Janice Gorzynski Smith University of Hawai i. Chapter 6. Modified by Dr. Juliet Hahn

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1 Organic Chemistry, Fifth Edition Janice Gorzynski Smith University of Hawai i Chapter 6 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 Kinetics and Energy Diagrams Kinetics is the study of reaction rates. E a is the energy barrier that must be exceeded for reactants to be converted to products. End class 10/6F 2

3 Factors Affecting Reaction Rates The higher the concentration, the faster the rate. The higher the temperature, the faster the rate. G, H, and K eq do not determine the rate of a reaction. These quantities indicate the direction of the equilibrium and the relative energy of reactants and products. 3

4 Rate Law A rate law or rate equation shows the relationship between the reaction rate and the concentration of the reactants. It is experimentally determined by measuring the decrease in concentrations of reactants or the appearance of products over time. Fast reactions have large rate constants, k. Slow reactions have small rate constants, k. 4

5 Rate equations The rate constant k and the energy of activation E a are inversely related; a high E a corresponds to a small k. A rate equation contains concentration terms for all reactants in a one-step mechanism. A rate equation contains concentration terms for only the reactants involved in the rate-determining step in a multistep reaction. The order of a rate equation equals the sum of the exponents of the concentration terms in the rate equation. 5

6 Rate equations for Two-Step Reactions A two-step reaction has a slow rate-determining step, and a fast step. In a multistep mechanism, the reaction can occur no faster than its rate-determining step. Only the concentration of the reactants in the rate-determining step appears in the rate equation. 6

7 Catalysts Some reactions do not proceed at a reasonable rate unless a catalyst is added. 7

8 A catalyst is a substance that speeds up the rate of a reaction. It is recovered unchanged in a reaction, and it does not appear in the product. A catalyst lowers the activation energy, thus increasing the rate of the catalyzed reaction. The energy of the reactants and products is the same in both the catalyzed and uncatalyzed reactions, the position of equilibium is unaffected. Figure 6.6 Role of the Catalyst 8

9 Enzymes Enzymes are biochemical catalysts composed of amino acids held together in a very specific three-dimensional shape. An enzyme contains a region called its active site which binds an organic reactant, called a substrate. The resulting unit is called the enzyme-substrate complex. Once bound, the organic substrate undergoes a very specific reaction at an enhanced rate. The products are then released. 9

10 Lactase a Biological Catalyst 10

11 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. 11

12 Alkyl Halides Alkyl halides are organic molecules containing a halogen atom bonded to an sp 3 hybridized carbon atom. The halogen atom in halides is often denoted by the symbol X. Alkyl halides are classified as primary (1 ), secondary (2 ), or tertiary (3 ), depending on the number of carbons bonded to the carbon with the halogen atom. 12

13 Naming Alkyl Halides 13

14 Polarity of Alkyl Halides Alkyl halides are weakly polar molecules. They exhibit dipole-dipole interactions because of their polar C-X bond. Since the rest of the molecule contains only C-C and C-H bonds, they are incapable of intermolecular hydrogen bonding. 14

15 Physical Properties of Alkyl Halides 15

16 Figure 7.3 Simple Alkyl Halides (for interest only) 16

17 Common Alkyl Halides 17

18 Reaction Types for Alkyl Halides 18

19 Substitution Reactions Three components are necessary in any substitution reaction. substrate 19

20 Nucleophiles in Substitution Reactions Nucleophiles are Lewis bases that can be negatively charged or neutral. Negatively charged nucleophiles like HO and HS are used as salts with Li +, Na +, or K + counterions to balance the charge. Since the identity of the counterion is usually inconsequential, it is often omitted from the chemical equation. 20

21 Neutral Nucleophiles When a neutral nucleophile is used, the substitution product bears a positive charge. The substitution product s positive charge is usually caused by a proton bonded to O or N. That proton is readily lost from this in a BrØnsted- Lowry acid-base reaction, forming a neutral product. 21

22 Drawing Products of Nucleophilic Substitution Reactions The overall effect of any nucleophilic substitution is the replacement of the leaving group by the nucleophile. To draw any nucleophilic substitution product: Find the sp 3 hybridized carbon with the leaving group. Identify the nucleophile, the species with a lone pair or π bond. Substitute the nucleophile for the leaving group and assign charges (if necessary) to any atom that is involved in bond breaking or bond formation. 22

23 The Leaving Group In a nucleophilic substitution reaction of R-X, the C-X bond is heterolytically cleaved, and the leaving group departs with the electron pair in that bond, forming X:. The more stable the leaving group X:, the better able it is to accept an electron pair. For example, H 2 O is a better leaving group than HO because H 2 O is a weaker base (more stable after leaving). 23

24 Nucleophiles and Bases Nucleophiles and bases are structurally similar: both have a lone pair or a π bond. They differ in what they attack. Bases attack protons. Nucleophiles attack other electron-deficient atoms (usually carbons). 24

25 Common Nucleophiles 25

26 S N 2 Reaction Mechanism The mechanism of an S N 2 reaction would be drawn as follows. Curved arrow notation is used to show the flow of electrons. H H H H H H H Leaving group Nu substrate 26

27 S N 2 Kinetics S N 2 reactions exhibit 2 nd order kinetics. The reaction is bimolecular both the alkyl halide and the nucleophile appear in the rate equation. Rate = k [substrate][nucleophile] End 10/9 Monday lecture 27