2 Isomerism (Organic Chem NJC)

National Junior College SH1 H2 Chemistry 1 2 ISOMERISM Note: bonds have to be broken in order for isomer A to be converted to isomer B. 2.1 INTRODUCTION Isomerism refers to the existence of two or more stable compounds with the same molecular formula but different arrangement of the atoms. These compounds are known as isomers. There are 2 major types of isomerism: constitutional (structural) isomerism and stereoisomerism.

National Junior College SH1 H2 Chemistry 2 2.2 STRUCTURAL ISOMERISM (CONSTITUTIONAL ISOMERISM) Success Criteria: • I can describe and draw constitutional (structural) isomers Structural isomers have the same molecular formula but different structural formulae; i.e., at least one atom is bonded to a different atom in one isomer compared to another. (i) Chain Isomers e.g. structural isomers of C5H12 Pentane (b.p. 36 C) Straight Chain 2,2-dimethylpropane (b.p. 10 C) Branched Both share the same molecular formulae but differ in the longest continuous hydrocarbon chain. (ii) Positional Isomers e.g. pentan-3-one vs pentan-2-one CH3CH2CCH2CH3 O pentan-2-one pentan-3-one Both share the same molecular formulae but different location of C=O group on the longest continuous hydrocarbon chain. Chain and positional isomers have different physical properties such as density, solubility, melting and boiling points as the surface area of contact between molecules are different. These isomers have similar chemical properties as they have the same functional groups. However, the shape of a molecule often determines its chemical reactivity. An easy- to-reach functional group or other side chain reacts more readily than one that experiences steric hindrance. (i.e., one that is “buried” in the middle of a long C chain or “protected” by bulky groups). (Concept of steric hindrance will be covered in details in later chapter.)

National Junior College SH1 H2 Chemistry 3 (iii) Functional group isomers There are 4 common pairs of functional group isomers. • alcohols and ethers with general formula CnH2n+2O CH3CH2OH, ethanol and CH3OCH3, dimethyl ether • aldehydes and ketones with general formula CnH2nO • alkenes and cycloalkanes with general formula CnH2n CH3CH2CH2CH=CH2 • Carboxylic acids and esters with general formula CnH2nO2 Functional group isomers have different physical and chemical properties. Note: Consider 1) chain isomerism - vary no. of C atom in longest chain by moving C atoms to become substituent (e.g. branch) 2) positional isomerism - vary positional number of principal functional group and substituents 3) functional group isomerism -vary arrangement of atoms into a different functional group Checkpoint 1 (a) Draw all the structural isomers of molecular formula C6H14. (b) Draw all the structural isomers of an ester of molecular formula C4H8O2.

National Junior College SH1 H2 Chemistry 4 2.3 STEREOISOMERISM Stereoisomers have the same molecular and structural formulae. The corresponding atoms are bonded to the same atoms but differ in the spatial (3D) arrangements of their bonds. There are two types of stereoisomerisms: cis-trans isomerism and optical isomerism (enantiomerism). 2.3.1 Cis-trans Isomerism Success Criteria: • I can identify the two structural features required for an alkene to undergo cis - trans isomerism, and explain it in terms of restricted rotation due to the presence of π bonds. • I can name and draw cis- and trans- isomers. Two conditions are required for cis-trans isomerism to exist: • restricted rotation about a bond can be due to (i) C=C bonds in a straight chain) (ii) a ring structure (not in H2 syllabus) • two different groups on each of the carbon atoms with restricted rotation. e.g. C C a b a b C C d b a b C C d e a b The compound below does not exhibit cis -trans isomerism as it has two methyl groups attached to one of the C atoms in the C=C bond. C C Cl H CH3 CH3 Note: Free rotation about the single C−C bond may cause spatial arrangement of atoms across 2 C to differ but they are not isomers of each other as bond breaking process is not involved to convert 1 arrangement to the other. (i) Restricted rotation about single bonds versus double bonds Single bond Double bond Free rotation is possible about σ bonds. Rotation does not cause any bond to be broken. Free rotation is not possible about a double bond. Conversion of cis-isomer to trans-isomer requires the  bond to be broken

National Junior College SH1 H2 Chemistry 5 Note: Alkenes with four different substituents across C=C also exhibit cis-trans isomerism. Another naming convention (E / Z) is used instead. **E/Z naming is not in H2 syllabus. *Cyclic alkenes do not exhibit cis-trans isomerism despite satisfying both structural features required. (see 2.3.1.) (ii) Naming of cis-trans Isomers cis-isomers trans-isomers have identical atoms / groups of atoms on the same side of the double bond have identical atoms / groups of atoms on diagonally opposite sides of the double bond e.g. e.g. (iii) Properties of cis-trans isomers Success Criteria: • I can explain why cis- and trans- isomers have different boiling and melting points Cis-trans isomers have the same chemical but different physical properties. cis-isomer trans-isomer Structural Formula C C H Br Br H C C H Br H Br Net Polarity C C H Br Br H polar molecule C C H Br H Br non-polar molecule Boiling Point has a higher boiling point as more energy is needed to overcome stronger permanent dipole- permanent dipole interactions between the molecules has a lower boiling point as less energy is needed to overcome weaker intermolecular instantaneous dipole- induced dipole interactions Melting Point has lower melting point due to poorer packing efficiency (lesser number of molecules per unit volume in solid state), leading to less intermolecular forces to overcome per unit volume has a higher melting point due to better packing efficiency of molecules (packs better in a crystalline lattice due to its more symmetrical shape), leading to more intermolecular forces to overcome per unit volume

National Junior College SH1 H2 Chemistry 6 (iv) Molecules containing two or more cis-trans C=C In general, a molecule with “m” cis-trans C=C has a maximum of 2m stereoisomers. For a compound with three cis-trans C=C , eight (2 3) stereoisomers are expected. 2.3.2 Optical Isomers Success Criteria: • I can identify a chiral centre and explain what it means. • I can use suitable diagrams to illustrate optical isomerism and state the properties of optical isomers. • I can explain the similarities and differences in physical and chemical properties trends of enantiomers. • I understand the term racemic mixture and able to explain why it is optically inactive. • I can deduce whether a molecule is chiral by checking for the (i) presence of chiral centre and (ii) plane of symmetry. Optical isomerism exists when molecules contain at least one chiral centre causing them to lack both a point and plane symmetries. (i) Chirality A molecule is described to contain a ‘ chiral centre’ if it has a carbon atom attached with four different atoms / groups of atoms. As a result, the mirror images of the molecules are non-superimposable. A chiral carbon is usually labelled with an asterisk (*). Enantiomers refer to a pair of compounds with the same connectivity but opposite three-dimensional shapes. Enantiomers are non-superimposable mirror images of each other. e.g. a molecule with a chiral centre with its enantiomer