Problems in Physical Organic Chemistry

57 Pages·1998·31.15 MB·english

by  Butler A.R.

PROBLEMS
IN
PHYSICAL ORGANIC CHEMISTR y
~
~\ ANTHONY R. BUTLER
University 01St~Andrews
JOHN WILEY & SONS
London . New York. Sydney . Toronto
Preface
Copyright @1972John Wiley&SonsLtd. AlIRights Physicalorganicchemistryisasubjectofincreasingresearchactivityandthis
Reserved. No part oí this publication may be re- activityisreflectedinthewaythat organicchemistryistaught. Astudy ofthe
produced. stored in a retrievalsystem.or transmitted.
use of physical measurements in elucidating reaction mechanisms is an
in any forroor by any means.electronic, mechanical
photo-copying, recording or otherwise, without the important part of any advanced course in organic chemistry, and such a
prior writtenpermissionoftheCopyrightowner. study is well suited to instruction by the discussion of examples from the
chemicalliterature. This method has been usedfor a number ofyears with
LibraryofCongresscatalogcardnumber72-617
undergraduates at the University of StoAndrews, and it is hoped that the
ISBN O471 126802 examplesusedwiIlbeofvalue to other students ofthe subject.
Thanks must begivento many investigators allayer the world who have
provided so muchilluminatingand instructivematerial. 1would liketotake
tbisopportunity ofthanking Mr. H. R. Rore, whodid somuchto stimulate
myinterestwhileatschool,Professor LordTedderforreadingthemanuscript
and writing a foreword, and finally Miss Helen Wallace, who carefully
checkedeach problem and made manyvaluable suggestions.Tbe remaining
errors and omissionsare thesale responsibiIity offue author.
October, 1971 Anthony R. Butler
St.Andrews
Printed inGreat Britain by J. W.Arrowsmith Ltd., Bristo13
Foreword
An understanding ofphysicalsciencecan only beacbievedbyparticipating.
Learning bynote mayenablea student to acquire an assemblyoffacts and
lawsbut comprehension cannot beattained this way.Full participation can
onlybeacbievedbytakingpart inresearch and fuenearerthestudent can be
brought to problems confronting fue research worker fue fuller will be bis
understanding of fue subject. In fue present book, Dr. Butler, himself an
active researcher in fue field of Physical Organic Chemistry, has brought
together a collection of problems in this field.The problems are of graded
difficultythemoreadvancedbeingtaken directlyfreIDtheresearchliterature.
Astudentwhoworksthrough thisbookwillindeedhaveparticipated andwill
havegainedanunderstanding ofthisimportant branchofOrganicChemistry.
A straight reading of problem and answer will prov~ a valuable exercise
though itishoped that most readers willseekto completetheir ownanswer
before comparing it with the one provided. This book will help not only
students but activeworkers who wiIl1am SUtefindtheir ideasclarified,as 1
did.
Tedder
- --
Introduction
This book of problems is intended as an aid to students taking courses in
physical organic chemistry. As fue solutions to the problems are given, the
book isnot suitable for semi~arsor tutorials, although somequestions have
been left partly 'open-ended' to permit their use in discussion. The first
part of the book is a seriesof straight forward exerciseson specifictopics
and each section is preceded by a short discussion, with references to the
literature afilie subject.Inusingfueproblemsitissuggestedthat fuestudent
coversthe solution with a sheet ofpaper, works through the problem, and
thenchecksbis answeragainstthat given.Nearly alltheproblems aretaken
freIDoriginal research papers and should anyaspect ofthe problem interest
or puzzlethe student, hecan take recourse to the original paper for further
information and discussion.
The second part of the book is a collection of more general problems
involvingseveraltopics comingwithin the general fieldof physicaIorganic
chemistry,and maybe found usefulbystudents preparing for examinations
involving'problem' papers or at fueconclusion of acourseof lectures.This
part afilie book should.beusedinthe samewayasthefirst.Problems which
maybe found difficult,orinvolveconsiderable calculation, are indicated by
an asterisk.
Undergraduates rarely have occasion toconsult research papers during
their studies and this is a serious omission. In severalexercises,therefore,
suchconsultation isnecessarybefore theproblem canbesolved.Thishas the
disadvantageofmaking theproblem moretimeconsumingandhasbeenused
sparingly.
The main difficultyexperienced in preparing this collection of problems
has been defining fue term 'physical organic chemistry'. There are already
severalcollectionsofproblemsonstructure determination, andalsospectros-
copy,and thesetopicshavenot beenincluded. What topicsare rightlycalled
'physicaIorganic' is,inthefinalanalysis,a matter oftheauthor' schoicebut
it ishoped that thestudent willnot findthe scopetoa limitedto beofvalue.
During the preparation of this book the second edition of Hammett's
Physical Organic Chemistry appeared and, broadly, the topics discussed
there have been included in tbis present volume. As Professor Hammett
did somuch to initiate study of the subject, it isnot unreasonable to usebis
textasdefinitive.Ingeneral,theproblemspermitfueelucidationofareaction
mechanism byfueuseofquantitative data.
)
Contents
Part1
The Literature of Physical Organic Chemistry ........ ..... .. .... .. 3
......................... 4
Inductive, Resonance, and Steric Effects .
16
Hammett Relationship. .. .... .... ... ........ .......... ... ... ...
ProductAnalysis.............................................. 16
Kinetics..................................................... 20
Activation Parameters . ........ ... ........ .......... ... ........ 25
Salí and Solvent Effects .. ........ ... ..... ............... .... ... 30
Isotopes..................................................... 34
42
Acid-Base Catalysis . . .. . . . .. . .. . .. . .,. .. . .. .. . .. . . . .. . .. . .. . . . .
Acidity Functions .'........... ... ...... .. ..... ................ . 49
Brensted Catalysis Law .... ... ....... .............. ........ .... 53
ss
Complex Formation. . .. . .. . . . . .. . .. .. . . . . .. .. . .. .. . . . . . . .. . .. .
57
Optical Activity. .. . . . . .. . .. . .. . . . . . . . . .. . . . .. . .. . .. .. . .. . .. .. .
Conservation ofOrbital Symmetry .............................. 59
Part2
Miscellaneous Problems. ...... ........... ................ ...... 65
°" Index ~....... 103
~~t-Sr-t- ~
,
The Literature of Physical Organic
Chernistry
Itisimpossibleto mentían allthe books whichmight beincludedunder this
heading but the followingare amongst those most"commonlyin use.In this
collection ofproblems they willbe referred to by the names ofthe authors.
More specializedtexts,and reviewarticles,willbe mentioned in the appro-
priate places.
R.W.Alder,R. Baker,and John M. Brown,MechanisminOrganicChem-
istry, Wiley-Interscience, London, 1971.R. P. Bell,Acid-Base Catalysis,
OxfordUniversityPress,Oxford,1941.E.S.Gould,MechanismandStructure
inOrganicChemistry,Holt, Rinehard, and Winston, NewYork, 1959.L.P.
Hammett, Physical OrganicChemistry,2nd ed., McGraw-Hill, New York,
1970.1.Hine,PhysicalOrganicChemistry,2nded.,McGraw-Hill, NewYork,
1962C.K.Ingold~StructureandMechanisminOrganicChemistry,2nded.,G.
Bel~London, 1970.E. M. Kosower, An lntroduction lOPhysical Organic
Chemistry,Wiley,New York, 1968.J.E. LefHerand E.Grunwald, Rates and
Equilibria of OrganicReactions, Wiley, New York, 1963.K. B. Wiberg,
PhysicalOrganicChemistry,Wiley,NewYork, 1964.
3
s
INDUCTIVE, RESONANCE,ANDSTERIC EFFECTS
Solution. The strength ofan acid depends upon the stability ofthe anion
Inductive, Resonance, and Steric Effects
formedonionization and this,inturn, dependsupon theextentofdeloca1i-
zation of*e negativecharge.
Thesetopicsarediscussedinallthe textsonphysicalorganic chemistryand,
(i) The methyl group iselectron-repellingand (b)isweakerthan (a).
indeed, in most books on organic chemistry. In view of the monumental
(ii) The nitro group is strongly electron-accepting and (d)is stronger
contribution made to the study of these effectsupon cher11icalreactivity,
than (c).
Ingold's book must remain the source book for subsequent reviewsbut it
O~ 0-
may be too detailed for most undergraduate courses. The I, T, and M
nomenclature of Ingold has not been adopted. A clear and descriptive 6
terminology isthat due to Tedder and Nechvatal (BasicOrganicChemistry,
Part 2.Wiley,London, 1967)and willbe used here. The inductive effectis
described as 'electron-attracting' or 'electron-repelling' and the mesomeric
N
effectas 'electron-accepting' or 'electron-donating'. There isan interesting ~
0"'"
article byG. V.Calder and T.]. Barton [J.Chern.Ed.,48,338(1971)]which
indicates that thesimpleaccounts giveninmanytextbooks arenot inagree- (iii) The mesomericeffectofthe acetyl group iselectron-accepting but
ment with alltheexperimental data. thiscannot be relayedfrom the m-position,so (f)isstronger than (e).
1. Whichisthestronger acid ofthe followingpairs and why?
O~ 0-
(i) (a) C02H and (b) C02H 6
0 0
CH3 C
~
H3C"'"
(ii) (c) C02H and (d) C02H
0 0 (iv) The second dissociation ofmalonic acid ismuch lessthan the first
as it involves separation of a proton from a species which is already
negativelycharged sothat (g)isa stronger acidthan (h).
N02
(v) The hydroxy group is electron-attracting so U)is stronger than (i).
pKavaluesforalltheseacidscanbefoundinA.Albertand E.P.Serjeant,
(iii) (e) C02H and (f) CO2H Ionization Cons?'antsof Acids andBases,Methuen, London, 1962.
0 2. DiscussthepKavaluesofthecarboxylicacidsgiven:(AlowerpK. indicates
OCOCH3 COCH3 a stronger acid.)
C02H
0
(a) CH3CO2H (b) FCH2C02H
4-76 2.57
(iv) (g) / CO2H and (h) / CO2- (c) 4-17
H2C"- H2C"- (d) 0C02H (e) C02H (f) C02H (g) CO2H
CO2H CO2H OF 0
aCMe
F
OMe
(v) (i) CH3CO2H and (j) HOCH2CO2H 4-14 3.27 4-09 4-47
v
4
6 PROBLEMSIN PHYSICALOllOANlC CHEMISTRY INDUCTIVE, REsoNANCE, ANDSTERIC EFFECTS 7
Solution. Electron-attracting groups like tluoro delocalize the negative -
(e) Predominant attack is at the position 0 to the NMez group,
charge on the anion and so (b)isa stronger acid than (a).This is true of -
illustrating thedominance oftheolpdirectingproperties of NMez over
p-tluorobenzoicacidbut themesomericeffectactsintheopposite sense,so the lesspowerfulchlorine.
a p-tluoro substituent has little effecton the pKa of benzoic acid.There
(f) Attack is0and p to the methyl group showingthat the effectofthe
may also be a further important factor known as the I1Ceffect: this is
methylgroup (whichisactivating as wellasolpdirecting)isstronger than
discussed by Tedder and Nechvatal (Basic Organic Chemistry.Part 2.
that ofthe nitro group (whichism-directingbut deactivating).
Wiley, London. p. 70).In the a-position the inductive effectis increased Data fortheseand similar reactions canbefound inP.B.D.dela Mare
becauseofthereduceddistanceand(e)isastronger acidthan benzoicacid.
and J. R.idd,AromaticSubstitution,Butterworth, London, 1959.
There may also be a steric factor, forcing the carboxyl group out ofthe
planeoftheringandreducingtheacid-weakeningproperties ofthebenzene 4. Squaric acid ionizesdirectly to the dianion and is a stronger acid than
sulphuric.
ring.The inductive effectofthe methoxygroup issufficientto make (f)a
slightlystronger acid than benzoic.However,an electron-donating meso-
2H+
meric effectcan be relayed from the p-position and the result is that p- HOn° ;: -One +
HO 0 -0 0
methoxybenzoic acid isweaker than benzoicacid. D. 1.G. Ivesand J. H.
Pryor, J. Chern.Soc.,I95S, 2104.J. F. J.Dippyand R.H. Lewis,J. Chern. Explain theseeffects.
Soc.,1936,644.
Solution. The strength ofan aciddepends largelyupon thestabilityofthe
3. Predict the distribution of isomers obtained by the electrophilic mono- anion. The dianionofsquaric acidisparticularly stableowingtoextensive
chlorination ofthe followingsubstances.
O'W,O
O~'O
CH3 Bul NO:z
0 0 0
(a) (b) (c) delocalization ofthe electrons to givea completelysymmetrical dianion.
G.Maahs and P.Hegenberg,Angew.Chern.Intern.Ed.Engl.,5,888 (1966).
5. Aromatic iodination may be effectedbyreaction with thallium trifluoro-
acetate in tritluoroacetic acid and subsequent treatment with aqueous
potassium iodide,allat room temperature.
CI NMe:z Me
0 0
(d) (e) (f) 0
ONO:z
CI + TI(0:zCCF3)3 ;: OTI(0:zCCF3):Z
11\" 1KI
Solution. (a) The methyl group is ortho/para directing and gives approxi-
01
matelythedistribution expectedonstatisticalgrounds(60%0and 40%p).
(b) With the t-butyl group the amount of a-substitution is reduced,
probably for steric reasons (22%0 and 76%p). The followingisomerratios wereobtained for anumber ofcompounds:
(c) The nitro group isalmost exclusively m-directing. Isomerdistribution
(d) Chlorine donates electrons mesomerically but attracts them by %0 %m %p
induction. The former effect makes chlorine o/p directing but the ring is PhCH2OH 100 0 0
deactivated. As the inductive effect decreases with distance the a-position PhCH2OCH3 100 0 0
ismoredeactivatedthanthep-positionandsoahighproportionofp-iso-' PhCH2CH2OCH3 85 3 12
PhCH2CH2CH2OH 12 9 79
mer isobtained (33%0and 55% p). PhCH2CH2CH3 3 6 91
INDUCflVE, REsoNANCE, ANDSTERlC EFFECTS 9
8 PROBLEMS IN PHYSICAL ORGANIC CHEMISTRY
Comment onthesevalues(considercarefullythedefinitionoftheterm'partial
In the last case,ifthe reaction mixture isreftuxed during thallation, the isomer
rate factor').
distribution is changed to 9%0, 78%m,and 13%p.
Suggest reasons for (a) exclusive ortho attack with benzyl alcohol and Solution. The I-position of biphenylene is unusual in being activated
benzyl methyl ether, (b) an increase in the amount of p-isomer as the chain (with respect to a position in benzene) towards hydrogen exchange, but
length is increased, and (c)the change to rn-substitution on reftuxing. deactivatedin protodesilylation. This probably indicates that the simple
picture ofthe easeand orientation ofelectrophilic substitution depending
Solution. Exclusive ortho attack may be explained by complexing of
upon resonance stabilization of the Wheland intermediate is an over
thallium at a basic site in the side chain
simplification of the situation. Activation, or deactivation, of a position
dependsalsoupon thedemand forresonance stabilizationofthetransition
CH3
I state which, with biphenylene, appears to be much greater for hydrogen
CH-0
exchange than for protodesilylation. J. M. Blatchly and R. Taylor, J.
Chern.Soc.(B),1964,4641.R.Taylor, J. Chern.Soc.(B),1971,536.
V
0:-'t,(00CCF3b
- 7. Explain thevariation ofpartial rate factor obtained in the nitration ofa
seriesofalkylbenzenes.
I 0 m p
and subsequent attack at theo-position.Asthebasicsiteismovedfurther toluene 49.7 1.3 60.0
fromtheringthisnolongeraffectsthesiteofthallation andthep-compound ethylbenzene 31.4 2.3 69.5
is formed.Thallation isa reversible process and, while normally kinetic isopropylbenzene 14-8 24 71.6
factors decide the position of thallation in the absence of a side chain t-butylbenzene 4-5 3.0 75.5
containing a basic site, reftuxing produces the thermodynamically most Solution. The rneta/para ratio changes very little along the series showing
stableisomer(i.e.thern-compound).E.C.Taylor,F. Kienzle,R.L.Robey,
that the polar effects of the alkyl groups are very similar. However, there
and A.McKillop, J. Arner.Chern.Soc.,92,2175(1970). is a dramatic decrease in the amol\nt of o-substitution and this is due to
steric factors. J. R. Knowles, R.O. C. Norman, and G. K. Radda, J. Chern.
6. Two reactions frequentlyusedinmeasuringthe reactivity ofan aromatic
Soc., 1960,4885.
compound towards electrophilic attack are (a) hydrogen exchange in tri-
ftuoroaceticacid(protodetritiation), and (b)protodesilylation. Bothofthese 8. Alkyl'substitution normally increases the basicity (pK. is larger) of
havebeenexamined with respectto the I-position ofbiphenylene. pyridinebut thefollowingresultswereobtained for2,6-di-t-butylpyridinein
threedifferentsolvents.
- 20%aq.methanol 20%aq.ethanol 20%aq.2-propanol
(a) (0) T~ ()1) ~ pyridine 5.12 5-09 4-96
" I I I I
+ CF3C02H + CF3C02T 2-r-butylpyridine 5.65 5.61 5.56
~",:::- ",:::-"':::- 2,6-di-t-butylpyridine 5-06 4-81 4-61
The weakening effect\pfthe second t-butyl group may be due to (a) steric
(b) s~e3 - ()1) ~ strainonthebound proton [H.C.Brownand B.Kanner,J.Arner.Chern.Soc.,
CI oOI I I 88,986(1966»,or (b)steric inhibition ofsolvation (E.E. Condon, J. Arner.
+ H+ + SiMa;
",:::-"':::- ",:::-"':::- Chern. Soc., 87, 4494 (1965)].
Assuming that the effectof alkyl substituents should be additive, show
whichoftheseexplanations isconsistent with the abovedata.
The followingpartial rate factors forthetwo reactions wereobtained:
Solution. The differencebetween the calculated pK., assuming that the
104 0-52 effectofthe second t-butyl group is the same as the first,and the experi-
(a) ~
~ (b)~ mentallydetermined onevarieswiththe solvent,sofavouringexplanation
~ (b).D. H. McDaniel and M. Ozcan, J. argoChern.,33,1922(1968).