Oxygen carbonyl group

Isoamyl acetate is the common name of the substance most responsible for the characteris tic odor of bananas Wnte a structural formula for isoamyl acetate given the information that it IS an ester in which the carbonyl group bears a methyl substituent and there is a 3 methylbutyl group attached to one of the oxygens... 

To understand the effect of a carbonyl group attached directly to the ring consider Its polarization The electrons m the carbon-oxygen double bond are drawn toward oxy gen and away from carbon leaving the carbon attached to the nng with a partial posi tive charge Using benzaldehyde as an example... 

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

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... 

Neither sodium borohydride nor lithium aluminum hydride reduces isolated carbon-carbon double bonds This makes possible the selective reduction of a carbonyl group m a molecule that contains both carbon-carbon and carbon-oxygen double bonds... 

The reaction of alcohols with acyl chlorides is analogous to their reaction with p toluenesulfonyl chloride described earlier (Section 8 14 and Table 15 2) In those reactions a p toluene sulfonate ester was formed by displacement of chloride from the sulfonyl group by the oxygen of the alcohol Carboxylic esters arise by displacement of chlonde from a carbonyl group by the alcohol oxygen... 

A major difference between alcohols and thiols concerns their oxidation We have seen earlier m this chapter that oxidation of alcohols gives compounds having carbonyl groups Analogous oxidation of thiols to compounds with C=S functions does not occur Only sulfur is oxidized not carbon and compounds containing sulfur m various oxida tion states are possible These include a series of acids classified as sulfemc sulfimc and sulfonic according to the number of oxygens attached to sulfur... 

The structural features especially the very polar nature of the carbonyl group point clearly to the kind of chemistry we will see for aldehydes and ketones in this chapter The partially positive carbon of C=0 has carbocation character and is electrophilic The planar arrangement of its bonds make this carbon relatively uncrowded and susceptible to attack by nucleophiles Oxygen is partially negative and weakly basic... 

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... 

IS a two step process m which the first step is rate determining In step 1 the nucleophilic hydroxide ion attacks the carbonyl group forming a bond to carbon An alkoxide ion is the product of step 1 This alkoxide ion abstracts a proton from water m step 2 yielding the gemmal diol The second step like all other proton transfers between oxygen that we have seen is fast... 

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 reaction of ketones with peroxy acids is both novel and synthetically useful An oxygen from the peroxy acid is inserted between the carbonyl group and one of the attached car bons of the ketone to give an ester Reactions of this type were first described by Adolf von Baeyer and Victor Vilhger m 1899 and are known as Baeyer—Villiger oxidations... 

The product (6 hexanohde) is a cyclic ester or lactone (Section 19 15) Like the Baeyer-Vilhger oxidation an oxygen atom is inserted between the carbonyl group and a carbon attached to it But peroxy acids are not involved m any way the oxidation of cyclohexanone is catalyzed by an enzyme called cyclohexanone monooxygenase with the aid of certain coenzymes... 

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... 

Increased single bond character in a carbonyl group is associated with a decreased carbon—oxygen stretching frequency Among the three compounds benzaldehyde 2 4 6 trimethoxybenzaldehyde and 2 4 6 trinitrobenzaldehyde which one will have the lowest frequency carbonyl absorption" Which one will have the highest" ... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

Additionally sp hybridization of the hydroxyl oxygen allows one of its unshared electron pairs to be delocalized by orbital overlap with the tt system of the carbonyl group (Figure 19 1) In resonance terms this electron delocalization is represented as... 

Lone pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron rich site to be the oxygen of the carbonyl group and the most electron poor one to be as expected the OH hydrogen... 

The resonance effect of the carbonyl group Electron delocalization expressed by resonance between the following Lewis structures causes the negative charge in acetate to be shared equally by both oxygens Electron delocalization of this type IS not available to ethoxide ion... 

Activation of the carbonyl group by protonation of the carbonyl oxygen... 

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... 

Acid anhydrides The carbonyl group of an acid anhydride is better stabilized by electron donation than the carbonyl group of an acyl chloride Even though oxygen... 

Esters Like acid anhydrides the carbonyl group of an ester is stabilized by elec tron release from oxygen Because there is only one carbonyl group versus two m anhydrides esters are stabilized more and are less reactive than anhydrides... 

Amides Nitrogen is less electronegative than oxygen therefore the carbonyl group of an amide is stabilized more than that of an ester... 

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