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Encyclopedia Britannica - Main :: AJA-ALL |
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ALCOHOLS , in organic chemistry, a class of compounds which may be considered as derived from hydrocarbons by the replacer meat of one or more hydrogen atoms by hydroxyl groups. It isconvenient to restrict the term to compounds in which the hydroxyl group is attached to an aliphatic residue; this excludes such compounds as the hydroxy-benzenes, naphthalenes, &c., which exhibit many differences from the compounds derived from the aliphatic alkyls. Alcohols are classified on two distinct principles, one depending upon the number of hydroxyl groups present, the other on the nature of the remaining groups attached to the carbon atom which carries the hydroxyl group. Monatomic or monohydric alcohols contain only one hydroxyl group; diatomic, two, known as glycols (q.v.) ; triatomic, three, known as glycerols (q.v.); and so on. The second principle leads tp alcohols of three distinct types, known as primary, secondary and tertiary. The genesis and formulation of these types may be readily understood by considering the relation which exists between the alcohols and the parent hydrocarbon. In methane, CH4, the hydrogen atoms are of equal value, and hence only one alcohol, viz. CH3OH, can be derived from it. This compound, methyl alcohol, is the simplest primary alcohol, and it is characterized by the grouping CH2OH. Ethane, C2H6
In r86o Hermann Kolbe predicted the existence of secondary and tertiary alcohols from theoretical considerations. Regarding methyl alcohol, for which he proposed the name carbinol, as the simplest alcohol, he showed that by replacing one hydrogen atom of the methyl group by an alkyl residue, compounds of the general formula
formula
RNH2 (12, R2) NH (RiR2123)N RCH2OH (R1R2)CHOH (RIR2R3)COH Primary. Secondary. Tertiary. - To distinguish Primary, Secondary and Tertiary Alcohols.Many reactions serve to distinguish these three types of alcohols. Of chief
original
intense blue solution, while the tertiary compound does not act with nitrous acid. The reactions outlined above may be thus represented: RCH2OH >RCH2I ~R CH2 NO2 ~RrC\N0 R>CHOH ~R1>CHI SR>CHNO2 >C (R,R2R)COH-(R,R2R,)CI>(R1R2R3)CNO2. Tertiary alcohol. By heating to the boiling point of naphthalene (218) tertiary alcohols are decomposed, while heating to the boiling point of anthracene (36o) suffices to decompose secondary alcohols, the primary remaining unaffected. These changes can be followed out by determinations of the vapour density, and so provide a method for characterizing alcohols (see Compt. Rend. 1904, 138, P. 9841. Alcohols may be readily prepared from the corresponding alkyl haloid by the action of moist silver oxide
oxide
CH3CHO ~CH3CH< 2znC2H6 >CH3CH HC ->HCCH 3 >HCCH3 >HCCH3 \QC2H5 \OC2H5 \CH3 \CHa Formic ester. Isopropyl alcohol. Tertiary alcohols may be synthesized by a method devised by A. Butlerow in 1864, who thus discovered the tertiary alcohols. By reacting with a zinc alkyl (methyl or ethyl) on an acid chloride, an addition compound is first formed, which decomposes with water to give a ketone. If, however, a second molecule of a zinc alkyl be allowed to react, a compound is formed which gives a tertiary alcohol when decomposed with water. R. C/0 --> RCOZnCH 3--3RCEOZnCH3~RCcOH3 \CI \Cl \CH3 \CH3 Acid chloride. Tertiary alcohol. It is interesting to note that, whereas zinc methyl and ethyl give tertiary alcohols, zinc propyl only gives secondary alcohols. During recent
The alcohols are neutral in reaction, and the lower members possess the property of entering into combination with salts, in which the alcohol plays the role of water of crystal- Pro- lization. Sodium or potassium dissolves in them parties. with the formation of alcoholates, the hydrogen of the hydroxyl group being replaced by the metal. With strong acids water is split off and esters are formed. The haloid esters of the paraffin alcohols formed by heating the alcohols with the halogen acids are the monohaloid derivatives of the paraffins, and are more conveniently prepared by the action of the phosphorous haloid on the alcohol. Energetic dehydration gives the olefine hydrocarbons, but under certain conditions ethers
The physical properties of the alcohols exhibit a gradation with the increase of molecular weight. The lower members are colourless mobile liquids, readily soluble in water and exhibiting a characteristic odour and taste. The solubility decreases as the carbon content rises. The normal alcohols containing i to 16 carbon atoms are liquids at the ordinary temperatures`; the higher members are crystalline, odourless and tasteless solids, closely resembling the fats in appearance. The boilingpoints of the normal alcohols increase regularly about for each CH3 increment; this is characteristic of all homologous series of organic compounds: Of the primary, secondary and tertiary alcohols having the same empirical formula, the primary have the highest, and the tertiary the lowest boiling point; this is in accordance with the fairly general rule that a gain in symmetry is attended by a fall in the boiling point.The following monatomic alcohols receive special
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