Lecture 11: Geometrical Isomerism (PDF)

Lecture 11 1  2- Geometrical isomerism Two stereoisomers (diastereomers) have the same molecular formula but differ in spatial arrangement due to one of 2 reasons: Open-chain alkanes undergo rotations about their carbon–carbon single bonds, while, 1- Alkenes, there is restriction f...

Lecture 11 1  2- Geometrical isomerism Two stereoisomers (diastereomers) have the same molecular formula but differ in spatial arrangement due to one of 2 reasons: Open-chain alkanes undergo rotations about their carbon–carbon single bonds, while, 1- Alkenes, there is restriction for the free rotation around double bond. 2- Cycloalkanes. there is restriction for the free rotation. The 2 stereoisomers have different physical & chemical properties. H Cl **Example H H Cl H Cl Cl Cis-dichloroethene Trans -dichloroethene  Z, E Nomenclature In case of different groups bonded on double bonded carbon atom, the E/Z system of nomenclature has to use. Z (Zusammen) and used when the higher priority groups are orientated on the same side across the double bond. The priority of group determine by using the Cahn-Ingold-Prelog rules. The E ( Entgegen) and is used when the higher priority groups are orientated on different sides across the double bond. The 2 groups around each carbon on the double bond ordered according to the priority rule. 1 CH3 1 e.g.1 Cl... the geometeric isomers is Z H 2 CH 2Cl 2 2 Cl 1 Cl e.g.2 Br... the geometeric isomers is E 1 H 2  H e.g.3 CH3 H not geometericisomers H If one carbon of the two carbons of the double bond has 2 identical substituents... no geometrical isomers Geometrical isomers in cyclic compounds:- Cycloalkanes are alkanes that contain rings of carbon atoms, For example, the cycloalkane with four carbon atoms in a ring is called cyclobutane. Cycloalkanes are similar to alkenes, as there is restriction for the free rotation around the C-C bond. So cycloalkane has two distinct faces. If two substituents point toward the same face, they are cis. If they point toward opposite faces, they are trans. H H CH3 H H3 C CH3 H CH3 cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentane  Cl Cl OCH3 OCH3 trans-1-chloro-3-methoxycyclopentane cis-1-chloro-3-methoxycyclopentane Br Br Cl Cl cis-1-bromo-3-chlorocyclobutane trans-1-bromo-3-chlorocyclobutane up CH 3 a a e e a e e down Br a e Cl e up a a Cl down cis-1-Bromo-3-chlorocyclohexane trans- 1-Cholro-2-methylcyclohexane a = axial, e = equatorial  Exercise???? I- Determine the type of geometrical isomerism of the following compounds: Cl CH3 Br H Br H H H Cl CH3 CH3 H H Br H H COOH Cl H NH2  H CHO H3C CH3 Cl H H H Cl Cl H H CH3 H H CH3  Cl H Br H Br Cl H H H CH3 H OH H H CH3 CH3 3-CONFORMATIONAL ISOMERISM (ROTAMERS) Conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds. Such isomers are generally referred to as conformational isomers or conformers and, specifically, as rotamers. Types of conformational isomerism: Ethane conformers are either eclipsed or staggered. What is a dihedral angle? Angle between bonds on adjacent atoms. Conformations of Butane Naturally, the most stable conformer would be one in which the bulkiest groups are as far as possible (staggered form or anti). While, in a molecule such as ethylene glycol intramolecular hydrogen bonding is possible in gauch form but not in staggered so, the gauch form is more stable than the staggered. Gauch form of ethylene glycol (CH2OH-CH2OH).  Remember: Dihedral angle θ:- the angle between the C-H bonds on the front carbon atom and the C-H bonds on the back carbon in the Newman projection. Eclipsed conformation:- Any conformation with bonds directly lined up with each other, one behind the other, in the Newman projection. The conformation with θ = 0o is an eclipsed conformation Skew conformation:- The conformation with θ between 0 and 60o is an skew conformation. Staggered conformation:- Any conformation with the bonds equally spaced in the Newman projection. The conformation with θ = 60° is a staggered conformation. Torsional strain:- The strain associated with eclipsing of bonds in the ring.