The Nitrogen Rule

Nitrogen is an A + 1 element, while the natural abundance of 15N isotope constitutes about 0.4% of 14N. Since besides nitrogen only carbon contributes notably to the intensity of the A + 1 peak, it is possible to estimate the number of nitrogen atoms in the molecule on the basis of the nitrogen mle and the differences in the abundances of 13C ii 15N isotopes. 

The formulas analogous to that used for carbon may be applied to calculate the intensities of the isotopic peaks due to the presence of nitrogen atoms. The general formula for the intensity of the A + m peak (as a percentage of the A peak) for a compound containing n nitrogen atoms is represented by Equation 5.13  

When the nature and number of A + 2 and A + 1 elements in the molecule is known it is easy to derive conclusions about monoisotopic elements. As the masses of these elements are rather unique, it is not a difficult task. 

Establishing the elemental composition based on the isotopic peaks may be problematic if, for example, the sample contains impurities with the masses in the region of the molecular ion cluster. In the El mass spectra of amines, alcohols, acids, and some other classes of organic compounds there is often a peak of [M + H]+ ion. It distorts the isotopic picture. It is worth mentioning as well that in real experimental conditions the peak intensity may vary slightly in each 

Unknown 9. Estimate the number of carbon atoms in the following compounds using the intensities of the isotopic peaks. In the majority of cases the spectra are represented as m/z value (intensity to the base peak in the spectrum, %). The molecular peak is the first in the row. 

If you can establish the elemental composition for the proposed molecular ion, the rings-plus-double-bonds formula will show immediately if the ion is an odd- or even-electron species. For the general formula C Hj,N O , the value of X — + jz + 1 will be a whole number for any odd-electron ion, and end in  

For most elements encountered in organic compounds, there is a fortunate correspondence between the mass of an element s most-abundant isotope and its valence either both are even-numbered, or both are odd-numbered, with nitrogen as the major exception. This leads to the so-called nitrogen rule, which can be stated as follows  

If a compound contains no (or an even number of) nitrogen atoms, its molecular ion will be at an even mass number. An examples, the following molecules yield even-mass molecular ions  

An odd number of nitrogen atoms causes M to be at an odd mass number  

Thus if the ion of highest m/z is at an odd mass number, to be the molecular ion it must contain an odd number of nitrogen atoms. 

Another fact that can be used in determining the molecular formula is expressed as the Nitrogen Rule. This rule states that when the number of nitrogen atoms present in the molecule is odd, the molecular mass will be an odd number when the number of nitrogen atoms present in the molecule is even (or zero), the molecular mass will be an even number. The Nitrogen Rule is explained further in Chapter 3, Section 3.6. 

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A carbohydrate was isolated from a sample of cow s milk. The substance was found to have a molecular mass of 342. The unknown carbohydrate can be hydrolyzed to form two isomeric compounds, each with a molecular mass of 180. Using the Rule of Thirteen, determine a molecular formula and an index of hydrogen deficiency for the unknown and for the hydrolysis products. (Hint Begin by solving the molecular formula for the 180-amu hydrolysis products. These products have one oxygen atom for every carbon atom in the molecular formula. The unknown is lactose. Look up its structure in The Merck Index and confirm its molecular formula.) 

O Neil, M. J., et al., eds. The Merck Index, 15th ed., London, England Royal Society of Chemistry, 2013. Pavia, D. L., G. M. Lampman, G. S. Kriz, and R. G. Engel, Introduction to Organic Laboratory Techniques A Small Scale Approach, 3rd ed., Behnont, CA Brooks-Cole Cengage Learning, 2011. 

Lampman, G. S. Kriz, and R. G. Engel, Introduction to Organic Laboratory Techniques A Micro- 

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A relatively simple example is the nitrogen rule A molecule with an odd number of nitrogens has an odd molecular weight a molecule with only C H and O or with an even number of nitrogens has an even molecular weight... 

B In light of the nitrogen rule mentioned in molecular formula of pyridine, M+ = 79 ... 

The nitrogen rule of mass spectrometry says that a compound with an odd number of nitrogen atoms has an odd-numbered molecular weight. Thus, the presence of nitrogen in a molecule is detected simply by observing its mass spectrum. An odd-numbered molecular ion usually means that the unknown... 

The unknown gave a molecular ion at m/z 193 with fragment ions at m/zs 174, 148, and 42. From the abundance of the molecular ion, it is probably aromatic, and according to the Nitrogen Rule, contains at least one nitrogen atom. From accurate mass measurement data and an examination of the isotopic abundances in the molecular ion region, the molecular formula was found to be CnH15N02. 

In addition to the study of the isotopic cluster, useful information on analyte identity can be obtained by the nitrogen rule. It states that if a molecule contains an odd number of nitrogen atoms, its molecular weight must be an odd number. However, if an even number of nitrogen atoms or no nitrogen is present, its molecular weight must be an even number. 

Try to identify the molecular ion peak. Is it present in the spectrum Use four necessary conditions to consider a peak as representing M+. Do not forget about the nitrogen rule. 

Restricting to the more common elements in organic mass spectrometry (H, B, C, N, O, Si, S, P, F, Cl, Br, I, etc.), a simple rule holds valid With the exception of N, all of the above elements having an odd number of valences also possess an odd mass number and those having an even number of valences have even mass numbers. This adds up to molecular masses fulfilling the nitrogen rule (Tab. 6.7). 

Examples To rationalize the mass spectrum of methane, reactions 6.2-6.6 were proposed. They all obey the mle. You should check the mass spectra and fragmentation schemes throughout this chapter for additional examples of the nitrogen rule ... 

Note The first decimal of the m/z values of carbenium ions continuously rises with increasing m/z as a result of mass sufficiency of hydrogen (Chap. 3.3). Theoretically from C32H6s onwards m/z 449.5081), rounding of the m/z value causes a shift to the next integer m/z, and thus result in confiision due to the nitrogen rule. Therefore, integer m/z values should not be used above m/z 400 instead use of the decimal is recommended for LR-MS. 

The derived molecular formula must obey the nitrogen rule (Chap. 6.2.5). An odd-numbered m/z value of the molecular ion requires 1, 3, 5,. .. nitrogens to be contained, whereas an even m/z value belongs to 0, 2, 4,... nitrogens. 

If the compound contains or may contain nitrogen, the nitrogen rule is applicable. According to this rule, any molecule with an odd number of nitrogen atoms has an odd mass. For example, in the compound N(CH3)3 the mass is 59 g/mole. 

Don t forget to consider the nitrogen rule when interpreting the mass spectra of amides. (See Chapter 5 to review the nitrogen rule.)... 

Many peaks can be ruled out as possible molecular ions simply on grounds of reasonable structure requirements. The nitrogen rule is often helpful. It states that a molecule of even-numbered molecular weight must contain either no nitrogen or an even number of nitrogen atoms an odd-numbered molecular weight requires... 

For the nitrogen rule to hold, only unit atomic unit masses (i.e., integers) are used in calculating the formula masses. 

In many laboratories, the El spectrum and the Cl spectrum (with methane or isobutane) are obtained routinely since they are complementary. The Cl spectrum will frequently provide the [M + H]+ peak when the El spectrum shows only a weak or undetectable M + peak. The [M - H]+ peak may also appear in the Cl spectrum by hydride abstraction. The Cl fragmentation pattern is usually difficult to predict or rationalize. Note that the nitrogen rule (see Section 2) does not apply to the [M + H]+ or the [M - H]+ peaks neither does it apply to the Cl fragmentation ions. 

A very simple and useful rule to aid in the interpretation of the MS data of organic compounds relates to the fact that nitrogen has an even mass but an odd valency, whereas the valency and mass for the most abundant isotope of other elements are either both odd or both even. This gives us a very simple rule that we can immediately apply to any mass spectral data the nitrogen rule which is given in Box 5.2. 

The nitrogen rule helps us propose compositions for molecular ions If a compound has an odd number of nitrogen atoms—in addition to any number of C, H, halogens, O, S, Si, and P—then M+ has an odd nominal mass. For a compound with an even number of nitrogen atoms (0, 2, 4, and so on), M+ has an even nominal mass. A molecular ion at mlz 128 can have 0 or 2 N atoms, but it cannot have 1 N atom. 

In the chemical ionization mass spectrum of pentobarbital in Figure 22-4, the peak with the most significant intensity at the high end of the mass spectrum at mlz 227 is suspected to be MH+ If this is so. then the nominal mass of M is 226. The nitrogen rule tells us that a molecule with an even mass must have an even number of nitrogen atoms. If you know from elemental analysis that the compound contains only C, H. N. and O. how many atoms of carbon would you suspect are in the molecule ... 

The nitrogen rule requires that the molecular mass is always even when the number of nitrogen atoms is even or zero. This results from the fact that nitrogen has a different mass parity and valence electrons parity mass 14 u, five peripheral electrons. Both of these parities are identical in the case of any other atom. It should be noted that this holds only if we consider the mass of the predominant isotope. Thus, the chemical mass of bromine is 80 u, an even number, but its predominant isotope is that of mass 79 u, an odd mass. In the same way, isotopically labelled compounds do not always obey this rule. 

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