ASRJC H2 Chem 8b. Isomerism

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 1 ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY ISOMERISM Contents • Isomerism: constitutional (structural); cis–trans; enantiomerism Learning Outcomes (LOs) Candidates should be able to: (a) describe constitutional (structural) isomerism (b) describe cis–trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of  bonds [use of E, Z nomenclature is not required] (c) explain what is meant by a chiral centre (d) deduce whether a given molecule is chiral based on the presence or absence of chiral centres and/or a plane of symmetry (e) recognise that an optically active sample rotates plane–polarised light and contains chiral molecules (f) recognise that enantiomers have identical physical properties except in the direction in which they rotate plane– polarised light [usage of the term diastereomers is not required] (g) recognise that enantiomers have identical chemical properties except in their interactions with another chiral molecule (h) recognise that different stereoisomers exhibit different biological properties, for example in drug action (i) deduce the possible isomers for an organic molecule of known molecular formula (j) identify chiral centres and/or cis–trans isomerism in a molecule of given structural formula References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Organic Chemistry, McMurry, Brooks/ Cole 3. Understanding Advanced Physical Inorganic Chemistry, Jeanne Tan and Kim Seng Chan

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 2 I. INTRODUCTION • Isomerism is the phenomenon in which two or more compounds possessing the same molecular formula exist in different forms due to different arrangements of atoms in their molecules. These compounds are known as isomers. • Isomerism in organic compounds can be classified into 2 broad categories which in turn can further differentiate into 5 types as shown: (A) Constitutional (structural) isomerism • Constitutional ( structural) isomers are compounds with the same molecular formula but different structural formulae. • Structural isomers can be members of different homologous series (i.e. functional groups), or they may be members of the same series. As structural isomers have different structures, they have different properties. • There are 3 types of structural isomerism. 1. Chain isomerism • Chain isomers have the same number of carbon atoms, but their carbon backbones are different. • Example of chain isomers with the formula C4H10 butane CH3 CH2 CH2 CH3 2−methylpropane CH3 CH CH3 CH3 Isomerism Stereoisomerism Constitutional (structural) Isomerism Chain isomerism Positional isomerism Functional group isomerism Cis–trans isomerism Enantiomerism

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 3 Checkpoint 1 Consider structures A and B A B CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH3 Is A a chain isomer of B? (Hint: try using the rules that you’ve learnt in nomenclature to suggest the names for both structures. Answer: A is ______ 1-methylpropane! Both A and B are the _______ molecule, _________! • When we consider the longest continuous carbon chain, both structures have 4 carbon atoms and no branching! • There is complete free rotation about any C−C single bond, so the two diagrams are just different ways of representing butane. One can be formed from the other simply by rotating about a single bond. 2. Positional isomerism • Positional isomers have the same carbon backbone, but the positions of their functional group(s) differ. • Examples of pairs of positional isomers Notice the difference in positions of Cl atom. CH2 CH2 ClCl 2 1 1,2−dichloroethane vs CH3 C H Cl Cl 2 1 1,1−dichloroethane Notice the difference in positions of C=C 2 1 CH3−CH2−CH=CH2 but−1−ene vs 2 1 CH3−CH=CH−CH3 but−2−ene Notice the difference in positions of CH3- group CH3 CH3 1,2−dimethylbenzene vs CH3 CH3 1,3−dimethylbenzene 1 1 2 3

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 4 3. Functional group isomerism • Functional group isomers contain different functional groups. They often have very different reactions from one another. • Example of functional group isomers with the same formula C3H6O2. CH3CH2CO2H propanoic acid CH3CO2CH3 methyl ethanoate CH3COCH2OH hydroxypropanone CH3CH2 C OH O CH3 C O O CH3 CH3 C CH2 OH O Checkpoint 2 Compound X is a non-cyclic alkane with a molecular formula of C5H12. Draw the structural formulae for all its constitutional isomers. Checkpoint 3 How many alcohols, including only constitutional isomers, can have the molecular formula C4H10O?

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 5 What is a systematic way to derive the number of isomers? Step 1: Start with the longest C chain with functional group of high priority on C1. Obtain the first isomer. Step 2: Continue with the longest C chain, shift the position of the functional group to adjacent C atom to get another isomer. Step 3: Repeat Step 2 until you don’t get more isomers with the same longest C chain. No more isomers. Putting –OH group on C3 will get the same structure as putting –OH on C2. Step 4: Using the isomer from Step 1, shorten the C chain by shifting the last C to C2. Step 5: Similar to Step 2 and 3, using the isomer from Step 4 and shift the position of the functional group to adjacent C atom to get another isomer. Step 6: Check if Step 4 - 5 can be repeated to derive different isomers of even shorter carbon chains. No longer possible for this molecule. Step 7: As a final check, you can try to name all the isomers drawn. You should have different names for all the structures. If you arrive at the same name, this means that you have drawn the same molecule! E.g. and CH3 C C CH3 OH H H H are both butan-2-ol! They are identical molecules, not isomers! You can try the steps above yourself and practice with other molecules of different molecular formula!

2024 JC1 H2 Isomerism ©2024ASRJC/CHEM 6 (B) Stereoisomerism • Stereoisomers contain the same atoms bonded to one another , and the bonding and functional groups are identical (i.e. they have the same structural formula! They are NOT structural isomers of each other). • Stereoisomers differ only in the way the atoms are arranged in 3−dimensional space. • There are 2 types of stereoisomerism: cis–trans and enantiomerism 1. Cis−trans isomerism • Cis–trans isomerism occurs with alkenes that do not have two identical groups on both ends of the C=C double bond. • It arises because, unlike the case of a C−C single bond, it is not possible to rotate freely about a C=C double bond. Doing so involves breaking the  bond and this requires more energy than is available at normal temperatures. Free rotation about this single bond NO free rotation about this double bond Hence, structure on the right can be created from structure on the left by a simple rotation of C-C. Both structures are the same molecule. Hence, structures on the right CANNOT be created from structure on the left. The two structures are different molecules! • The following two forms of but−2−ene are therefore distinct and separate isomers where the atoms are arranged differently in the 3D space. cis−but−2−ene C C CH3 HH CH3 trans−but−2−ene C C H H CH3 CH3 –CH3 on same side of C=C OR H on same side of C=C –CH3 on different sides of C=C OR H on different sides of C=C • Cis−trans isomers differ in their physical properties but have similar chemical properties. • The following compounds do not exhibit cis–trans isomerism because one end, or