Nitrogen Rule Mass Spectrometer

University:
Kentucky State University
Course:
CHE 414 | Instrumental Analysis
Academic year:

2019
Views:

119

Pages:

29
Author:

Jerynclessyncpa6

Mass spectrometer An instrument that generates ions from molecules and measures their masses. The essential components of a mass spectrometer: 18 Electrical field accelerates particles toward magnetic region magnetic field separates particles according to their mass-to-charge ration 19 Electron impact EI • Vapor of sample is bombarded with electrons typically 70 eV • Advantages • inexpensive , versatile and reproducible • fragmentation gives structural information • large databases if El spectra exist and are searchable • Disadvantages • fragmentation at expense of molecular ion • sample must be relatively volatile 20 Resolution • • Resolution: A measure of how well a mass spectrometer separates ions of different mass. • low resolution: Refers to instruments capable of separating only ions that differ in nominal mass; that is ionș that differ by at least 1 or more atomic mass units. • high resolution: Refers to instruments capable of separating ions that differ in mass by as little as 0.0001 atomic mass unit. • A molecule with mass of 44 could be C,H. C,H,O, CO,, or CN H, If a more exact mass is 44.029. pick the correct structure from the table 21 C3H6O and C3H8O have nominal masses of 58 and 60, and can be distinguished by low-resolution MS. C3H8O and C2H4O2 both have nominal masses of 60. Distinguish between them by high-resolution MS. High resolution MS can replace elemental analysis for chemical formula confirmation 22 22 Most elemants have more than one stable isotope 23 Relative Isotope abundance of common elements 24 Easily recognized elements in ms Most elements occur naturally as a mixture of isotopes. The presence of significant amounts of heavier isotopes leads to small peaks that have masses that are higher than the parent ion peak. M+1 = a peak that is one mass unit higher than M M+2 = a peak that is two mass units higher than M+ M+4 -a peak that is four mass units higher than M+ The elements like. Cl. Br. S.I and N can be easily deducted from MS 25 Molecular ions and interpreting a mass spectrum nitrogen rule Molecular Ions and Interpreting a mass spectrum The only elements to give significant M + 2 peaks are Cl and Br. If no large M + 2 peak is present, these elements are absent. Is the mass of the molecular ion odd or even? Nitrogen Rule: If a compound has zero or an even number of nitrogen atoms, its molecular ion wilI have an even m/z value. an odd number of nitrogen atoms, its molecular ion will have an odd m/z value 26 Chlorine and bromine The most common elements giving rise to significant M +2 peaks are chlorine and bromine. Chlorine in nature is 75.77% Cl and 24.23% "CI. A ratio of M to M + 2 of approximately 3:1 indicates the presence of a single chlorine in a compound. 27 bromine Bromine in nature is 50.7%"Br and 49.3% "Br. A ratio of M to M +2 of approximately 1:1 indicates the presence of a single bromine in a compound. 28 sulfur Sulfur is the only other element common to organic compounds that gives a significant M + 2 peak. 2S = 95.02% and 4S = 4.21% ratio; M: M+2= 4:1 29 The presence of heavier isotopes The presence of heavier Isotopes tell us: Hydrocarbons contain 13C,1.1% will be a small M+1 peak. Br is present, M+2 is equal to M Cl present, M+2 is one-third of M. iodine is present, peak at 127, large gap. N is present, M will be an odd number S is present, M+2 will be 4% of M+. 30 Molecular formula as a clue to structure nitrogen rule Molecular Formula as a Clue to Structure It is difficult to assign an entire structure based only on the mass spectra. However, the mass spectra gives the mass and formula of the sample which is very important information Nitrogen rule: In general, "small" organic molecules with an odd mass must have an odd number of nitrogens. Organic molecules with an even mass have zero or an even number of nitrogens If the mass can be determined accurately enough, then the molecular formula can be determined (highresolution mass spectromeiry) 31 Determination of molecular formula from ms 32 Alkanes Strong molecular ion Fragmentation often splits off simple alkyl groups: - Loss of methyl - Loss of ethyl - Loss of propyl - Loss of butyl - Branched alkanes tend to fragment forming the most stable carbocations 33 34 More stable carbocations will be more abundant 35 Alkenes Fragmentation • Fairly prominent M+ • Fragments ion of • Terminal alkenes lose allyl cation if possible to form resonancestabilized allylic cations 36 Aromatic hydrocarbons fragmentation • Molecular ion usually strong • Alkyl benzene will often form intense ions am m/z 91 • Tropylium ion • 7-membered ring favored • Tropylium ion van fragment by successive losses of acetylene • Phenyl ions decompose the same way 37 Alcohol fragmentation • Fragment easily resulting in very small or missing parent ion peak May lose hydroxyl radical or water • M- 17 or M* - 18 • Commonly lose an alkyl group attached to the carbinol carbon forming an Oxonium in (largest R group lost as radical). • 1 alcohol usually has prominent peak at m/z 31 corresponding to H,C=OH • Molecular ion strength depends on substitution • primary alcohol secondary alcohol tertiary alcohol weak M VERY weak M* M' usually absent 38 39 Carbonyl compounds Fragmentation • Dominant fragmentation pathway are : • Cleavage • McLafferty rearrangement 40 aldehyde • Useful in distinguishing between aldehydes and ketones 41 • Fragmentation of aldehydes may form acylium ion 42 Fragmentation leads to formation acylium ion 43 • Mc-Lafferty Rearrangement • A hydrogen on a carbon 4 atoms away from the carbonyl oxygen is transferred • The "1,5 shift" in carbonyl-containing ions is called the McLafferty rearrangement • Creates a distonic radical cation (charge and radical separate) • 6-membered intermediate is sterically favorable • Such rearrangements are common • Once the rearrangement is complete, molecule can fragment by any previously described mechanism 44 45 46

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