Hydrate of an aldehyde

FIGURE 17 6 Potential en ergy diagram for base catalyzed hydration of an aldehyde or ketone... 

When treating the overall transformation kinetics of an organic compound as we have done for the hydrolysis of benzyl chloride (Eq. 12-11), we assume that the reverse reaction (i.e., the formation of benzyl chloride from benzyl alcohol) can be neglected. For many of the reactions discussed in the following chapters we will make this assumption either because the reverse reaction has an extremely small rate constant (i.e., the reaction is practically irreversible), or because the concentration ) of the reactant(s) are very large as compared to the concentration(s) of the product(s). There are, however, situations in which the reverse reaction has to be taken into account. We have already encountered such a reaction in Illustrative Example 12.1. To demonstrate how to handle the reaction kinetics in such a case, we use the hydration of an aldehyde to yield a diol (Fig. 12.3). This example will also illustrate how the equilibrium reaction constant, Kn is related to the kinetic rate constants, kY and k2, of the forward and reverse reaction. 

The hydration of an aldehyde RCHO can be expressed with two (pseudo-first-order rate laws consisting of an elimination (minus sign) and a production (plus sign) term ... 

Homogeneous system fall Ja are zero). An example of a two-dimensional homogeneous system is the first-order reversible reaction between two chemical species discussed in Chapter 12 using the example of the hydration of an aldehyde (see Eq. 12-16). Again, matrix theory provides us with a very useful mle which states that for such systems the resulting matrix is singular (that is, its determinant is zero, see Box 21.8) and thus at least one eigenvalue must be zero. Furthermore, in Eq. 21-48 all ) , are zero. 

A direct metal involvement in the hydration of an aldehyde is seen in the ruthenium(n) complex of the hydrated form of 2-pyridinecarbaldehyde (3.10). 

We can prove this assertion by using several alternative reaction mechanisms and by invoking the principle of microscopic reversibility, which states that when an overall reaction is at equilibrium, each step in the mechanistic sequence must also be at equilibrium (i.e., the rate of forward reaction equals the rate of the reverse reaction). Three alternative mechanisms for the hydration of an aldehyde can be considered as follows ... 

FIGURE 17.4 The mechanism of hydration of an aldehyde or ketone in basic solution. Hydroxide ion is a catalyst it is consumed in the first step, and regenerated in the second. 

Hydration of an aldehyde can also be catalyzed by hydroxide ion. Propose a mechanism for hydroxide-ion-catalyzed hydration. 

Hydration of an Aldehyde or Ketone in Basic Solution THE OVERALL REACTION ... 

Steps 1-3 Acid-catalyzed addition of water to the carbonyl group These three steps are the same as those for acid-catalyzed hydration of an aldehyde or ketone (Mechanism 17.2). Steps 1 and 3 are proton transfers between oxygens and are fast. Water acts as a nucleophile in step 2. Step 2 is rate-determining. 

Hydration of an aldehyde or a ketone is readily reversible, and the diol can eliminate water to regenerate the aldehyde or ketone. In most cases, equilibrium strongly favors the carbonyl group. For a few simple aldehydes, however, the hydrate is favored. For example, when formaldehyde is dissolved in water at 20°C, the position of equilibrium is such that it is more than 99% hydrated. 

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