioxane, the reaction proceeds with complete inversion, but in 1,1,1,3,3,3 hexafluoro-2-propanol with 100% retention. In acetic acid, the reaction occurs mainly with inversion (83%), but in formic acid the amount of retention (40%) is comparable to the amount of inversion (60%). Discuss these results, particularly with respect to the change of product composition and stereochemistry as a function of solvent. 

The mechanism of this degradation has received considerable attention, and for some species the reaction is equivalent to a nonenzymatic Baeyer-Villager reaction, producing first the 17j8-acetate. This functionality can then be hydrolyzed and oxidized to the ketone and may undergo a second Baeyer-Villager reaction to produce a lactone ... 

The reaction of the iV-acetylenanime with perbenzoic acid at low temperature is much faster than attack at a A -double bond, so the latter can be preserved without prior protection. In addition, the A-acetyl compound is relatively stable to weak alkali and to Oppenauer conditions permitting, e.g., hydrolysis of a 3-acetate group and conversion to a 3-keto-A" function. These latter compounds also react selectively with perbenzoic acid at the A -double bond. 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

However, for the preparation of derivatives which contain a functional group directly attached to position 6, the application of the foregoing cyclization method is considerably limited by the availability or existence of the required derivatives of -keto acids and may also be affected by differences in their reactivity. Cyclization of thiosemicarbazones was, therefore, used for these substances only in the case of the 6-carboxylic acid (see also Section II,B,2,a). Of the other derivatives known, the 6-acetic acid ester should be mentioned. Recently some further derivatives of dioxotriazine-6-carboxylic acid were reported. ... 

A wide range of polymer networks are constructed in this manner. Poly(vinyltrichloacetate) was used as the coinitiator with styrene, MMA and chloroprene as cross-linking units. Polycarbonates, polystyrene, N-haloge-nated polyamide, polypeptides, and cellulose acetate, suitably functionalized, have been used as a coinitiator... 

Acetal (Section 19.10) A functional group consisting of two -OR groups bonded to the same carbon, R2C(OR )2-Acetals are often used as protecting groups for ketones and aldehydes. 

Plot an appropriate volume unit vs. mole fraction of acetic acid. Determine the partial molar volumes of water and acetic acid at X2 = 0, 1, and several intermediate compositions (at least three). Plot V and V2 as a function of. Y2. 

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