20.3 Stereoisomerism 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 R1R2C=CR3R4 (R1 ≠ R2, R3 ≠ R4) where neither R1 nor R2 need to be different to R3 or R4.

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.

STEREOISOMERISM Conformational Configurational Remember that structural isomers have the same molecular formula but different arrangements of atoms. Stereoisomers have molecules attached together in the same order but differ in their three dimensional arrangements. Two types: Conformational Configurational

Conformational Isomerism These are isomers that spontaneously interconvert by rotating around sigma bonds. These are very difficult to isolate.

Cyclohexane- Conformational 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 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 Br has higher priority. C2H5 has higher priority.

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

Identify E/Z isomers

(Z)- form

Draw and name E/Z conventions for stereoisomer of 3-methylpent-2-ene. CH3 –CH=C-CH2CH3 CH3

PHYSICAL PROPERTIES Physical properties of cis-tran and E/Z isomers depend upon: The polarity of the molecules The shape or symmetry of the molecules Boiling point is mostly affected by the polarity and the strength of the intermolecular forces.

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

CHEMICAL PROPERTIES The chemical properties of cis-tran and E/Z isomers are usually very similar.

Stereoisomerism

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 !

The same is true for optical isomers they are known as entantiomers. 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 Video Thalidomide Link Explanation Optical Isomerism

ENANTIOMERS Chiral molecules have no plane of symmetry. Enantiomers are two optical isomers of chiral molecules which are non-superimposable which means they do not line up when put together. Mixtures containing the same amount of the two enantiomers are known as racemic mixtures.

DIASTEREOMERS Diastereomers have opposite configurations at some but not all chiral centers and are not mirror images of each other. Diastereomers have different chemical and physical properties.

Diastereomers are not mirror images of each other.

Example- Diastereomers of Sugar

Representing optically active molecules

You have two minutes to solve it

Identify Chiral Centre Look first at the carbon atom labeled 2. Is this a chiral Centre? No, it isn't. Two bonds (one vertical and one to the left) are both attached to methyl groups

Identify Chiral Centre What about the number 3 carbon atom? This has a methyl group below it, an ethyl group to the right, and a more complicated hydrocarbon group to the left. Plus, of course, a hydrogen atom to make up the 4 bonds that have to be formed by the carbon. That means that it is attached to 4 different things, and so is a chiral Centre.

Identify Chiral Centre Carbon 1 has a hydrogen, an -OH and two different hydrocarbon chains (actually bits of rings) attached. Check clockwise and anticlockwise, and you will see that the arrangement isn't identical in each direction. Four different groups means a chiral centre.

Identify Chiral center Carbon 2 has a methyl and three other different hydrocarbon groups. If you check along all three bits of rings , they are all different - another chiral centre. This is also true of carbon 6.

Identify Chiral center Carbons 3, 4, 5 and 7 are all basically the same. Each is attached to a hydrogen and three different bits of rings. All of these are chiral centres.

Identify Chiral center Finally, carbon 8 has a hydrogen, a methyl group, and two different hydrocarbon groups attached. Again, this is a chiral centre.

* Optical activity The polarimeter A B 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 If the light appears to have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY 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

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 two enantiomers of a chiral compound rotate plane-polarized light in equal and opposite directions. A racemic mixture does not rotate light so it is said to be optically inactive.

REACTIVITY WITH OTHER CHIRAL MOLECULES Resolution is the process by which two enantiomers are separated from a racemic mixture. When a racemic mixture is reacted with a single enantiomer of another chiral compound, the two components of the mixture, the (+) and (-) enantiomers react to produce different products. These products have different chemical and physical properties which can be separated easily.

Citations International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015. 2 Chemistry for IB students ISBN 9781107622708 Most of the information found in this power point comes directly from this textbook. The power point has been made to directly complement the Higher Level Chemistry textbook by Steve and is used for direct instructional purposes only.