Chapter 8 Naming Organic Compounds（2）

Organosulfur and Organoselenium Compounds Organosulfur and organoselenium compounds are chemical compounds containing carbon-to-sulfur and carbon-to-selenium chemical bonds, respectively. Thiols (RSH) and selenols (RSe H) are the sulfur and selenium equivalents of alcohols. These compounds are relatively unstable and usually have an unpleasant smell. Phenylselenol or selenaphenol, Ph Se H (p Ka=5.9), is more acidic than thiophenol (p Ka=6.5), and it also oxidizes more easily to the diselenide. Diselenides R-Se-Se-R’ are the selenium equivalents of peroxides and disulfides, and they are used in organic chemistry as starting materials for selenols and selenenyl halides such as RSe-Cl or R-Se-Br. Selenides R-Se -R’ are the selenium equivalents of ethers and thioethers, and they are also the organic counterparts of inorganic Selenides. These compounds are ambiphilic due to the identical electronegativities of carbon and selenium, and they can react either as a nucleophile or as an electrophile. Selenoxides R-Se(O) -R’ are the selenium equivalents of sulfoxides and can be further oxidized to selenones R-Se(O)-OR’, which are similar to sulfones with sulfur replaced again by selenium.

Nomenclature of Heterocyclic Compounds compounds because there exists many common names in addition to systematic nomenclature that is internationally agreed-upon. Many heterocycles, in particular, amines, were identified early on, and given trivial names which are still in use preferably. The following chart shows some monocyclic compounds of this kind. The common (trivial) name is in black, and a systematic name based on the Hantzsch-Widman system is also given beneath it in blue. The types of heteroatoms present in a ring are indicated by prefixes; in particular, “oxa-”, “thia-”, “aza-”, “selena-”, “phospha-”, “sila-” and“bora-” denote oxygen, sulfur, nitrogen, selenium, phosphorous, silicon and boron atoms, respectively. The number of heteroatoms of a particular kind is indicated by numerical prefixes joined to the heteroatom prefixes, such as “dioxa-” and “triaza-”. If there are different kinds of heteroatoms, they are indicated by combining the above prefixes, with the following order of preference:“oxa-” first, followed by “thia-”, and then “aza-”.

Polycyclic compounds containing one or more heterocyclic rings are well-known. A few of these are shown in the following diagram. Similarly, common names appear in black and systematic names in blue.

Carbohydrate Nomenclature “-ose” denotes a sugar and “-ulose” is the suffix for a keto sugar. The monosaccharides with 3, 4, 5, 6, and 7 carbons use prefixes such as“tri-”, “tetra-”, “penta-”, “hexa-”, and “hepta-” to denote the number. “Aldo-” refers to the aldehyde and “keto-” to the ketone group in monosaccharides. The term aldopentose denotes two structural features, chain length and carbonyl group. Ribose is an aldopentose while fructose is a ketohexose.

Aldo sugars always have the “carbonyl” group on C-l, while keto sugars have it on C-2. This terminology does not indicate the orientation of the -OH groups. What distinguishes one sugar from another is the right-left orientations of internal -OH groups. D-glucose, an aldohexose, is shown in the first diagram of the following figure. The sticks represent the internal -OH groups. Because a - CH2OH group is common to all sugars, it is not necessary to draw this group each time.

The left/right orientation of the -OH group on the last chiral carbon shown by the thick stick line determines if it is a D- or L-sugar. L-glucose is the mirror image of D-glucose. The sugars can be learned by focusing on chiral center orientations. The only way to represent keto sugars is by sticks, with a double bond to show the keto group, as shown for D-fructose in the above figure.

The orientation of the -OH group is used to pinpoint a specific sugar’s name. Internal configurations have prefixes of their own, such as gluco, manno, galacto, fructo, etc. Note that D-glucose is different from D-galactose by orientation around C-4 and from D-mannose at C-2. In the pentoses, ribose has all of its -OH groups on the same side. In the tetroses the variation is in the orientation around C-2. Fructose stands out in that the keto group is on C-2, with C-3 to C-6 of fructose having the same configuration as glucose.

Recall, aldohexoses have 4 chiral centers, or 16 (24) stereoisomers possible. Does that mean 16 individual sugar names? No. When we denote aldohexoses as D or L, only 8 (23) D-steroisomers are possible. Because the D, L designation fixes one of the centers, for 8 D and 8 L of the 16, they will have the same name. Common aldohexoses are D-glucose, D-mannose, and D-galactose. Know these and D-fructose. Based on the same rule, there are four D-aldopentoses and two D-aldotetroses. The common D-aldopentoses are D-ribose and D-xylose, which is pronounced zy-lose. The aldotetroses are represented by D-erythrose and D-threose. Similar to D-ribose, D-erythrose has all-OH groups on the right, while D-threose has one internal-OH on the right and one on the left.

Oligosaccharides are carbohydrates that are composed of 2-10 monosaccharide residues joined through glycosidic linkage, which can react with acid and be hydrolyzed to produce the constituent monosaccharide units. Some oligosaccharides, e.g., maltose, lactose, and sucrose, were trivially named before their chemical constitution was determined, and these names are still used today.

Oligosaccharides are named from left to right as glycosyl [glycosyl]n glycoses or glycosyl [glycosyl]n glycosides, depending on whether the reducing end is a free hemiacetal group. Appropriate symbols are used, for example, a or (3 to indicate the stereochemistry of the glycosidic bonds, D or L to indicate the configuration of the monosaccharide residue, and O to indicate the substitutions at oxygen atoms.

Synthetic Polymers Naming of polymers (or oligomers) is done with the prefix poly (or oligo) followed in parentheses or brackets by the name of the constitutional repeating unit (CRU). For example, if the name of the CRU is “ABC”, the corresponding name for the polymer (or oligomer) is poly(ABC) or oligo(ABC).

Note that if it is necessary to specify the chain length of an oligomer, it is possible to use the appropriate Greek prefix “deca-”, “docosa-”, etc. [e.g.-(-OCH2CH2-)10, deca(oxyethylene)].

The names of CRU can be formed by using the names of its subunits, including substituents (substituted subunits), citing them in order from left to right as they appear in the CRU.