Models Illustrating Rotational
H. J. G. Hayman The
H e b r e w University
Isomerism in Aliphatic Chains
Jerusalem, l s r a e l
staggered configuration by means of the usual snapfastener, the rivet-heads (3/16-in.diameter) fitted into these depressions. Figure 2a shows the use of two such modified carbon atom models to illustrate the stability of the staggered configuration of the ethane molecule. Since the height of the rivet-heads was only 1.1 mm, the two methyl groups could be rotated rels, tive to each other from one staggered configuration to the next without becoming separated in the process. The standard snap-fastener retaining springs in these models proved strong enough to give ample rigidity to the staggered configurations. The above modifications to the standard Fisher-Hirschfelder-Taylor models were carried out using a rimmed triangular template which could be fitted over any desired face of a carbon atom model. In order to prepare models illustrating the rotational isomers of aliphatic hydrocarbon chains it was of course necessary to modify t v o faces of each carbon atom model, except for those a t the ends of the chains. We have found thew models to be an invaluable aid in understanding the configurational problem and exStuart, H . A,, Z. phyaik. C h m . (Leipig), B27, 350 (1034). cluded volume effect in normal paraffins, considered 2 Obtainable from Fisher scientific CO., 633 ~ ~ ~ ~s t .~, ~ , . i ~ h recently by Smith and M ~ r t e n s e n . ~Figures 2b, 2c and New York 14, N. Y . MIZUSHIMA, S., "Structure of Molecules and Internnl Rota2d show the three allorved t v ~ e sof rotational isomers tion," Academic Press, h e . , Yew York, 1954, p. 53. of n-uentane discussed bv these authors
The usual atom models of the Stuart type1 such as the Fisher-Hirschfelder-Taylor models2 are not suitable for demonstrating rotational isomerism in aliphatic hydrocarbon chains since these models have free rotation about single bonds. We describe here a simple modification of the Fisher-Hirschfelder-Taylor models, which, without abolishing the possibility of rotation, gives additional stability to the staggered (trans or gauche) configurations of such hydrocarbon chains; the behavior of these models thus resembles that of the actual molecules in which the trans and gauehe configurations are separated by potential hamiers.3 Three holes (2 mm diameter and 8 mm deep) were drilled symmetrically into one face of an aliphatic carbon atom model (Fig. la) and three round-headed aluminum rivets (S/64-in. diameter and L/&-in. long) were inserted (Fig. lb). Three conical depressions (5 mm diameter) were then drilled into one of the faces of the second carbon atom model (Fig. lc) such that when the two models were connected tocether in a
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Figure 1 . Modification of the facer of the Firher-HirrchfelderToylor rorbonatom molecvler
208 / Journal of Chemical Education
Figure 2. Staggered conflgurationr of pmroffin moleculer; a, ethane; b, c, d, the three allowed types of rotational isomers of o-pentane; o, type 01; b, type 22; c, type l I . (The nomenclature is tho? d Smith and Morten~en.~]
