Organic Chemistry Lesson 14 Geometric isomerism (HL)

Your notes and mind-map must be ready for me to inspect. Refresh Which formula represents a polyamide? ( CH2–CHCl ) n ( NH–(CH2)6–NH–CO–(CH3)4–CO ) n ( CF2–CF2 ) n ( O–(CH2)2–O–CO–--–CO ) n Reviewing Your Notes You should spend 60 seconds reviewing your notes from last lesson before attempting this. Your notes and mind-map must be ready for me to inspect.

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Lesson 15: Geometrical Isomerism Objectives: Understand the term stereoisomerism Understand and identify geometrical isomerism Understand the chemical significance of cis-trans isomerism

Molecular Modelling Make a molecule of but-2-ene Compare your model with your neighbours

Stereoisomerism Stereoisomers are compounds with the same structural formula but different 3D arrangement of atoms There are two types of stereoisomerism: Geometrical isomerism Optical isomerism

Geometrical (E/Z or cis-trans) isomerism Geometric isomerism involves the arrangement of groups around a double bond Or a single bond that can’t rotate freely such as in a cyclic compound It happens because double bonds are not free to rotate Each C=C carbon must have two different groups attached to it. In the trans isomer, the substituents (the – CH3 groups) are on opposite sides of the double bond In the cis isomer, the substituents are on the same side of the double bond

Properties Chemical and physical properties will often be similar but there can be important differences as you are about to see….

To do (individually, not pairs): Build, draw and label the cis and trans isomers of: Pent-2-ene 2,3-dichloro-but-2-ene 1,2-dichlorocyclopropane 1,2-dichlorocyclobutane Research the melting/boiling points of cis/trans but-2-ene, hex-3-ene and oct-4-ene. What pattern do you notice between the cis and trans isomers? Why do you think this happens? Draw the cis and trans isomers of 1,2-dichloroethene. Predict, with a reason, which you think will have the higher boiling points. Research online to determine whether you were right and think of a reason if you were wrong.

E/Z or cis/trans? Cis/trans is useful to understand the concept of geometrical isomerism, but there are problems as compounds get more complicated. In this example, which isomer is cis and which is trans? The E-Z system gets around this problem - but makes things slightly more difficult for the simple examples we usually meet on the course.

E/Z or cis/trans? Z ≈ cis (from zusammen, German for the same). E ≈ trans (from entgegen, German for opposite). The difference in the systems lie in that we now have some rules to assign priority Z isomer E isomer

E/Z rules Z ≈ ze zame zide E ≈ enemies are on the opposite side Rules (2 and 3 are beyond IB chemistry) The atom which has the higher atomic number is given the higher priority e.g. C higher than H, BR higher than Cl. If you have isotopes, such as D and H, the highest atomic mass as priority. For alkyl groups, the carbons attached are equal so judge based on the priority of what is attached to those carbons.

Chemical properties of geometric isomers Can be different For example: Why do you think this happens? Hint: think about the previous couple of lessons

Research Research online the importance of geometrical isomerism Find three examples of commercial compounds (medicines, plastics etc) where geometrical isomerism plays a key role in their properties

Key Points Cis isomers: the substituents are on the same side Trans isomers: the substituents are on opposite sides Cis alkenes have lower mp/bp than trans due to ‘rounder’ shape Cis halogenoalkenes have higher mp/bp due to polarity Chemical properties can be different where cis brings groups close enough to react