Janice Gorzynski Smith University of Hawai i. Chapter 7. Modified by Dr. Juliet Hahn

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1 Organic Chemistry, Fifth Edition Janice Gorzynski Smith University of Hawai i Chapter 7 Modified by Dr. Juliet Hahn Copyright 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 1

2 S N 2 Reaction in Adrenaline Synthesis End Exam III 2

3 Organic Synthesis Organic synthesis is the systematic preparation of a compound from a readily available starting material by one or many steps. Nucleophilic substitution reactions, especially S N 2, are used to introduce a wide variety of functional groups into a molecule, depending on the nucleophile. Organic synthesis has produced many useful compounds (e.g., pharmaceuticals, pesticides, and polymers used in everyday life). Chemists may rely on synthesis to prepare useful substances such as a natural product produced by organisms, but in only minute amounts (e.g., Taxol used in cancer treatment). 3

4 Organic Synthesis Using Alkyl Halides 4

5 Thinking Backwards in Organic Synthesis To carry out the synthesis of a particular compound, we must think backwards, and ask ourselves the question: What starting material and reagents are needed to make it? If a nucleophilic substitution is being used, determine what alkyl halide and what nucleophile can be used to form a specific product. 5

6 Approaches Used in Organic Synthesis To determine the two components needed for synthesis, remember that the carbon atoms come from the organic starting material, in this case, a 1 alkyl halide. The functional group comes from the nucleophile, HO in this case. With these two components, we can fill in the boxes to complete the synthesis. 6

7 Organic Chemistry, Fifth Edition Janice Gorzynski Smith University of Hawai i Chapter 8 Modified by Dr. Juliet Hahn Copyright 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 7

8 General Features of Elimination Elimination reactions involve the loss of elements from the starting material to form a new π bond in the product. 8

9 Elimination of HX In both example reactions a base removes the elements of an acid, HX, from the organic starting material. 9

10 Dehydrohalogenation Removal of the elements HX is called dehydrohalogenation. Dehydrohalogenation is an example of β elimination. The curved arrow formalism shown below illustrates how four bonds are broken or formed in the process. 10

11 Common Bases for Dehydrohalogenation The most common bases used in elimination reactions are negatively charged oxygen compounds, such as HO and its alkyl derivatives, RO, called alkoxides. 11

12 Drawing Products of Dehydrohalogenation Find the α carbon. Identify all β carbons with H atoms. Remove the elements of H and X from the α and β carbons and form a π bond. 12

13 Alkenes are hydrocarbons containing a carbon-carbon double bond. Each carbon of the double bond is sp 2 hybridized. The alkene carbons are trigonal planar. The bond angles are 120 o. Alkenes 13

14 Alkene Structure The double bond of an alkene consists of a σ bond and a π bond. 14

15 Classifying Alkenes Alkenes are classified according to the number of carbon atoms bonded to the carbons of the double bond. Figure

16 Recall that even though there is free rotation around single bonds, rotation about double bonds is restricted. Figure 8.2 Restricted Rotation About Double Bonds 16

17 Stereoisomers of Alkenes Because of restricted rotation, two stereoisomers of 2- butene are possible. cis-2-butene and trans-2-butene are diastereomers (i.e., non-mirror image stereoisomers). 17

18 Alkene Diastereomers Whenever the two groups on each end of a carboncarbon double bond are different from each other, cistrans isomers are possible. 18

19 Stability of Trans Alkenes In general, trans alkenes are more stable than cis alkenes because the groups bonded to the double bond carbons are further apart, reducing steric interactions. 19

20 Stability in Alkenes The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases. 20

21 Stability in Alkenes The higher the percent s-character, the more readily an atom accepts electron density. Thus, sp 2 carbons are more able to accept electron density and sp 3 carbons are more able to donate electron density. Increasing the number of electron donating groups on a carbon atom able to accept electron density makes the alkene more stable. 21

22 Relative Stability of Butenes The 2-butenes (disubstituted) are more stable than 1-butene (monosubstituted). trans-2-butene is more stable than cis-2-butene (less crowding). 22

23 Elimination Mechanisms There are two mechanisms of elimination E2 and E1, just as there are two mechanisms of substitution, S N 2 and S N 1. The E2 mechanism is called bimolecular elimination. The E1 mechanism is called unimolecular elimination. The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation, analogous to the S N 2 and S N 1 mechanisms. E2 and S N 2 reactions have some features in common, as do E1 and S N 1 reactions. 23

24 E2 Mechanism The most common mechanism for dehydrohalogenation is the E2 mechanism. It exhibits second-order kinetics, and both the alkyl halide and the base appear in the rate equation. The reaction is concerted all bonds are broken and formed in a single step. 24

25 E2 Mechanism There are close parallels between E2 and S N 2 mechanisms in how the identity of the base, the leaving group, and the solvent affect the rate. 25

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27 End class 10/20/17F 27