Oxidation, alcohols aldehydes

The most widely occurring terpenes are the smallest molecules, that is, the monoterpenes, CioHjg, and their oxygenated derivatives such as ketones, aldehydes, alcohols, oxides, phenols, along with simple hydrocarbons. Their properties are determined by functional groups—oxygen-containing radicals attached to the carbon skeleton. 

Precaution Eye protection, gloves rec. avoid temps. > 200 C Hazardous Decomp. Prods. Complete combustion COj incomplete combustion CO, hydrocarbon oxidation products including organic acids, aldehydes, alcohols, oxides of sodium... 

It IS often necessary to prepare ketones by processes involving carbon-carbon bond formation In such cases the standard method combines addition of a Gngnard reagent to an aldehyde with oxidation of the resulting secondary alcohol... 

Mechanistically these reactions probably proceed through the hydrate of the aldehyde and follow a course similar to that of alcohol oxidation... 

Sofid sodium permanganate monohydrate has been shown to be a selective synthetic reagent (156). It is typically used in hexane for the heterogeneous oxidation of aldehydes, alcohols, and sulfides. Synthetic methodology based on crystal surfaces exhibited greater selectivity, higher yield, and easier work-up as compared to aqueous permanganate reactions. 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

Commercially, pure ozonides generally are not isolated or handled because of the explosive nature of lower molecular weight species. Ozonides can be hydrolyzed or reduced (eg, by Zn/CH COOH) to aldehydes and/or ketones. Hydrolysis of the cycHc bisperoxide (8) gives similar products. Catalytic (Pt/excess H2) or hydride (eg, LiAlH reduction of (7) provides alcohols. Oxidation (O2, H2O2, peracids) leads to ketones and/or carboxyUc acids. Ozonides also can be catalyticaHy converted to amines by NH and H2. Reaction with an alcohol and anhydrous HCl gives carboxyUc esters. 

Secondary alcohols are oxidized at room temperature to ketones in high yields by HOCl generated in situ from aqueous NaOCl and acetic acid (109,110). Selective oxidation in the presence of a primary alcohol is possible. In methanol, aldehydes are oxidized to methyl esters (110). Under the proper conditions, alcohols can be esterified with HOCl forming isolable alkyl hypochlorites. 

Octyl Alcohol.—This is the primary normal alcohol of the formula CH3(CH2)gCH20H. It has an odour recalling that of opoponax, and is useful in the blending of perfumes of this type. It boils at 196° to 197°, and has a specific gravity 0"8278. It yields octyl aldehyde on oxidation, whose naphtho-cinchoninic acid compound melts at 234°. 

We ve already discussed two methods of aldehyde synthesis oxidation of primary alcohols and oxidative cleavage of alkenes. 

I Oxidation of a primary alcohol or an aldehyde yields a carboxylic acid (Sections 17.7 and 19.3). Primary alcohols are often oxidized with C1O3 in aqueous acid, and aldehydes are oxidized with either acidic Cr03 or basic silver oxide (Tollens reagent). 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

Various experimental conditions have been used for oxidations of alcohols by Cr(VI) on a laboratory scale, and several examples are shown in Scheme 12.1. Entry 1 is an example of oxidation of a primary alcohol to an aldehyde. The propanal is distilled from the reaction mixture as oxidation proceeds, which minimizes overoxidation. For secondary alcohols, oxidation can be done by addition of an acidic aqueous solution containing chromic acid (known as Jones reagent) to an acetone solution of the alcohol. Oxidation normally occurs rapidly, and overoxidation is minimal. In acetone solution, the reduced chromium salts precipitate and the reaction solution can be decanted. Entries 2 to 4 in Scheme 12.1 are examples of this method. 

Aldehydes can be oxidized to carboxylic acids by both Mn(VII) and Cr(VI). Fairly detailed mechanistic studies have been carried out for Cr(VI). A chromate ester of the aldehyde hydrate is believed to be formed, and this species decomposes in the rate-determining step by a mechanism similar to the one that operates in alcohol oxidations.209... 

Effective catalysts for heterogeneous oxidations using 02 are mainly Pt and Pd with some activity by Ir70 and Ru.71 Much work has gone into alcohol oxidations that are dehydrogenations to ketones or aldehydes. Also, oxygen may be inserted at allylic positions of alkenes and these may be dehydrogenated to ketones or aldehydes.72 In the case of aldehydes, additional oxidation may be accomplished to produce acids.72,73... 

Since aldehydes are notoriously polymerizable and difficult to manipulate, the products of periodate oxidation are oftentimes further oxidized, with hypohalite, to carboxylic acids, or are reduced to the corresponding alcohols. Oxidation has been more usually employed than reduction, since acids frequently form crystalline salts and other conveniently prepared derivatives. A process of oxidation of these aldehydic products by hypo-bromite, in the presence of barium carbonate or strontium carbonate, was developed and used extensively by Hudson and his coworkers.107 110 194-199,90s Their method can best be illustrated by an example the further oxidation of the dialdehyde, VI, shown previously (see p. 16) to be obtained by the oxidation of the methyl a-D-aldohexopyranosides. The isolation of... 

Oxidative damage to membrane polyunsaturated fatty acids leads to the formation of numerous lipid peroxidation products, some of which can be measured as index of oxidative stress, including hydrocarbons, aldehydes, alcohols, ketones, and short carboxylic acids. 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

Alcohols retard the oxidation of aldehydes. The parameters of aldehyde co-oxidation with cycloolefins, alcohols, and aldehydes are collected in Table 8.6. 

Parameters of Aldehydes Co-oxidation with Hydrocarbons, Alcohols, and Aldehydes... 

James An early process for making mixed oxygenated organic compounds by the catalytic oxidation of petroleum fractions. The products were aldehydes, alcohols, and carboxylic acids. Developed by J. H. James at the Carnegie Institute of Technology, Pittsburgh. 

This complex oxidizes primary or secondary alcohols in DCE at 50° with little difference in the rate. The aldehydes or ketones are obtained in nearly quantitative yield. In this solvent, epoxidation of double bonds does not compete with alcohol oxidation. 

Oxidation of primary alcohols leads to aldehydes and oxidation of secondary alcohols leads to ketones. This oxidation also involves the loss of two hydrogen atoms. However, unlike the oxidations discussed so far in this chapter that are mediated almost exclusively by cytochromes P450, the major enzyme involved in the oxidation of ethanol is ALD (discussed earlier in this chapter) (74). Although ALD is the major enzyme involved in the oxidation of ethanol and most other low molecular-mass alcohols, cytochromes P450, especially 2E1, can also oxidize ethanol and this enzyme is induced in alcoholics. Although comprehensive studies have not been published, it appears that cytochromes P450 are often the major enzymes involved in the oxidation of higher molecular mass alcohols. 

Aldehydes are oxidized to carboxylic acids. A major enzyme responsible for this oxidation is aldehyde dehydrogenase (see Aldehyde Dehydrogenases section in this chapter) (79) however, other enzymes such as AO and cytochromes P450 can also mediate the oxidation of aldehydes as discussed (discussed earlier in this chapter). Ketones are not substrates for aldehyde dehydrogenase for the same reason that tertiary alcohols cannot be oxidized by ALD. Unlike the oxidation of alcohols, the oxidation of aldehydes is irreversible. Aldehydes are usually toxic and therefore there are aldehyde dehydrogenases in virtually all cells and in most compartments within cells. 

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