Aldehydes are more reactive than ketones

The second feature is a steric consideration. During nucleophilic addition, the planar sp system of the carbonyl compound (bond angle 120°) is converted 

This crowding is more severe with two alkyl substituents (from ketones) than with one alkyl and the much smaller hydrogen (from aldehydes). A consequence of this change is that the planar aldehyde or ketone can be attacked from either side of the plane with essentially equal probability. If the substituents are all different, then this will result in the creation of a chiral centre but, since both enantiomers will be formed in equal amounts, the product will be an optically inactive racemate (assuming no other chiral centres are present in the R groups) see Section 3.4.1. 

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Electronically, aldehydes are more reactive than ketones because of the greater polarization of aldehyde carbonyl groups. To see this polarity difference, recall the stability order of carbocations (Section 6.9). A primary carbocation is higher in energy and thus more reactive than a secondary carbocation because... 

How might you carry out the following selective transformations One of the two schemes requires a protection step. (Recall from Section 19.5 that aldehydes are more reactive than ketones toward nucleophilic addition.)... 

Reactivity factors in additions to carbon-hetero multiple bonds are similar to those for the tetrahedral mechanism of nucleophilic substitution. If A and/or B are electron-donating groups, rates are decreased. Electron-attracting substituents increase rates. This means that aldehydes are more reactive than ketones. Aryl groups are somewhat deactivating compared to alkyl, because of resonance that stabilizes the substrate molecule but is lost on going to the intermediate ... 

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. 

Ketones are less reactive towards the nucleophile. In Organic Chemistry 1, you saw that alkyl groups are electron donating. In ketones, the presence of the two alkyl groups attached to the carbonyl do a better job at compensating for the 5+ on the carbon atom than do one alkyl group and a hydrogen atom in an aldehyde. For this reason, aldehydes are more reactive than ketones. 

Aldehydes are more reactive than ketones. Two factors that make aldehydes more reactive than ketones are electronic and steric effects. Ketones have two alkyl groups, whereas aldehydes have only one. Because alkyl groups are electron donating, ketones have their effective partial positive charge reduced more than aldehydes. The electrophilic carbon is the site where the nucleophile approaches for reaction to occur. In ketones, two alkyl groups create more steric hindrance than one in aldehydes. As a result, ketones offer more steric resistance toward the nucleophilic attack than aldehydes. 

Aldehydes are more reactive than ketones. Therefore, aldehydes react with ethylene glycol to form acetals preferentially over ketones. Thus, aldehydes can be protected selectively. This is a useful way to perform reactions on ketone functionalities in molecules that contain both aldehyde and ketone groups. 

Ketones and aldehydes are similar in structure, and they have similar properties. There are some differences, however, particularly in their reactions with oxidizing agents and with nucleophiles. In most cases, aldehydes are more reactive than ketones, for reasons we discuss shortly. 

Any other portions of the molecule that get in the way of (or, in other words, that cause steric hindrance to) the Biirgi-Dunitz trajectory will greatly reduce the rate of addition and this is another reason why aldehydes are more reactive than ketones. The importance of tire Biirgi-Dunitz trajectory will become more evident later—particularly in Chapter 34. 

Homoallylic alcohols, The reagent reacts with carbonyl compounds in the presence of BF, etherate to form homoallylic alcohols. In general, aldehydes are more reactive than ketones, and methyl ketones are more reactive than internal ketones. 

KF is ineffective in the reaction, even when activated by a crown ether. This allylation apparently involves cleavage to a and b, which adds to the carbonyl group to give c, and is possible because of the high Si—F bond energy. Aldehydes are more reactive than ketones. 

Aldehydes are more reactive than ketones towards nucleophilic attack for both steric and electronic reasons. 

There is no doubt that aldehydes are more reactive than ketones toward nucleophiles. However, both carbonyl substrates are functionalized by activated nucleophiles e.g. RLi or RMgX (X=halogen), with poor chemoselectivity. For example, benzaldehyde is not a dominant species to be alkylated in the coexistence of acetophenone. Reetz and co-workers addressed these difficulties by systematic studies on ligand effects in carbonyl addition reactions of RMgL (L=relatively bulky ligand) [31]. Upon reacting a 1 1 mixture of PhCHO and PhCOMe with 1 equiv of RMgL in a competition experiment, the aldehyde reacted essentially exclusively to form adduct 2-H (Table 2-2, entries 2-A). 

Cu(BF4)2vvH20, MeOH, trimethylorthoformate, rt, 78-95% yield. Aldehydes are more reactive than ketones but with insufficient chemoselectivity to be useful. ... 

If we again assume that the reaction products, and their orientation, are determined by kinetic rather than thermodynamic considerations, then the two important features of the condensation are the pH of the reaction mixture and the difference in reactivity of the two carbonyl groups in the a-dicarbonyl compound. In general aldehydes are more reactive than ketones, which are in turn more reactive than carboxylate groups. Thus applying the arguments used in Section III, at neutral pH the products 16... 

In the following reaction, the aldehyde reacts with the diol because aldehydes are more reactive than ketones. The Grignard reagent will now react only with the keto group. The protecting group can be removed by acid-catalyzed hydrolysis. 

Aldehydes also react smoothly to afford adducts in 75-85% yield (equation IV). In fact aldehydes are more reactive than ketones. 

Miscellaneous Alkylations.—Chromic chloride is readily reduced by Li AIH4 to give a salt, presumably Cr", which can be used for the Grignard-type carbonyl addition to allyl halides [equation (18)]. Aldehydes are more reactive than ketones and can... 

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