Acetyl groups

NHCOCH3. White crystals, m.p. 18l-l83"C. Soluble sulphacetamide is the sodium salt which is soluble in water. Il is prepared by acetylating sulphanilamide and hydrolysing one acetyl group. Being more soluble than most of the sulphonamides it is used in treating infections of the urinary tract and of the conjunctiva. 

For the determination of the number of acetyl groups in pentacetyl-glucose, see p. 456,... 

Note that p-acetamidobenzenesulphonyl chloride will similarly react with primary and secondary amines, and the products, after hydrolysis of the acetyl group, may furnish notable drugs e.g., the condensation products with 2-amino-pyridine and 2-aminothiazole, after remo al of the acetyl groups, provide the drugs commonly known as sulphapyridine (M B 693) and sulphathidzole respectively. 

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

Estimation of the Number of Acetyl Groups in an Acetyl Ester. 

I. DETERMINATION OF NUMBER OF ACETYL GROUPS IN TRIACETIN (TRIACETYL-GLYCEROL) C3H6(O COCH3)3. Molecular Weight, 218. 

The a-carbon atom of the phenylacetyl group is more susceptible to attack by the basic catalyst (triethylamine) than the acetyl group hence a-phenyl-cinnamic acid, but no cinnamic acid, is obtained. 

The acetyl group decreases the basicity of the corresponding 2-aminothiazole by about 2 pK units (71, 474), and it increases the acidity... 

Because the position of electrophilic attack on an aromatic nng is controlled by the direct ing effects of substituents already present the preparation of disubstituted aromatic com pounds requires that careful thought be given to the order of introduction of the two groups Compare the independent preparations of m bromoacetophenone and p bromoace tophenone from benzene Both syntheses require a Friedel-Crafts acylation step and a bromination step but the major product is determined by the order m which the two steps are carried out When the meta directing acetyl group is introduced first the final product IS m bromoacetophenone... 

The key here is to recognize that an ethyl substituent can be introduced by Fnedel-Crafts acylation followed by a Clemmensen or Wolff-Kishner reduction step later in the syn thesis If the chlorine is introduced prior to reduction it will be directed meta to the acetyl group giving the correct substitution pattern... 

Amide resonance within the N acetyl group competes with delocalization of the nitro gen lone pair into the ring... 

All the individual steps are catalyzed by enzymes NAD" (Section 15 11) is required as an oxidizing agent and coenzyme A (Figure 26 16) is the acetyl group acceptor Coen zyme A is a thiol its chain terminates m a sulfhydryl (—SH) group Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A... 

We can descnbe the major elements of fatty acid biosynthesis by considering the for mation of butanoic acid from two molecules of acetyl coenzyme A The machinery responsible for accomplishing this conversion is a complex of enzymes known as fatty acid synthetase Certain portions of this complex referred to as acyl carrier protein (ACP), bear a side chain that is structurally similar to coenzyme A An important early step m fatty acid biosynthesis is the transfer of the acetyl group from a molecule of acetyl coenzyme A to the sulfhydryl group of acyl carrier protein... 

When both building block units are m place on the acyl carrier protein carbon-carbon bond formation occurs between the a carbon atom of the malonyl group and the carbonyl carbon of the acetyl group This is shown m step 1 of Figure 26 3 Carbon-carbon bond formation is accompanied by decarboxylation and produces a four carbon acetoacetyl (3 oxobutanoyl) group bound to acyl earner protein... 

Step 1 An acetyl group is transferred to the a carbon atom of the malonyl group with evolution of carbon dioxide Presumably decarboxylation gives an enol which attacks the acetyl group... 

Cellulose Acetate, Propionate, and Butyrate. Cellulose acetate is prepared by hydrolyzing the triester to remove some of the acetyl groups the plastic-grade resin contains 38 to 40%... 

Cellulose triacetate is obtained by the esterification of cellulose (qv) with acetic anhydride (see Cellulose esters). Commercial triacetate is not quite the precise chemical entity depicted as (1) because acetylation does not quite reach the maximum 3.0 acetyl groups per glucose unit. Secondary cellulose acetate is obtained by hydrolysis of the triacetate to an average degree of substitution (DS) of 2.4 acetyl groups per glucose unit. There is no satisfactory commercial means to acetylate direcdy to the 2.4 acetyl level and obtain a secondary acetate that has the desired solubiUty needed for fiber preparation. 

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