ORGANIC CHEMISTRY
ORGANIC CHEMISTRY GAMSAT Study Guide GradMed
CONTENTS
ORGANIC CHEMISTRY Chapter 1 Structure and Bonding in Organic Chemistry Structure Drawing Chemical Bonding Predicting Molecular Polarity Atomic and Molecular Structure – Orbitals S Orbitals P Orbitals Hybrid Orbitals
Page 1 2 3 5 6 7 8 9
2 Organic Nomenclature Bicyclic Nomenclature
11 15
3 The Alkanes Rotamers Reactions of Alkanes Free Radical Halogenation
19 20 23 24
4 Alkenes Reaction of Alkenes / Electrophilic Addition Carbocation Rearrangement Alkene Hydrogenation Isomerism in Alkenes and the Cahn Ingold Prelog Rules Alkenes in Nature – Terpenes Other Reactions of Alkenes Oxidation States in Organic Chemistry Polymerisation Diels Alder Reaction
27 28 31 31 31 34 36 36 38 40
5 Alkynes Functional Groups
42 43
6 Stereochemistry and Stereoisomers Absolute Configuration Diastereomers Why Enantiomers Matter Atypical Chirality The Fischer Projection Sugars Haworth Projections
48 50 53 54 55 56 58 61
7 The Haloalkanes Antibonding Orbitals Nomenclature Substitution SN1 Reactions SN2 Reactions Elimination Grignard Reactions
66 66 66 67 67 68 69 70
8 The Oxygen Family Oxidation Level 1 – Alcohols and Ethers Oxidation Level 2 – Aldehydes and Ketones Reaction with Grignard Reagents Enolates The Aldol Reaction Addition of HCN Oxidation Level 3 – Carboxylic Acids and Derivatives Fatty Acids Saponification Naming Esters
72 72 75 76 76 77 79 80 82 83 84
9 Aromatic Chemistry Derivatives of Benzene The Electrophilic Substitution Mechanism Friedel-Crafts Reactions Directing Effect of Groups on Benzene Ring Side Chain Reactions
95 97 98 99 101 106
10 Nitrogen Heterogroups Forming Amines Diazonium Salts Elimination Reactions of Note Amides and Nitriles Hofmann Degradation Nitriles
107 108 108 109 110 111 112
11 Electrocyclic Reactions and Sigmatropic Rearrangements Diels Alder Stereochemistry 1,3 Dipolar Cycloadditions Sigmatropic Rearrangements
113 113 114 116 117
12 Spectroscopy Light as Particles The Appearance of Spectra The Beer Lambert Law Vibrational Spectroscopy Infrared Spectra Infrared Bending Frequencies in Aromatic Compounds Nuclear Magnetic Resonance Spectroscopy 1H NMR Spectra – Proton Spectroscopy
120 120 120 121 123 124 129 131 143
13 Introduction to Basic Chromatographic Principles
148
Structure and Bonding
CHAPTER 1
CH HAPTER 1 Strructure and a Bond ding in Organic O Chemistr C ry The chemistry of o life is made e up of rema arkably few of o the 103 kn nown elements. In fact most o only six ele ements. The ese six make e up the com mmon amino a acids, glycos sides life is made up of e. and nucleotides that are the major constiituents of life Organic chemistry w was called such ectly) because it was assumed (incorre o manufa acture ce ertain that to compoun nds was impo ossible, unle ess it was done e by a living g organism. The term hass now come to mean simply the chem mistry of carbo on compounds. pecial So what has made ccarbon so sp hemistry of life? Carbon can in the ch form strong bonds to itself and ca arbon g bonds to other o can also form strong atoms.
Bu uckminsterrfullerene
arbon has a valency of four, Finally ca meaning that it alwayys forms a tottal of ds in stable compounds.. It is four bond this com mbination of properties that makes carbon un nique amo ongst ad of elements, and meanss that a myria possible compounds can be formed. An exam mple of two o of the more m complex structures fo ormed by ca arbon wn here. are show
NH2
ook a little de eeper into why w carbon We shall now lo o special, and at a few off its compoun nds. is so o get over The biggest hurdle for most people to en considerin ng Organic chemistry iss grasping whe the basics of molecules. m W What shape they are, at makes them t react, what makes them wha diffe erent from on ne and anotther. Withou ut this you are just learning g tables of reactions wiith no real e as to what is going on or o why. clue n of this boo ok will be considering c The first section basiics of moleccular structurre and looking at how this affects the e shape of molecules and their perties. An understand ding here is a key prop foun ndation for the developm ment of more complex GAM MSAT conce epts.
N
N
N
N
HO O
H
H
H
O OH
H
H
De eoxyribonu ucleoside
© GradMed Limited All rightsLimited reserved–2015 © Gr-radMed All rights rese erved 2015
1
1
Structure Drawing Over the years, molecules have been drawn in many ways. This leads to confusion sometimes because different conventions are taught at different levels. If you last did organic chemistry many years ago then you may be used to a structural representation like the one shown here on the left.
If this is the case, then you MUST get this convention out of your head. It became almost completely obsolete in 1890 and is remarkably counterproductive. I guarantee you will never meet this system in medical school, and very rarely in GAMSAT. Organic chemists currently use the skeletal convention to represent molecules. To the right is a representation of the same molecule in skeletal format. The rules of skeletal drawing are simple: • • • •
C-C bonds are shown by a line C atoms are not shown C-H bonds are not shown. Carbon is assumed to have a valency of four. Any group containing other atoms is labelled in full (e.g. OH, CN, NO2)
The most frequent problem encountered with respect to these structures is the idea that each line is a carbon-carbon bond, and that the hydrogens are invisible. Carbon always has 4 bonds in organic chemistry (unless something very bad has happened to it and then it will have charges or dots next to it) and so if you can only see two bonds then there must be two others you cannot see. These will be the bonds to hydrogens. Shown below are two representations of the same molecule to help you get the idea.
I cannot overstress the importance of this system. You need to automatically “see” the hydrogens or nothing else in the book will make much sense. The skeletal system also has room for some 3D effects. Organic chemistry is a 3 dimensional science, after all. Put simply, these are called wedge and dash notation. The idea is that a straight line shows bonds, which are by convention in the plane of the page.
2
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© GradMed Limited - All rights 2 reserved 2015
The reason for this is simp ple. Bulky groups will interact with h each othe er and repel one onformer thatt forces them m closer toge ether will be high h energy b because of this. anotther. Any co In Cycloalkanes C these rotam mers can take e up one of tw wo different minimum en nergy position ns or confformers: the e chair and th he boat. Of these two th he chair is byy far the more e stable, as all of its bonds b are sta aggered and d there is no overlap betw ween any off its substitue ents. In the boat form m two bondss are eclipse ed and two groups g have e significant overlap. Most cycloalkanes preffer to be in the chair fo ormat, but th here is free interconverrsion via a rring flip at room r temp perature.
axial H
H
H
H
H
H H
equatoria al H
H
H
H H
H
axia al
e different a alignments within w The the chair and the boat are wn here. show The difference betw ween eclipse ed and stagg gered can be qu uite significant. Cycclohexanes are almost alwa ays chairs in solution.
H
Ch Chair
Boa at
All of these stab ble or unstable forms are only relative: the etic differencce between n the energe forms is not great enough for there t activation en nergy to be a significant a barrierr. But the e relationship in space does make a real difference as far as reactionss are concerrned, as we will see laterr.
t type of visualisation v ed you, don’t panic. The e key If the idea of rottamers and this has confuse g from a GA AMSAT persp pective is to be able to lo ook at the re elative positio on of groups s and thing relatte this to the e energy con ntent of the rotamer invo olved. If large groups are e forced clos se to each h other this will result in a higher en nergy form. Hence H the eclipsed e form m above is higher enerrgy than the e anti-form, where w the groups g are fu urther apart.. In GAMSA AT they are most likelyy to give you u a graph off these energ gies and ask k you to inte erpret this. An example set s of thesse questions is given in th he sample pa apers, which h accompanyy this book.
© GrradMed Limited – All rights rese erved 2015
22
22 © GradMed Limited - All rights reserved 2015
Haworth Projections As you will read more on in Biology, sugars are able to go from the straight chain form we have been representing above and form into a ring structure. This is where one of the OH groups from the chain, reacts with the aldehyde group at the end of the chain. Where the straight chain form is in equilibrium with the cyclic form we describe the process as mutarotation. The resulting compound will normally be a five or six membered ring, which contains an oxygen atom from the alcohol group, which reacted. We use the Haworth projection to show the arrangement of the groups bonded to the ring in space. CH2OH
H OH H
H
HO HO
H
OH
H OH
H
O
H
O
H OH
OH OH H
OH
HOHC H HO H
OH H OH
H
O CH2OH
Above are three representations of glucose in three different systems. See if you can transform between them. Remember the Haworth projection shows the sugar as an idealised flat hexagon, and uses a single vertical line to show the ring substituents. The cyclic Fischer shown at bottom is typical of a sugar representation. In GAMSAT you may well be asked to convert between these structures. To convert it into a Haworth, first determine whether the ring goes to the left or right in the Fischer. If to the right then it is a natural or D sugar. Next draw a perspective hexagon like this.
CH 2OH
By convention the back right is the ring oxygen and the CH2OH group goes up for D sugars. So draw those in.
O
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61
61
Problem 6.2
Problem 6.3
Problem 6.4 CH2OH
H OH H
O O
HO H
H
H OH
OH
OH
OH OH
OH
CH2OH
H OH
O
OH O HO H
H
OH
H H
OH
OH
OH OH
OH
CH2OH
H OH
O
OH O HO HO
OH
H
OH
H H
H
OH
OH
OH
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OH
65
65
Electrocyclic Reactions and Sigmatropic Rearrangements
CHAPTER 11
CHAPTER 11
Electrocyclic Reactions and Sigmatropic Rearrangements This area has been tested in GAMSAT a number of times, as it is a good way to assess if you can really ‘see’ what is happening during organic processes. A few general points can be made which relate to the bulk of the electrocyclic reactions we encounter. • • • • •
Electrons move in a ring during reaction Reactions are often thermally or photochemically driven Rearrangements involve one molecule and the constituent parts of that molecule Classified by the number of atoms moving or by the chain numbers at the start and finish of reaction Include cycloaddition reactions.
Diels Alder Perhaps the most commonly asked example of such electrocyclic reactions is the Diels-Alder reaction. We have seen this previously when we were looking at the alkenes and our standard example is repeated here.
2 3
a
1
a 5
4
b
1 2
b 4
3
5
To understand the electrocyclic nature of the reaction we need to look at the movement of electrons, which takes place as the reaction proceeds. As you will see below the reaction is concerted, with all of the bond forming and breaking taking part at the same time. In total six electrons are involved in the cyclic transition.
In this reaction the diene reacts with an alkene (the dienophile) to form a new cyclic product, with the loss of one double bond. The reaction is often classified as a 4π + 2π reaction, relating to the pi electrons in the double bonds being used. The yield of the reaction can be improved when a strong electron-donating group is present on the diene. This pushes in electrons and so increases the reactivity of the diene, examples would include OH and NH2. The reverse situation is true for the dienophile, where electron-withdrawing groups increase the reactivity. Examples might include C=O, CN and NO2. To have an effect the groups must be directly bonded to the diene or dienophile. Some examples with such substituent groups are given in the examples below.
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113
113
ome reaction ns it is possiible for the cis c dienophile e to lead to the t formation of two diffe erent In so prod ducts, depen nding on the arrangemen nt of the ring gs in space. These two fforms are terrmed the endo and th he exo produ uct and are illustrated be elow. The endo product iis the one, which w ms, although the exo prod duct is actuallly the more stable as it has h less steriic crowding. form
s the end do form, rem member the substituent groups form m the dienop phile will end up To spot closer to the dou uble bond in the ring. It iss worth for GAMSAT learrning this exa ample, as it is the nsidering thiss particular point p of stere eochemistry. classic when con
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115
115
Problem 11.1 Here are some further examples. See if you can understand what is happening in each. The answers are given below.
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118
118 © GradMed Limited - All rights reserved 2015
Inffrared Sp pectra Infra ared spectra a give inform mation aboutt the functional groups present p with hin the molecule. Thiss is because e the bending and stretcching frequencies that th he spectrum shows rela ate to particular kinds of bond pre esent within the molecu ule. So a C= =O bond wiill stretch at one w stretch at another. frequency and a C-N bond will c wh hat kind of fu unctional gro oups are present, Thiss makes the system veryy useful for checking but not n very handy for finding g other inform mation such as which iso omer it mightt be. That ha aving been n said the sp pectra are ve ery easy to in nterpret, and because of this have be een introduce ed as quesstions occassionally. Wha at they will expect you to o do is be ab ble to look att a spectrum m and iden ntify from a table t of data a the type of functional group g presen nt within a m molecule. You u will not be b expected to memorize e specific values at which h bonds give e rise to peakks. oblem with IR is getting used to the e look of the e spectrum. IR spectra a are The biggest pro mally arrange ed with frequ uency along the bottom, and the vertical axis bein ng transmitta ance. norm Although frequency is norma ally measured in Hz the bottom b axis on o an IR spectrum is norm mally cm-11, which is in truth closelyy analogous. mittance is ussed, the IR spectrum s ap ppears as a series of tro oughs rather than Because transm e baseline is at the top an nd you can te ell the bandss as they han ng down from m the peakks. Thus the base eline. The sp pectrum is diivided up into o different re egions, with different d type es of deformation occu urring in diffe erent regionss. he regions of ave been discussed abov ve. Below yo ou will find th o the The types of defformation ha on between them. t specctrum these refer to and the correlatio
hows the gen neral regionss you might expect e to fin nd different fu unctional gro oups. Thiss diagram sh Rath her than ana alyse it in detail however the t best thing is probablyy to look at ssome spectra a and see how the diffe erent functional groups re elate to it. a GAMSA AT questionss you will always a be given g a table of data, which gives s the In any wavvenumbers (ccm-1) where different bon nds absorb IR. Your job is to use the e table of da ata to iden ntify specific bonds within n an IR specttrum. This is simple detecctive work, a and as always the factss may not be e black and white. w It is oftten possible to find a pea ak, which cou ed by uld be cause more than bond d. In cases liike this GAM MSAT will no ormally make e it fairly cle ear which typ pe of d they are assking you to identify. Let’s look at a ta able of data relating r to IR R frequencies s. bond
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124
124 © GradMed Limited - All rights reserved 2015
GAMSAT Study Guide
ORGANIC CHEMISTRY From basic nomenclature to advanced visualization in three dimensions, this guide will prepare you for all aspects of Organic Chemistry demanded by GAMSAT. Written in such a way as to teach theory and then show you its application, this guide covers all of the commonly set themes and topic areas explored by the examination. This is then reinforced by constant application to sample GAMSAT problems and an analysis of the skills they require. GRADMED expert GAMSAT preparation since 2002.
GRADMED Copyright @ 2015 GradMed Limited. All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise except as noted below, without the permission of GradMed Limited.
CONTENTS
ORGANIC CHEMISTRY Chapter 1 Structure and Bonding in Organic Chemistry Structure Drawing Chemical Bonding Predicting Molecular Polarity Atomic and Molecular Structure – Orbitals S Orbitals P Orbitals Hybrid Orbitals
Page 1 2 3 5 6 7 8 9
2 Organic Nomenclature Bicyclic Nomenclature
11 15
3 The Alkanes Rotamers Reactions of Alkanes Free Radical Halogenation
19 20 23 24
4 Alkenes Reaction of Alkenes / Electrophilic Addition Carbocation Rearrangement Alkene Hydrogenation Isomerism in Alkenes and the Cahn Ingold Prelog Rules Alkenes in Nature – Terpenes Other Reactions of Alkenes Oxidation States in Organic Chemistry Polymerisation Diels Alder Reaction
27 28 31 31 31 34 36 36 38 40
5 Alkynes Functional Groups
42 43
6 Stereochemistry and Stereoisomers Absolute Configuration Diastereomers Why Enantiomers Matter Atypical Chirality The Fischer Projection Sugars Haworth Projections
48 50 53 54 55 56 58 61
7 The Haloalkanes Antibonding Orbitals Nomenclature Substitution SN1 Reactions SN2 Reactions Elimination Grignard Reactions
66 66 66 67 67 68 69 70
8 The Oxygen Family Oxidation Level 1 – Alcohols and Ethers Oxidation Level 2 – Aldehydes and Ketones Reaction with Grignard Reagents Enolates The Aldol Reaction Addition of HCN Oxidation Level 3 – Carboxylic Acids and Derivatives Fatty Acids Saponification Naming Esters
72 72 75 76 76 77 79 80 82 83 84
9 Aromatic Chemistry Derivatives of Benzene The Electrophilic Substitution Mechanism Friedel-Crafts Reactions Directing Effect of Groups on Benzene Ring Side Chain Reactions
95 97 98 99 101 106
10 Nitrogen Heterogroups Forming Amines Diazonium Salts Elimination Reactions of Note Amides and Nitriles Hofmann Degradation Nitriles
107 108 108 109 110 111 112
11 Electrocyclic Reactions and Sigmatropic Rearrangements Diels Alder Stereochemistry 1,3 Dipolar Cycloadditions Sigmatropic Rearrangements
113 113 114 116 117
12 Spectroscopy Light as Particles The Appearance of Spectra The Beer Lambert Law Vibrational Spectroscopy Infrared Spectra Infrared Bending Frequencies in Aromatic Compounds Nuclear Magnetic Resonance Spectroscopy 1H NMR Spectra – Proton Spectroscopy
120 120 120 121 123 124 129 131 143
13 Introduction to Basic Chromatographic Principles
148
Structure and Bonding
CHAPTER 1
CH HAPTER 1 Strructure and a Bond ding in Organic O Chemistr C ry The chemistry of o life is made e up of rema arkably few of o the 103 kn nown elements. In fact most o only six ele ements. The ese six make e up the com mmon amino a acids, glycos sides life is made up of e. and nucleotides that are the major constiituents of life Organic chemistry w was called such ectly) because it was assumed (incorre o manufa acture ce ertain that to compoun nds was impo ossible, unle ess it was done e by a living g organism. The term hass now come to mean simply the chem mistry of carbo on compounds. pecial So what has made ccarbon so sp hemistry of life? Carbon can in the ch form strong bonds to itself and ca arbon g bonds to other o can also form strong atoms.
Bu uckminsterrfullerene
arbon has a valency of four, Finally ca meaning that it alwayys forms a tottal of ds in stable compounds.. It is four bond this com mbination of properties that makes carbon un nique amo ongst ad of elements, and meanss that a myria possible compounds can be formed. An exam mple of two o of the more m complex structures fo ormed by ca arbon wn here. are show
NH2
ook a little de eeper into why w carbon We shall now lo o special, and at a few off its compoun nds. is so o get over The biggest hurdle for most people to en considerin ng Organic chemistry iss grasping whe the basics of molecules. m W What shape they are, at makes them t react, what makes them wha diffe erent from on ne and anotther. Withou ut this you are just learning g tables of reactions wiith no real e as to what is going on or o why. clue n of this boo ok will be considering c The first section basiics of moleccular structurre and looking at how this affects the e shape of molecules and their perties. An understand ding here is a key prop foun ndation for the developm ment of more complex GAM MSAT conce epts.
N
N
N
N
HO O
H
H
H
O OH
H
H
De eoxyribonu ucleoside
© GradMed Limited All rightsLimited reserved–2015 © Gr-radMed All rights rese erved 2015
1
1
Structure Drawing Over the years, molecules have been drawn in many ways. This leads to confusion sometimes because different conventions are taught at different levels. If you last did organic chemistry many years ago then you may be used to a structural representation like the one shown here on the left.
If this is the case, then you MUST get this convention out of your head. It became almost completely obsolete in 1890 and is remarkably counterproductive. I guarantee you will never meet this system in medical school, and very rarely in GAMSAT. Organic chemists currently use the skeletal convention to represent molecules. To the right is a representation of the same molecule in skeletal format. The rules of skeletal drawing are simple: • • • •
C-C bonds are shown by a line C atoms are not shown C-H bonds are not shown. Carbon is assumed to have a valency of four. Any group containing other atoms is labelled in full (e.g. OH, CN, NO2)
The most frequent problem encountered with respect to these structures is the idea that each line is a carbon-carbon bond, and that the hydrogens are invisible. Carbon always has 4 bonds in organic chemistry (unless something very bad has happened to it and then it will have charges or dots next to it) and so if you can only see two bonds then there must be two others you cannot see. These will be the bonds to hydrogens. Shown below are two representations of the same molecule to help you get the idea.
I cannot overstress the importance of this system. You need to automatically “see” the hydrogens or nothing else in the book will make much sense. The skeletal system also has room for some 3D effects. Organic chemistry is a 3 dimensional science, after all. Put simply, these are called wedge and dash notation. The idea is that a straight line shows bonds, which are by convention in the plane of the page.
2
© GradMed Limited – All rights reserved 2015
© GradMed Limited - All rights 2 reserved 2015
The reason for this is simp ple. Bulky groups will interact with h each othe er and repel one onformer thatt forces them m closer toge ether will be high h energy b because of this. anotther. Any co In Cycloalkanes C these rotam mers can take e up one of tw wo different minimum en nergy position ns or confformers: the e chair and th he boat. Of these two th he chair is byy far the more e stable, as all of its bonds b are sta aggered and d there is no overlap betw ween any off its substitue ents. In the boat form m two bondss are eclipse ed and two groups g have e significant overlap. Most cycloalkanes preffer to be in the chair fo ormat, but th here is free interconverrsion via a rring flip at room r temp perature.
axial H
H
H
H
H
H H
equatoria al H
H
H
H H
H
axia al
e different a alignments within w The the chair and the boat are wn here. show The difference betw ween eclipse ed and stagg gered can be qu uite significant. Cycclohexanes are almost alwa ays chairs in solution.
H
Ch Chair
Boa at
All of these stab ble or unstable forms are only relative: the etic differencce between n the energe forms is not great enough for there t activation en nergy to be a significant a barrierr. But the e relationship in space does make a real difference as far as reactionss are concerrned, as we will see laterr.
t type of visualisation v ed you, don’t panic. The e key If the idea of rottamers and this has confuse g from a GA AMSAT persp pective is to be able to lo ook at the re elative positio on of groups s and thing relatte this to the e energy con ntent of the rotamer invo olved. If large groups are e forced clos se to each h other this will result in a higher en nergy form. Hence H the eclipsed e form m above is higher enerrgy than the e anti-form, where w the groups g are fu urther apart.. In GAMSA AT they are most likelyy to give you u a graph off these energ gies and ask k you to inte erpret this. An example set s of thesse questions is given in th he sample pa apers, which h accompanyy this book.
© GrradMed Limited – All rights rese erved 2015
22
22 © GradMed Limited - All rights reserved 2015
Haworth Projections As you will read more on in Biology, sugars are able to go from the straight chain form we have been representing above and form into a ring structure. This is where one of the OH groups from the chain, reacts with the aldehyde group at the end of the chain. Where the straight chain form is in equilibrium with the cyclic form we describe the process as mutarotation. The resulting compound will normally be a five or six membered ring, which contains an oxygen atom from the alcohol group, which reacted. We use the Haworth projection to show the arrangement of the groups bonded to the ring in space. CH2OH
H OH H
H
HO HO
H
OH
H OH
H
O
H
O
H OH
OH OH H
OH
HOHC H HO H
OH H OH
H
O CH2OH
Above are three representations of glucose in three different systems. See if you can transform between them. Remember the Haworth projection shows the sugar as an idealised flat hexagon, and uses a single vertical line to show the ring substituents. The cyclic Fischer shown at bottom is typical of a sugar representation. In GAMSAT you may well be asked to convert between these structures. To convert it into a Haworth, first determine whether the ring goes to the left or right in the Fischer. If to the right then it is a natural or D sugar. Next draw a perspective hexagon like this.
CH 2OH
By convention the back right is the ring oxygen and the CH2OH group goes up for D sugars. So draw those in.
O
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61
61
Problem 6.2
Problem 6.3
Problem 6.4 CH2OH
H OH H
O O
HO H
H
H OH
OH
OH
OH OH
OH
CH2OH
H OH
O
OH O HO H
H
OH
H H
OH
OH
OH OH
OH
CH2OH
H OH
O
OH O HO HO
OH
H
OH
H H
H
OH
OH
OH
© GradMed Limited – All rights reserved 2015 © GradMed Limited - All rights reserved 2015
OH
65
65
Electrocyclic Reactions and Sigmatropic Rearrangements
CHAPTER 11
CHAPTER 11
Electrocyclic Reactions and Sigmatropic Rearrangements This area has been tested in GAMSAT a number of times, as it is a good way to assess if you can really ‘see’ what is happening during organic processes. A few general points can be made which relate to the bulk of the electrocyclic reactions we encounter. • • • • •
Electrons move in a ring during reaction Reactions are often thermally or photochemically driven Rearrangements involve one molecule and the constituent parts of that molecule Classified by the number of atoms moving or by the chain numbers at the start and finish of reaction Include cycloaddition reactions.
Diels Alder Perhaps the most commonly asked example of such electrocyclic reactions is the Diels-Alder reaction. We have seen this previously when we were looking at the alkenes and our standard example is repeated here.
2 3
a
1
a 5
4
b
1 2
b 4
3
5
To understand the electrocyclic nature of the reaction we need to look at the movement of electrons, which takes place as the reaction proceeds. As you will see below the reaction is concerted, with all of the bond forming and breaking taking part at the same time. In total six electrons are involved in the cyclic transition.
In this reaction the diene reacts with an alkene (the dienophile) to form a new cyclic product, with the loss of one double bond. The reaction is often classified as a 4π + 2π reaction, relating to the pi electrons in the double bonds being used. The yield of the reaction can be improved when a strong electron-donating group is present on the diene. This pushes in electrons and so increases the reactivity of the diene, examples would include OH and NH2. The reverse situation is true for the dienophile, where electron-withdrawing groups increase the reactivity. Examples might include C=O, CN and NO2. To have an effect the groups must be directly bonded to the diene or dienophile. Some examples with such substituent groups are given in the examples below.
© GradMed Limited – All rights reserved 2015 © GradMed Limited - All rights reserved 2015
113
113
ome reaction ns it is possiible for the cis c dienophile e to lead to the t formation of two diffe erent In so prod ducts, depen nding on the arrangemen nt of the ring gs in space. These two fforms are terrmed the endo and th he exo produ uct and are illustrated be elow. The endo product iis the one, which w ms, although the exo prod duct is actuallly the more stable as it has h less steriic crowding. form
s the end do form, rem member the substituent groups form m the dienop phile will end up To spot closer to the dou uble bond in the ring. It iss worth for GAMSAT learrning this exa ample, as it is the nsidering thiss particular point p of stere eochemistry. classic when con
© GrradMed Limited – All rights rese erved 2015 © GradMed Limited - All rights reserved 2015
115
115
Problem 11.1 Here are some further examples. See if you can understand what is happening in each. The answers are given below.
© GradMed Limited – All rights reserved 2015
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Inffrared Sp pectra Infra ared spectra a give inform mation aboutt the functional groups present p with hin the molecule. Thiss is because e the bending and stretcching frequencies that th he spectrum shows rela ate to particular kinds of bond pre esent within the molecu ule. So a C= =O bond wiill stretch at one w stretch at another. frequency and a C-N bond will c wh hat kind of fu unctional gro oups are present, Thiss makes the system veryy useful for checking but not n very handy for finding g other inform mation such as which iso omer it mightt be. That ha aving been n said the sp pectra are ve ery easy to in nterpret, and because of this have be een introduce ed as quesstions occassionally. Wha at they will expect you to o do is be ab ble to look att a spectrum m and iden ntify from a table t of data a the type of functional group g presen nt within a m molecule. You u will not be b expected to memorize e specific values at which h bonds give e rise to peakks. oblem with IR is getting used to the e look of the e spectrum. IR spectra a are The biggest pro mally arrange ed with frequ uency along the bottom, and the vertical axis bein ng transmitta ance. norm Although frequency is norma ally measured in Hz the bottom b axis on o an IR spectrum is norm mally cm-11, which is in truth closelyy analogous. mittance is ussed, the IR spectrum s ap ppears as a series of tro oughs rather than Because transm e baseline is at the top an nd you can te ell the bandss as they han ng down from m the peakks. Thus the base eline. The sp pectrum is diivided up into o different re egions, with different d type es of deformation occu urring in diffe erent regionss. he regions of ave been discussed abov ve. Below yo ou will find th o the The types of defformation ha on between them. t specctrum these refer to and the correlatio
hows the gen neral regionss you might expect e to fin nd different fu unctional gro oups. Thiss diagram sh Rath her than ana alyse it in detail however the t best thing is probablyy to look at ssome spectra a and see how the diffe erent functional groups re elate to it. a GAMSA AT questionss you will always a be given g a table of data, which gives s the In any wavvenumbers (ccm-1) where different bon nds absorb IR. Your job is to use the e table of da ata to iden ntify specific bonds within n an IR specttrum. This is simple detecctive work, a and as always the factss may not be e black and white. w It is oftten possible to find a pea ak, which cou ed by uld be cause more than bond d. In cases liike this GAM MSAT will no ormally make e it fairly cle ear which typ pe of d they are assking you to identify. Let’s look at a ta able of data relating r to IR R frequencies s. bond
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124 © GradMed Limited - All rights reserved 2015
GAMSAT Study Guide
ORGANIC CHEMISTRY From basic nomenclature to advanced visualization in three dimensions, this guide will prepare you for all aspects of Organic Chemistry demanded by GAMSAT. Written in such a way as to teach theory and then show you its application, this guide covers all of the commonly set themes and topic areas explored by the examination. This is then reinforced by constant application to sample GAMSAT problems and an analysis of the skills they require. GRADMED expert GAMSAT preparation since 2002.
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