M. Jerome Bigelow Idaho State University Pocatello 83201
I
I
Textbook Errors,
92
The Structure of Solid Aluminum Chloride
A
census of topics considered in standard textbooks should indicate with some precision those points which are considered most important by the most active teachers and writers in the field. Thus, the inclusion of the "bridge" structure for the aluminum chloride dimer (Fig. 1) in 18 out of a sample of 28 standard one-year general chemistry textbooks puhlished since 1963 indicates significant interest. The
Layer lattices hardly need be described as "complex." Many anhydrous halides such as CdC12, CrCI3, NiCI?, and FeCh crystallize in this manner. The resemblance of these layer structures to a familiar layer structure, that of graphite, is made more striking when it is noted that the electrical conductivity of solid aluminum chloride is much greater than that of aluminum chloride in t,he liquid st,ate. Aluminum chloride nearly doubles in volume on melting. This information, coupled with the low melting point of aluminum chloride, 192"C, indicates to us the "dilemma" facing this compound at low temperatures. On the one hand, efficient packing in the solid requires octahedral symmetry; on the other, stronger bonding is possible wit.h tetrahedral symmetry. Physical Properties of Aluminum Halides
Figure 1. The AIsCls molecule. Open circles represent chlorine otomr; closed dots in the center of the tetrahedra, represent aluminum otom..
tendency to dimerize points to the strength of the AIC13 molecule as a Lewis acid. The importance of the "octet rule" as a principle for predicting the reactions of small atoms may be invoked. And, of course, aluminum chloride is one of the most important of inorganic compounds to the organic chemist, and therefore deserves attention. Unfortunately, most of t,he texts which include the bridge structure of aluminum chloride did not complete the story with a correct statement regarding the structure of the solid. Some stated t,hat t,he liquid and the gas a t low t,emperatures were dimeric mole.cules, hut omitted ment.ion of t,he solid. Some implied that all thmc states were molecular. Some were cont,ent, wit,h: "The solid, on the ot,her hand, is ionic," or "The nt,ruct,ure of t,he solid is complex." Several aut,hors included balanced eauations with Al&ls as a reactant. A correct description of t,he structure of solid aluminum chloride could be found in only onc text,, that of Mastert,on and Slowinski (1). The bond between aluminum and chlorine at,oms must he primarily covalent (9, 3). I n liquid and gaseous aluminum chloride, (AI2Cls,-Fig.I ) , each aluminum atom is surrounded by a slightly distorted t,etrahedron of clilorine atoms. However, in solid aluminum chloride, each aluminum atom is surrounded octahedrally by six chlorine atoms in a layer latt,ice similar to that. illustrated in Figure 2 (4. ... 6). . Aluminum atoms o c c u. .~ y two-thirds of the oct,ahedral holes between two closepacked layers of chlorine atoms, the whole being slightly distort,cd t,o cryst,allize in monoclillic crystals, Bondiw may bc described as partly ionic, partly covalent (spad2hyhridization on aluminum).
Compound
Solid Lattice
Aluminum fluoride Aluminum chloride
ionic "ionic"
Aluminum bromide Aluminum iodide
Liquid and Gas
...
AlnCla molecules
ALBs molecules AMr molecr~les
?E
E!
1290 192 under press. 87.5 179.5
1291 180 sobl. 255 381
The table shows how physical properties of related substances (aluminum halides) change near the borderline between ionic and molecular lattices. Aluminum fluoride possesses the high melting point and boiling point expected for a primarily ionic compound. Aluminum bromide and aluminum iodide, which are dimeric molecules even in the solid, have low melting and hoiling points. The aluminum atom is simply too small to allow a coordination number larger than 4 with the
~
Figure 2,
A
of a loye,
for a
MNI,
The large open
circler represent upper-layer chlorine otoms; the smaller open circler represent lower-layer chlorine atoms; the closed dob represent olvminum atoms which occupy two-third, of the octahodrol holes between layers. ~h~ lattice is a distorted of thir ideolired loyet structure.
Volume 46, Number 8, Augurf 1969 / 495
large bromine and iodine atoms, and the electronegativity difference is too small to lead to much ionic character in the bonding. Aluminum chloride is of intermediate nature; a t low temperatures a relatively unstable octahedral packing is possible, but with relat.ively little thermal agitation molecules are formed and the substance melts or sublimes. To avoid misleading t,he student reader, a t,ext which gives the bridge struct,ure of AI2Cl6could, for example, add as a minimum qualifying statement AI.CL molecnles exist onlv in the lianid and low-temoeratore atomn; the bonding is polar covalent,.
There would seem to he no justification for using the formula A1?CI6 in equations for react,ions in aqueous media, since the react.ant is presumably solid aluminum chloridc. On the other hand, solutions in nou-donor solvents contain the AllC16 species. Thus, for the dis-
496 / Journal o f Chemical Educafion
solution and first-step hydrolysis of solid aluminum chloride we write AlClds)
+ 6Hn0
-
AI(OH)(H10).2+
+ H + + 3CI-
but for the Friedel-Crafts methylation of benzene the following is preferable CaH,
+ CHIC]
AlrCll
+ HCI
CeHsCHa
Literature Cited (1) MASTERTON, W.L., A N D SLOWINSKI, E. J., "Chemical Principles," W. B. Saunders Co., Philadelphia, 1966, p. 367. (2) SANDERSON, It. T., "Chemical Periodicity," Reinbold Publishing Carp., New York, 1960, p. 253. (3) PAULING, LINUS, "The Nature of the Chemical Bond" (3rd Ed.), Cornell Univemit.~Press, Ithaca, New York, 1960, pp. SP-0. J. A. A,, ILI.\CGILLAVRY, C. H., .\NDRENBs, P. A,, (4) KETEL~AR, Rec. Trav. chim. Pays-Bas 66, 601 (1947), ef. Chem. Abslrads 42, 434 (1948). (5) HWCKEL,W., "Structural Chemistry of Inorganic Campounds," Elsevier Publishing Co., New York, 1950, pp. 16&9; 475-6.
