Oxygen carbonyl

Fig. 12.20 4-Acetamido benzoic acid. Triangle smoothing predicts that the lower bound distance between the amide nitrogen and the carbonyl oxygen is equal to the sum of the van der Waals radii. The actual distance is about 6.4A.

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

Because the acylated product has a delocahsed lone pair and is less reactive than PhNHi. You may have been surprised that LiAlHi reduction completely removes the carbonyl oxygen atom. To help explain this, please draw the likely intermediate. 

In order for the transferability of parameters to be a good description of the molecule, force fields use atom types. This means that a sp carbon will be described by different parameters than a. sp - carbon, and so on. Usually, atoms in aromatic rings are treated differently from sp atoms. Some force fields even parameterize atoms for specific functional groups. For example, the carbonyl oxygen in a carboxylic acid may be described by different parameters than the carbonyl oxygen in a ketone. 

Reductive coupling of carbonyl compounds to yield olefins is achieved with titanium (0), which is freshly prepared by reduction of titanium(III) salts with LiAIH4 or with potassium. The removal of two carbonyl oxygen atoms is driven by T1O2 formation- Yields are often excellent even with sensitive or highly hindered olefins. (J.E. McMurry, 1974, 1976A,B). 

A regioselective aldol condensation described by Biichi succeeds for sterical reasons (G. Biichi, 1968). If one treats the diaidehyde given below with acid, both possible enols are probably formed in a reversible reaaion. Only compound A, however, is found as a product, since in B the interaction between the enol and ester groups which are in the same plane hinders the cyclization. BOchi used acid catalysis instead of the usual base catalysis. This is often advisable, when sterical hindrance may be important. It works, because the addition of a proton or a Lewis acid to a carbonyl oxygen acidifies the neighbouring CH-bonds. 

Step 1 Hydride (hydrogen + two electrons) is transferred from boron to the positively polarized carbon of the carbonyl group The carbonyl oxygen bonds to boron... 

The carbonyl oxygen of aldehydes and ketones can form hydrogen bonds with the pro tons of OH groups This makes them more soluble m water than alkenes but less solu ble than alcohols... 

Overall the reaction is classified as an addition Water adds to the carbonyl group Hydrogen becomes bonded to the negatively polarized carbonyl oxygen hydroxyl to the positively polarized carbon... 

The mechanism of this reaction is outlined m Figure 17 8 It is analogous to the mech anism of base catalyzed hydration m that the nucleophile (cyanide ion) attacks the car bonyl carbon m the first step of the reaction followed by proton transfer to the carbonyl oxygen in the second step... 

Under conditions of acid catalysis the nucleophilic addition step follows protonation of the carbonyl oxygen Protonation increases the carbocat ion character of a carbonyl group and makes it more electrophilic... 

The most stable conformation of acetone has one of the hydrogens of each methyl group eclipsed with the carbonyl oxygen Construct a model of this conformation... 

Construct a molecular model of cyclohexanone Do either of the hydrogens of C 2 eclipse the carbonyl oxygen" ... 

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

Step 1 A proton is transferred from the acid catalyst to the carbonyl oxygen... 

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

Notice too that the carbonyl oxygen of the carboxylic acid is protonated m the first step and not the hydroxyl oxygen The species formed by protonation of the car bonyl oxygen is more stable because it is stabilized by electron delocalization The pos itive charge is shared equally by both oxygens... 

Step 1 The carboxylic acid is protonated on its carbonyl oxygen The proton donor shown in the equation for this step is an aUtyloxonium ion formed by proton transfer from the acid catalyst to the alcohol... 

The positive charge m this cation cannot be shared by the two oxygens it is localized on one of them Because protonation of the carbonyl oxygen gives a more stable cation that cation is formed preferentially... 

The transition state involves the carbonyl oxygen of one carboxyl group—the one that stays behind—acting as a proton acceptor toward the hydroxyl group of the carboxyl that IS lost Carbon-carbon bond cleavage leads to the enol form of acetic acid along with a molecule of carbon dioxide... 

Protonation of the carbonyl oxygen activates the carbonyl group toward nucleophilic addition Addition of an alcohol gives a tetrahedral inter mediate (shown m the box m the preceding equation) which has the capacity to revert to starting materials or to undergo dehydration to yield an ester... 

A key feature of the first stage is the site at which the starting ester is protonated Protonation of the carbonyl oxygen as shown m step 1 of Figure 20 4 gives a cation that IS stabilized by electron delocalization The alternative site of protonation the alkoxy oxygen gives rise to a much less stable cation... 

Protonation of the carbonyl oxygen as emphasized earlier makes the carbonyl group more susceptible to nucleophilic attack A water molecule adds to the carbonyl group of the protonated ester m step 2 Loss of a proton from the resulting oxonium ion gives the neutral form of the tetrahedral intermediate m step 3 and completes the first stage of the mechanism... 

In an extension of the work described m the preceding section Bender showed that basic ester hydrolysis was not concerted and like acid hydrolysis took place by way of a tetrahedral intermediate The nature of the experiment was the same and the results were similar to those observed m the acid catalyzed reaction Ethyl benzoate enriched m 0 at the carbonyl oxygen was subjected to hydrolysis m base and samples were isolated before saponification was complete The recovered ethyl benzoate was found to have lost a por tion of Its isotopic label consistent with the formation of a tetrahedral intermediate... 

The amide is activated toward nucleophilic attack by protonation of its carbonyl oxygen The cation produced m this step is stabilized by resonance involving the nitro gen lone pair and is more stable than the intermediate m which the amide nitrogen is protonated... 

Protonation of carbonyl oxygen Protonation of amide nitrogen... 

The O acylation of phenols with carboxylic acid anhydrides can be conveniently catalyzed m either of two ways One method involves converting the acid anhydride to a more powerful acylatmg agent by protonation of one of its carbonyl oxygens Addi tion of a few drops of sulfuric acid is usually sufficient... 

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