Relative reactivity aldehydes ketones

Aldehyde or ketone Relative reactivity Aldehyde or ketone Relative reactivity ... 

AH ahphatic aldehydes and most ketones react to form cyanohydrins. The lower reactivity of ketones, relative to aldehydes, is attributed to a combination of electron-donating effects and increased steric hindrance of the second alkyl group in the ketones. The magnitude of the equiUbrium constants for the addition of hydrogen cyanide to a carbonyl group is a measure of the stabiUty of the cyanohydrin relative to the carbonyl compound plus hydrogen cyanide ... 

Reductions by NaBKt are characterized by low enthalpies of activation (8-13kcal/mol) and large negative entropies of activation (—28 to —40eu). Aldehydes are substantially more reactive than ketones, as can be seen by comparison of the rate data for benzaldehyde and acetophenone. This relative reactivity is characteristic of nearly all carbonyl addition reactions. The reduced reactivity of ketones is attributed primarily to steric effects. Not only does the additional substituent increase the steric restrictions to approach of the nucleophile, but it also causes larger steric interaction in the tetrahedral product as the hybridization changes from trigonal to tetrahedral. 

The carbonyl carbon of a ketone bears two electron-releasing alkyl groups an aldehyde carbonyl group has only one. Just as a disubstituted double bond in an alkene is more stable than a monosubstituted double bond, a ketone carbonyl is more stable than an aldehyde carbonyl. We ll see later in this chapter that structural effects on the relative stability of carbonyl groups in aldehydes and ketones are an important factor in their relative reactivity. 

As an example of enolate-ion reactivity, aldehydes and ketones undergo base-promoted o halogenation. Even relatively weak bases such as hydroxide ion are effective for halogenation because it s not necessary to convert the ketone completely into its enolate ion. As soon as a small amount of enolate is generated, it reacts immediately with the halogen, removing it from the reaction and driving the equilibrium for further enolate ion formation. 

Kinetic studies established that tetra-n-butylammonium borohydride in dichloromethane was a very effective reducing agent and that, by using stoichiometric amounts of the ammonium salt under homogeneous conditions, the relative case of reduction of various classes of carbonyl compounds was the same as that recorded for the sodium salt in a hydroxylic solvent, i.e. acid chlorides aldehydes > ketones esters. However, the reactivities, ranging from rapid reduction of acid chlorides at -780 C to incomplete reduction of esters at four days at 250 C, indicated the greater selectivity of the ammonium salts, compared with sodium borohydride [9], particularly as, under these conditions, conjugated C=C double bonds are not reduced. 

Aldehydes are generally more reactive than ketones in nucleophiUc addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophUicity of the carbonyl carbon more effectively than in former. 

Relative reactivities of aldehydes, ketones, esters, acid chlorides and anhydrides. 

Because of the relative inertness of functional groups other than alcohols to Dess-Martin conditions, a Dess-Martin oxidation is a good choice when an in situ reaction of the resulting aldehydes or ketones is desired. It is particularly common to use Dess-Martin periodinane in order to generate very reactive aldehydes or ketones that suffer in situ concerted reactions, such as Diels-Alder additions,67 oxy-Claisen reactions,68 pericyclic processes69 and concerted hydrogen shifts.70... 

Hypervalent silicon componnds have found wide utility in organic synthesis. In general, pentacoordinated anionic silicates are more reactive toward nucleophiles than are tetracoordinated silanes. For example, Mes2SiF2 is nmeactive toward water, while (the 18-crown-6 potassium salt of) Mes2SiF3 is completely hydrolyzed within minutes. Similarly, the pentacoordinate anion HSi(OEt)4 is an effective reducing agent for aldehydes, ketones, and esters at or below room temperature (Scheme 2) no snch reaction occurs with HSi(OEt)3. The difference in relative reactivities of hypervalent and nonhypervalent species is relevant to the intermediates proposed in Section 7.6. 

As a result, aldehydes and ketones react with nucleophiles. The relative reactivity of the carbonyl group is determined by the number of R groups bonded to it. As the number of R groups around the carbonyl carbon increases, the reactivity of the carbonyl compound decreases, resulting in the following order of reactivity ... 

The reaction of diazomethane and its derivatives with ketones has been a rich source of chemistry. The major part of the developmental work with regard to reagents and mechanisms emerged from extensive studies in this area. Relative to aldehydes, aliphatic ketones are much less reactive to diazomethane and... 

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions for both steric and electronic reasons. Sterically, the presence of only one relatively large substituent bonded to the C=0 carbon in an aldehyde versus two large substituents in a ketone means that an attacking nucleophile is able to approach an aldehyde more readily. Thus, the transition state leading to the tetrahedral intermediate is less crowded and lower in energy for an aldehyde than for a ketone (Figure 19.3). 

---