Thomas Rees Phillios Academv ~ndove;,MA 01810
I I
Tetrahedral Bonding in CH, An alternative explanation
In the usual description of the spSorbitals in methane, one of the 2s electrons in the carbon atom is promoted to the 2p level, and the 2s12p12p'2p' orbitals are hybridized togive four equivalent spa orhitals. The carbon atom can then react with four hydrogen atoms togive a CH, molecule with bond angles of 109°28'. Now that the Valence Shell Electron Pair Renulsion (VSEPR) theory has become popular, there is an alternative "thought-experiment" which can give the same final model of the methane molecule. Two hydrogen atoms are allowed to react with a carbon atom in its ground state, Is"s22p'2p'. In the resulting molecule, :CH2, the valence shell of the carbon atom has three pairs of electrons, of which one pair is nonbonded. ~ e c a u s kthe non-bonded pair exerts a stronger repulsion than the honded pair, the VSEPR theory predicts that the H-C-H angle will he less than 120'. If a hydrogen molecule is now dissociated into H+ and H:-, the H+ can form a hond through coordination with the :CH2 to give CH:I+.The VSEPR theory predicts the trigonal planar orientation of bonds in this ion. When the H:- ion is added to the CHs+, the H:- will approach from above or below the plane of the CHB+ions, repelling the bonds during the process of furming the CH4 molecule. If the students are well grounded in the study of Lewis acids and hases and also the VSEPR theory, the students can be guided by means of leading questions to the final shape of the CH4 molecule. Before presenting this "thought-experiment" to the students it helps to discuss the reaction of the fluoride ion, F-, with BFs. Good students, without having seen the above discussion of the formation of CH,, can answer the following questions in an hour test. The questions look painfully simple, but we must remember that these are young students venturing into stranee territorv. 1. Two hydrogen atoms arereacted ;ith a carbon atom in its ground state, ls22s22p12p'.
(a) Draw an electron dot diagram of the resulting molecule. (h) According to the VSEPR theory what will be the resulting hond angle, approximately? Explain.
2. A hydrogen atom is dissociated into H+ and H:-.The Hi is then reacted with the CH2 molecule. (a) What is the Lewis acid in this reaction? (h) What is the Lewis base? ( e ) Draw an electron dot diagram of the product. What is the shape of the mulecule and the predicted hond angle?
3. The molecule is then reacted with the H:- ion. aeid in this reaction? (b) What is the Lewis hase? (c) Make a sketch showing the approach of the Lewis hase toward the lnwis aeid and predict what the final hond angles will be. (a) What is the Lewis
From the students' points of view this "thought-experiment" has two advantages over the usual discussion of hybridization. First of all. the treatment is intuitivelv understandable. It is doubtful whether a student could" ever be suided to hvhridization hv means of leadine ouestions. Seck d l y , this discussion corr&ponds more closiyto the VESPR theory, which is receiving greater attention in today's textbooks. This discussion and the hybridization concept both suffer i r w u rhe same s e r ~ ilaas. ~ s St~lrlrntsa r t all ton likrly 11, hcliwr that methane is rrdly i o r ~ ~ ~ine this d manner ~ ~ n l wr es~ caution them that this is not so. A more serious shortcoming is that we leave the students ignorant of the fact that the tetrahedral orientation was fully established by the elegant work of Pasteur, KekulB, van't Hoff and LeBel, and Emil Fischer long before the electronic theories were devised. Today's students have no way of knowing that the elecronic theories do not give an original proof of tetrahedral orientation but only confirm and strengthen a concept which was already well established.
Volume 57, Number 12, December 1980 / 899
