Skeletal Models for Organic Molecules Constructed Easily and Inexpensively S.Y. Lam-Leung and Albert W. M. Lee Hong Kong Baptist College, 224, Waterloo Road, Kowloon, Hong Kong
Many articles have appeared in this Journal reporting the use of some inexpensive materials to construct simple geometric and molecular models. Likewise, some simple devices made with inexpensive or recycled material have been reported recently for the student laboratory. Certainly, using the skeletal model of an organic molecule is helpful for students, especially beginners, to understand the spatial relationships of atoms in the molecule. In this paper, we would like to describe how the skeletal models of simple organic molecules can be constructed by using flexible drinking straws (FDS) readily available from local supermarkets.
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b
t
.
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.1
Preparation of Skeletal Atoms The three main types of skeletal atoms in organic molecules, namely carbon, oxygen, and nitrogen, may be distingushed easily by using FDSs of different colors.
The sp3 SkeletalAtom The FDSs are first cut to a n equal arn-length configuration. The length of the two arms "a" is made proportional to the covalent radius of the specified atom. The recommended lengths for carbon, oxygen, and nitrogen are shown in the table. Male joints are created by making 20mm lonrritudinal cuts a t both ends. The resultant unit (fragment A) is the basic fragment for building the chosen skeletal atom. A sp3skeletal atom can be built by joining two fragment A's in the way described below. First, each basic fragment-Ais bent to an acute angle of 45". Apair of vertical cuts of 213-in. depth is made along the left-third and also the right-third trough of the bent flexible zone of each of the fragment-A's. The resultant unit with two slits are called fragment-B's (Fig. 1).Second, to one of the fragment-B portions, an additional longitudinal cut is made to divide the cut area into two halves. The reRelative Length of the Straight Plastic Tube for Building the Carbon, Nitrogen, and Oxygen Skeletal Atomsa
Atom Type
Length (in mm)
Carbon
Nitrogen
Arm "a"
3 ~ . 5 ~
29.0
27.0
sp2
Arm " b
27.5
25.3
21 .O
(double bond)
(double bond)
(double bond)
sp2
Arm "c"
31.5~
29.0
27.0
(lone pair)
(lone pair)
Tube E
67.5b
61.5
SO -r
Tube F .--
30.0
27.5
-
(tipe wnd) ;rope bond)
. ...... 7 n e recommenoea snglns in m metes are proponona la lne covalent
,.,
.=". ,-"., ! 7 . ?ha C-H band distances insp3,sp2,andsp sigma bonds areaflscled by the amount of the scharacler in the concerned hybrid orbital(2).
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Journal of Chemical Education
sultant unit with a pair of jaw-like parts is called fragment-C (Fig. 1). Finally, the shape of fragments B and Cis bent to nearly a 109.5°. Now, each of the two jawlike parts of fragment C can be inserted completely into one of the slits of the fragment B (Fig. 1).Then, a suitable amount ofthe adhesive is applied to cover the whole surface of the flexible zone of the two joined fragments. After the adhesive has set, a sp3 skeletal atom results. Aphotograph of the built sp3carbon fragment is shown in Figure 2a. For sp3 atoms with loneelectrons such as s$ oxygen or sp3 nitrogen, the existence of the lone-paifis) can be indicated by coloring the corresponding nonlinked arm(s).
Oxygen
sp3
sp
Figure 1. Construction of the sp3-skeletalatom.
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-..
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Figure 2. Photograph ofthe built modelsof skeletal atoms: (a)the sp3 fragment; (b) the sp2fragment; (c)the sp fragment.
The sp2 SkeletalAtoms
Fragments D for the specified atom are prepared by cutting the rigid tube of the FDS's to the recommended lengths, b and c, as shown in the table. Male joints are then created by making 20-mm longitudinal cuts at both ends of fragment D. An sp2 skeletal atom is built by using two fragment D's of the same kind (table). First, a 120"bend is made to each of the two basic fragments. A thin layer of adhesive is then applied to cover the flexible zone to fm the bending angle. The short arms "bnof the two 120"-bent basic fragments are f m d together by using transparent mending tape that orovides the rieiditv of the double bond to be formed. A photograph of the b h t sp2skeletal carbon is shown in Figure 2b. The sp Skeletal Atom
One long fragment (tube E) and two short fragments (tube F) are needed to build the sp-skeletal atom. Their suggested lengths for different atoms are given in the table. One long tube E is placed in between two short tube F's. They are then held together by using transparent mending tape as shown in Figure 2c. Male joints are then created a t both the single joint and the multi-joint end of the sp skeletal atom by making appropriate 20-mm longitudinal cuts on the tubes. Preparation of Terminal Atoms All terminal atoms, such as hydrogen and halogen atoms, are prepared by using tubes with appropriate lengths. The suggested length of the tube for the atom concerned is calculated by using the formula given below.
The length of tube (mm) = the covalent radius (I) of the concerned atom in nm x 500 + 15 Construction of Skeletal Model for Simple Organic Molecules The skeletal models of simple organic molecules are constructed by joining the required skeletal atoms together with the help of connectors. The connectors, which can be prepared bywing the previously cut off tubes, are 30 mm in lewth. One male joint of each of the two concerned skeletal a&ms is inserted into the 30-mm connector until the ends of the joints hit one amther. The distance between the two joined atoms is then proportional to the covalent bond length. The carbon-carbon backbone of a cyclohexane moleeule is built by using six 30 mm-connectors for joining six sp8carbon fragments. To represent the carbon-hydrogen bonds, 30-mm conneetors are then added to each of the 12 uncovered male joints of the backbone structure. The skeletal model of the cyclohexanemolecule is shown in Figure 3. This skeletal model is useful in studying the relationship between the chair and boat forms of cyclohexane. Each single joint represents one sigma bond and can be rotated freely.
Figure 3. A photograph of the constructed model of oydohexans. Construction of Skeletal M O M S for Some SDeclal Molecules For organic molecules containing phosphorus, sulfur, and any other elementthe suggested length of the tube for the concerned atom can be oalculated by using the appropriate formula given below.
or 8p4skeletal atom, For qS
,
the length of straight tube remained iu FDS (mm) =the covalent radius ( I )in nm x 500 - 6
For sp skeletal atom, the length of the tube E (mm) = the triple bended oovalent radius (1)of the concerned atom innmx500 and the length of the hhe F (inm) =the sum of the triple- and the siog1e-bonded covalent radii (1) ofthe concerned atom in mu x 500
Acknowledgment We wish to express our appreciation to Allen K.Y. Cheung and K P. Kwok of our Centre for Educational Development for their technical assistance in preparing the 5gures.
Volume 70 Number 10 October 1993
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