Chapter_2 Molecular Representations

7/23/2019 Chapter_2 Molecular Representations

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There are many ways to representmolecules:

If you were representing a large molecule

with 20 or more atoms, which structurewould be most time consuming to draw?

Which structures give you the most

information about the structure?

2. !epresenting "olecules

#opyright 202 $ohn Wiley %&ons, Inc.

Klein, Organic Chemistry 1e2-1


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'iven that there are three isomers of

propanol (below), which structures above areade*uate to represent only isopropanol andnot its isomers?

+ractice with &I--/I-1! 2..





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To draw large molecules *uicly, a di3erenttype of representation is needed.

#onsider the antibiotic amo4icillin. Its -ewis

structure loos cluttered, and it would bevery time consuming to draw.





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The bond5line structure is easier to read andto draw:

2.2 ond5line &tructures



vs.


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It may seem lie a foreign language at 6rstbecause many of the atoms are not labeled.

This type of representation is the "7I8 way

that chemists communicate, so it is alanguage you "/&T master to be successfulin organic chemistry.





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-ie -ewis structures, lines are drawnbetween atoms to show covalent bonds:

7toms are bonded at angles (9ig9ag) thatrepresent the actual geometry of the bondangles. What is the bond angle for sphybridi9ed carbon?

#arbon atoms are not labeled, but a carbonis assumed to be located at every corner or

endpoint on the 9ig9ag.





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-ie -ewis structures, lines are drawnbetween atoms to show covalent bonds:

#arbon;hydrogen bonds are omitted. W


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+ractice identifying the location of the carbonatoms and counting the number of carbonatoms in the structures below.





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ouble bonds and triple bonds arerepresented as you might e4pect:

Why is a triple bond written without9ig9agging?





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=ou must also be able to use the bond5linestructure language to interpret the numberand location of < atoms in a molecule.

< atoms are not shown, but we can assumethere are enough to complete the octet (fourbonds) for each carbon:





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=ou should practice bond5line structures untilit becomes natural for you to see all of thecarbon and hydrogen atom locations.


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If you are given a -ewis structure orcondensed structure, you must also be ableto draw the corresponding bond5linestructure: !epresent the bond angles with 9ig9ags:

>ollow &1+! and spread out the electron pairs ona central atom:





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If you are given a -ewis structure orcondensed structure, you must also be ableto draw the corresponding bond5linestructure: &ingle bonds are a4es of rotation, so be aware

that they can rotate.

'ive alternative bond5line structures for themolecule below.





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If you are given a -ewis structure orcondensed structure, you must also be ableto draw the corresponding bond5linestructure:


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raw bond5line representations for thefollowing -ewis structures:





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ond5line structures allow chemists to*uicly e4amine how a chemical reaction haschanged a molecule.

#ompare the condensed formula with thebond5line structure below for the samereaction.

Which representation maes it more

2. Indentifying >unctional'roups




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When certain atoms are bonded together in speci6carrangements, they undergo speci6c chemicalreactions.

&uch arrangements of atoms are called functional

groups. W/8#TI@87-?




d if i i l


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"ore functional groups are listed in table 2.




+ractice with#@8#1+T/7-#


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>ormal charge (&ection .B) a3ects thestability and reactivity of molecules, so youmust be able to identify formal charges inbond5line representations.

-abel all of the formal charges in thefollowing molecule.

+ractice with #@8#1+T/7- #ormal #harge



2 B d li & Wi h


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"ost carbon atoms will have >@/! covalentbonds and no lone pairs to avoid carrying aformal charge. &ometimes carbon will have a C charge. In such

cases, the carbon will only have T

Is there anything especially unstable about#7!@#7TI@8&?

2.B ond5line &tructures With>ormal #harge



2 B d li & Wi h


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"ost carbon atoms will have >@/! covalentbonds and no lone pairs to avoid carrying aformal charge. &ometimes carbon will have a 5 charge.

Is there anything especially unstable about#7!78I@8&?

If carbon carries a charge in a molecule, the5

2.B ond5line &tructures With>ormal #harge



2 E d li &t t d


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&ometimes lone pairs are omitted from bond5line structures. >or e4ample:

=ou canDt determine the formal charge on the 8atom unless you now how many electrons thereare on the 8.

It could be:

=ou must 7-W7=& draw formal charges on a

bond5line structure to eliminate confusion.

2.E ond5line &tructures and-one +air 1lectrons



2 E d li &t t d


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If the formal charge is indicated on an atom,you can determine how many lone pairs arepresent.

To calculate the number of lone pairelectrons for an atom, compare the numberof valence electrons that &


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=ou can also determine the formal charge onan @ atom by matching its bonding patternwith its formal charge according to Table 2.2.

+ractice with &I--/I-1! 2.B.




2 E d li &t t d


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The formal charge on an 8 atoms can becalculated the same way, or by matching itsbonding pattern with its formal chargeaccording to Table 2..

+ractice with &I--/I-1! 2.E.


#opyright 202 $ohn Wiley %&ons, Inc. Klein, Organic Chemistry 1e2-26


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The vast maority of molecules are 5dimensional (), but it is diJcult torepresent a molecule on a 25dimensional(2) piece of paper or blacboard.

We will use dashedand solidwedges toshow groups that point backinto thepaper or out of the paper.

2.F ond5line &tructures



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Imagine the thin lines as being Kat in theplane of the boardLpaper: Dashedand solidwedges to show groups that

point backinto the paperor out of the paper.

We will focus on this type of representationin #hapter E.

2.F ond5line &tructures



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rawing lines between atoms inade*uatelyrepresents covalent bonds in molecules withresonance. !emember from general chemistry: what is

resonance?

#onsider the allyl carbocation:


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-etDs loo at thehybridi9ation of thecarbons in the allylcarbocation: #alculate the steric number

(M of N bonds C lone pairs).

When the steric number is ,it is sp2hybridi9ed.

2.H !esonance



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If all of the carbons haveunhybridi9ed p orbitals,they can overlap.

7ll three overlapping porbitals allow the electronsto move throughout the

overlapping areasimultaneously.

ThatDs !1&@878#1.

2.H !esonance



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>rom a molecularorbital point of view,when the T

T

carbocation?

2.H !esonance



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The allyl carbocationhas a charge of C.Which "@ is themissing electronmissing from?

Where is the positivecharge located?

2.H !esonance



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ecause neither of the contributors e4ists(loo at "@s), the average or hybrid is muchmore appropriate.


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!esonance maes a molecule "@!1 stablethrough: elocali9ation of electrons:

1lectrons in resonance e4ist in orbitals that span a

greater distance giving the electrons more freedom.

elocali9ation of charge: The charge is spread out over more than one atom. The

resulting partial charges are more stable than a full Ccharge.

2.H !esonance


#$ #$

resnance hy!ri"

2 O #urved 7rrows in


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Throughout organic chemistry, we will beusing curved arrows to show electronmovement. The sooner you master this sill, the easier the

course will be.

#urved arrows generally show electronmovement for +7I!& of electrons:

The arrow starts where the electrons are currentlylocated.

The arrow ends where the electrons will end upafter the electron movement.

2.O #urved 7rrows in!esonance




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!ules for using curved arrows to show!1&@878#1:

. 7void breaing a single bond.

& &ingle bonds can brea, but 8@T in !1&@878#1.

& !esonance occurs for electrons e4isting inoverlapping p orbitals, while electrons in single

bonds are overlapping sp, sp

2

, or sp

(sigma)orbitals.





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2. 8ever e4ceed an octet for 2ndrow elements (,#, 8, @, >)

& 7toms in the 2ndrow can only have four 2ndenergy level orbitals holding a ma4. of Oelectrons.

& 14amples of arrows that violate rule 2.





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. 2ndrow elements (, #, 8, @, >) will rarely, butsometimes, have -1&& than an octet.

What will the resonance hybrid loo lie for



2 P >ormal #harge in


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When using curved arrows to show!1&@878#1, structures will often carry aformal charge that must be shown.

raw the resonance contributor indicated bythe arrows below.

7re any of the rules violated?

&how any formal charges on the

contributors.

2.P >ormal #harge in!esonance


2 P >ormal #harge in


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In the resonance, the arrows tell us how tomove the electrons to obtain the othercontributor.

raw arrows showing the resonance in the

reverse direction.

=ou can also thin of the arrows as showingthe direction that charge will Kow.

+ractice with &I--/I-1! 2.H.

2.P >ormal #harge in!esonance



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There are E main bonding patterns in whichresonance occurs. !ecogni9e these patternsto predict when resonance will occur:

. 7llylic lone pairs

2. 7llylic positive charge

. -one pair of electrons adacent to a positivecharge

B. 7 pi bond between two atoms with di3erentelectronegativities

E. #onugated pi bonds in a ring

We will see many e4amples in the ne4t few

slides.

2.0 +atterns in !esonance



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I8=- and 7--=-refer to positions directlybonded to or one atom away from a #G#double bond.

-abel the vinylic chlorides and the allylicchlorides.




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. Identifying allylic lone pairs:

#ircle all of the allylic lone pairs.

raw arrows on each structure to showresonance.




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. Identifying allylic lone pairs: >or each of the following, show the resulting

resonance contributor and all formal charges.

& +ractice with #@8#1+T/7- #


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2. ealing with allylic positive charge: @nly one curved arrow is needed.

If there are multiple conugated double bonds,then multiple contributors are possible. &howthe resonance contributors and curved arrowsbelow.

raw a resonance hybrid.



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. 7 lone pair adacent to a positive charge: @nly one arrow is needed.

14plain how the formal charges are a3ected bythe electron movement in the following

e4amples.




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. 7 lone pair adacent to a positive charge: #onsider the resonance in the 8IT!@ group.

Why canDt @81 arrow be used to cancel out theformal charge and create a resonance

contributor?

& raw all possible resonance contributors



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B. 7 pi bond between atoms of di3erentelectronegativity:

The pi electrons will be more attracted to themore electronegative atom.

& 14plain how the formal charges are createdby the electron movement in the followinge4amples.



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E. #onugated pi bonds in a ring: 1ach atom in the ring "/&T have an

unhybridi9ed p orbital that can overlap with itsneighbors.

1lectrons can be shown to move clocwise orcounterclocwise.

What type of motion do the electrons actuallyhave?



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&ummary ; >igure 2.E:

+ractice with #@8#1+T/7- #


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&how all of the resonance contributors for thefollowing molecule.

8otice that carbons with B bonds (sp) isolate areas of

resonance from one another.




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!emember our nectarine analogy. Is anectarine more lie a peach or more lie aplum?

What if we made a fruit that was FEQ peach,

BQ plum, and Q iwi?


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ormal charge generally 1#!17&1&stability.

2. 7 Ccharge on an electronegativeatom or 5

on a low electronegativityatom is especiallyunstable.

. #@"+-1T1 @#T1T& I8#!17&1 stability.

raw the three resonance contributors foracetic acid.

7ssess the stability of each contributor, and

2. &tability of #ontributors


bili f ib


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. >ormal charge generally 1#!17&1& stability.2. 7 C charge on an electronegative atom or 5

on a low electronegativity atom is especiallyunstable.

. #@"+-1T1 @#T1T& I8#!17&1 stability. raw the three resonance contributors for

the acetate ion.

7ssess the stability of each contributor, anddraw a resonance hybrid.



2 & bili f # ib


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. >ormal charge generally 1#!17&1& stability.2. 7 C charge on an electronegative atom or 5

on a low electronegativity atom is especiallyunstable.

. #@"+-1T1 @#T1T& I8#!17&1 stability. raw the resonance contributors for the

following molecule.

7ssess the stability of each contributor, and



2 & bili f # ib


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The octet rule is usually a bigger factorthan formal charge when assessing stability.

>or each structure, assess the stability ofeach contributor, and draw a resonancehybrid.

+ractice with &I--/I-1! 2.O.



2.2 elocali9ed vs.


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-ocali9ed electrons are 8@T in resonance. elocali9ed electrons 7!1 in resonance.

elocali9ation increases stability.

There are a couple ways to recogni9eelectrons that are delocali9ed throughresonance?

. To be delocali9ed, electrons must e4ist in an

/8


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To allow for delocali9ation (resonance), someatoms that would normally be spwill become sp2hybridi9ed.

#alculate the steric number and predict the

hybridi9ation for the nitrogen atom of an amide.

If the nitrogen atom were sp, its lone pair of electronscouldnDt engage in resonance.

The nitrogen of an amide will be sp2hybridi9ed. In this case, the stability of the delocali9ation outweighs

the stability di3erence between sp2and sp.

Why is sp2

generally less stable than sp

?

2.2 elocali9ed vs.-ocali9ed


2.2 elocali9ed vs.


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oes the delocali9ation of the electrons inthe amide create a more or less stablecontributor?

elocali9ation must be a very stabili9ingforce if it can favor sp2hybridi9ation over

speven when the delocali9ation doesnDt



2.2 elocali9ed vs.


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The sp2hybridi9ationof the nitrogen atomcauses it to betrigonal planar

rather thantetrahedral.

To be delocali9ed, allthree atomsinvolved "/&T havep orbitals

overlapping.



2.2 elocali9ed vs.


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'enerally, lone pars adacent to a #G#

double bond are capable of resonance, butnot in this case.

The electron movement above does notviolate any of our rules, so why canDt thenitrogenDs lone pair be delocali9ed?



2.2 elocali9ed vs.


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The electron movement above does not violate anyof our rules, so why canDt the nitrogenDs lone pairbe delocali9ed?

!ecall that delocali9ed electrons must e4ist in anunhybridi9ed p orbital overlapping with p orbitals onneighboring atom.

The nitrogenDs lone pair is positioned perpendicular to theplane, where the other pi electrons reside.

+ractice with &I--/I-1! 2.P.


Chapter_2 Molecular Representations

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