Oxygen hydroxyl group

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

Overall the reaction leads to addition of two hydroxyl groups to the double bond and IS referred to as hydroxylation Both oxygens of the diol come from osmium tetraox ide via the cyclic osmate ester The reaction of OSO4 with the alkene is a syn addition and the conversion of the cyclic osmate to the diol involves cleavage of the bonds between oxygen and osmium Thus both hydroxyl groups of the diol become attached to the same face of the double bond syn hydroxylation of the alkene is observed... 

Section 15 14 The hydroxyl group of an alcohol has its O—H and C—O stretching vibrations at 3200-3650 and 1025-1200 cm respectively The chemical shift of the proton of an O—H group is variable (8 1-5) and depends on concentration temperature and solvent Oxygen deshields both the proton and the carbon of an H—C—O unit Typical... 

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 hydroxyl group slabilizes Ihe carbocalion by delocalizalion of one of Ihe unshared eleclron pairs of oxygen... 

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... 

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

Although a hydroxyl group strongly activates an aromatic ring toward electrophilic attack an oxyanion substituent is an even more powerful activator Electron delocaliza tion m phenoxide anion leads to increased electron density at the positions ortho and para to oxygen... 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

The anomeric carbon of a furanose or pyranose form of a ketose bears both a hydroxyl group and a carbon substituent In the case of 2 ketoses this substituent is a CH2OH group As with aldoses the anomeric carbon of a cyclic hemiacetal is readily identifi able because it is bonded to two oxygens... 

A point to be emphasized about glycoside formation is that despite the presence of a number of other hydroxyl groups m the carbohydrate only the anomenc hydroxyl group IS replaced This is because a carbocation at the anomenc position is stabilized by the nng oxygen and is the only one capable of being formed under the reaction conditions... 

Protic solvent (Section 8 12) A solvent that has easily ex changeable protons especially protons bonded to oxygen as in hydroxyl groups... 

A completely dehydroxylated surface consists essentially of an array of oxygen atoms the Si-0 linkages are essentially covalent so that the silicon atoms are almost completely screened by the much larger oxygen atoms. Such a surface represents the extreme case and, even on samples ignited at 1100°C, a minute residue of isolated hydroxyl groups will be present. 

Castor oil (qv) contains a predominance of ricinoleic acid which has an unusual stmcture inasmuch as a double bond is present in the 9 position while a hydroxyl group occurs in the 12 position. The biochemical origin of ricinoleic acid [141-22-0] in the castor seed arises from enzymatic hydroxylation of oleoyl-CoA in the presence of molecular oxygen. The unusual stmcture of ricinoleic acid affects the solubiUty and physical properties of castor oil. 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

The unshared pairs of electrons on hydroxyl oxygens seek electron deficient centers. Alkylphenols tend to be less nucleophiUc than aUphatic alcohols as a direct result of the attraction of the electron density by the aromatic nucleus. The reactivity of the hydroxyl group can be enhanced in spite of the attraction of the ring current by use of a basic catalyst which removes the acidic proton from the hydroxyl group leaving the more nucleophiUc alkylphenoxide. 

This addition is general, extending to nitrogen, oxygen, carbon, and sulfur nucleophiles. This reactivity of the quinone methide (23) is appHed in the synthesis of a variety of stabili2ers for plastics. The presence of two tert-huty groups ortho to the hydroxyl group, is the stmctural feature responsible for the antioxidant activity that these molecules exhibit (see Antioxidants). 

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