Chapter 8 Predicting Mlecular Gemetries 8-1 Mlecular shape The Lewis diagram we learned t make in the last chapter are a way t find bnds between atms and lne pais f electrns n atms, but are nt intended t say anything abut the three-dimensinal shape f the mlecule. IN fact they can cmpletely mislead yu! Take C2Cl2 ::Cl: :::Cl-C-Cl::: -C-Cl::: I II The abve Lewis diagram leads yu t believe that there are tw structures fr C2Cl 2. Bth structures appear t be planar with 90 bnd andles between atms. Further, structure I is nnplar because the plarity f the C-Cl bnds cancel ut, but structure II is plar because the plar bnds are riented s they can add tgether instead f canceling ut. Key Cncept: Mlecules with the same chemical frmula but a different 3-D arrangement f atms are called stereismers Thus yu wuld think that there are tw stereismers fr C2Cl2 WRONG, n s many levels!! There is nly ne structure fr C2Cl 2 nt tw. The mlecule is NOT planar, it des NOT have 90 bnd angles, and it has in intermediate plarity. 8-2 The Tetrahedrn What is it then? A tetrahedrn. What is a tetrahedrn? Mdels and Figure 8.2 Bth Methane C 4 and C2Cl 2 share this structure, as des every cmpund that cntains a C with 4 bnds t it S hw d we bridge the gap between the flat, tw-dimensinal Lewis frmula and a realistic 3-D structure?
2 8-3 VSEPR Thery VSEPR stands fr Valence-Shell Electrn-Pair Repulsin A big muthful f wrds, but a simple enugh cncept Key Cncept: VSEPR means that yu arrange electrn pairs arund a central atm in such a way as t minimize their cntact with each ther. This gives rise t linear, trignal planar, tetrahedral, trignal bipyramid and ctahedral electrn gemetries 1. BeCl 2 Lewis structure: : : :Cl:Be:Cl: : : There are nly 2 pairs f electrns arund the Be atm. S there are 2 electrn regins w can we get them as far apart as pssible frm each ther? Place at 180 apart. We call this structure linear, and, since there are n lne pairs, the mlecular and electrnic structure are bth the same, linear. 2. BF 3 : :F: : : : Nte here is the example f an electrn deficient B we had earlier :F:B:F: There are three electrn regins, and the way t get these 3 bnds : : away frm each ther is t put them at 120 apart. We call this structure Trignal planar. Again there are n lne pairs s the mlecular and electrnic structures are the same. 8-4 Predictin f Mlecular Gemetry Thse tw were easy. Nw n t the tetraheral 3. C 4 4 single bnds, 4 electrn regins. If the mlecule was planar like this piece f paper, yu get fur 90's. -C- wever, the structure is 3 dimensinal and has 109.5 and get further spread. We call this structure a Tetrahedral S whenever 4 pairs f electrns arund an atm, shuld be Tetrahedral
3 4. PCl 5 5 electrn regins. w d we get these away fr each ther? One straight up, ne straight dwn, 3 even abut the equatr We call this structure a trignal bipyramid There are 90 angles between the up and dwn Cl s and the ne at the equatr, and there are 120 angles between the atms at the equatr. 5. PCl - 6 6 electrn regins ctahedral electrn and mlecular gemetry
4 8-5 Lne pairs and shape In all f the abve structures we nly had bnding pairs f electrns arund the central atm. What happens when we mix in lne pair f electrns? Key Cncept: A lne pair cunts as 1 electrn regin, but distrts the gemetry slightly. Als lne pairs f electrns DO NOT appear in the mlecular structure 6. N 3 I the lne pair cunts as an electrn regin, s 4 electrn regins -N: This makes ur electrnic structure tetrahedral, but what abut the I Mlecular structure? In the mlecular structure we ignre the lne pairs (We can nly see atms, nt electrns) That makes this mlecular structure a trignal pyramid. Technically the lne pair pushes a little harder (abut 2 ) than the electrns in a bnding pair, s this distrts the tetrahedral a bit. This makes the angles actually <109.5 (Actually 107.3 ) but I m nt very picky abut this fine pint. 7. 2O : 4 electrn regins, electrnic structure = tetrahedral -O- Ignring the lne pairs, the mlecular structure is V-shaped : Again the bnd angles are again <109.5 (actually104.5 ) Befre we lk at lne pairs in ther gemetries, let s lk as hw we handle multiple bnds 8-6 VSEPR and multiple bnds Key Cncept: In VSEPR Thery we cunt duble r triple bnds as 1 electrn regin 8. CO 2 ::O=C=O:: 9. CN -C N: 2 duble bnds cunt as 2 regins Linear electrn and mlecular structure 1 duble bnd, 1 triple bnd, bth cunt as a single regin Linear electrn and mlecular structure
5 10. SO 2 : : O =S-O::: : Lking at the S, the duble bnd cunts as ne electrn regin, the duble bnd as anther electrn regin, and the lne pair as a third electrn regin, fr a ttal f 3 electrn regins. Again ur electrnic structure is a trignal pyramid. wever, what is the mlecular structure? In the mlecular structure we ignre the lne pairs (We can nly see atms, nt electrns) That makes this mlecular structure V-shaped. Due t the extra electrns in a duble bnd, duble bnds will push extra hard n electrns arund them and distrt the gemetries slightly. Fr example the bnd angle in 2CO \ 3 electrn regins C=O:: Trignal planar gemetry / Bnd angle shuld be 120 but is actually 116 Again, I dn t fret abut this small difference 8-7 Trignal Bipyramids Lne pairs in trignal bipyramids present an interesting prblem. Where d yu put them? Abve and belw r arund the equatr? If yu put a lne pair pinting up r dwn, it is 90 t three ther bnds If yu put it n the equatr, then it is at 90 t nly tw bnds. This is the preferred cnfiguratin because it minimizes electrn repulsin Key Cncept: Always put lne pair f electrns arund the equatr f a trignal bipyramid
6 11. SeF 4 One lne pair, s 5 electrn regins, again a trignal bipyramid electrn structure. But what abut the mlecular structure? We have tw nnequivalent places t put the lne pair. Tp/bttm r arund the equatr. If we put the lne par at the tp r bttm, then it has 3 F s lcated at 90. If we put the lne pair n the equatr, then we have nly 2 F s at 90, and 2 at 120. This is slightly better. S as a rule yu always put the lne pars arund the equatr in a trignal bipyramid. We call this mlecular structure a See-Saw (Yu need t see a dem t see this!) 12. ClF 3 Again 5 electrn regins fr a trignal bipyramid electrnic structure, but with 2 lne pairs arund the equatr, the mlecular structure is called T-shaped (Ops errr in figure One f the equitrial F s shuld be :) 13. I - 3 Nte: the entire structure has a - charge. 5 electrn regins, trignal bipyramid electrnic structure, but a linear mlecular structure!
8-8 Octahedra Putting the first lne pair in ctahedra is n prblem because all the psitins are equivalent. But where d yu put the secnd pair? Since lne pairs f electrns push harder than bnding pairs, we want t put them as far apart as pssible. That puts them n ppsite sides in the ctahedral structure. Key Cncept: If there are tw lne pairs f electrns in an ctahedral, the pairs are placed ppsite each ther in the structure 7 14. IF 5 6 electrn regins ctahedral electrn gemetry the lne pair can g any place, there is n difference. The mlecular gemetry is call square pyramid (Put the lne pair n the bttm t it lks flat, what yu have left is a pyramid) 15. XeF4 6 electrn regins Octahedral electrn gemetry The lne pairs g n ppsite sides s they dn t have t cntact each ther. The resulting mlecular structure is called square planar Lucky fr yu n ctahedral mlecules have been fund with >2 lne pairs!
8 8-9 Structure and Diples At the end f the last chapter we talked abut plar bnds and plar mlecules Just t remind yu Key Cncept: In rder t has a mlecular diple (be plar) a mlecule must: 1. ave plar bnds 2. ave an asymmetric arrangement f these plar bnds G ver the structures we just did and decide if the mlecules are plar r nt 1. BeCl 2 Plar bnds - linear symmetric structure - nnplar 2. BF3 Plar bnds - symmetric structure - nnplar 3. C4 Nnplar bnds - Mlecule can t be plar 4. PCl5 Plar bnds - symmetric structure - nnplar - 5. PCl6 Plar bnds - symmetric structure - nnplar 6. N3 Plar bnds - asymmetric mlecule - plar 7. 2O Plar bnds - asymmetric mlecule - plar 8. CO Plar bnds - symmetric structure - nnplar 2 9. CN Brderline plar bnds - asymmetric arrangement - plar 10. SO Plar bnds - asymmetric mlecule - plar 2 11. SF4 Plar bnds - asymmetric mlecule - plar 12. ClF3 Plar bnds - asymmetric mlecule - plar - 13. I3 Nnplar bnds - Mlecule can t be plar 14. IF 5 Plar bnds - asymmetric mlecule - plar 15. XeF Plar bnds - symmetric structure - nnplar 4 8-10 Ismers We have already discussed resnance, where yu keep the atms in place, but arrange electrns differently. Anther thing yu will see is the ability t make different structures be rearranging atms. Take, fr instance, the frmula C38O :O: -C-C-C-O::- -C-C-C-
Key Cncept: Structural ismers are mlecules with the same chemical frmula but a different arrangement f bnds cnnecting the atms. One structural ismer will have different physical and chemical prperties than anther structural ismer, s they are truly a different mlecules It is als pssible t ismers in which the spatial arrangement f atms is different, but the atm-t-atm cnnectivity is the same, as shwn in SF4Cl 2. Mlecules with different spatial arrangements f the same chemical bnds are called stereismers. ere we will talk abut tw kinds f stereismers, Gemetric Ismers and Optical Ismers Key Cncept: Gemetric ismers are mlecules with the same chemical cnnectivity between atms, but a different gemetric arrangement f these bnds. One gemetric ismers will have different physical and chemical prperties than an ther gemetric ismer, s they are truly a different mlecules Optical Ismers ccur when ne stereismer is the mirrr image f anther stereismer. The physical and chemical prperties f tw ptical ismers are usually almst identical, but the bichemical prperties in a living cell can be radically different. Fr instance all but 1 f the 20 different amin acids that make up the prteins f yur bdy are stereismers. And ALL terrestrial rganisms use the L-frm f these amin acids Figure 8-26 D and L ismers f Alanine Added figure Thalidmide The upper ismer is a sedative, and is a great treatment fr mrning sickness. The bttm ismer is a tetratgen and cause birth defect and bne cancers 9