DEGREE OF UNSATURATION

Because of its double bond, an alkene has fewer hydrogens than an alkane with the same number of carbons—C H2n for an alkene versus C H2 +2 for an alkane—and is therefore referred to as unsaturated. Ethylene, for example, has the formula C2H4, whereas ethane has the formula C2H6. 

In general, each ring or double bond in a molecule corresponds to a loss of two hydrogens from the alkane formula C H2 +2. Knowing this relationship, it s possible to work backward from a molecular formula to calculate a molecule s degree of unsaturation—the number of rings and/or multiple bonds present in the molecule. 

Similar calculations can be carried out for compoimds containing elements other than just carbon and hydrogen. 

Copyii t2010Cengage Learning. All Rigjils Reserved. Maynotbeco[Hed,scanned,ordiq)licated,in whole or in part. Due to electronic rights, sane third party content may be sui ressed fron the eBook and/or eChapter(s). Editoial review has deemed diat any suppressed content does not materially affect the overall leamii experience. Cei age Leamii reserves the right to remove additional content at any time if subsequent rights restrictions require it. 

Calculate the degree of unsaturation in each of the following formulas, and then draw as many structures as you can for each  

Traditionally, the molecular formula of a compound was derived from elemental analysis and its molecular weight which was determined independently. The concept of the degree of unsaturation of an organic compound derives simply from the tetravalency of carbon. For a non-cyclic hydrocarbon i.e. an alkane) the number of hydrogen atoms must be twice the number of carbon atoms plus two, any deficiency in the number of hydrogens must be due to the presence of unsaturation, i.e. double bonds, triple bonds or rings in the structure. 

The degree of unsaturation can be calculated from the molecular formula for all compounds containing C, H, N, O, S or the halogens. There are 3 basic steps in calculating the degree of unsaturation  

Step 1 - take the molecular formula and replace all halogens by hydrogens 

Step 3 - for each nitrogen, omit the nitrogen and omit one hydrogen 

After these 3 steps, the molecular formula is reduced to CnHm and the degree of unsaturation is given by  

The task of drawing isomers for a particular formula can be made easier by comparing the number of hydrogens with the number of carbons in the formula. Such a comparison allows the calculation of the degree of unsaturation, which furnishes useful information about possible structures that will fit the formula (such as whether double bonds can be present) and provides a starting point for drawing isomers. 

Now let s see what happens to the number of hydrogens in a compound if a double bond is present. To form a double bond, a hydrogen must be removed from each of two adjacent carbons. Therefore, a compound with one double bond has two fewer H s than the maximum. 

Similarly, to form a ring from a chain of carbons, one H must be removed from both end C s so that they can be bonded together. A compound with a ring also has two fewer H s than the maximum number. 

Any compound whose formula has two hydrogens less than the maximum number possible (2n + 2) must contain one double bond or one ring. The total number of multiple bonds plus rings is called the degree of unsaturation (DU) and is equal to 1 for this case. The DU of a compound can be calculated by using the following formula  

The DU is very useful when drawing isomers. Let s look at a simple example. Suppose we are asked to draw the constitutional isomers with the formula C5H 0- The DU for C5H o is [(2(5) + 2) — 10]/2 = 1. Therefore, this compound must have one double bond or one ring. Any compound that has five C s and one double bond or one ring will fit the formula. Although C5H 2 has only three isomers, having two fewer H s actually increases the number of isomers, because there is now a double bond or a ring to vary. 

---

The term vitamin K2 was applied to 2-methyl-3-difarnesyl-l,4-naphthoquinone, m.p. 54 C, isolated from putrefied fish meal. It now includes a group of related natural compounds ( menaquinones ), differing in the number of isoprene units in the side chain and in their degree of unsaturation. These quinones also appear to be involved in the electron transport chain and oxidative phosphorylation. 

The tendency for a solvent to form deposits by polymerization of impurities such as olefins is measured by the test for potential gums . Olefin content can also be represented by the bromine index , which is a measure of the degree of unsaturation (see paragraph 3.4.1). 

The iodine number of fats and oils provides a quantitative measurement of the degree of unsaturation. A solution containing a 100% excess of IGl is added to the sample, reacting across the double-bonded sites of unsaturation. The excess IGl is converted to I2 by adding KI. The resulting I2 is reacted with a known excess of Na2S203. To complete the analysis the excess 8203 is back titrated with coulometrically generated I2. 

Hydrocarbon resins based on CPD are used heavily in the adhesive and road marking industries derivatives of these resins are used in the production of printing inks. These resins may be produced catalyticaHy using typical carbocationic polymerization techniques, but the large majority of these resins are synthesized under thermal polymerization conditions. The rate constants for the Diels-Alder based dimerization of CPD to DCPD are weU known (49). The abiHty to polymerize without Lewis acid catalysis reduces the amount of aluminous water or other catalyst effluents/emissions that must be addressed from an environmental standpoint. Both thermal and catalyticaHy polymerized DCPD/CPD-based resins contain a high degree of unsaturation. Therefore, many of these resins are hydrogenated for certain appHcations. 

Unsaturation Value. The reaction temperature, catalyst concentration, and type of counterion of the alkoxide affect the degree of unsaturation. The tendency for rearrangement of PO to aHyl alcohol is greatest with lithium hydroxide and decreases in the following order (100) Li+ >... 

Siace the mid 1950s, tall oil fatty acids (TOFA) have become available ia good quaUties and large quantities (see Carboxylic acids, fatty acids from TALL oil). Refined grades of TOFA have degrees of unsaturation rivaling that of soya acids. Siace it is a year-rouad by-product from the paper iadustry, its... 

Steroids (1) are members of a large class of lipid compounds called terpenes that are biogenicaHy derived from the same parent compound, isoprene, C Hg Steroids contain or are derived from the perhydro-l,2-cyclopentenophenanthrene ring system (1) and are found in a variety of different marine, terrestrial, and synthetic sources. The vast diversity of the natural and synthetic members of this class depends on variations in side-chain substitution (primarily at C17), degree of unsaturation, degree and nature of oxidation, and the stereochemical relationships at the ring junctions. 

The prostaglandins (qv) constitute another class of fatty acids with aUcycHc structures. These are of great biological importance and are formed by i vivo oxidation of 20-carbon polyunsaturated fatty acids, particularly arachidonic acid [27400-91-5]. Several prostaglandins, eg, PGE [745-65-3] have different degrees of unsaturation and oxidation when compared to the parent compound, prostanoic acid [25151 -18-9]. 

An important chemical characteristic of unsaturated acids is the iodine value (IV), which indicates the average degree of unsaturation. It is equal to the number of grams of iodine absorbed under standard conditions by 100 g of the unsaturated acid. 

The determination of iodine value (IV), AOCS Tg 1-64, is sometimes used to determine the extent of unsaturation. Because the tertiary aUyflc hydrogen ia the compounds is capable of substitution by halogen atoms, this only approximates a value for the degree of unsaturation. 

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. NMR is perhaps the most important physical method to ascertain the structure, especially the conformational statics and dynamics, of large heterocycles. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Details are found in Chapters 5.16-5.22. Some data on seven-membered rings are included in Table 10. 

BF4. In large rings the fate of the onium ions depends mostly on the structure and degree of unsaturation of the particular compound, and the onium salts range from completely stable to highly unstable. 

One of the most common modes of characterization involves the determination of a material s surface chemistry. This is accomplished via interpretation of the fiag-mentation pattern in the static SIMS mass spectrum. This fingerprint yields a great deal of information about a sample s outer chemical nature, including the relative degree of unsaturation, the presence or absence of aromatic groups, and branching. In addition to the chemical information, the mass spectrum also provides data about any surface impurities or contaminants. 

Forms of BR and polyisobutylene. The properties of butyl rubber and polyisobutylene depend on their moleeular weight, degree of unsaturation, nature of the stabilizer incorporated during manufacture and, in some cases, chemical modification. It is common to produce halogenated forms of butyl rubber to increase polarity and to provide a reactive site for alternate cure mechanisms [6],... 

Commercial data sheets generally provide the following properties for the resins softening point colour degree of unsaturation acid number saponification number density ash content odour. 

Degree of unsaturation. Unsaturation accounts for the existence of carbon-carbon double bonds in resins. It is generally indicated by the bromine or iodine number. Both methods are based on the halogen addition to the double carbon-carbon bonds. Because the different reactivity of bromine and iodine, both numbers cannot be compared. The bromine or iodine number does not necessarily correlate with the reactivity of the resin, for instance in the ageing process. However, within a given resin series of the same structure, relative comparisons can be made. 

Most rubbers used in adhesives are not resistant to oxidation. Because the degree of unsaturation present in the polymer backbone of natural rubber, styrene-butadiene rubber, nitrile rubber and polychloroprene rubber, they can easily react with oxygen. Butyl rubber, however, possesses small degree of unsaturation and is quite resistant to oxidation. The effects of oxidation in rubber base adhesives after some years of service life can be assessed using FTIR spectroscopy. The ratio of the intensities of the absorption bands at 1740 cm" (carbonyl group) and at 2900 cm" (carbon-hydrogen bonds) significantly increases when the elastomer has been oxidized [50]. 

The primary air flow rate per jet necessary for smokeless combustion depends on the molecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percent unsaturates, from a minimum of 20 % excess air for a flare gas containing 0 % unsaturates to 35 % excess air for a gas containing 67 mol % unsaturates. Based on this relationship and a gas flow rate of 72.2 mVh per jet, the required primary air flow rate can be computed directly from the gas composition, or approximated conservatively from the following equation ... 

As is true in the case of other phenolic lipids, urushiol is also a mixture of components varying mostly in the degree of unsaturation. Thus, the urushiol from Rhus vernicefera has structures shown in Fig. 6 [139]. Rhus toxicodendron is also known to give urushiol, but its... 

The classic chemical technique for measuring the degree of unsaturation in diene polymers is iodometry (iodine value) [102]. Kubo et al. [103] extensively measured the iodine value to determine the amount of residual double bonds present in the HNBR. However, this method exhibited significantly poorer precision as compared with IR and NMR spectroscopies [99-101]. Acid... 

These unsaturated alcohols act as monofunctional initiators, giving rise to terminally unsaturated PO-EO diblock impurities, which may be quantified by determining the degree of unsaturation in the final product. 

Biodiesel does not present any special safety concerns. Pure biodiesel or biodiesel and petroleum diesel blends have a higher flash point than conventional diesel, making them safer to store and handle. Problems can occur with biodiesels in cold weather due to their high viscosity. Biodiesel has a higher degree of unsaturation in the fuel, which can make it vulnerable to oxidation during storage. 

Although alkadienes have a higher degree of unsaturation than alkenes, their chemical behavior is similar to alkenes, and their physical properties are similar to alkanes containing the same number of carbon atoms. Common alkenyl groups include... 

ThomsonMOW Click Organic Interactive to practice calculating degrees of unsaturation. 

Let s assume that we want to find the structure of an unknown hydrocarbon. A molecular weight determination on the unknown yields a value of 82, which corresponds to a molecular formula of CfcHjo Since the saturated Q alkane (hexane) has the formula C61-114, the unknown compound has two fewer pairs of hydrogens (H]4 - H l() = H4 = 2 H2), and its degree of unsaturation is two. The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There s still a long way to go to establish structure, but the simple calculation has told us a lot about the molecule. 

---