VJC Isomerism Lecture Notes

VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT ISOMERISM ______________________________________________________________________________ LECTURE OUTLINE 1 Constitutional (Structural) Isomerism 2 Stereoisomerism 2.1 Cis-trans Isomerism 2.2 Chirality and Enantiomerism 3 Molecules Exhibiting Multiple Isomerism 4 Differences between Properties of Enantiomers ASSESSMENT OBJECTIVES 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 Scan here for to ask question related to the lecture

2 ISOMERISM Isomerism is the occurrence of two or more compounds with the same molecular formula but having different structures or spatial orientation. There are two main types of isomerism. Firstly, c onstitutional (structural) isomerism , which is further divided into chain isomerism, positional isomerism and functional group isomerism. Secondly, stereoisomerism, which is further divided into cis-trans isomerism and enantiomerism. 1 Constitutional (Structural) Isomerism Candidates should be able to: (a) describe constitutional (structural) isomerism It refers to isomers containing the same number of each kind of atom but differing in regard to how the atoms are linked to one another, i.e. they have the same molecular formula but different structural formula . There are three types: chain, position al and functional group isomerism.  Chain isomerism The isomers differ in the way carbon atoms are linked together. They possess the same functional group or homologous series. E.g. C5H12 CH3–CH2–CH2–CH2–CH3 C CH3 CH3 H CH2 CH3  Positional isomerism The isomers have the same functional group but differ in its position along the same carbon chain length. E.g. C3H8O CH3–CH2–CH2–OH E.g. C7H7Cl (contains benzene ring) CH3 Cl CH3 Cl CH2Cl Exercise Draw all possible positional isomers for a six -carbon chain alkene with molecular formula C6H12.  Functional group isomerism The isomers have different functional groups. E.g. C2H6O CH3–CH2–OH CH3–O–CH3 alcohol ether

3 E.g. C3H6O CCH3 CH3 O aldehyde ketone E.g. C3H6O2 CH3 C O O CH3 carboxylic acid esters Exception: Alkene and cycloalkane share the same molecular formula, C nH2n. They are constitutional isomers but are not classified as functional group isomers as alkane is not regarded as a functional group. E.g. C3H6: CH3–CH=CH2 propene cyclopropane  Determination of constitutional isomers from given molecular formula Candidates should be able to: (i) deduce the possible isomers for an organic molecule of known molecular formula Step 1: arrangement of carbon atoms Consider the longest possible carbon chain followed by subsequent removal of one C atom from the chain and re -attach it to any C atom except an end carbon (as this will restore the original chain length). Step 2: possible functional groups For example, the presence of one oxygen atom indicates that alcohol, ether, aldehyde or ketone could be present. If the compound is saturated with C nH2n+2 ratio, alcohol or ether could be present. For each arrangement of carbon atoms in step 1, attach the functional group to all possible positions that give rise to a different structure. Note: (i) The greater the number of atoms in a molecule, the greater the number of isomers. (ii) Isomers with the same functional groups (i.e. they belong to the same homologous series) have similar chemical properties but different physical properties. (iii) Isomers with different functional groups (i.e. they belong to different homologous series) have different physical and chemical properties. E.g. C4H10O (alcohol or ether) Alcohols: Ethers:

4 Exercise Draw the skeletal formulae of all the possible constitutional isomers with one functional group with the molecular formula C4H8O2. Exercise Determine all the possible constitutional isomers for a compound with molecular formula C3H5Br. 2 Stereoisomerism It refers to isomers having the same molecular and constitutional (structural) formulae but differs in the way the atoms are arranged in 3-dimensional space. There are two types of stereoisomerism: cis-trans isomerism and enantiomerism. 2.1 Cis-trans isomersim Candidates should be able to: (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]  Cis–trans isomerism occurs in compounds in which free rotation is prevented by the presence of double bonds, a ring structure or steric factors. A single bond can be easily rotated but not a double bond as rotation reduces the extent of lateral overlap of p orbitals. If a multiple bond is rotated through 90 o, p orbital overlap will be reduced to zero which implies the non-existence of the pi bond.  In alkenes, cis–trans isomerism arises from the restricted rotation about the C=C bond due to the presence of  bond. Each carbon atom of the C=C bond is bonded to two different atoms / groups. cis isomer: same atoms / groups are on the same side of the C=C bond trans isomer: same atoms / groups are on opposite sides of the C=C bond E.g. E.g. For above molecules with four different atoms / groups, the isomers are designated using the E, Z nomenclature (not in H2 syllabus) instead of cis and trans.

5  Cis-trans isomerism cannot exist if either carbon of the C=C bond is bonded to two identical groups. E.g.  Cyclic alkenes with 3 to 7 carbon atoms are fixed in the cis structure. A trans structure for a double bond can exist in cyclic compounds only when a large ring is present , otherwise the ring strain in the compound is very high. Hence, C=C bond in a ring does not show cis-trans isomerism due to ring strain in the trans isomer.  For a given structure which exhibits only cis-trans isomerism, no. of cis-trans isomers = 2n, where n = no. of C=C bond with cis-trans isomerism  Cyclic compounds can also exhibit cis-trans isomerism due to restricted rotation about the single bonds in cyclic systems. The substituents of the cis isomer are on the same side of the ring while the substituents of the trans isomer are on opposite sides of the ring.  Restricted rotation about C=C bond prevents spontaneous inter –conversion of cis isomer into trans isomer, i.e. they can be isolated as two different compounds.  Cis-trans isomers contain the same functional groups and hence show similar chemical properties. However, chemical properties are not identical as the structures are neither identical nor mirror images . They react with the same reagents but perhaps at different rates.  Cis-trans isomers have dif