Hydrogenation carbon-oxygen double bonds

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... 

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... 

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. 

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. 

Alternative mechanisms for carbon-oxygen double bond hydrogenation and methanol synthesis are shown in Schemes 4 and 5. 

Certain electrophilic carbon-carbon and carbon-oxygen double bonds can undergo an addition reaction with alkenes in which an allylic hydrogen is transferred to the... 

Catalytic hydrogenation of carbon-oxygen double bonds to the corresponding alcohols proceeds readily in the presence of various metal catalysts, usually under ambient conditions. Two reviews were published1,315 in 1987 and 1991, respectively, since our monograph appeared in 19852. In the latter book this field is summarized in detail with reference to earlier monographs. The present review shortly summarizes the results published recently and discusses the basic conclusions arrived at on the hydrogenation of oxo compounds. 

Examination of the molecular formula C14H2602 reveals that the compound has an index of hydrogen deficiency of 2. Because we are told that the compound is a carboxylic acid, one of these elements of unsaturation must be a carbon-oxygen double bond. The other must be a carbon-carbon double bond because the compound undergoes cleavage on ozonolysis. Examining the products of ozonol-ysis serves to locate the position of the double bond. 

Compound A (C4H602) has an index of hydrogen deficiency of 2. With two oxygen atoms and a peak in the infrared at 1760 cm-1, it is likely that one of the elements of unsaturation is the carbon-oxygen double bond of an ester. The H NMR spectrum contains a three-proton singlet at 8 2.1 ppm, which is consistent with a CH3C unit. It is likely that compound A is an acetate ester. 

The main reactions of the carbonyl group are nucleophilic additions to the carbon-oxygen double bond. As shown below, this addition consists of adding a nucleophile and a hydrogen across the carbon-oxygen double bond. 

The ene reaction,3 6360-365 the addition of a carbon-carbon or carbon-oxygen double bond with concomitant transfer of an allylic hydrogen, can allow for chirality transfer.366-369 The reaction has similarities to the Diels-Alder reaction in that a o-bond is formed at the expense of a 7t-bond. In addition, the use of a Lewis acid as a catalyst allows for control of the relative stereochemistry (Scheme 26.14).370-372 Large-scale reactions will be complicated by the need to use either high temperatures or Lewis acids. In addition, thermal and Friedel-Crafts-type degradation products may be problematic with the use of these conditions.361373... 

A reaction of an achiral molecule may introduce a chirality center, producing a chiral product. For example, reaction of the following ketone with hydrogen in the presence of a catalyst results in addition of the hydrogen to the carbon-oxygen double bond, producing 2-butanol ... 

When the catalytic hydrogenation reaction is run under relatively mild conditions (room temperature and a pressure of hydrogen gas of several atmospheres or less), the reaction is very selective. Carbon-carbon double bonds of alkenes and carbon-carbon triple bonds of alkynes react readily, whereas carbon-carbon double bonds of aromatic rings and carbon-oxygen double bonds are usually inert under these reaction conditions. Some examples are provided in the following equations. Note that the stereochemistry of the addition reaction makes no difference in the first two examples. In the last example the major product results from syn addition. 

Reduction of aldehydes and ketones usually occurs by the addition of hydrogen across the carbon-oxygen double bond to yield alcohols, but reductive conversion of a carbonyl group to a methylene group requires complete removal of the oxygen, and is called deoxygenation. 

The difficult step in all this is breaking the carbon-hydrogen bond this is made possible by the synchronous departure of HCr03 , in what is really an E2 elimination—but here with the formation of a carbon-oxygen double bond. 

Catalytic hydrogenation of a,jS-unsaturated aldehydes and ketones yields saturated alcohols, addition of hydrogen occurring both at carbon-carbon and at carbon-oxygen double bonds. It is for the purpose of ultimately preparing saturated alcohols that the aldol condensation is often carried out. For example, /i-butyl alcohol and 2-ethyl-1-hexanol are both prepared on an industrial scale in this way ... 

This type of compound is called an enol, and the reorganisation of the carbon/ oxygen double bond in the carbonyl group to the carbon/carbon double bond in the enol compound is called the keto/enol tautomerism. Notice, also, that this is accompanied by a hydrogen atom changing place, from the carbon to the oxygen. Tautomers are different from canonical structures in that the former... 

The general formula for a molecule that contains a carbon/oxygen double bond is R R C=0. If either or both of the substituents is a hydrogen atom, the molecule is an aldehyde if both substituents are alkyl groups, the molecule is a ketone. The C=0 group itself is called a carbonyl group. 

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