Mechanism resonance contributors

Previously, it has been noted that the solvolysis rates of both 3-methyl-l-bromoadamantane and 3,5-dimethyl-1-bromoadamantane are slower than 1-bromoadamantane itself. This result was felt to be inconsistent with any mechanism which distributed positive charge to all bridgehead positions of the adamantyl cation under solvolysis conditions since the introduction of tertiary resonance contributors (cf. Eq. (49)) should enhance rather than retard solvolysis rates 56,164 ... 

Write the steps in the mechanism of acetal formation and hydrolysis. Draw the structures of resonance contributors to intermediates in the mechanism. 

Complete the mechanism for this acid-catalyzed transesterification by drawing out all the individual steps. Draw the important resonance contributors for each resonance-stabilized intermediate. 

Draw the important resonance contributors for both resonance-stabilized cations (in brackets) in the mechanism for acid-catalyzed hydrolysis of an amide. 

The authors proposed a mechanism similar to the one in Scheme 1 (see p. 213). The excited singlet state reacts to give both the in-cage ion pair, 34, and the radical pair, 35, but the question of whether these are two separate species or two resonance contributors was left open. Toluene and the methylether are derived from this singlet pair the quantum yields are 0.19 and 0.11, respectively, in 50 50 t-butyl alcohol/water for the unsubstituted bromide. The triplet radical pair is formed from the excited triplet state with a quantum yield of 0.2 in 50 ... 

All of these electrophilic aromatic substitution reactions take place by the same two-step mechanism. In the first step, benzene reacts with an electrophile (Y ), forming a carbocation intermediate. The structure of the carbocation intermediate can be approximated by three resonance contributors. In the second step of the reaction, a base in the reaction mixture pulls off a proton from the carbocation intermediate, and the electrons that held the proton move into the ring to reestablish its aromaticity. Notice that the proton is always removed from the carbon that has formed the new bond with the electrophile. 

To make the mechanisms easier to understand, only one of the three resonance contributors of the carbocation intermediate is shown in this and subsequent illustrations. Bear in mind, however, that each carbocation intermediate actually has the three resonance contributors shown in Section 15.9. In the last step of the reaction, a base (=B) from the reaction mixture removes a proton from the carbocation intermediate. The following equation shows that the catalyst is regenerated ... 

Alkenes react as Br0nsted-Lowry bases in the presence of strong mineral acids, HX. The reaction of alkenes and mineral acids (HX) generates the more stable carbocation, leading to substitution of X at the more substituted carbon. This constitutes the mechanism of the reaction, but it is given the name Markovnikov addition 1, 2, 3, 4, 5, 6, 65, 66, 67, 69, 71, 72, 75, 82, 83, 99,100,116. More highly substituted carbocations are generally more stable. An increase in the number of resonance contributors will increase the stability of a carbocation 7, 61, 68. 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

This map indicates that the acyl carbon and the carbon atom at the end of the C=C unit (resonance contributors 37B and 37C) are most likely to react with chloride ion to form a product. If chloride ion reacts at the terminal carbon (37C), the product is an enol, which tautomerizes to the final product, 39. Keto-enol tautomerization was introduced in Chapter 10 (Section 10.4.5) and discussed again in Chapter 22 (Section 22.1). An alternative mechanism is possible in which HCl reacts with the C=C unit of 10 to generate 38 directly, and subsequent reaction with chloride ion gives 39. Experimental evidence suggests that 37 is the more likely intermediate that leads to 39. 

Nevertheless, for simphcity, when drawing mechanisms, we will often draw the less significant resonance contributor of the enolate, in which the negative charge is on the a carbon. That is, C-attack will be drawn like this ... 

The present author can only reiterate his conclusion, stated in the Introduction, based on the evaluation of theory and experiment as given above There is no one mechanism at the root of SERS however, there is a mechanism which, in the large majority of systems, is the main contributor to the surface enhancement effect. That mechanism is a resonance mechanism. It is felt there is not enough evidence, yet, to determine which of the mechanisms belonging to this group is the important one, or which can be ruled out. The LFE mechanism certainly has a role, but a more minor one. Note, however, that a minor factor in SERS is a factor of a 100 or so, which may be the difference between a detectable and a nondetectable signal ... 

QCM measurements, we also recorded elasticity maps of MDCK cells before and after fixation with PFA and GA by scanning force microscopy (SFM). Applying the commonly used Hertz model to the recorded raw data, we obtained a median Young s modulus of 2.5 0.3 kPa for native MDCK cells that was increased to 3.7 0.9 kPa after PFA fixation. The highest median values of 25 3 kPa were found after a 30 min fixation with GA. Thus, the well-established SFM measurements indicate that there is a graded and individual stiffening of the cells when different fixatives are used. Consistent with the QCM experiments, PFA was found to be less efficient in cell stiffening than GA. In SFM studies the cortical actin cytoskeleton is considered to be the dominant contributor to the mechanical properties of the cell membrane [46]. Since the QCM readout correlates with SFM measurements, the conclusion may apply that the cortical actin cytoskeleton is also predominantly responsible for the acoustic load of the resonator. 

We encountered enols earlier as intermediates in the hydration of alkynes (see Mechanism 9.2). Enolates, represented as a hybrid of the resonance structures shown, are the conjugate bases of enols. The major enolate contributor is the structure with the negative charge on oxygen. It is, however, the carbanionic character of the a carbon that is responsible for the importance of enolates in organic synthesis, and we will sometimes write the enolate in the form that has the negative charge on carbon to emphasize this. 

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