Equilibrium constants cyanohydrin formation

Table 3. Equilibrium Constants for Formation of Cyanohydrins from Hydrogen Cyanide Plus Carbonyl Compounds ...

The equilibrium constant for formation of the cyanohydrin of cyclopentanone and HCiN is 67, while for butan-2-one and HCN it is 28. Explain,... 

Stabilization of the carbonyl group decreases the equilibrium constant for formation of the cyanohydrin. K is greatest for formaldehyde, which has the least stabihzed carbonyl, and... 

Cyanohydrin formation is somewhat unusual because it is one of the few examples of the addition of a protic acid (H—Y) to a carbonyl group. As noted in the previous section, protic adds such as H20, HBr, HC1, and H2S04 don t normally yield carbonyl addition products because the equilibrium constants ate unfavorable. With HCN, however, the equilibrium favors the cyanohydrin adduct. 

However, such addition is not likely to facilitate formation of cyanohydrin because it represents a competitive saturation of the carbonyl double bond. Indeed, if the equilibrium constant for this addition were large, an excess of hydroxide ion could inhibit cyanohydrin formation by tying up the ketone as the adduct 1. 

Exercise 16-10 One possible way of carrying out the cyanohydrin reaction would be to dispense with hydrogen cyanide and just use the carbonyl compound and sodium cyanide. Would the equilibrium constant for cyanohydrin formation be more... 

Exercise 16-12 Explain what factors would operate to make the equilibrium constant for cyanohydrin formation 1000 times greater for cyclohexanone than for cyclo-pentanone. Why What would you expect for cyclobutanone relative to cyclopenta-none Why ... 

The constant of equilibrium 22 may be calculated by several methods. A competitive method was used211 to evalute the equilibrium constant in the formation of cyanohydrin anions of substituted benzaldehydes in DMSO (equilibrium 24). 

The nucleophilic reaction of the cyanide ion on the carbonyl group is facilitated by protonat-ing the latter to a carboxonium ion. The addition of acid promotes the formation of cyanohydrins, but mainly for a thermodynamic reason. Under acidic conditions cyanohydrins equilibrate with the carbonyl compound and HCN. Under basic conditions they are in equilibrium with the same carbonyl compound and NaCN or KCN. The first reaction has a smaller equilibrium constant than the second, that is, the cyanohydrin is favored. So when cyanohydrins are formed under acidic or neutral (see Figure 9.8) instead of basic conditions, the reversal of the reaction is suppressed. 

Explain why p-me tho xyacetophenone has a smaller equilibrium constant for cyanohydrin formation than does acetophenone. 

The methoxy group donates electrons to the carbonyl carbon by resonance, making it less electrophilic and less reactive. Therefore, the equilibrium constant for cyanohydrin formation is larger for acetophenone than for p-methoxyacetophenone. 

Explain which compound has the larger equilibrium constant for cyanohydrin formation O O... 

Arrange these compounds in order of increasing equilibrium constant for cyanohydrin formation and explain your reasoning ... 

Explain the difference in the equilibrium constants for cyanohydrin formation for these compounds ... 

Cyanohydrin formation is reversible, and the equilibrium constant may or may not favor the cyanohydrin. These equilibrium constants follow the general reactivity trend of ketones and aldehydes ... 

Formaldehyde reacts quickly and quantitatively with HCN. Most other aldehydes have equilibrium constants that favor cyanohydrin formation. Reactions of HCN with ketones have equilibrium constants that may favor either the ketones or the cyanohydrins, depending on the structure. Ketones that are hindered by large alkyl groups react slowly with HCN and give poor yields of cyanohydrins. 

Rate constants for cyanohydrin formation have been calculated for the limited range of compounds where both rate and equilibrium constants were available. The mechanistic model involved bond making and geometry change as reaction dimensions, with allowance for desolvation of cyanide ion when it was in contact with the... 

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