Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/579

ALDEHYDES.] CHEMISTRY 567 of phosphorus pentasulphide (5C 2 H 5 .HO + P 2 S 5 = 5C 2 H 5 .HS + P 2 O 5 ). The sulphur analogues of the phenols and dihydric phenols are prepared by a special general method. The sulphonic acid (see p. 561) of the corre sponding hydrocarbon is converted into a sulphonic chloride by the action of PC1 5 , and the sulphonic chloride is then acted on by nascent hydrogen : C 6 H 5 (SO 2 C1) + 3H 2 = C 6 H 5 .HS 20H + HC1 Benzene-sulphonic chloride. Phenyl hydrosuJphide. C 6 H 4 (SO 2 C1) 2 + 6H 2 = C 6 H 4 (HS) 2 + 40H 2 + 2HC1. Benzene disulphonic Phenylene dihydro- chlortde. sulphide or thioresorein. The sodium derivatives of these thio-phenols yield com pound thio-ethers by the action of haloid ethers : C 6 H 5 .NaS + C 2 H 5 I = C 6 H 5 .S.C 2 H 5 + Nal . Sodium thiophenate. Phenyl-ethyl sulphide. Compounds intermediate between alcohols and thio- alcohols are known : C 3 H 5 (HO) 2 C1 + KHS = C 3 H 5 (HO) 2 (HS) + KC1 . The compounds of this family are mobile or oily liquids or crystalline solids. Most of them possess characteristic and offensive odours. They are susceptible of the same isornerism as their oxygen analogues. The thio-alcohols combine energetically with alkali metals, and with certain metallic oxides and salts, to form derivatives analogous to the metallic alcohol derivatives, hence the name mer cap- tans (corpus mercurio aptum) sometimes given to these bodies. Certain lead mercaptides, when heated in the dry state, yield the corresponding thio-ethers. VI. ALDEHYDES. The relations between these compounds and the primary alcohols are clearly brought out by the mode of formula tion adopted for these alcohols (p. 562). Thus : H GEL C B H 3B . , H CH CH 2n+1 CH 3 CH 3 CH, Methane. Ethane. Paraffins. CLH fl . CH.:, CH 2 OH CH 2 OH CH 2 OH COH Methyl Primary carbinol. alcohol. GHoOH ai Carbinol. Aldehydes. COH Benzene. Methyl"benzene. Benzyl alcohol. Benzoic aldehyde. It will be seen from these formulae that aldehydes are derived from alcohols by the elimination of H. 2 from the group CH 2 OH, i.e., 2 hydrogen atoms are withdrawn from the typical carbon atom, leaving the group (COH) (com pare with definition previously given, p. 553) ; hence the generic name (alcohol dehijdrogenatum). It is thus possible for any alcohol to furnish an aldehyde, although great numbers of these compounds have yet to be discovered to complete the various series. The following formulae will illustrate the derivation of aldehydes from polyhydric alcohols : CH, CH 2 OH COH CH || ; C 6 H 6 C 6 H 4 <^ s C 6 H 4 COH /1 3 Aldehyde . Dimethyl- Aldehyde CH., CH 2 OH CH 2 OH COH Ethene. Glycol. (glyoxal). Benzene. benzene. (phthalic). The aldehydes of monohydric alcohols are metameric with the oxides (ethers) of dyad radicles. Thus, ethyl aldehyde, CH 3 .COH, is metameric with ethene oxide (C H 4 )"O ; benzoic aldehyde would be metameric with methyl-phenylene oxide [C 6 H 3 (CH 3 )]"O . The aldehydes are liable also to the isomerism of their contained hydro carbon radicles, while aromatic aldehydes of dihydric alcohols are susceptible of the isomerism incident to the relative positions of the COH groups. By the action of oxidizing agents aldehydes are con verted into acids containing the same number of carbon atoms ; thus, R being the monatomic radicle R R I +0=| COH COOH Aldehyde. Acid. Starting, therefore, from the parent hydrocarbons, the primary alcohols are the first results of the introduction of hydroxyl into the methyl group. The next step in oxida tion removes the two remaining hydrogen atoms from this group with the formation of aldehydes, and the final result of the oxidation is to convert COH into carboxyl COOH, with the formation of acids. Aldehydes thus occupy a position intermediate between alcohols and acids : CH, CH 3 Ethane or methyl methane. CH, CH 2 OH Methyl- carbinol. CH 3 COH Ethyl or acetic aldehyde. CH, COOH Acetic acid Ethidene dichloride or dichloraldehyde Aldehydes take up nascent hydrogen, reproducing alco hols R .COH + H 2 =R .CH 2 OH. The acid sulphites of the alkali metals combine directly with aldehydes, forming crystalline compounds, which, on treatment with a mineral acid, yield the aldehyde unaltered. Aldehydes corresponding to eleven of the primary alcohols of the C n H 2n+1 .HO series are known, and are generally prepared by the oxidation of these alcohols : R .CH 2 OH-H 2 = R .COH. The contained C n H 2n+1 radicles are in some cases normal primary, and in others iso-primary. The first member, methyl or formic aldehyde, H.COH, is gaseous; the succeeding terms are liquid, and hexdecyl aldehyde is a crystalline solid. The liquid alde hydes are colourless, transparent, and possessed of pungent ethereal odours ; their mobility decreases, and their boiling- points rise as the series is ascended. Their solubility in water decreases in the same manner. By tha action of PC1 5 aldehydes lose their oxygen, and C1 2 is substituted. These chloraldehydes are isomeric with the dichlorides of the defines, and (with the exception of the ethyl com pound), with the chlorides of monochlorinated C n H 2n+1 radicles. Thus : CH 2 C1.CH 2 C1 Ethene dichloride. (See also p. 559.) The aldehydes of the present series combine directly with ammonia, forming aldehyde-ammonias of the general formula R . C(OH)(NH 2 )H , which, by losing the elements of water, condense into basic substances, termed aldines and oxaldines. When acted on by chlorine in large excess these aldehydes yield the corresponding chlorides of acid radicles (R CO) Cl. Trichloraldehyde, CC1 3 .COH, or chloral, can be obtained by the action of chlorine on ethyl alcohol. The acetals are compounds formed by the combination of aldehydes and alcohols of the C n H 2n+1 HO series, with elimination of water. By the action of zinc chloride, &c., acetic aldehyde yields crotonic aldehyde : 2C 2 H 4 - OH 2 = C 4 H 6 . The aldehydes oxi dize with the greatest readiness to the corresponding acids ; thus, silver oxide is reduced to the metallic state when heated with an aldehyde and water. A drop of acetic aldehyde let fall on blue litmus paper shows an acid reaction on mere exposure to the air, owing to the forma tion of acetic acid. Aldehydes are characterized by their extreme readiness to undergo polymeric modification. Thus the presence of a trace of certain reagents converts acetic aldehyde under some conditions into paraldehyde, C 6 H 12 O 3 = 3C 2 H 4 O, and under other conditions into metaldehyde, n(C 2 H 4 0). The aldehydes corresponding to the C n H. 2n _ 1 .HO alco hols are acrylic aldehyde or acrolein C 9 H 3 .COH, and crotonic aldehyde, previously mentioned. These aldehydes

do not yield compounds analogous to aldehyde-ammonia.