Unit 4 organic chemistry 1. Isomerism – geometric and optical (stereoisomerism) 2. Reactions of carbonyl compounds 3. Mechanisms of carbonyl reactions 4. Properties and reactions of carboxylic acids 5. Preparation of carboxylic acids 6. Preparation and reactions of carboxylic acid derivatives 7. Commercial and life importance of esters and their reactions

Isomerism L.O. To recap functional groups To be able to distinguish between different isomers

Activity Make the following models and name them: Can you spot the pattern in number of hydrogen atoms when 1 extra carbon is added to a saturated backbone? C C O H CH3 C2H5 C O H C3H7 C O H H H C Cl C N H C4H9 C5H11 H H Can you make and name this compound? CH3CH2CH(Cl)CH2CH2OH 3-Chloropentanol

ISOMERS EMPIRICAL FORMULA from % by mass composition (Simplest ratio of atoms) eg 24.2% C ; 4.0%H ; 71.8% Cl  CH2Cl MOLECULAR FORMULA using empirical formula & Mr from mass spectrometer (Actual number of atoms) eg Mr = 99  C2H4Cl2 Cl H Cl H STRUCTURAL FORMULAE C C or (Shows bonding of individual atoms) ISOMERS Molecules with SAME MOLECULAR FORMULA but DIFFERENT STRUCTURAL FORMULAS

ISOMERS STRUCTURAL ISOMERS STEREOISOMERS = molecules with the same molecular formula but different bonding patterns of atoms = isomers resulting from different spacial arrangements of same atoms in the same structural formula

STEREOISOMERISM Molecules have the SAME MOLECULAR FORMULA but the atoms are joined to each other in a DIFFERENT SPACIAL ARRANGEMENT - they occupy a different position in 3- dimensional space. There are two types... • GEOMETRICAL ISOMERISM • OPTICAL ISOMERISM

    GEOMETRICAL ISOMERISM How to tell if it exists Two different atoms/groups attached Two different atoms/groups attached  GEOMETRICAL ISOMERISM  Two similar atoms/groups attached Two similar atoms/groups attached Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism Two similar atoms/groups attached Two different atoms/groups attached  Two different atoms/groups attached Two different atoms/groups attached  GEOMETRICAL ISOMERISM

E/Z nomenclature The traditional system for naming the geometric isomers of an alkene, in which the same groups are arranged differently, is to name them as cis or trans. However, it is easy to find examples where the cis-trans system is not easily applied. The general strategy of the E-Z system is to analyze the two groups at each end of the double bond. Then, see whether the higher priority group at one end of the double bond and the higher priority group at the other end of the double bond are on the same side (Z, from German zusammen = together) or on opposite sides (E, from German entgegen = opposite) of the double bond.

C H C H “Z” isomer “E” isomer C H CH3 C H CH3 Z-but-2-ene E-but-2-ene Geometric isomers C H C H Non-rotatable C=C “Z” isomer “E” isomer C H CH3 C H CH3 Z-but-2-ene E-but-2-ene

Questions Show the alkenes that are the structural isomers of C4H8 Show the E-Z isomers of C4H8

OPTICAL ISOMERS - KEYPOINT This is shown by chiral molecules. These are molecules that are not superimposable on their mirror image. In most cases (and all you will come across in A2) the chiral centre is an asymmetric carbon atom. This is a carbon atom with 4 different substituent groups attached. W mirror W C C Y Y X X Z Z

OPTICAL ISOMERS - KEYPOINT Molecules that display optical isomerism show optical activity. Two optical isomers (ENANTIOMERS) will rotate the plane of polarisation of plane-polarised light by the same amount BUT in opposite directions. W mirror W C C Y Y X X Z Z

OPTICAL ISOMERISM - DEFINITION Optical isomerism is shown by chiral molecules. Optical isomers are non-superimposable mirror images that rotate the plane of polarisation of plane polarised light by the same amount BUT in opposite directions.

Chirality and drug synthesis A synthetic amino acid, made in the lab, is optically INACTIVE: It contains equal amounts of each isomer – a RACEMIC mixture A natural amino acid, made in nature, is optically ACTIVE: It contains only 1 of the optical isomers (enantiomers). This single optical isomer (enantiomer) has the ability to rotate the plane of polarised light (monochromatic).

Chiral compounds in the news…... Thalidomide: One optical isomer helps morning sickness (+). The other one causes birth defects (-). Thalidomide occurs as a racemic mixture!

Chiral compounds in the news…... L-Dopa: Parkinson’s disease treatment (Dopamine)

Questions Use the model kits to build the following and their enantiomers. In each case identify the chiral carbon and draw both enantiomers. C4H9OH CH3CH(NH2)COOH Extension: