HOMEWORK PROBLEMS: STRUCTURE, BONDING & HYBRIDIZATION 1. The molecule shown below is Griseofulvin, an antifungal compound.

a. Give the hybridization of carbons 1, 2, 3 and oxygen 4. C1: sp3 C2: sp3 C3 sp2 O4 sp3 b. What is the geometry at each of these atoms? (Ignore distortions.) C1: tetrahedral C2: tetrahedral C3 trigonal planar O4 tetrahedral c. Fill in all nonbonding electrons and H's on the structure below. (Assume neutral charge.)

2. For the molecule shown give the information corresponding to each letter as indicated below. a. give the hybridization of these atoms. b. give these bond angles.

OCH3

O

Cl

H3CO

O OCH3

O

H3C 1

23

4

OCH3

O

Cl

H3CO

O OCH3

O

H3C

HH

H HH

OH

O

HO

O

OH

sp2

sp2

Prostaglandin E2

120°

120°þ

109.5°

sp3sp3

3. Consider acetylene anion, shown below.

a. Give the hybridization of all carbon atoms.

b. In what type of orbital does the nonbonding pair of electrons reside? sp orbital 4. Draw reasonable skeletal (line/angle) formulas for the following condensed formulas/structures. Next, draw good 3D structures by adding wedges and dashes to your simple skeletal structures. The answers include the 3D of the skeletal and an expanded idea of the 3D structure with the hydrogens added back in (essentially hybrids of the skeletal and Lewis). a. CH3CH2CH2CH(CH3)2

b. CH3CH2CHBrCH2C(CH3)3

c.

C CH

sp sp

C CH

H

HH

H

HH

HH

H HH

H

H H

Br

H

BrH H

H H

H

H H H

HH

H

HH

H HH

Br

H3C CH2 CH C

O

OH

H3C

d.

5. Convert the following skeletal structures to Lewis structures. Make sure to include all atoms and lone pairs.

6. Convert the following Lewis structures to skeletal structures.

OH

O

OH

OH

HH H

H

H

HH

H

OH

O

CH2

CH2

CH2

CCH

CH

CH2H

H

H

H

HH

H

H

H H

C

CC

C

CC

CC

CC

C

OCl

C

CC

C

CC

C

O

C

O

N

HH

H HH H

H H

H

HH H HH

H

H

H HHH

H

H

NH

O OH

NH2

OH

O

O

O

7. In the following molecule, identify all atoms that have a trigonal planar geometry. Can you make any correlations between obvious structural features and the geometries? Explain.

All the atoms here that are invovlved in double bonds have trigonal planar geometry and will be best described as sp2-hybridized. This trend is generally true (note allenes as special cases). The key feature overall is that large regions of the molecule are flat. Interconnected groups of trigonal planar atoms lead to planar regions of a molecule. Moreover, as we think about the orbital pictures, we recognize that these planar regions have electrons residing above and below them because the p-orbitals used to make the pi bonds lie perpendicular to the plane with the sigma bonds. Interesting chemistry will result from these electron clouds.

N

O

OH

Cl

carbonger, aC-C

1_

1L 1 l

8. (a) Provide the ground state electron configuration and the number of valence electrons for carbonand silicon. Describe how these two atoms are similar. (b) Which bond do you think is stronger, a C-(bond or si-si? Explain'

I g"o, c on,l 5, hc,vr sc,n{ j

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'* | L

1 1 lo'*; t\r*'n 5' -5-' qt ;t \ " lL

L ( r ' - lrng or)r,hl: k nr, Z"l lt',erltr a,

'lt y1 v | , r 2 ( o ' I

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ll 4v [ (,*".*h,,r, hfie',r a"'rcr-lqa t:1" j

I s k

Carbun I l-.,iruc |s,,r,,|ur s;,zzJ ocb,.lau)

\ 5 ilicovr9. (a) What hybridization do you predict for the carbon atom in CH:n, CH:-, and :CHz? (b) What do yrpredict for the H-C-H bond angle for each of these species? (b) What do you predict for the moleculargeometry?

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i f

Fl rl

h1b.iJ X' s?t s('

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rroo)ec....r)*t- t*1onol tt\yon.o,l , b<"ltryom,a\

' fi',o,2+: (1fan:'dal

CHEM24 Dr. Spence

10. Draw the Lewis/Kekule structures for SiH+, PHg and HzS. Provide the electronic geometry and the

molecular geometry for each as predicted by VSEPR theory.

i1* ?.- H

*trahcdrE I

IH - 5; -t{

I

r-leo 9gorrr :

-ittr'qheJr".\

nt _l<w lc /st{-uhr dml9a'* / -

H - P - Ht

l-lieTc.,heJ{" I

tnqu,r,alginm,J"l

g+nt

I t

tl-p'HIl"t

CHEM24 Dr. Spence

11. Consider the Lewis/Kekule structure for PH3. (a) What value do you predict for the H-p-H bondangle? (b) Is P-H a stronger bond or weaker bond than an N-H bond in NH3? Explain. (c) Ishybridization necessary to explain the trivalent nature of phosphorus? Explain. If it does undergohybridization, what hybridization state would PH3 have and why would it undergo hybridization?

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? r{ a*}',t,\ (€t*- N lb,Lil) ,ryrth H n5 N{o.rhrb,l (Zl sre|)

(C) N1i. t){rhlbnJ'zr/ ? cc4n &rrn 3 banJs (r*a., € cwfigrrql'oo)

(r)) tQ oo hlfi,di"qion ,10" bw,J 1n*S $ uueqlst bo*Js (g,u'rer o'^:wlug"

wl p-orbrto!/ 5"(' hlVnrlizolron lr-'r-t 51<amryr bnnJs anJbq,5" VanJ c;nglns t Sr,l1>F-f v1E?R theof/

lZ.Draw anorganic molecule which contains^the followingtype of bonds:(a) a Csp'- Osp' sigma bond (c) Nsp'- C.p' sigma and pi bond

CHr - orf' I

(b) Csp3 - Crpt sigma bond

/->' l

13. Why is an sp hybridized C-H bond stronger than an sp'C-H bond? ,\ rr

Rn 5f arb',fql ,: rrao'rt dire ."t\rlo* I (l4ten.''t) Siott- 'f '> 5Oy" S cnrnpred

fo 2{z trrqA i: wl1""%'L S". A rnort Jrtcfiooq/ orbifuI rrfr)I

ort( laf b<ilsr rrr rth H ls orbthi 7b grtL q Skooopl- bo"d,

14. Are the hydrogen atoms in the molecule H2C=C=Q=CHz in the same plane or in planesperpendicular to each other? Explain using the hybridization of each carbon atom.

ftll ,', fcam<- plc,r{. t" r'rtvq) can?ir'l.oc kJ')"tft 17 baalS a{L /rrrlel

,-\'ZN -Yl\

(d) Nsp2 - Crp' sigma bond (with no pi bondlfc* lea:t nut be1#6f.a;

*rt'O

\ At t^nt'l L /

H-.."0,4rffi.,o.il

,,o'(4784\1f VoacJl

ctlt l1'5 art

t|.r p)rtn\,

V<c guodr cvlr^r'

fu ?"{

CHEM24 Dr. Spence

15. When energy is supplied to the n bond of ethylene, one electron is promoted from a molecularorbital of lower energy to one of higher energy. (a) Show what this excited state MO diagram wouldlook like. (b) What happens to the n bond if this occurs (consider bond order before and after)? (c)Based on values of bond strengths, how much energy is needed for this excitation to occur?

t"-_tonh b'"Jin1

- - t 1 ' ' ' 1

/? L'., 'j-- 7("'#j

bon,J''nryrn0

i ll r l l

rL \(i.'11\DTv'lrfi ()

,-1". I;-

- . t

\ ' 1 , ' .r | . r ,Y2onc'l\flc,\

Ia

rO)

h r 1

(Y1,tfi conriir:b( (":ooqnc€ Foot^ ( to7;t Z )

po n OO

) Z^' fet. Forr^ rvt,tif co,.tlr,WtU s'

,X;-tr +-> p"/=.f,.tt ( , ,t s{ h-t'br;J.rr) conol g}anc^r.'r{

h I ttt st ^ 6a"rJ rc: Fw'{"l'mwl irrx- s{;n qI-

fv'la;of fe5. dlrnof fts \ af tX"n - (trnev/lbtr Wt CwrtftvY nai:'ft olutnl

crntr.,Wtuc C"c#,h\rr ) ?or fes h>rtn s )17. Can boron satisfy the octet rule by forming covalent bonds? Explain why or why not'

//ot c/;rt *H &o{oro ha > *t\ 3 vq f$nrt e }t rkooE d,

fYtur< t ltilny 7 co'v,;b.*1t" Y;aoJS ",nJ 6 eltc{t'6"t ,?s} g'

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C =C tpontl *trtn3\h = &tX F5)lmer)

C -t b,on) r$nnrth = i5o P5l"ml

f,t> 7l bonrl = %i trSlmot '" ener1l ow'l'zJ $ +cil-t

16. For the following molecule, all the atoms are in the same plane, i.e. the plane of the paper. Is thisexpected? Why or why not? If it is not expected, what does this tell you about this compound?

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{"6"-t 7

CHEM 24 Dr. Spence

18. Given below is the molecular orbital diagram for CHa. Mathematical combination of the carbon 2sand 2p orbitals with the four hydrogen 1s orbitals generates the eight molecular orbitals as shown (note,recall these orbitals and their energy levels must be determined through calculations and cannot bepredicted). In this diagram, fill in the valence electrons in the atomic and molecular orbitals and labelbonding and antibonding molecular orbitals. What does MO theory predict for the number of bonds (orbond order) for CH+. Is this consistent with the way we draw the Lewis structure?

\ \\ '

\\ 4 1 4 ' l1 f I I t/-

4 X H l s o r b i t a l s

1 1 tt t ,

c2p - -,

1l-." ".C2s --1 \\ \\ \\ . ' .

4 r' - !Lt

t a

a/a ,

a t

t t '

4 t n l , ' , 'I r " lL . . '

I

t )eonJ orJ+C\ z' . .

, , ' 4 -f '> , i [

'Jo., l i ;k ,^ \\ t t r . ' U v

- l l I - '

\"J-L" -z : L l \ lewnlnStroclwt-)

For your information: rt - c-H

The MO diagram for CH+ illustrates that of the 8 bonding electrons in this molecule, two'arelower in energy than the other six. This result cannot be predicted based on Lewis structures or valencebond theory, and the question becomes is this truly the case? In a technique called photoelectronspectroscopy, the ionization potential (the energy needed to remove an electron from a molecule) of CH+can be measured. If all electrons had the same energy value, photoelectron spectroscopy would give asingle value for the ionization potential. This is not the case. Photoelectron spectroscopy provides twovalues for the ionization potential of CH+ (depending on which electrons are ejected) as predicted fromMO theory - since there are electrons at two different energy levels, they require a different amount ofenergy to be removed.

MO is a more accurate theory to describe bonding, but it is very mathematical and not as easy tounderstand as valence bond theory. For this course, valence bond theory and Lewis structures only run

into trouble when no one single Lewis structure can be drawn and the concept of resonance is used.

Consider ozone,the only difference in the two major resonance structures is in the location of the n bond(or n electrons) and nonbonding electrons. The atoms are in the same position, and the geometry is the

same in each case. As a result, valence bond theory gives a good description of the o bonds but a poor

description of the n bonds, where the electrons are delocalized over the molecule. This delocalization is

precisely what MO theory does best! During this course we will primarily utilize valence bond theory to

describe the electrons in o bonds (or bonds forming in a reaction) as localized between the two atoms,

and in later chapters use MO theory to show how the n electrons of certain molecules (and reactions) are

delocalized over the entire molecular structure.

@/o__o

@-O- <-+

o oo o

)