HL only

Learning outcomes Understand: There are two sub-classes of stereoisomers - conformational isomers and configurational isomers Conformational isomers interconvert by rotation about a σ bond. Configurational isomers can only interconvert by breaking and reforming a bond.

Learning outcomes Configurational isomers are further subdivided into E/Zand cis/trans isomers and optical isomers. Cis/trans isomers can occur in alkenes or cycloalkanes (or heteroanalogues) and differ in the positions of atoms (or groups) relative to a reference plane. E/Z isomerism refers to alkenes of the form R 1 R 2 C=CR 3 R 4 (R 1 ≠ R 2, R 3 ≠ R 4 ) where neither R 1 nor R 2 need to be different to R 3 or R 4.

An asymmetric (joined to four different atoms or groups) carbon atom is called a chiral carbon atom. Optically active compounds can rotate the plane of plane- polarized light as it passes through a solution of the compound. Optical isomers are known as enantiomers. Enantiomers are non-superimposable mirror images of each other. Diastereomers are not mirror images of each other. A 50:50 mixture of two enantiomers is known as a racemic mixture (or racemate) and is optically inactive.

Apply their knowledge to: Construct real or virtual 3-D models of different stereoisomers. Explain stereoisomerism in non-cyclic alkenes and in three and four membered cycloalkanes. Compare the physical and chemical properties of enantiomers. Describe and explain optical isomers in simple organic molecules. Use a polarimeter to distinguish between optical isomers.

Configurational Isomerism Can be converted only by breaking bonds Cis-Trans and E/Z isomerism Where there are double bonds ( Restricted rotation) Optical Isomerism Chirality exists where there is asymmetric carbon atoms

Cis- Trans Isomerism This Isomer arise when there are two or more different groups attached to double bonds or ring structure Same groups on one side cis Different groups - trans 1. Cis trans Isomerism in butene

Cis-Trans Isomerism in cyclic compounds

Practice- Draw cis Trans Isomers of butenedioic acid (HOOC-CH=CH-COOH )

Can you name this? Cis trans failed here, we will use E/Z Isomerism

E/Z Isomerism E/Z isomerism is based on priority rules. Group with high atomic number gets priority Longer chain hydrocarbons gets priority

E/Z Isomerism High priority groups same side- Z High priority groups opposite side –E

Draw and name E/Z conventions for stereoisomer of 3-methylpent-2-ene. CH 3 –CH=C-CH 2 CH 3 CH 3

Net dipole Boilingpoint 60 C Meltingpoint -80 No Net dipole Boilingpoint 48 C Meltingpoint -50 the trans form has free hydrogen bonds (intermolecular forces) thus more energy is need to separate trans hydrogen bonds Melting point trans acid (287 C) Melting than cis acid (139 C). The –COOH groups in cis form intranolecular hydrogen bonds

Optical Isomers A carbon atom attached to four different atoms or groups is known as asymmetric. The four groups can be arranged in two different three-dimensional configurations, which are mirror images of each other. This is known as optical Isomerism. Optical Isomerism refers to the ways in which the isomers interact with plane polarize light. They are said to be Chiral molecules and have no plane of symmetry Let’s see how it works !

Video Thalidomide If you look at your hands they are mirror images, when you put you hands on top of each other they do not line up, the are non- superimposed. The same is true for optical isomers they are known as entantiomers. A mixture of two equal enantiomers is known as racemic mixture such a mixture is said to be optical inactive.. Video optical Isomerism Link Explanation Optical Isomerism

Diastereomers are not mirror images of each other.

Example- Diastereomers of Sugar

Representing optically active molecules

You have two minutes to solve it

Properties of optical isomers Optical Isomers, the enantiomers, have identical physical and chemical properties with two important exceptions * Optical activity * Reactivity with other chiral molecules

The polarimeter If the light appears to have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY AB C D E F A Light source produces light vibrating in all directions B Polarising filter only allows through light vibrating in one direction C Plane polarised light passes through sample that changes the angle. D If substance is optically active it rotates the plane polarised light F Direction of rotation is measured coming towards the observer * Optical activity The amount of rotation depends on: - Length of sample tube - Concentration of enantiomers

Both isomers present, rotation cancels out called: racemic mixture * Reactivity with other chiral molecules When a racemic mixture is reacted with a single enantomer of another chiral compound, the (+) and (-) react different to produce different products. The method to separate the two enantomers is called resolution