Water Proton transfer

Step 2 In a solvent such as water proton transfers convert the dipolar intermediate to the carbmolamme... 

The mechanism of Scheme 8 is compatible with this observation. At low concentrations of hydroxide ion the rate of collapse of the tetrahedral intermediate to reactants must be faster than its reaction with hydroxide ion ( -1 At2[OH ]) the observed rate constant is dependent upon the concentration of hydroxide ion with the diffusion-controlled step, being rate-limiting, The calculated pAT,-values for the protonated amine of the tetrahedral intermediates are well below that for water. Proton transfer from the tetrahedral intermediate to hydroxide ion is therefore in the thermodynamically favourable direction and it is to be expected that the rate-limiting step for this process is the diffusion-controlled encounter of the proton donor and acceptor. 

Following is a structural formula for imidazole, a building block of the essential amino acid histidine (Chapter 27). It is also a building block of histamine, a compound all too familiar to people with allergies and takers of antihistamines. When imidazole is dissolved in water, proton transfer to it gives a cation. Is this cation better represented by structure A or B Explain. 

The Key Event Formation of Water Proton Transfer in Acid-Base Reactions Acid-Base Titrations... 

Modeling of polymer membrane structure through dynamics simulations is generally coupled with the water/proton-transfer properties. Yana et al. conducted molecular dynamics (MD) simulations of Krytox-Silica-Nafion composite membranes in comparison to the pure Nation. A 5 wt% of carboxylic acid terminated perfluoropolyether hybrid with silica (Krytox-Silica) in Nation composite polymer was used in the modification of a PEM in order to improve its efficiency at high operating temperatures. MD simulations were carried out in order to understand the microscopic properties of two systems, Krytox-Silica in Nation and pure... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

The other halides dissociate at lower temperatures and, if put into water, all are decomposed, the proton transferring to water which is a better electron pair donor ... 

Fig. 1. The rate-determining step in the neutral hydrolysis of paramethoxy-phenyl dichloroacetate. In the reactant state (a) a water molecule is in proximity of the carbonyl carbon after concerted proton transfer to a second water molecule and electron redistribution, a tetrahedral intermediate (b) is formed.

Drukker, K., Hammes-Schiffer, S. An analytical derivation of MC-SCF vibrational wave functions for the quantum dynamical simulation of multiple proton transfer reactions Initial application to protonated water chains. J. Chem. Phys. 107 (1997) 363-374. 

We saw m Chapter 1 especially m Table 1 7 that alcohols resemble water m respect to their Brpnsted acidity (ability to donate a proton/rom oxygen) They also resemble water m their Brpnsted basicity (ability to accept a proton on oxygen) Just as proton transfer... 

Furthermore a substance such as HCl that dissociates completely when dissolved m water also dissociates completely when dissolved m an alcohol Many important reactions of alco hols involve strong acids either as reactants or as catalysts In all these reachons the first step IS formation of an alkyloxonium ion by proton transfer from the acid to the alcohol... 

Proton transfers from strong acids to water and alcohols rank among the most rapid chemical processes and occur almost as fast as the molecules collide with one another Thus the height of the energy barrier the activation energy for proton transfer must be quite low... 

Bromide ion forms a bond to the primary carbon by pushing off a water molecule This step IS bimolecular because it involves both bromide and heptyloxonium ion Step 2 IS slower than the proton transfer m step 1 so it is rate determining Using Ingold s ter mmology we classify nucleophilic substitutions that have a bimolecular rate determining step by the mechanistic symbol Sn2... 

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

Because acetylene is a far weaker acid than water and alcohols these substances are not suitable solvents for reactions involving acetylide ions Acetylide is instantly converted to acetylene by proton transfer from compounds that contain —OH groups... 

These reactions are usually performed in water or alcohols as solvents and the alkox ide ion intermediate is rapidly transformed to an alcohol by proton transfer The other involves acid catalysis Here the nucleophile is often... 

Step 3 Proton transfer to water completes the reaction and regenerates the acid catalyst... 

IS a two step process m which the first step is rate determining In step 1 the nucleophilic hydroxide ion attacks the carbonyl group forming a bond to carbon An alkoxide ion is the product of step 1 This alkoxide ion abstracts a proton from water m step 2 yielding the gemmal diol The second step like all other proton transfers between oxygen that we have seen is fast... 

Step 2 Proton transfer from water to the intermediate formed m step 1... 

Step 3 Proton transfer from the conjugate acid of the geminal diol to a water molecule... 

Step 5 The nitrogen stabilized carbocation is the conjugate acid of the imine Proton transfer to water gives the imine... 

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

The slow step m base catalyzed enolization is formation of the enolate ion The second step proton transfer from water to the enolate oxygen is very fast as are almost all proton transfers from one oxygen atom to another... 

There is another reaction available to the enediol intermediate Proton transfer from water to C 1 converts the enediol not to an aldose but to the ketose d fructose... 

Step 2 Proton transfer from water to give neutral form of tetrahedral intermediate 0 --------------------OH... 

Step 3 Proton transfer from water to nitrogen of tetrahedral intermediate... 

Table 8.7 Proton Transfer Reactions of Inorganic Materials in Water at 25°C 8.18...

---