Alpha-Cleavage

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

Cleavage Alpha to Carbon Bearing a Nitrogen Atom... 

As may be expected from previous discussion, aldonic acid spectra show a number of fragmentations due to carbon chain cleavages alpha to TMS-ether groups. Charge retention on the non-carbonyl fragment gives a series of ions m/z = 103 + n X 102). The first member of the series, m/z 103, may indicate a terminal hydroxyl on the chain, but can be confused with m/z 103 arising from... 

A typical phenol plant based on the cumene hydroperoxide process can be divided into two principal areas. In the reaction area, cumene, formed by alkylation of benzene and propylene, is oxidized to form cumene hydroperoxide (CHP). The cumene hydroperoxide is concentrated and cleaved to produce phenol and acetone. By-products of the oxidation reaction are acetophenone and dimethyl benzyl alcohol (DMBA). DMBA is dehydrated in the cleavage reaction to produce alpha-methylstyrene (AMS). 

The neutralized cleavage product, consisting of acetone, phenol, water, hydrocarbons, and trace organic impurities, is separated in a series of distillation columns. Also in this section alpha-methylstyrene is either recovered as a product or hydrogenated to cumene. 

KRdHNKE - ORTOLEVA PyndiniumSalts Formation oi kelo pyndinium salts by reaction of alpha halo katona darrvatives with pyndine and th cleavage to caiboxytic acids... 

The biologically active form of vitamin Bg is pyridoxal-5-phosphate (PEP), a coenzyme that exists under physiological conditions in two tautomeric forms (Figure 18.25). PLP participates in the catalysis of a wide variety of reactions involving amino acids, including transaminations, a- and /3-decarboxylations, /3- and ") eliminations, racemizations, and aldol reactions (Figure 18.26). Note that these reactions include cleavage of any of the bonds to the amino acid alpha carbon, as well as several bonds in the side chain. The remarkably versatile chemistry of PLP is due to its ability to... 

Nitrophenyl)ethyl Carbamate. The photolytic removal of this group occurs twice as fast as does the 2-nitrobenzyl carbamate. Additionally, substitution at the alpha carbon increases the rate of cleavage even more. 

Examine the geometry of the most stable radical. Is the bonding in the aromatic ring fuUy delocalized (compare to model alpha-tocopherol), or is it localized Also, examine the spin density surface of the most stable radical. Is the unpaired electron localized on the carbon (oxygen) where bond cleavage occurred, or is it delocalized Draw all of the resonance contributors necessary for a full description of the radical s geometry and electronic structure. 

Loss of C3H7 (M+-43) by alpha cleavage gives a peak of mass 59. 

Both fragmentation modes are apparent in the mass spectrum of 1-butanol (Figure 17.14). The peak at ni/z = 56 is due to loss of water from the molecular ion, and the peak at mjz = 31 is due to an alpha cleavage. 

Figure 17.14 Mass spectrum of 1-butanol (M+ = 74). Dehydration gives a peak at mjz - 56, and fragmentation by alpha cleavage gives a peak at m/z = 31.

N. T., Lhomme J., Helene C. Sequence-specific recognition, photocrosslinking and cleavage of the DNA double helix by an oligo-[alpha]-thymidylate covalently linked to an azidoproflavine derivative. Nucleic Acids Res. 1987 15 7749-7760. 

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