Alkene Loss Research Articles

ChemInform Abstract: The Gas‐Phase Acidities of the Alkanes

The gas-phase acidities of 15 simple alkanes have been determined in a flowing afterglow-selected ion flow tube (FA-SIFT) by a kinetic method in which alkyltrimethylsilanes were allowed to react with hydroxide ions to produce a mixture of trimethylsiloxide ions by loss of alkane and alkyldimethylsiloxide ions by loss of methane. The reaction is proposed to proceed by addition of hydroxide ion to the silane to form a pentacoordinate siliconate ion intermediate which decomposes through two transition states, one in which negative charge is placed on a methyl group and the other in which negative charge is placed on the alkyl group. The ratio of siloxide ions produced is proposed to be correlated to the relative basicity of the methyl and alkyl anions. The method is calibrated by making use of the known acidities of methane (/Delta/H/sub acid/) = 416.6 kcal/mol and benzene (/Delta/H/degree//sub acid/ = 400.7 kcal/mol). In general, methyl substitution is found to stabilize alkyl anions in the gas phase except that the ethyl anion is found to be more basic than the methyl anion. By combining the gas-phase acidities with the bond dissociation energies, the electron affinities (EA) of the corresponding alkyl radicals can be calculated. Many simplemore » alkyl radicals are found to have negative EA's. The results for the alkyl groups studied are given.« less

The gas-phase acidities of the alkanes

The gas-phase acidities of 15 simple alkanes have been determined in a flowing afterglow-selected ion flow tube (FA-SIFT) by a kinetic method in which alkyltrimethylsilanes were allowed to react with hydroxide ions to produce a mixture of trimethylsiloxide ions by loss of alkane and alkyldimethylsiloxide ions by loss of methane. The reaction is proposed to proceed by addition of hydroxide ion to the silane to form a pentacoordinate siliconate ion intermediate which decomposes through two transition states, one in which negative charge is placed on a methyl group and the other in which negative charge is placed on the alkyl group. The ratio of siloxide ions produced is proposed to be correlated to the relative basicity of the methyl and alkyl anions. The method is calibrated by making use of the known acidities of methane (/Delta/H/sub acid/) = 416.6 kcal/mol and benzene (/Delta/H/degree//sub acid/ = 400.7 kcal/mol). In general, methyl substitution is found to stabilize alkyl anions in the gas phase except that the ethyl anion is found to be more basic than the methyl anion. By combining the gas-phase acidities with the bond dissociation energies, the electron affinities (EA) of the corresponding alkyl radicals can be calculated. Many simplemore » alkyl radicals are found to have negative EA's. The results for the alkyl groups studied are given.« less

Ion–dipole complexes in the unimolecular reactions of isolated organic ions. Effect of N-methylation on olefin and amine loss from protonated aliphatic amines

The slow unimolecular fragmentation reactions of 18 gaseous protonated aliphatic amines of general formula R1NH+R2R3(R1= Prn, Pri, Bun, Bui, Bus, or But; R2,R3= H,CH3) are reported and discussed. Two decomposition routes are observed for metastable ions R1NH+R2R3. The first involves elimination of a neutral amine, R2R3NH, and formation of a carbocation, R1+, via a mechanism involving an incipient cation bound to the developing amine by an ion–dipole attraction. Rearrangement of the cation, to give thermodynamically more stable isomers, is feasible in these ion–dipole complexes. Further reorganization of the complexes leads to a species in which an incipient olefin [R1– H] and an amine [R2R3NH] are co-ordinated to a common proton. Dissociation of these proton-bound complexes, with retention of the proton by the developing amine, results in olefin loss, which is the second reaction undergone by metastable ions R1NH+R2R3. The relative abundance of amine expulsion is greater for protonated amines containing a primary alkyl group, R1, than is the case for isomeric ions containing secondary or tertiary alkyl groups. Progressive methylation of the nitrogen atom decreases the relative abundance of amine loss from R1NH+R2R3, regardless of the nature of the principal alkyl group. These two trends are explained in terms of the energetics of the intermediates and products involved in the decomposition of the protonated amines.

Gas-phase studies of alkane oxidation by transition-metal oxides. Selective oxidation by CrO+

The gas-phase reactions of CrO/sup +/ with alkanes have been studied by using ion beam reactive scattering techniques. CrO/sup +/ undergoes facile reactions with alkanes larger than methane. CrO/sup +/ selectively oxidizes ethane to form ethanol. In addition to the possibility of alcohol formation, reactions with larger alkanes are more complex, yielding products in which dehydrogenation and loss of alkenes and alkanes occur. In reactions with cyclic alkanes, cyclopropane and cyclobutane yield products characteristic of C-C bond cleavage. In contrast, reactions with cyclopentane and cyclohexane mainly involve dehydrogenation and elimination of H/sub 2/O. A series of hydrogen abstraction reactions are examined to determine the bond dissociation energy D/sup 0/ (CrO/sup +/-H) = 89 +/- 5 kcal mol/sup -1/. This bond energy has implications for the reaction mechanisms of CrO/sup +/ with alkanes, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add across the Cr/sup +/-O bond as an initial step. This is supported by an examination of the reactions of Cr/sup +/ with alcohols.

Mechanistic and kinetic study of alkane activation by titanium(I) and vanadium(I) in the gas phase. Lifetimes of reaction intermediates

The reactions of Ti/sup +/ and V/sup +/ with several deuterium-labeled alkanes are studied by using an ion beam apparatus. The dominant reactions observed for both of these metal ions are single and double dehydrogenations. Alkane loss reactions are also observed for Ti/sup +/ but may be due to electronically excited states. The dehydrogenation mechanisms are investigated by using partially deuterated alkanes. The results are consistent with 1,2-eliminations for both V/sup +/ and Ti/sup +/, where deuterium scrambling may occur in the latter case. It is proposed that some 1,3-elimination of hydrogen also occurs in the reaction of Ti/sup +/ with n-butane. Although the dehydrogenation reactions of V/sup +/ and Ti/sup +/ appear to be similar to those of Ru/sup +/ and Rh/sup +/, there are some important differences in the reactivity of V/sup +/. Extensive adduct formation and large deuterium isotope effects are consistent with reaction intermediates which are relatively long-lived for V/sup +/ in comparison to Ti/sup +/, Ru/sup +/, and Rh/sup +/. Collisional stabilization studies are used to estimate dissociation rates of reaction intermediates formed when Ti/sup +/ and V/sup +/ interact with n-butane. The measured upper limits to the unimolecular decomposition rates are 1.47 x 10/supmore » 5/ s/sup -1/ and 1.23 x 10/sup 7/ s/sup -1/ for V/sup +/ and Ti/sup +/, respectively. Model RRKM calculations are able to reproduce these rates and provide an explanation of isotope effects observed when n-butane-d/sub 10/ is employed as the neutral reactant.« less

Gas-phase reactions of MS + ions (M = Fe, Co, Ni) with alkanes. An FTMS study

The gas-phase reactions of MS + ions (M = Fe, Co, Ni) with small alkanes are reported. MS + reacts with hydrocarbons primarily by H 2S loss from the collision complex. This complex can further dissociate by dehydrogenation, creating a metal-polyolefin complex, or by loss of small alkenes. These latter dissociations are similar to the collision-induced dissociation decomposition pathways for the metal-olefin complex and suggest that H 2S loss leaves an internally excited metal-olefin complex. As has been observed for a variety of mono-ligated metal species, MS + shows a preference for CH bond insertion, leading to dehydrogenation, compared with the bare metal ions which predominantly attack CC bonds. The sulfur ligand does not activate reactions as much as an oxygen ligand, which may be attributed to the fact that H 2S loss is less exothermic than H 2O loss. Photodissociation threshold measurements yield D° (FE + −S) = 65 ± 5 kcal mol −1, D°(Co + −S) = 62 ± 5 kcal mol −1, and D°(Ni + −S) = 60 ± 5 kcal mol −1, from which additional thermochemical information is derived.

Sulphonium ions: Collision‐activated dissociation using fast atom bombardment ionization and a triple quadrupole mass spectrometer

AbstractThe low‐energy collision‐activated dissociation of sulphonium cations, including symmetrical trialkyl R3S+, dimethylalkyl (CH3)2RS+, diphenylalkyl Ph2RS+ and cyclic (CH2)nS+R, has been recorded using fast atom bombardment ionization and a triple quadrupole mass spectrometer. The general trends are easy loss of sulphide to give [R]+, except for R  CH3, and loss of alkene to give protonated sulphide if β‐hydrogens are available. Loss of alkane, generally found in ammonium compounds, is not observed.

Photochemistry of (alkene)pentacarbonyltungsten(0) complexes in rigid alkane at low temperature: relative importance of dissociative loss of alkene and carbon monoxide

Irradiation UV (254 ou 213 nm) de complexes W(CO) 5 (alcene) avec alcene=C 2 H 4 , C 3 H 6 et pentene-1 dans le methylcyclohexane. La perte relative de l'alcene est deux fois plus importante a 313 nm qu'a 254 nm. En outre la perte en compose ethylenique est triplee selon l'ordre C 2 H 4 <C 3 H 0 <C 5 H 10 . Mecanisme. Spectres IR, UV, visible

Infrared multiphoton and collision-induced dissociation studies of some gaseous alkylamine ions

Collision-induced dissociation and infrared multiphoton dissociation of ions formed in di- and tri-ethylamine, di- and tri-n-propylamine, and di-isopropylamine were investigated by Fourier-transform ion-cyclotron resonance mass spectrometry. Molecular ions of all amines except di-n-propylamine produced similar fragment ions when subjected to either dissociation technique. The initial fragmentation involved C αC β bond cleavage, loss of an alkyl radical, and formation of an immonium ions. Subsequent fragmentations of the immonium ions produced by both dissociation mechanisms involved McLafferty-type rearrangements and loss of alkenes. The molecular ion of di-n-propylamine fragmented by a different mechanism when subjected to infrared irradiation. Protonated molecules of di- and tri-n-propylamine yielded C 3H 6 and an ammonium ion upon infrared multiphoton dissociation, while protonated molecules of the other amines did not dissociate when this technique was applied. In contrast, collision-induced dissociation produced fragmentation for all protonated molecules. Explanation of the different fragmentations observed for the two dissociation techniques is given in terms of a mechanism involving a tight transition state for protonated di- and tri-n-propylamine dissociation.

Alkane eliminations from radical cations through ion-radical complexes

It is concluded from published isotope effects and energetic measurements that the losses of alkanes from a variety of radical cations take place in the gas phase through intermediate ion-radical complexes formed by simple bond cleavages.

Survey of reaction types in low-energy colliosional activation of protonated methyl alkyl ketones

In contrast to high-energy collisions, where simple cleavages are commoner, rearrangements typically account for 96–100% of products in low-energy collisions of the [M + 1] + ions of aliphatic methyl ketones. Rules for predicting low-energy helium collsional-activated decomposition (CAD) spectra of the ketones are based on proton affinities of fragments formed by simple rearrangements. The commonest reaction is equivalent to the energetically improbable four-center 1,3-H or 1,3-R shift; other rearrangements equivalent to processes with five-, six-, or seven-center activated complexes are less important. In larger ions, loss of water followed by loss of alkene dominates. O-Protonated enol forms either lose water to give a carbonium ion that rearranges to forms capable of losing olefin fragments, or rearranges to an intermediate in the formation of acetyl ion and an alkane. O-Protonated keto forms rearrange to alkanes and protonated smaller carbonyl compounds. The ion kinetic energies necessary to produce several intense daughter ions at threshold establish the order of sequential fragmentations. When helium is the collisional gas and the ion energy is 30 eV in the laboratory frame (the maximum value studies), the ion energy is only 0.8–1.9 eV in the center-of-mass frame depending on its size. Only a fraction of this kinetic energy is converted to internal energy, so that onsets of reaction channels differing by several tenths of an electron-volt are easily studied. Some isomers of methyl ketones can be easily distinguished by He-CAD spectra of their [M + 1] + ions.

Survey of reactions of protonated acetate esters induced on low-energy collisions with helium and nitrogen

The major fragmentations induced by collisions of 30-eV (laboratory coordinate) protonated acetate esters with helium and nitrogen are the rearrangements observed in earlier high-energy collisional activation studies, except for the diagnostic loss of alkane observed in the high-energy process, which is missing. The kinetic energy dependence of fragmentation is reflected in peak intensities at 30 eV. As proposed earlier, helium collisions are useful for categorization of the compounds, and nitrogen for their identification.

Alkane loss from the molecular ions of alcohols, ketones, and amines in the μsec timeframe

Neutral alkane molecules are eliminated in the low-energy reactions of many low molecular weight secondary alcohols, ketones and sec-alkyl primary amines. The alkane molecule consists of one α-alkyl group plus a hydrogen atom from the first carbon of the ‘other’ α-alkyl (formally a simple 1,2-elimination). Alkane loss competes inefficiently with other intramolecular hydrogen abstraction reactions (e.g., the McLafferty rearrangement), and is only observed for compounds with relatively short alkyl groups. When an α-substituent is methyl exceptional behavior is sometimes encountered: alkane loss is not important for methyl ketones (except acetone), and methane loss from α-methyl amines occurs only for isopropylamine. Tertiary alcohols have not been observed to eliminate alkane molecules, and alkane loss does not occur for secondary and tertiary amines.

Unimolecular reactions of isolated organic ions: olefin elimination from immonium ions R1R2NCH2

The slow unimolecular reactions of numerous tertiary immonium ions of general formula R1R2NCH2 are reported and discussed. Two distinct processes involving olefin loss are observed; the first reaction results in elimination of an olefin having the same number of carbon atoms as R1(or R2), whilst the second proceeds with expulsion of a smaller olefin, having one less carbon atom than R1(or R2). Evidence is presented to show that the latter reaction proceeds via transfer of a γ-hydrogen atom from R1 or R2 to the isolated CH2 group, followed by σ-cleavage in the resulting open-chain carbonium ion. Whenever this process involves a secondary or tertiary cation, it dominates over the alternative route, for energetic reasons. However, when γ-hydrogen transfer produces a primary cation, both classes of olefin elimination occur in comparable abundance. Loss of olefins from R1 and/or R2, in competition, is likewise controlled by the nature of the intermediate cation.

Gaseous odd- and even-electron ions

The principal collision-induced fragmentations of simple protonated ketones, aldehydes, ethers, amines, sulphides, alcohols, acids, nitriles and halides are discussed. These protonated molecules decompose mainly by loss of alkane, alkene and RX (R = alkyl, H; X = OH, SH, NH 2, Br, I). Substantial radical losses are only observed for small protonated molecules. Deuterium-labelling demonstrates that the XH bond is particularly strong. The fragmentation of (MH) + ions is compared with that of the corresponding (M) +− ions. The spectra of the (M) +− ions are dominated by direct bond cleavages, in particular α-cleavages, as a result of both the stability of the ionic fragment and the loose transition state. In (MH) + ions direct bond cleavages lead to energetically less favourable products. Thus rearrangement reactions play a more important role in the decomposition of these ions. (MH) + ions are more stable relative to fragmentation than (M) +− ions.

Loss of alkanes from ionized branched chain ethers. Metastable decomposition from an electronically excited state?

Loss of alkanes from ionized branched chain ethers. Metastable decomposition from an electronically excited state?

Potential energy profiles for unimolecular reactions of isolated organic ions: some isomers of C4H10N+ and C5H12N+

The unimolecular reactions of two isomers of C4H10N+ and four isomers of C5H12N+, of general formula RNH+CH2(R = C3H7 or C4H9), are discussed in terms of a potential energy profile approach. Two main decomposition channels are observed, both of which involve loss of an olefin from the original alkyl side chain. Hydrogen transfer from the alkyl chain to nitrogen can occur in an intermediate in which the incipient carbonium ion R+ is weakly co-ordinated to NHCH2. This process results in the formation of +NH2CH2 and expulsion of an olefin containing the same number of carbon atoms as R. Alternatively, a 1,5-hydride shift can take place, from a γ-carbon atom in RNH+CH2, to the isolated methylene group. This rearrangement may lead to loss of an olefin containing one less carbon atom than R, thus giving rise to CH3NH+CH2 as the daughter ion. The γ-hydrogen transfer process is analogous to the McLafferty rearrangement which is well known for ionised carbonyl compounds of sufficient chain length. Evidence is presented which shows that the 1,5-hydride shift is not synchronously concerted with olefin loss; instead, the γ-carbon atom acquires at least a partial positive charge before the olefin is expelled, with kinetic energy release.

Mass spectrometry of compounds of the type [Fe(CO)2(η-C5H5)R](R = alkyl)

The electron-impact mass spectra of a series of compounds of the type [Fe(CO)2(η-C5H5)R](R = alkyl) have been obtained. Three major fragmentation routes commence with loss of alkyl (R), alkene (R–H), or carbonyl groups. Particularly intense peaks are also formed by loss of one, two, or three molecules of H2 from the [M– 2CO]+ ions, leading to species which presumably contain η3-allylic and η5-pentadienyl moieties.

Fragmentation of even electron ions. Protonated ketones and ethers

AbstractA systematic study of the types of fragmentation undergone by internally excited protonated molecules is presented. Two representative functional groups, ethers and ketones, are selected, and both aliphatic and aromatic substituents are examined. The ions are formed by protonation or alkylation in a chemical ionization source, and caused to fragment by collision at high relative kinetic energy. The prevalence of 1,3‐rearrangements and the breakdown of the even electron fragmentation rule highlight the ion chemistry uncovered here. Specific findings include: (i) electron unpairing reactions associated with alkyl radical loss are common. These are probably energetically less favorable simple bond cleavages and are observed in competition with entropically less favorable elimination reactions. (ii) Alkane elimination is a characteristic reaction, which often occurs by a four‐centered mechanism. Several variations on this reaction type are encountered. (iii) Alkene elimination is another ubiquitous reaction which occurs by a four‐centered mechanism in the systems studied here. Competition between alkene and alkane loss is extremely sensitive to the particular system in hand.

The mass spectrometric fragmentation of n‐alkanes

AbstractThe fragmentation of n‐hexane, n‐nonane and n‐tetradecane under electron impact has been investigated, using 13C labelled compounds. The mechanism of the formation of the alkyl radical ions is quantitatively explained by using a method of calculation developed in an earlier publication for n‐heptane. It is assumed that these ions are formed either by a direct C‐C bond cleaveage or by a secondary olefin loss from an alkyl radical ion. In the latter case the probability for a particular carbon to be lost in the neutral fragment is assumed to be random. The probability for a direct cleavage to an alkyl ion is about 80% for an ion containing at least half of the number of carbon atoms of the molecular ion and 15% for the smaller ions. The [MH]+ ion seems to be a special case not yet clearly understood. Former results about the loss of methyl from the molecular ion are confirmed.