Carbon—oxygen double bond

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

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 most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

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... 

FIGURE 17 2 Both (a) ethylene and (b) formal dehyde have the same num ber of electrons and carbon IS sp hybridized in both In formaldehyde one of the carbons is replaced by an sp hybridized oxygen Like the carbon-carbon double bond of ethylene the carbon-oxygen double bond of formaldehyde is com posed of a (T component and a TT component... 

In these and numerous other simple cases the keto form is more stable than the enol by some 45-60 kJ/mol (11-14 kcal/mol) The chief reason for this difference is that a carbon-oxygen double bond is stronger than a carbon-carbon double bond... 

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... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

This suggests sp hybridization at carbon and a ct + tt carbon-oxygen double bond analogous to that of aldehydes and ketones... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

This makes a carbon-oxygen double bond Note If you cannot see which va lence is the double valence then rotate the model first... 

Conjugated with a carbon-carbon double bond —C=C—C=C— Conjugated with a carbon-oxygen double bond —C=C—C=0 Cumulative —C=C=C— or —C=C=0 Triple bond... 

Cumulative carbon-carbon-oxygen double bonds H—C—C=C=0 108(1)... 

The carbon-oxygen double bond of the carbonyl group is opened, and the hydrogen sulfite radical is added. An increase in temperature reverses the reaction more easily for ketones than for aldehydes. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

Both the carbon-carbon and carbon-oxygen double bonds of fluoroketenes can take part in [2+2] cycloadditions, but with cyclopentadiene, only cyclo butanones are produced via concerted [2 +2 ] additions [J34] (equation 58) Cycloadditions involving the carbon-oxygen double bonds to form oxetanes are discussed on page 855 Difluoroketene is veiy short lived and difficult to intercept but has been trapped successfully by very electron rich addends to give 2 2 di fluorocyclobutanones m moderate yields [/55] (equation 59)... 

Commonly used in biology as a tissue preservative, formaldehyde, CIDO, contains a carbon -oxygen double bond. Draw the line-bond structure of formaldehyde, and indicate the hybridization of the carbon atom. 

The carbon-oxygen double bond of a carbonyl group is similar in many respects to the carbon-carbon double bond of an alkene. The carbonyl carbon atom is s/ 2-hybridized and forms three valence electron remains in a carbon p orbital and forms a tt bond to oxygen by overlap with an oxygen p orbital. The oxygen atom also has two nonbonding pairs of electrons, w hich occupy its remaining two orbitals. 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

When a carbonyl group is bonded to a substituent group that can potentially depart as a Lewis base, addition of a nucleophile to the carbonyl carbon leads to elimination and the regeneration of a carbon-oxygen double bond. Esters undergo hydrolysis with alkali hydroxides to form alkali metal salts of carboxylic acids and alcohols. Amides undergo hydrolysis with mineral acids to form carboxylic acids and amine salts. Carbamates undergo alkaline hydrolysis to form amines, carbon dioxide, and alcohols. 

C O , that is, the normal covalent carbon-oxygen double bond,- the estimated bond energy 6.60 v.e., then the ketones would show a resonance energy of 1.11 v.e. arising from the + —... 

It has been found that a carbon-oxygen double bond decreases the single-bond radius of the carbon atom involved Pauling and Brockway, paper to be submitted to This Journal. 

The carbon-oxygen double bond in all ketones involves resonance of the. . +. ... 

Active Substrate. If a new stereogenic center is ereated in a molecule that is already optically active, the two diastereomers are not (except fortuitously) formed in equal amounts. The reason is that the direction of attack by the reagent is determined by the groups already there. For certain additions to the carbon-oxygen double bond of ketones containing an asymmetric a carbon. Cram s rule predicts which diastereomer will predominate (diastereo-selecti vity). ... 

The addition of a negative ion to a carbon-oxygen double bond does not give a carbanion, since the negative charge resides on the oxygen. 

No rate enhancement of the enantioselective hydrogenation pathway is expected, in the manner adduced for the Pt-catalysed reaction, because the process is not one of simple H-atom addition across a carbon-oxygen double bond. 

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