Dehydration mechanism for

Rosenow, M. A., Huffman, H. A., Phail, M. E., and Wachter, R. M. (2004). The crystal structure of the Y66L variant of green fluorescent protein supports a cyclization-oxidation-dehydration mechanism for chromophore maturation. Biochemistry 43 4464 1472. 

Then, during the acid-catalyzed reaction, the weight equivalent of another 0.75 moles of water per mole of glucose was lost. After this stage, all samples were dark brown. Feather and Harris (11) describe a number of intra- and intermolecular dehydration mechanisms for carbohydrates in acidic solution. In that study, some of those mechanisms were aided by the presence of amines. 

Thomke has used model alcohols very effectively in his studies of dehydration mechanisms. For several oxides including thoria a number of deuteriated butanols were employed including d,l erthro(threo)-[3- Y i -butan-2-ol(a), [l,l,l,2,3,3- H6]butan-2-ol (b), and [2- Hi]butan-2-ol (c). 

The relationship between the structure and dehydration mechanism for the pentahydrate series has been discussed by Niinisto et al. (1982). A further case of an obvious structure-thermal-stability correlation is provided by the dehydration of the highly hydrated selenate phase La2(Se04)3 I2H2O (Karvinen and Niinisto, 1986). Its dehydration starts around 50°C and at 80°C a distinct plateau in the weight loss curve is reached which corresponds to the formation of the more stable octahydrated phase. 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

IS reversible with respect to reactants and products so each tiny increment of progress along the reaction coordinate is reversible Once we know the mechanism for the for ward phase of a particular reaction we also know what the intermediates and transition states must be for the reverse In particular the three step mechanism for the acid catalyzed hydration of 2 methylpropene m Figure 6 9 is the reverse of that for the acid catalyzed dehydration of tert butyl alcohol m Figure 5 6... 

Figure 17 10 presents the mechanism for the reaction between benzaldehyde and methylamine given m the first example The first two steps lead to the carbmolamme the last three show the dehydration of the carbmolamme to the imine Step 4 the key... 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

FIGURE 5.6 The El mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. 

As noted eaiTier (Section 4.10) primary caibocations aie too high in energy to be intennediates in most chemical reactions. If primaiy alcohols don t fonn primary caibocations, then how do they undergo elimination A modification of our general mechanism for alcohol dehydration offers a reasonable explanation. For primary alcohols it is... 

In Problem 5.17 (Section 5.13) we saw that acid-catalyzed dehydration of 2,2-dimethyl-cyclohexanol afforded 1,2-dimethylcyclohexene. To explain this product we must write a mechanism for the reaction in which a methyl shift transforms a secondary carbocation to a tertiary one. Another product of the dehydration of 2,2-dimethylcyclohexanol is isopropylidenecyclopentane. Write a mechanism to rationalize its formation. 

The mechanism for the conversion of the A -oxide (94) to the o-methylaminophenylquinoxaline (96) involves an initial protonation of the A -oxide function. This enhances the electrophilic reactivity of the a-carbon atom which then effects an intramolecular electrophilic substitution at an ortho position of the anilide ring to give the spiro-lactam (98). Hydrolytic ring cleavage of (98) gives the acid (99), which undergoes ready dehydration and decarboxylation to (96), the availability of the cyclic transition state facilitating these processes. ... 

When 2-methyl-2<5-pentanedioJ is treated with sulfuric acid, dehydration occurs and 2,2-dimethyltetrahydrofuran is formed. Suggest a mechanism for this reaction. Which of the two oxygen atoms is most likely to be eliminated, and why ... 

Problem 29.4 Write a mechanism for the dehydration reaction of /3-hydroxybutyryl ACP to yield crotonyl ACP in step 7 of fatty-acid synthesis. 

The amino acid leucine is biosynthesized from n-ketoisocaproate, which is itself prepared from -ketoisovalerate by a multistep route that involves (1) reaction with acetyl CoA, (2) hydrolysis, (3) dehydration, (4) hydration. (5) oxidation, and (6) decarboxylation. Show lhe steps in the transformation, and propose a mechanism for each. 

Phenolic networks are well known for their excellent thermal and thermo-oxidative stabilities. The mechanisms for high-temperature phenolic degradation include dehydration, thermal crosslinking, and oxidation, which eventually lead to char. 

FeCl3 6H20 turned out to be the catalyst of choice for this reaction, since the presence of water improved the yield. However, high yields of the desired ketones were obtained for electron-rich alkynes with anhydrous FeCla at room temperature. Alcohols that are sensitive to acid-catalyzed dehydration were also tolerated under the present conditions (R = Me or Et). Based upon experimental observations a mechanism for this reaction was proposed (Scheme 14). 

The reason for heat resistance is thought to lie in the fact that the core or spore cytoplasm becomes dehydrated during sporulation. The mechanism for this dehydration... 

Scheme L A mechanism for the formaldehyde catalyzed dehydration of p-hydroxynitrosamines.

An alkene can accept a proton to generate a carbocation in a process that is essentially the reverse of the deprotonation step in the El mechanism for dehydration of an alcohol. 

The use of the dehydrating agent 2-chloro-l,3-dimethylimidazolinium chloride (DMC) in combination with DMSO for the preparation of 1,2,4-thiadiazoles via a type A synthesis has been published. Excellent yields >90% are reported along with a mechanism for the transformation <1999JOC6989>. 

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