Addition to the Carbonyl Group

Only rarely are the hydrates sufficiently stable to be isolated, but the hydration reaction allows an examination of the factors that influence addition to the carbonyl group. It is common to represent the equilibrium between an aldehyde or ketone and the corresponding hydrate in terms of a dissociation constant, K, as shown in equation 7.60. 

Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed. Cornell University Press Ithaca, NY, 1969 p. 994 U. 

TABLE 7.6 Equilibrium Constants for Dissociation of Hydrates of Carbonyl Compounds  

Values of for several hydrates of interest are shown in Table The 

The Ki values shown in Table 7.6 are defined (reference 122) as Kj = (RiR2C=0]/[RiR2C (0H)2] and are therefore dimensionless because the concentration of water (55 M, with activity assumed to be unity) is included in the equilibrium constant. Literature values for equilibrium constants in which the concentration of water is not incorporated into the Kd term have been divided by 55.5 to make the values in Table 7.6 comparable. 

---

Aldehydes and Ketones Nucleophilic Addition to the Carbonyl Group... 

ALDEHYDES AND KETONES NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP... 

The mechanism for formation of benzaldehyde diethyl acetal which proceeds m two stages is presented m Figure 17 9 The first stage (steps 1-3) involves formation of a hemiacetal m the second stage (steps 4-7) the hemiacetal is converted to the acetal Nucleophilic addition to the carbonyl group characterizes the first stage carbocation chemistry the second The key carbocation intermediate is stabilized by electron release from oxygen... 

The characteristic reactions of aldehydes and ketones involve nude ophihc addition to the carbonyl group and are summarized m Table 17 5 Reagents of the type HY react according to the general equation... 

Section 17 14 Nucleophilic addition to the carbonyl group is stereoselective When one direction of approach to the carbonyl group is less hindered than the other the nucleophile normally attacks at the less hindered face... 

---