Bond breakage

The hydroxyl group (— OH) can be separated from the carboxyl group — C—OH by the cleavage of the C —O bond. 

The R— C— group is called the acyl group. If a different functional group is attached to the acyl group other than the — OH group, the compound is called an acid derivative. 

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The Fischer cyclization is usually carried out with a protic or Lewis acid which functions both to facilitate the formation of the cnchydrazine by tautomerization and also to assist the N N bond breakage. The mechanistic basis of the Fischer cyclization has been discussed in recent reviews[l,2]. 

When cyclohexane is nitrated to produce nitrocyclohexane [1122-60-7] the foUowiag techniques minimise undesired C—C breakage ia the ring low temperature nitrations with NO2 (31), carefiil control of the reactor temperatures, and use of halogen additives. Oxygen iacreases the level of C—C bond breakage. 

The mechanism of subcritical crack growth is the reaction of the corrosive medium with highly stressed bonds at the crack tip. In siUca, in the absence of stressed bonds, the rate of the reaction between the bonds and corrosive media such as water is very low. The introduction of strain energy into crack tip bonds increases the activity of the bond. For siUca glass in water, attack and bond breakage occurs by the following reaction (47) ... 

Heterolytic bond breakage (Section 5.2) The kind of bond-breaking that occurs in polar reactions when one fragment leaves with both of the bonding electrons A B - A+ + B . ... 

Chemical reactions involve rearrangements of atoms Some chemical bonds break, and others form. Bond breakage always requires an input of energy, and bond formation always results in a release of energy. The balance between these opposing trends determines the net energy change for the reaction. 

Step t Vaporintkon Step 2 Bond breakage St p I lomutiOA Step 4t Electron a tlschmeAl Step S Condrn aiiion EMtgy OP reaciloh... 

Draw molecular pictures showing the bond breakage and formation that must occur in the course of this reaction. Suggest how the catalyst might make it easier for these reactions to occur. 

The complex hierarchy of native protein structure may be disrupted by multiple possible destabilizing mechanisms. As has been described in the foregoing, these processes may disrupt noncovalent forces of interaction or may involve covalent bond breakage or formation. A summary of the processes involved in the irreversible inactivation of proteins is illustrated in Fig. 3 and described briefly in the following section. Detailed discussions of mechanisms of protein desta-... 

The first step in this reaction is connected with breaking of the O—O bond, with a markedly unsymmetrical energy profile (a 0.2). Other reactions connected with a similar bond breakage have an analogous course, e.g. the reduction of cystine at a mercury electrode. 

The pronounced proclivity of phosphoric monoester monoanions to eliminate POf is not always recognizable from the characteristic pH profile of Fig. 1. The hydrolysis rate maximum at pH w 4 may be masked by a faster reaction of the neutral phosphoric ester, as in the case of a-D-glucose 1-phosphate63) or on hydrolysis of monobenzyl phosphate 64). In the latter case, the known ability of benzyl esters to undergo SN1 and SN2 reactions permits fast hydrolysis of the neutral ester with C/O bond breakage. The fact that the monoanion 107 of the monobenzyl ester is hydrolyzed some 40 times faster than the monoanion 108 of the dibenzyl ester at the same pH again evidences the special hydrolysis pathway of 107, rationalized by means of the metaphosphate hypothesis. 

This all suggests slow, rate-limiting breaking of the C—H bond to form the stabilised carbanion intermediate (54), followed by fast uptake of D from the solvent D20. Loss of optical activity occurs at each C—H bond breakage, as the bonds to the carbanion carbon atom will need to assume a planar configuration if stabilisation by delocalisation over the adjacent C=0 is to occur. Subsequent addition of D is then statistically equally likely to occur from either side. This slow, rate-limiting formation of a carbanion intermediate, followed by rapid electrophilic attack to complete the overall substitution, is formally similar to rate-limiting carbocation formation in the SNi pathway it is therefore referred to as the SE1 pathway. 

The sonolysis of Mn2(CO)10 makes for an interesting comparison (186), since either metal-metal (as in photolysis) (187) or metal-carbon (as in moderate temperature thermolysis) (188) bond breakage could occur. Ligand substitution will occur from either route producing the axially di-substituted Mn2(CO)8L2. Using benzyl chloride as a trap for the possible intermediacy of Mn(CO)5, the sonochemical substitution of Mn2(CO)10 has been shown to follow the thermal, rather than the photochemical, pathway of dissociative CO loss. 

Our objective has been to develop methods that allow the calculation of various electronic parameters such as partial atomic charge, q, electronegativity, polarizability, a, for each atom of a molecule. In this way, the values assigned to an atom not only reflect the type of the atom, but also the particular molecular environment into which this atom is embedded (Fig. 18). The electronic parameters assigned to the atoms of a bond will then be used to arrive at a quantitative value for this bond which reflects its reactivity. A detailed description of a reaction will also have to include parameters characteristic of the reagent in order to account for its influences on bond breakage and formation. 

Fig. 29. Bond breakages calculated by EROS for the reaction of hydroxynorbomene 4 with base. Enthalpies are in kcal/mol in parentheses, relative to 4...

In the case of CH3CD3, various other dissociation modes exist, including C—C and C—H bond breakage, either separately or in various combinations (Freeman, 1968). A few other examples follow of dissociation of excited inorganic and organic molecules ... 

In a-methyl-benzylcalcium and a-Me3Si-benzylcalcium complexes, energy barriers for inversion of the chiral benzylic carbon (17-19 kcal mol-1) are concentration independent, suggesting that a dissociative mechanism is involved that involves Ca-Ca bond breakage. The a-methyl-benzylcalcium compounds are less stable and show a... 

Carbon-Carbon Bond Breakage with Oxidation to Aldehydes... 

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