Doc Brown's Advanced A Level Organic Chemistry Revision Notes - Help in Revising Advanced Organic Chemistry
PART 14 ORGANIC ISOMERISM and Stereochemistry Revision Notes
Part 14.2 Organic Stereoisomerism - Introduction and E/Z isomerism
(was called cis-trans, geometric or geometrical isomerism)
E/Z stereoisomerism is explained and examples of E/Z isomers fully described including how to name them and assign the E or Z prefix to the specific isomer name. Case studies are described covering the molecular structure of the E/Z isomers, how to name them and explaining any differences in physical and chemical properties. A few details of differences in physical and chemical properties are pointed out where it seems appropriate. Note for simple cases modern E = old trans isomers AND modern Z = old cis isomers. Cis and trans notation is still to be found in older textbooks and even some contemporary literature.
Case studies are discussed concerning structure, naming, formation, properties and stereochemical consequences of E/Z stereoisomerism
14.2 to 14.6 Stereoisomerism-Stereochemistry - Definition and Introduction
Stereoisomerism occurs when two or more molecules have identical molecular formula AND the same structural formula (i.e. the atoms are arranged in the same order), BUT differ in their 2D or 3D spatial arrangements of their bonds - which means different spatial arrangement of the atoms - even though they are bonded in the same order!
There are two main sub-divisions of stereoisomerism:
(a) E/Z isomerism (old names: cis/trans geometrical/geometric isomerism, in specific cases E = trans, Z = cis isomers)
(b) R/S optical isomerism (optical isomerism)
A 3rd section (c), looks at other stereochemical examples which do not readily fit into (a) or (b). However, before these sections, you need to read to know about 'priority rules' to assign the correct nomenclature to any absolute stereoisomer configuration.
14.2(a) Priority Rules for designating the precise isomer configuration in stereoisomerism
In order to specify exactly which stereoisomer you are referring to in e.g. E/Z (geometrical/geometric isomerism) and R/S optical isomerism (molecules exhibiting chirality) you need rules to account for the different groups of atoms. These are known as the Cahn-Ingold-Prelog priority sequence rules and their importance can only be fully understood when dealing with the isomerisms described in sections E/Z Isomerism and R/S optical Isomerism. A brief description of the first two rules are given below, which is all you need for pre-university courses.
Below is an explanation of how to apply the Cahn-Ingold-Prelog priority rules devised in the 1960s by Robert Cahn, Chris Ingold and Vladimir Prelog and now accepted by the IUPAC nomenclature system for naming isomers.
You need a periodic table to get the relevant atomic numbers.
When dealing with a C-X bond grouping, the priority of X is given by ....
Sequence priority Rule 1: The higher the atomic number of an atom the higher the priority it is assigned.
Sequence priority Rule 2: If the relative priority of two groups cannot be decided by Rule 1, it shall be determined by applying Rule 1 to the next atom or sequence of atoms in the group 'X'.
These two rules will be applied below in the case of the notation for E/Z isomerism and on a separate page for R/S optical isomerism (R/S notation for enantiomers) and this rules section will be referred back to as examples of these isomerisms are described.
Please note that for UK pre-university chemistry courses, no knowledge of the Cahn-Ingold-Prelog priority sequence rules is expected beyond the two rules above. In many cases just knowing -H has the lowest priority is sufficient to assign the E or Z isomer in E/Z stereoisomerism. Some pre-university examinations may require a working knowledge of the R/S notation for optical stereoisomerism?
14.2(b) E/Z Stereoisomerism (geometrical - cis/trans isomerism) - examples in applying the assignment rules
Molecules of the same molecular formula, exhibit E/Z stereoisomerism, that is spatially different molecules (E/Z isomers) because of the inhibited/restricted rotation about at least one bond due to too high an energy requirement.
E/Z isomers are molecules that are fixed into their different spatial positions with respect to one another due to restricted rotation about a bond.
Apart from the two sequence priority rules above which also apply to R/S optical isomerism, the specific E/Z isomerism positional assignment rule is as follows, and essential for understanding the rest of the page and is actually quite simple to apply!
For each pair of substituent groups on each carbon of the C=C double bond you need to assign a higher and lower priority based on the atomic number or sequence of atomic numbers (as described above).
(a) If the two highest priority groups are on the same side of the >C=C< bond network planarity that gives you the Z isomer (old notation - cis in limited cases if lower = H). So, in terms of higher and lower priorities, for the Z isomer we have ...
Or you can say the two lowest priority atoms/groups are on the same side of the plane of the double bond >C=C<
(b) If the two highest priority groups are diagonally on opposite sides of the >C=C< bond network planarity, that gives you the E isomer (old notation - trans in limited cases if lower = H). So, in terms of higher and lower priorities, for the E isomer we have ...
Or you can say the two lowest priority atoms/groups are diagonally on the opposite sides of the plane of the double bond >C=C<.
An example of an alkene exhibiting E/Z isomerism with four different groups attached to the C=C double bond.
NOTE: The IUPAC recommend that the terms geometrical/geometric isomerism are NOT used but to use the stereoisomerism classification terms E/Z stereoisomerism or E-Z isomerism. The E/Z notation is replacing the limited cis/trans notation in assigning names to a particular stereoisomer. However cis/trans nomenclature is in widespread use so it will be acknowledged in parallel with the E/Z convention where appropriate.
Note: cis-trans isomerism is sometimes defined as a special type of E/Z isomerism in which there is a non-hydrogen group and a hydrogen atom attached to each C of a C=C double bond.
A simple example: but-2-ene
However, in order for E/Z stereoisomers to exist there must be two different atoms/groups attached to both carbon atoms of the C=C carbon double bond or two adjacent carbons in a substituted cycloalkane.
Again, this is why but-1-ene , with two hydrogens on the right-hand carbon of the double bond, cannot exhibit E/Z isomerism, but it is a structural isomer of the E/Z isomers of but-2-ene and methylpropene.
The three most common situations you are likely to encounter are >C=C< or a >C=N- double bond and around a C-C single bond in cycloalkanes. In both cases the energy required is too high to allow free rotation around the double bond BUT free rotation is possible around single bonds (C-C, C-O etc.) e.g. alkyl groups around the C-C single bonds in non-cyclo linear/branched alkanes.
If two identical atoms/groups are attached to the same carbon atom of a C=C double bond, you cannot have geometrical isomers e.g. those with R2C=C< or R2C=N- where R = R. See the diagram below.
The 'old' nomenclature term cis often means the same substituents are on the same side of the double bond and trans when they are on opposite sides. Under the E/Z notation cis is now Z and trans is now E. In a sense cis/trans isomers were a special case of a substituent and a hydrogen atom on each carbon of the C=C double bond. E/Z configuration assignment is absolutely necessary when there 3 or 4 different substituents on the C=C group (again, see the diagram below)
Introductory exemplar diagrams to illustrate whether E-Z isomers can exist or not and how to use the modern E/Z isomerism notation-designation-assignment of absolute configuration.
Lower left example: (E)-1-bromo-1-chloropropene and (Z)-1-bromo-1-chloropropene
Lower right example: E-3-methylpent-2-ene and Z-3-methylpent-2-ene (repeated further down with skeletal formulae)
To understand the two lower left and right examples apply the Priority Rules to alkenes for E/Z ('geometrical') isomerism:
For each carbon of the double bond the higher priority atom/group is worked out.
The Z isomer is where both highest priority groups are on the same side of the double bond (includes all cis configurations of the old convention).
The E isomer is where the two highest priority atoms/groups are diagonally opposite each other on different sides of the plane of the double bond system (includes all trans isomers of the old convention).
(Note: In terms of the two highest priority atoms or groups, E, 'on opposite sides', comes from the German word entgegen, meaning 'opposite' and the Z 'on the same side' comes from the German word zusammen meaning 'together')
Some more alkene examples of E/Z isomerism
3-methylpent-2-ene, can be drawn as E/Z isomers using structural and skeletal formula
the E/Z isomers of
4-methylpent-2-ene, , has E/Z isomers:
4,4-dimethylpent-2-ene, E/Z isomers:
Further case studies of E/Z stereoisomerism
BUT, now including discussing the similarities & difference in physical & chemical properties of the E/Z isomers
Case study 2a.1 Isomers of C4H8, cis/trans or Z/E-but-2-ene and other alkenes
Ball and stick models for three isomers of C4H8
1. Z-but-2-ene (cis isomer), 2. E-but-2-ene (trans isomer), 3-methylpropene (cannot exhibit E/Z isomerism)
Priority order: -CH3 > H (since at. no. of carbon 6 > 1 for hydrogen)
(1) Z-but-2-ene (cis) (bpt 4oC) , , (Z-2-butene)
(2) E-but-2-ene (trans) (bpt 1oC), , (E-2-butene)
(1) and (2) are very similar physically (e.g. colourless gases and very low bpt) and chemically (e.g. alkene electrophilic addition reactions).
Note that there are four other physically similar isomers of C4H8 namely,
, , E-3-methylpent-2-ene
and , Z-3-methylpent-2-ene
BUT , 2-methyl-2-pentene does NOT exhibit E/Z isomerism because two identical (CH3) groups are attached to the same carbon atom of the double bond.
4,4-dimethylpent-2-ene, has two E/Z isomers:
Z-4,4-dimethylpent-2-ene (cis), and E-4,4-dimethylpent-2-ene (trans)
Case study 2a.2 trans/cis or Z/E-but-2-ene-1,4-dioic acid, HOOC-CH=CH-COOH
(butenedioic acids, old names given below). Substituent group priority -COOH > H
On heating the trans form (1) fumaric acid) now called E-but-2-ene-1,4-dioc acid, it proves difficult to change it into the cyclic anhydride (3) below.
Z-but-2-ene-1,4-dioc acid (2) cis form, maleic acid (3) + H2O
However, if the Z(cis) form (2) is heated, it readily changes into the cyclic acid anhydride (3). Not only does the restricted rotation about the C=C bond cause the existence of geometrical isomers, but in this case you can only readily get the elimination of water when the two -OH groups are on the same side of the planar >C=C< system, as in the cis form (2). In the trans form (1) the elimination reaction is stereochemically hindered because the -OH so far apart. However, both (1) and (2) undergo the same electrophilic addition reactions of the 'alkene' double bond, >C=C< and the same reactions of the carboxylic acid group -COOH.
Some physical differences.
*Sometimes the trans isomer has the higher symmetry and packs more closely into a crystal lattice, increasing the intermolecular forces, and this tends to increase melting points and density but decrease solubility as solvation is not as energetically favourable.
The physical differences may be partially explained by the different polarities of the molecules and the orientation of hydrogen bonding, either in the crystal lattice or when dissolved in water.
(1) The trans form: d = 1.64 gcm-3, solubility in water 0.7g/100 cm3 at 25 oC, melting point 287 oC,
(2) The cis form: d = 1.59 gcm-3, solubility in water 78.8g/100 cm3 at 25 oC, melting point 130 oC,
Case study 2a.3 Physical properties of cis/trans or Z/E-1,2-dichloroethene
Ball and stick models of halogenated ethene molecules.
1. 1,2-dichloroethene H2C=CCl2, 2. 2-bromo-1,1-dichloroethene CCl2C=CHBr (neither can exhibit E/Z isomerism)
3. E-1,2-dichloroethene (trans stereoisomer of ClCH=CHCl), 4. Z-1,2-dichloroethene (cis isomer of ClCH=CHCl)
The Cδ+-Clδ- bond is polar due to the difference in electronegativity between carbon and chlorine (Cl > C) and this partially accounts for the small differences in physical properties.
(1) Z-1,2-dichloroethene (cis) d = 1.265 gcm-3, mpt -80 oC, bpt 60 oC, dipole moment 1.89D, with both Cl's on the same side of the C=C bond, the combined effect of the two polar C-Cl bonds makes it a much more polar molecule and raises the bpt compared to (2), but not the mpt (not sure why, subtlety of the crystal structure?).
(2) E-1,2-dichloroethene (trans) d = 1.259 gcm-3, mpt -50 oC, bpt 48 oC, dipole moment 0.00D, the effect of the C-Cl polar bonds cancel each other out giving a relatively non-polar molecule,
and there is positional isomer (3) shown below.
(3) d = 1.218 gcm-3, mpt -? oC, bpt 32 oC, dipole moment 1.30D?, 1,1-dichloroethene, is a positional isomer of the two geometrical isomers and cannot exhibit E/Z isomerism because two identical groups (H's or Cl's) are attached to the same carbon of the double bond.
All three isomers are chemically similar e.g. the electrophilic addition reactions of alkenes.
Case study 2a.4 1,2- or 1,3-disubstituted cycloalkanes - based on cyclopropane and cyclobutane
E/Z isomers can exist in 1,2-disubstituted cyclopropanes* and cyclobutanes* because the -C-C- ring structure inhibits rotation about the C-C bonds.
If the 1,2-substituents are on the same side of the plane of the triangle/square of carbon atoms you get the Z (cis) form, if the are on opposite sides you get the E (trans) form.
(1) 1,2-dichlorocyclopropane can give E/Z isomers and the group priority is Cl > H
(2) 1,1-dichlorocyclopropane is a positional isomer of C3H4Cl2, and cannot exhibit geometrical isomerism.
(3) 1,2-dibromocyclobutane, likewise can give ...
(4) or (9) 1,1-dibromocyclobutane, is a positional isomer of C4H6Br2 and cannot exhibit E/Z (geometrical) isomerism because the two bromine atoms are attached to the same carbon.
(5) 1,3-dibromocyclobutane is also positional isomer of C4H6Br2 and can exhibit E/Z (geometric) isomerism.
Note 1: The molecular formulae C3H4Cl2 and C4H6Br2 can theoretically give rise to other functional group/positional/E/Z (trans/cis) isomers in the form of non-cyclic alkenes e.g.
Note 2: Strictly speaking the 4 carbon ring of cyclobutanes is NOT planar, in fact the 'V' of two of the carbon atoms is bent at 26o from the 'V' of the other two carbons. However, using planar projections it is possible to work out and illustrate the E/Z isomers of cyclobutanes.
Chlorocyclopentanes (ball and stick models below)
1. Chlorocyclopentane C5H9Cl: This has no stereoisomers, but is structurally isomeric with monochloropentenes.
2. 1,1-dichlorocyclopentane C5H8Cl2: This has a plane of symmetry and des not have any stereoisomers, but it is a positional structural isomer of C5H8Cl2.
For 3. and 4. the restricted rotation about the single C-C bonds allows E/Z stereoisomers to exist, they are also structural isomers of C5H8Cl2.
3. Z-1,2-dichlorocyclopentane C5H8Cl2: This is the Z isomer (cis in old terms). Both chlorine atoms are above the 'almost' flat plane of the ring of five carbon atoms.
4. E-1,2-dichlorocyclopentane C5H8Cl2: This is an E isomer (trans in old terms). One chlorine atom is above the plane of the carbon atom ring and the other chlorine is below the plane.
3. and 4. are formed when chlorine electrophilically adds to cyclopentene.
+ Cl2 ====>
More advanced note: The two C-Cl stereocentres give rise to E/Z isomerism, but they are also chiral carbon stereocentres which means there are R/S isomers.
Case study 2a.5 Isomerism in azo (-N=N-) and R2C=N-X compounds
Organic (or inorganic) compounds of the structure R-N=N-R' (e.g. aromatic azo dyes) can exist as cis and trans isomers in just the same way as alkenes, where R or R' = H, alkyl, aryl group etc. R can be different or the same as R'. Stereochemically, the lone pairs on the nitrogen effectively behave as an atom bonding pair of electrons in determining the trigonal planar orientation of the -N= bonds and the lone pair of electrons on the nitrogen. Three groups of electrons around an atom X, always give a trigonal planar arrangement around the central atom >X-. The double bond, N=N or C=N, ensures that too high an energy is required for ready rotation about the double bond. The cis and trans forms will have different physical properties such as melting/boiling points.
Examples of -N=N- systems:
(1) (Z/cis) and (2) (E/trans)
(3) (Z/cis) and (4) (E/trans)
Case Study 2a.6 Dienes can also exhibit E/Z stereoisomerism
, Z-buta-1,3-diene (cis), unstable, 2%, low activation of rotation to give the E isomer.
, E-buta-1,3-diene (trans), much more stable, 98%, in dynamic equilibrium with the Z isomer.
In this case the E/Z designation is based on the central C-C single carbon-carbon bond and therefore rotation is much easier and the E stereoisomer is the the more stable configuration.
Case study 2a.7 Trans fats
Many natural oils and fats are esters of propane-1,2,3-triol (glycerol) and long saturated and unsaturated chain fatty acids. Animal fats can be mainly saturated, but vegetable oils are unsaturated. Any unsaturated fats or oils can exist as E/Z isomers (trans/cis isomers). The terms mono, di or tri-unsaturated fatty acids refer to the number of alkene groups in the fatty acid.
Comparison of oil and fat structures not showing the stereochemical structure
Animal fats are mainly saturated fats with no carbon = carbon double bonds in the fatty acid chain and are low melting solids at room temperature.
The 'long–chain fatty acids' can be unsaturated, with one or more C=C double bonds, and so forming unsaturated oils or fats e.g. the triglyceride formed from oleic acid. Vegetable oils are usually viscous liquid at room temperature.
A simple (non-stereochemical) diagram showing several points of 'unsaturation' - the C=C carbon-carbon double bonds in the fatty acid chain.
Diastereomers (in case you come across the terms):
what is stereoisomerism in organic chemistry? what are cis/trans isomers? why can E/Z isomers exist? why is the interconversion of cis/trans isomers difficult? describe how to use the Cahn-Ingold-Prelog priority sequence rules for E/Z (sis/trans isomers), how do you define stereoisomerism for E/Z or cis/trans isomers of a given molecular formula? give the names and structures of the E/Z cis/trans isomers of molecular formula C4H8 butenes, give the names and structures of the E/Z cis/trans isomers of molecular formula C3H4BrCl, give the names and structures of the E/Z cis/trans isomers of molecular formula C6H12 alkenes, give the names and structures of the E/Z cis/trans isomers of molecular formula C7H14 heptenes, give the names and structures of the E/Z cis/trans isomers of molecular formula C4H4O4 unsaturated dicarboxylic acids, give the names and structures of the E/Z cis/trans isomers of molecular formula C2H2Cl2 dichloroethene, give the names and structures of the E/Z cis/trans isomers of disubstituted cycloalkanes of molecular formula C3H4Cl2 dichlorocyclopropane, give the names and structures of the E/Z cis/trans isomers of molecular formula C4H6Br2 dibromocyclobutane, give the names and structures of the E/Z cis/trans isomerism of the isomers of azo compounds advanced level chemistry Cahn-Ingold-Prelog Priority Rules for AQA AS chemistry, Cahn-Ingold-Prelog Priority Rules for Edexcel A level AS chemistry, Cahn-Ingold-Prelog Priority Rules for A level OCR AS chemistry A, Cahn-Ingold-Prelog Priority Rules for OCR Salters AS chemistry B, Cahn-Ingold-Prelog Priority Rules for AQA A level chemistry, Cahn-Ingold-Prelog Priority Rules for A level Edexcel A level chemistry, Cahn-Ingold-Prelog Priority Rules for OCR A level chemistry A, Cahn-Ingold-Prelog Priority Rules for A level OCR Salters A level chemistry B Cahn-Ingold-Prelog Priority Rules for US Honours grade 11 grade 12 Cahn-Ingold-Prelog Priority Rules for pre-university chemistry courses pre-university A level revision notes for Cahn-Ingold-Prelog Priority Rules A level guide notes on Cahn-Ingold-Prelog Priority Rules for schools colleges academies science course tutors images pictures diagrams for Cahn-Ingold-Prelog Priority Rules A level chemistry revision notes on Cahn-Ingold-Prelog Priority Rules for revising module topics notes to help on understanding of Cahn-Ingold-Prelog Priority Rules university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 11 grade 11 AQA A level chemistry notes on Cahn-Ingold-Prelog Priority Rules Edexcel A level chemistry notes on Cahn-Ingold-Prelog Priority Rules for OCR A level chemistry notes WJEC A level chemistry notes on Cahn-Ingold-Prelog Priority Rules CCEA/CEA A level chemistry notes on Cahn-Ingold-Prelog Priority Rules for university entrance examinations with advanced level chemistry
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