Again, the molecular ion peak is an odd number. The molecular ion at m/e=86 is weaker than that for hexane itself and the M-15 ion at m/e=71 is stronger. Loss of water from this gives a m/e=41 fragment, and loss of ethene from m/e=59 gives a m/z=31 fragmentThe molecular ion (m/e=90) is strong, and the presence of sulfur is indicated by a larger than usual M+2 (m/e=92) peak. On the other hand, the presence of Iodine is detected by the loss of 127 while the presence of flourine is observed by the loss of 19 (F+) or 20 (HF) amu. The molecular ion at m/e=84 is much stronger than the corresponding ions in acyclic compounds. The mass spectrum of cyclohexane has an abundant ion of m/e 56 arising from the loss of ethylene.The mass apectra of three different saturated hydrocarbons are displayed below. The fragmentation patterns of cycloalkanes may show mass clusters in a homologous series, as for the alkanes due to the presence of a straight chain hydrocarbon side chain.Thus the fragmentation of cycloalkanes is charecterized by the loss of ethylene from the parent molecule or from intermediate radical-ions. *Pentanal shosws a a typical fragmentation pattern of alkyl hydrocarbon chain (e.g. 8-Halides:As has ben discussed in details in lecture two, the presence of chlorine or bromine atoms is usually recognizable from isotopic peaks. The patterns resulting from aromatic ketones are almost identical to those governing aromatic aldehydes. The odd-electron rearrangement ion at m/e=62 results from loss of ethene. The molecular ion is the bas peak due to stabilization of the formed cation.4-Aldehydes and ketones:Major pfragmentation peaks of aliphatic carbonyls result from the cleavage of the C-C bons adjacent to the carboxyl group results in the loss of hydrogen (molecular ion less 1) or the loss of CHO (molecular ion less 29).Shown below are three example of aliphatic carbonyl compounds *Pentanal shosws a a typical fragmentation pattern of alkyl hydrocarbon chain (e.g. *molecular ion at m/e=86 is about as strong as in 2-pentanone. Energy sufficient to break chemical bonds: radical cation will usually However, the molecular ion often fragments further to smaller fragment which gives a fragmentaion pattern characterstic for each compound (assuming that ionization conditions are constant). The ArC≡O fragment loses CO to form the phenyl ion at m/z 77 that further degrades to give a peak at m/z 51. 4-Amides:The molecular ion is usually observable, and will be a good indication of the presence of an amide (the nitrogen rule is applicable). Orgenic compounds belonging to the same homologous series often exhibit simillar fragmentation patterns with slight variations arising from the difference in the substituents on the molecule. The directive effect of the carbonyl group on fragmentation is also apparent here. When observable, its odd mass (when an odd number of nitrogens is present) is a good indication of the presence of an amine (nitrogen rule). The fragment ion at m/e=55 is probably due to a methyl radical loss from the m/e=70 ion. Taking the Stress Out of Learning Science, Complete Summary of Organic Reactions (downloadable), All videos, study guides, and quizzes for chapters 1 and 2, DAT Practice Exams (free for a limited time), OAT Practice Exams (free for a limited time), Chad’s High School Chemistry Master Course, Chad’s Organic Chemistry Refresher for the ACS Final Exam, 14.6c Fragmentation Patterns of Ketones and Aldehydes, Chapter 1 – Electrons, Bonding, and Molecular Properties, 1.3 Valence Bond Theory and Hybridization, Chapter 2 – Molecular Representations and Resonance, 4.6 Cycloalkanes and Cyclohexane Chair Conformations, 5.2 Absolute Configurations | How to Assign R and S, 5.3 Molecules with Multiple Chiral Centers, 5.5 Determining the Relationship Between a Pair of Molecules, 5.6 Amine Inversion and Chiral Molecules Without Chiral Centers, Chapter 6 – Organic Reactions and Mechanisms, 6.1 Reaction Enthalpies and Bond Dissociation Energies, 6.2 Entropy, Gibbs Free Energy, and the Equilibrium Constant, 6.4 Nucleophiles, Electrophiles, and Intermediates, 6.5 Reaction Mechanisms and Curved Arrow Pushing, Chapter 7 – Substitution and Elimination Reactions, 7.4 Introduction to Elimination Reactions [Zaitsev’s Rule and the Stability of Alkenes], Chapter 8 – Addition Reactions to Alkenes, 8.1 Introduction to Alkene Addition Reactions, 8.3b Hydration Oxymercuration Demercuration, 8.4a Acid Catalyzed Addition of an Alcohol, 8.8 Predicting the Products of Alkene Addition Reactions, 8.9 Oxidative Cleavage Ozonolysis and Permanganate Cleavage, 9.5 Introduction to Addition Reactions of Alkynes, 10.2 Free Radical Chlorination vs Bromination, 10.3 The Mechanism of Free Radical Halogenation, 10.4 Allylic and Benzylic Bromination Using NBS, 10.5 Hydrobromination of Alkenes with Peroxide, 11.2 Increasing the Length of the Carbon Skeleton, 11.3 Decreasing the Length of the Carbon Chain or Opening a Ring, 11.4a Common Patterns in Synthesis Part 1, 11.4b Common Patterns in Synthesis Part 2, 11.4c Common Patterns in Synthesis Part 3, 11.4d Common Patterns in Synthesis Part 4, 12.1 Properties and Nomenclature of Alcohols, 12.3a Synthesis of Alcohols; Reduction of Ketones and Aldehydes, 12.3b Synthesis of Alcohols; Grignard Addition, Chapter 13 – Ethers, Epoxides, Thiols, and Sulfides, 13.1 Introduction to Nomenclature of Ethers, 13.7 Nomenclature, Synthesis, and Reactions of Thiols, 13.8 Nomenclature, Synthesis, and Reactions of Sulfides, Chapter 14 – IR Spectroscopy and Mass Spectrometry, 14.2b The Effect of Conjugation on the Carbonyl Stretching Frequency, 14.5 Isotope Effects in Mass Spectrometry, 14.6a Fragmentation Patterns of Alkanes, Alkenes, and Aromatic Compounds, 14.6b Fragmentation Patterns of Alkyl Halides, Alcohols, and Amines, 15.4 Homotopic vs Enantiotopic vs Diastereotopic, 15.5a The Chemical Shift in C 13 and Proton NMR, 15.5b The Integration or Area Under a Signal in Proton NMR, 15.5c The Splitting or Multiplicity in Proton NMR, 15.6d Structural Determination From All Spectra Example 4, 15.6e Structural Determination From All Spectra Example 5, 16.1 Introduction to Conjugated Systems and Heats of Hydrogenation, 16.2a Introduction to Pi Molecular Orbitals Ethylene, 16.2b Pi Molecular Orbitals 1,3 Butadiene, 16.2c Pi Molecular Orbitals the Allyl System, 16.2d Pi Molecular Orbitals 1,3,5 Hexatriene, 16.4 Addition Reactions to Conjugated Dienes, 16.5a Introduction to Diels Alder Reactions, 16.5b Stereoselectivity and Regioselectivity in Diels Alder Reactions, 16.5c Diels Alder Reactions with Cyclic Dienes, 16.5d Conservation of Orbital Symmetry in Diels Alder Reactions, 17.2b Aromatic vs Nonaromatic vs Antiaromatic, 17.3 The Effects of Aromaticity on SN1 Reactions and Acidity Basicity, 17.4 Aromaticity and Molecular Orbital Theory, Chapter 18 – Reactions of Aromatic Compounds, 18.1 Introduction to Aromatic Substitution Reactions, 18.2d EAS Friedel Crafts Alkylation and Acylation, 18.2e EAS Activating and Deactivating Groups and Ortho Para and Meta Directors, 18.2f EAS Predicting the Products of EAS Reactions, 18.3 Catalytic Hydrogenation and the Birch Reduction, 18.4a Side Chain Oxidation with Permanganate or Chromic Acid, 18.4c The Clemmensen and Wolff Kishner Reductions, 19.1 Nomenclature of Ketones and Aldehydes, 19.3 Introduction to Nucleophilic Addition Reactions, 19.5b Cyclic Acetals as Protecting Groups, 19.6a Addition of Primary Amines Imine Formation, 19.6b Addition of Secondary Amines Enamine Formation, 19.6c Mechanism for the Wolff Kishner Reduction, 19.9a Addition of Acetylide Ions and Grignard Reagents, 19.9b Addition of HCN Cyanohydrin Formation, Chapter 20 – Carboxylic Acids and Acid Derivatives, 20.1 Introduction to and Physical Properties of Carboyxylic Acids and Acid Derivatives, 20.3 Introduction to Nucleophilic Acyl Substitution, 20.4 Reaction with Organometallic Reagents, 20.6 Interconversion of Carboxylic Acids and Derivatives, 20.7 The Mechanisms of Nucleophilic Acyl Substitution, 20.9 Synthesis and Reactions of Acid Anhydrides, 20.11 Synthesis and Reactions of Carboxylic Acids, 20.13 Synthesis and Reactions of Nitriles, Chapter 21 – Substitution Reactions at the Alpha Carbon, 21.2 General Mechanisms of Alpha Substitution Reactions, 22.4b Synthesis of Amines Hofmann Rearrangement, 22.4c Synthesis of Amines Curtius Rearrangement and Schmidt Reaction, 22.4d Synthesis of Amines Gabriel Synthesis, 22.4e Synthesis of Amines Reductive Amination, 22.8a Reaction with Nitrous Acid and the Sandmeyer Reactions, 22.9 EAS Reactions with Nitrogen Heterocycles, FREE Trial -- Chad's Ultimate Organic Chemistry Prep. Two are isomeric hexanes and the third is cyclohexane.The following three examples are hydrocarbons having no functional groups. *The molecular ion at m/e=86 is more abundant than in the previous aldehyde spectrum. The presence of an [M – 1] peak helps to identify the aldehyde but the [M – R] peak (m/z 29) is not unique to this type of compound. Loss of CO from this fragment ion gives the aryl ion (m/e 77 in the case of acetophenones). These comprise the most prominent fragment ions. 6.11.4 Fragmentation of Aldehydes. The alkenyl cations at m/e=41 & 27 are stronger than the corresponding alkyl cations (m/e=43 & 29). Alkylated polyphenyls and alkulated polycyclic aromatic hydrocarbons exhibit the same beta-cleavage as alkyl benzenes. The m/e=57 butyl cation (M-29) is the base peak, and the m/e=43 and 29 ions are also abundant.Chain branching clearly influences the fragmentation of this isomeric hexane. The positive charge is usually retained by the smaller fragment, so we see a homologous series of hexyl (m/z = 85), pentyl (m/e = 71), butyl (m/e = 57), propyl (m/e = 43), ethyl (m/e = 29) and methyl (m/e = 15) cations. Thelargest substituent is expelled readily.Primary alcohols, in addition to the principle C-C cleavage nest to the oxygen atom, shows a homologous series of peaks of progressively decreasing intensity resulting from cleavage at C-C bonds successively removed from the oxygen giving a pattern simmilar to that produced by long chain hydrocarbons.A distinct and sometimes prominent peak can usually be found at M-18 due to loss of water.M-18 is frequently exaggerated by thermal decomposition of higher alcohols on hot interfaces.Elemination of water together with elemination of an olefin from primary alcohols, accounts for the presence of a peak at M-(olefin+H2O), that is a peak at M-46, M-74, M-102,....A peak at M-31 (see above) is quite diagnostic for a primary alcohol provided it is more intense than peaks at m/e 45, 59, etc.. • Cyclic ketones show complex fragmentation patterns • Aromatic ketones primarily lose R• upon α-cleavage, followed by loss of CO . An important fragmentation pattern involves alpha-cleavage (breaking either bond to the carbonyl carbon). Primary amides show base peak due to mcklaferty rearrangement as shown below. (m/e 31).Secondary and tertiary alcohols cleave analogously to give a prominent peak due to the fragment (m/e 45, 59, 73, etc.) Fragmentation of C-C bonds will take place to produce a mixture of alkyl radicals and alkyl carbocations. Fragment due to Alpha-cleavage (loss of an ethyl radical) forms the m/e=59 base peak. The mass spectrum of dodecane on the right illustrates the behavior of an unbranched alkane. Mass Spectrometry: Fragmentation Aliphatic Aldehydes pentanal H O MW = 86 M (86) mz = 29 85 M-1 mz = 43 mz = 44 McLafferty The presence of an [M – 1] peak helps to identify the aldehyde but the [M – R] peak (m/z 29) is not unique to this type of compound. Hexane shows the same fragmentation pattern as other unbranched alkanes. For this reason, the molecular ion is much less abundant than for straight-chain alkanes.

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