Chemistry 123: Physical and Organic Chemistry Topic 1: Organic Chemistry

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1 Topic 1: Mechanisms and Curved Arrows etc Reactions of Alkenes:.Similar functional groups react the same way. Why? Winter 2009 Page 73 Topic 1: Mechanisms and Curved Arrows etc Reactivity:.Electrostatic attraction is a primary driving force. ELECTRON RICH ATOMS OR MOLECULES ARE ATTRACTED TO ELECTRON DEFICIENT ATOMS OR MOLECULES. An Electron deficient compound is called an electrophile, while an electron rich compound is called an nucleophile. Electrophilic Addition Reaction. Winter 2009 From: Page 74 1

2 Topic 1: Mechanisms and Curved Arrows etc Arrow pushing" is a scheme for rationalizing and predicting the products of organic reactions. 1. An arrow represents movement of an electron pair. A chemical bond is an electron pair. The movement of atoms is not represented by arrows. 2. Single barded arrows are used for movement of single electrons 3. An arrow-push creates a new chemical bond or a new lone electron pair on one atom. Simultaneously, one bond or electron pair will vanish from the drawing. The total number of electrons does not change. 4. The atom at the origin of the arrow will become more positive by one charge and that at the head more negative by one. 5. All intermediates should be valid Lewis structures. Winter 2009 Page 75 Topic 1: Mechanisms and Curved Arrows etc Winter 2009 From: Page 76 2

3 More Alkenes and Alkynes Winter 2009 Page 77 Priority: Winter 2009 Page 78 3

4 Winter 2009 Page 79 H O O Winter 2009 Page 80 4

5 Winter 2009 Page 81 Reductions of Alkynes: Winter 2009 Page 82 5

6 Oxidation of Alkynes: Winter 2009 Page 83 Conformational isomerism is a form of stereoisomerism in which molecules with the same structural formula (same connectivity) exist as different conformational isomers or conformers in 3-D due to rotations about one or more σ bonds. Rotamers are conformers that differ by rotation about only a single σ bond. Conformers can differ by rotations about many sigma bonds or just one (i.e. a rotamer is a conformer). The rotational barrier, or barrier to rotation, is the activation energy required to convert from one rotamer to another rotamer. Winter 2009 Page 84 6

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9 If you make a model of cyclopropane or cyclobutane using one of the common plastic model kits you immediately realize that it can be difficult to take an alkane chain and transform it into a ring without breaking, or at least bending, the plastic tubes that represent the C-C bonds. The difficulty in making the models actually reflects a true instability in the compounds as a result of ring strain. These small rings are like loaded springs, they literally want to split open and release the ring strain, which is a form of stored energy. Winter 2009 Page 89 The reason for the ring strain is the fact that the models show carbon as being sp 3 hybridized and consequently preferring a tetrahedral geometry of atoms attached to the carbons. With tetrahedral geometry the ideal angle between C-C-C bonds is o In cyclopropane the angles between C-C-C bonds are 60 o and therefore these bonds are forced to be much closer together than in an open chain alkane. In butane the angle is 90 o, so there is less ring strain than in cyclopropane, but it is still significant. As we shall see, cyclohexane rings can adopt a conformation in which any C-C-C angle is almost o and consequently they have no ring strain. The C-C-C bond angles in a cyclopentane can also be made to be close to o and therefore there is only a small amount of ring strain. Winter 2009 Page 90 9

10 There is also another factor that makes small rings unstable, which is known as eclipsing (torsional) strain. If a cycloalkane ring is flat, then all the hydrogens on the ring will have an eclipsing relationship to each other and the resulting repulsion of all the eclipsing interactions forces the ring to want to twist into a conformation where some this interaction is reduced or even eliminated. Given that a plane is defined by three points, cyclopropane cannot change its conformation (its flat) so its hydrogen atoms are always eclipsed. Winter 2009 Page 91 Winter 2009 Page 92 10

11 In the case of cyclobutane, it can twist two oppoisite carbons in the ring up or down. The result is a more stable conformation that relieves some of the eclipsing hydrogen atoms interaction. Consequently, the most stable conformation of cyclobutane is not a flat ring, but a conformation known as puckered or sometimes it is called the butterfly conformation. The cyclobutane ring rapidly flips from one puckered conformation to the other. Winter 2009 Page 93 Winter 2009 From: Page 94 11

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14 We can measure the difference in free energy, ΔG o, (a measure of the relative stability) between the axial and equatorial conformers for a variety of substituents on a cyclohexane ring: Why would one axial substituent make the conformer less stable than another (e.g. H 3 C- versus (CH 3 ) 3 C-)?? Winter 2009 Page 99 Cyclopentane also removes some of its eclipsing interactions by adopting two different non-planar conformations, the envolope and the half-chair, which have similar stability: Winter 2009 Page

15 Cyclohexane is able to adopt a conformation in which all the C-C-C angles are close to o and where, at the same time, all the hydrogen atoms have a staggered relationship: This conformation is called the chair conformation of cyclohexane and it has no ring strain. Winter 2009 Page 101 Winter 2009 Page

16 Winter 2009 From: Page 103 When the methyl group is in the axial position it experiences what are called 1,3 diaxial interactions with each of the two axial hydrogen atoms that are on the same side of the ring. True for any axial substituent larger than a hydrogen: When the methyl is in the equatorial position it does not experience any such interaction, it represents the more stable conformation. Winter 2009 Page

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18 SUBSTITUTED CYCLOHEXANES: We will now apply our new found knowledge of conformational analysis to substituted cyclohexanes, starting with methylcyclohexane. By building a scale model and looking at atom to atom distances we realize that axial and equatorial methylcyclohexanes are not equivalent in energy. Winter 2009 Page Winter 2009 Page

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20 Topic 1: STEREOCHEMISTRY STEREOCHEMISTRY Winter 2009 Page