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ALCOHOLS
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ALCOHOLS , in organic See also:chemistry, a class of compounds which may be considered as derived from See also:hydrocarbons by the replacer See also:meat of one or more See also:hydrogen atoms by hydroxyl See also:groups. It isconvenient to restrict the See also:term to compounds in which the hydroxyl See also:group is attached to an aliphatic See also:residue; this excludes such compounds as the hydroxy-benzenes, naphthalenes, &c., which exhibit many See also:differences from the compounds derived from the aliphatic alkyls. Alcohols are classified on two distinct principles, one depending upon the number of hydroxyl groups See also:present, the other on the nature of the remaining groups attached to the See also:carbon See also:atom which carries the hydroxyl group. Monatomic or monohydric alcohols contain only one hydroxyl group; diatomic, two, known as See also: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 See also:primary, secondary and See also:tertiary. The See also:genesis and formulation of these types may be readily understood by considering the relation which exists between the alcohols and the See also:parent See also:hydrocarbon. In methane, See also:CH4, the hydrogen atoms are of equal value, and hence only one See also:alcohol, viz. CH3OH, can be derived from it. This See also:compound, methyl alcohol, is the simplest primary alcohol, and it is characterized by the grouping •CH2OH. Ethane, See also:C2H6, in a similar manner, can only give rise to one alcohol, namely See also:ethyl alcohol, CH3CH2OH, which is also primary. Propane, CH3CHaCH3, can give rise to two alcohols —a primary alcohol, CH3CH2CH2OH (normal propyl alcohol), formed by replacing a hydrogen atom attached to a terminal carbon atom, and a secondary alcohol, C113•CH(OH)•CH3 (isopropyl alcohol), when the substitution is effected on the See also:middle carbon atom. The grouping CH .OH characterizes the secondary alcohols; isopropyl alcohol is the simplest member of this class. Butane, C4H,6, exists in the two isomeric forms—normal butane, CH3•CH2•CH2•CH3, and iso-butane, CH(See also:CH3)3. Each of these hydro-carbons gives rise to two alcohols: n-butane gives a primary and a secondary; and iso-butane a primary; when the substitution takes See also:place in one of the methyl groups, and a tertiary, when the hydrogen atom of the i CH group is substituted. Tertiary alcohols are thus seen to be characterized by the group C•OH, in which the residual valencies of the carbon atom are attached to alkyl groups. In r86o See also:Hermann See also: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 See also:general See also:formula R•CH2.OH would result. These are the primary alcohols. By replacing two,of the hydrogen atoms, either by the same or different alkyls, compounds of the formula (R•R,)CH•OH (i.e. secondary alcohols) would result; while the replacement of the three hydrogen atoms would generate- alcohols of- the general formula (R•R,•R2)C•OH, i.e. tertiary alcohols. Furthermore, he exhibited a comparison between these three types of alcohols and the See also:amines. Thus:— - R•NH2 (12, See also:R2) NH (RiR2123)N R•CH2OH (R1R2)CH•OH (RIR2R3)C•OH Primary. Secondary. Tertiary. - To distinguish Primary, Secondary and Tertiary Alcohols.—Many reactions serve to distinguish these three types of alcohols. Of See also:chief importance is their , behaviour on oxidation. The primary alcohols are first oxidized to See also:aldehydes (q.v.), which, on further oxidation, yield acids containing the same number of carbon atoms as in the See also:original alcohol. Secondary alcohols yield See also:ketones (q.v.), which are subsequently oxidized to a mixture of two acids., . Tertiary alcohols yield neither aldehydes nor ketones, but, a mixture of two or more acids. Another method is based upon the different behaviour of the corresponding nitro-alkyl with nitrous See also:acid. The alcohol is first acted upon with See also:phosphorus and See also:iodine, and the resulting alkyl iodide is treated with See also:silver nitrite, which gives the corresponding nitro-alkyl. , The nitro-alkyl is then treated with See also:potassium nitrite dissolved in concentrated potash, and sulphuric acid is added. By this treatment a primary nitro-alkyl yields a nitrolic acid the potassium See also:salt of which forms an intense red See also:solution; a secondary nitro-alkyl forms a pseudo nitrol, which gives an intense See also:blue solution, while the tertiary compound does not See also:act with nitrous acid. The reactions outlined above may be thus represented: R•CH2OH —>R•CH2I ~R See also:CH2 NO2 ~RrC\N0<NO2H ' Primary alcohol. Nitrolic acid. R>CH•OH ~R1>CH•I —SR>CH•NO2 >C<NO Secondary alcohol. Pseudo nitrol. (R,R2R)C•OH-(R,R2R,)C•I—>(R1R2R3)C•NO2. Tertiary alcohol. By See also:heating to the boiling point of See also:naphthalene (218°) tertiary alcohols are decomposed, while heating to the boiling point of See also:anthracene (36o°) suffices to decompose secondary alcohols, the primary remaining unaffected. These changes can be followed out by determinations of the vapour See also: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 See also:action of moist silver See also:oxide (which behaves as silver hydroxide) ; by the saponification of their See also:esters; or by the reduction of polyhydric alcohols with hydriodic acid, and the subsequent See also:conversion of the resulting alkyl iodide into the alcohol by moist silver oxide. Primary alcohols are obtained by decomposing their sulphuric acid esters (from sulphuric acid and the olefines) with boiling See also:water; by the action of nitrous acid on primary amines; or by the reduction of aldehydes, acid chloride§ or acid anhydrides. Secondary alcohols result from the reduction of ketones; and from the reaction of See also:zinc alkyls on aldehydes or formic acid esters. •CH3CHO ~CH3•CH< 2znC2H6 >CH3•CH<SH Acetaldehyde. Methyl ethyl carbinol. /O /OZnCH3 /OZnCH3 /OH HC ->HC—CH 3 —>HC—CH3 >HC—CH3 \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 See also: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 --> R•C—OZnCH 3--3R•CEOZnCH3~R•CcOH3 \CI \Cl \CH3 \CH3 Acid chloride. Tertiary alcohol. It is interesting to See also:note that, whereas zinc methyl and ethyl give tertiary alcohols, zinc propyl only gives secondary alcohols. During See also:recent years (1900 onwards) many brilliant syntheses have been effected by the aid of See also:magnesium-alkyl-haloids. The alcohols are neutral in reaction, and the See also:lower members possess the See also:property of entering into See also:combination with salts, in which the alcohol plays the role of water of crystal- See also:Pro- lization. See also:Sodium or potassium dissolves in them parties. with the formation of alcoholates, the hydrogen of the hydroxyl group being replaced by the See also:metal. With strong acids water is split off and esters are formed. The haloid esters of the See also: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 See also:olefine hydrocarbons, but under certain conditions See also:ethers (see See also:ETHER) are obtained. The See also:physical properties of the alcohols exhibit a gradation with the increase of molecular See also:weight. The lower members are colourless See also:mobile liquids, readily soluble in water and exhibiting a characteristic odour and See also:taste. The solubility decreases as the carbon content rises. The normal alcohols containing i to 16 carbon atoms are liquids at the See also:ordinary temperatures`; the higher members are crystalline, odourless and tasteless solids, closely resembling the fats in See also:appearance. The boilingpoints of the normal alcohols increase regularly about for each CH3 increment; this is characteristic of all homologous See also: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 See also:rule that a gain in symmetry is attended by a fall in the boiling point. Additional information and CommentsThere are no comments yet for this article.
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