Carbon breaking

The most important cracking reaction, however, is the carbon-carbon beta bond scission. A bond at a position beta to the positively-charged carbon breaks heterolytically, yielding an olefin and another carbocation. This can be represented by the following example ... 

Six-coordination is obtained in [PtMe3(acac)]2 by bidentate (0,0 ) behaviour and by a bond to the 7-carbon, a situation maintained in solution at room temperature (on warming, the bond to the 7-carbon breaks). In the bipyridyl adduct, it is the bond to the 7-carbon that completes the octahedral coordination [192],... 

Find the carbonyl group of the Michael acceptor, and count three carbons away from the carbonyl group. The Michael donor forms the new bond to this carbon. Break this bond to identify the Michael donor and Michael acceptor. 

Then the bond between silicon and the bridgehead carbon breaks and the double bond shifts. The final step is an addition of a chlorine which then reacts with an excess of deuterated methanol to a methoxy group. This reaction route is best described analogous to a Sg-mechanism, which has already been published for other group 14 elements (Si, Ge, Sn) [5]. 

Mg and Ca oxide are easily prepared by igniting the carbonates in air. However, for the Sr and Ba carbonates breaking the C-0 is particularly difficult and decomposition of the carbonate is best accomplished by heating in the presence of hydrogen at temperatmes above 1200 °C, thus attacking the C-0 bond directly. 

Water as nucleophile then displaces mercury by back-side attack at the more highly substituted carbon, breaking the C-Hg bond. 

Aral and Nishiyama report that the pretreatment of carbon sipports significantly affects the temperature at which Pt films break up in vacuum to fc m metal crystallites. Their data summarized in Table 1 indicate that Pt films deposited on air- and H2-treated carbons break up at significantly higher temperatures (923 and 823 K) compared to that for untreated carbon (773 K). These results are consistent with those obtained for conventional Pt/caibcm catalysts showing that carbon pietreatirrent affects their sintering behavior. ... 

In this pathway electrons flow from the source to the multiply bonded carbon, break the pi bond, and produce a stable anion. An electronegative carbon atom, C-ewg, can replace an electronegative heteroatom, Y, and the electron flow does not change. When... 

This pathway (Fig. 7.15) is very similar to path AdN, with the difference that the nucleophile is poorer and is hydrogen bonded to a base when this pair collides with the polarized multiple bond. In this pathway the electron flow comes from the base to break the Nu-H bond, which in turn enhances the nucleophilicity of the nucleophile s lone pair. This lone pair attacks the multiply bonded carbon, breaks the pi bond, and produces a stable anion similar to path Adf. ... 

These trends agree with the frontier orbital analysis. In particular, the delivery of hydride from carbon breaks a relatively unpolarised bond, making the hydride notably soft, as we saw earlier in its capacity to attack pyridinium salts preferentially at the 4-position. The metal hydrogen bond will be more polarised, and metal hydrides should therefore be harder. Similarly, the delivery of hydride from boron will make it softer than when it is delivered from the more electropositive metal, aluminium. It also seems that, among a,(3-unsaturated carbonyl compounds, the susceptibility to conjugate reduction increases in the sequence ketones < esters < acids < amides but there are too few examples to be sure. 

A major reaction of nucleophiles is their attack at an acyl carbon (a carbonyl carbon), breaking the jt bond and forming a new bond to carbon. This class of organic reactions is called nucleophilic acyl addition. When the carbonyl contains a leaving group, acyl addition is followed by loss of that group to give a substitution... 

Considerable work has gone into differentiating between bound and free chlorides. As only the free chlorides contribute to corrosion that is ideally what we want to know. However, the binding of chlorides is a reversible and dynamic reaction, so attempts to remove and measure free chlorides will release bound chlorides. A further complication is that carbonation breaks down chloroaluminates thus freeing chlorides which proceed as a wave ahead of the carbonation front. 

A key question we shall want to address later in this chapter is this when does the bond between the leaving group and the carbon break Does it break at the same time that the new bond between the nucleophile and carbon forms, as shown below ... 

Now imagine that a negatively charged species (X) collides with the carbon of the C-Cl bond. In Figure 5.6 (note the use of the electron transfer arrows), the negative species (X) donates two electrons to the carbon, breaking the C-Cl bond. In effect, the X" unit is a Lewis base and the 6+ carbon atom is a Lewis acid, at least from the standpoint of electron donation and electron accepting abilities. As first defined in Chapter 2 (Section 2.6), when a species donates two electrons to carbon, it is known as a nucleophile. (Nucleophiles are discussed in more detail in Chapter 6, Section 6.7, and Chapter 11, Section 11.3.)... 

Ketones and aldehydes react with nucleophiles to give substituted alkoxides by acyl addition to the carbonyl to give alcohols in a two-step process (1) acyl addition and (2) hydrolysis. Nucleophilic acyl addition involves forming a new bond between the nucleophile and the acyl carbon, breaking the 7r-bond of the carbonyl with transfer of those electrons to the oxygen to give an alkoxide. Addition of an acid catalyst leads to an oxocarbe-nium ion that facilitates acyl addition. 

The polarization of the C=0 group, where the oxygen is 6- and the carbon in 6-1-, is responsible for the acid-base reactions, and also for the reaction of aldehydes and ketones with nucleophiles. Nucleophilic acyl addition occurs when a nucleophile X donates electrons to the carbonyl carbon (the acyl carbon), breaking the n-bond. The product of this reaction with an aldehyde or ketones is an alkoxide, 4. Several different nucleophiles react with aldehydes... 

Step 1 If the alcohol was added directly to the carbonyl (Make a bond), we would create an anionic oxygen on the ester carbonyl. Because the reaction is carried out in acid and the anionic oxygen is basic, this constitutes a mbced media error, and therefore is incorrect (Principle II). The OH group cannot depart from an sp carbon (Break a bond) because it would leave as hydroxide and we are in acidic media (Principle II). There are no protons that can be removed (Take a proton away Principle III). Hence, by process of elimination, the first step must be protonafion of the carbonyl oxygen to make structure II. Therefore, Add a proton. 

A strong nucleophile adds directly to the electrophilic acyl carbon, breaking the C=0 77 bond, thereby creating a tetrahedral addition intermediate. 

Consider two examples—a decarboxylation and an Sn2 reaction. The primary kinetic isotope effects (PKIE, ku ku for C relative to the natural abundance C, which is mostly C) at the methylene and carboxyl carbons in the decarboxylation of malonic acid are 1.076 and 1.065, respectively (Eq. 8.19). The values are very close. Indicating that a bond to each of these carbons breaks in the rate-determining step. The isotope effect (Icjs/in the displacement of chloride from benzyl chloride by cyanide is 1.0057, indicating that the bond to chlorine breaks in the rate-determining step (Eq. 8.20). 

A 77 bond forms between carbon and oxygen and, as the tt bond between the two carbons breaks, carbon picks up a proton. 

In a sigmatropic rearrangement, a a bond to an allylic carbon breaks in the reactant, a new a bond forms in the product, and the tt bonds rearrange. 

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