Rearrangement: a single reactant rearranges its

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1 Chapter 5: An overview of organic reactions 5.1 Kinds of organic reactions Even though there are hundreds of reactions to study, organic chemistry is governed by only a few key ideas that determine chemical reactivity. First, reactions can be organized by what kindsof reactions occur. Then, we can study howthose reactions occur. Bothneed to happen in order to fully understand organic chemistry. Four general types of organic reactions: 1. Addition 2. Elimination 3. Substitution 4. Rearrangement Addition: combination of two molecules into one. Elimination: one molecule splits into two. Substitution:two molecules exchange parts to give two new products. 4. Rearrangement: a single reactant rearranges its atoms to give an isomeric product. ch5 Page 1 ch5 Page 2

2 5.2 How organic reactions occur: Mechanisms In a clock, we see the hands move but the mechanism behind the face is what causes the movement. In an organic reaction, we see the transformation that has occurred. The mechanismdescribes the steps that cause the changes we observe. Mechanismsfor organic reactions are the series of steps in a reaction sequence from reactant to product Bond-making or bond-breaking Steps can occur one at a time or at the same time (concerted) Bond formation or breakage can be symmetrical or unsymmetrical. 5.3 Radical reactions Radical: highly reactive molecule or atom with an odd number of electrons (usually 7) in its valence shell (instead of the stable octet). Some example radicals: Fishhook arrowshows the movement of only one electron. Homolytic: symmetrical -electrons move one at a time - radical reactions Heterolytic: unsymmetrical -electrons move as a pair - polar reactions Radical substitution: Rad + A : B Radical addition: Rad + ch5 Page 3 ch5 Page 4

3 Steps in radical substitution There are three types of steps in a radical substitution reaction: 5.4 Polar reactions Differences in electronegativities make electron-rich and electron-poor regions in any polar molecule. 1. Initiation: homolytic formation of two reactive species with unpaired electrons -often initiated by light (hν) 2. Propagation: reaction of radical with molecul3e to make a new radical. Bond polarity can be increased by acid-base reactions: 3. Termination: combination of two radicals to form a stable product. Even though electronegativities are similar between C-S and C-I, these bonds are polar because the electrons in the large S and I atoms are polarizable-they can easily respond to other nearby charges. ch5 Page 5 ch5 Page 6

4 Nucleophiles and electrophiles Nucleophiles and electrophiles Because unlike charges attract, electron-rich sites seek out and react with electron-poor sites. Nucleophile: electron-rich (-or δ-) atom that seeks out an electron-poor atom (nucleus-loving) -Must have lone pair of electrons! Nucleophiles are Lewis bases. Electrophile: electron-poor (+ or δ+) atom that receives electrons from a nucleophile (electron-loving) Electrophiles are Lewis acids. When drawing a mechanism, a curved doubleheaded arrow is used to show the movement of a pair of electrons, from nucleophile to electrophile (Never the other way around!!) When the electrophile is an H, it's an acid-base reaction! CH 3 O - + H 3 O + ch5 Page 7 ch5 Page 8

5 5.5 Addition of HBr to ethylene Finding the nucleophile and electrophile is the key to almost every organic reaction. Let's start with a simple addition reaction. Mechanism of addition of HBr to ethylene The mechanism of this reaction happens in two steps: 1. The πelectrons from the nucleophilic double bond attack the electrophilic hydrogen on HBr, forming a new C-H σbond. Ethylene contains an electron-rich π bond The pair of electrons can act as a nucleophile when there is a strong acid present. HBris a strong acid and contains an electron-poor H. The δ+ H acts as a strong attractor for electrons from another molecule. The H is the electrophile in this reaction. 2. This leaves the other carbon (formerly of the πbond) with only 6 electrons and an empty p orbital -a positively charged carbocation. Simultaneously, two electrons from the H-Br σbond move onto bromine, making a bromide ion. The bromide ion donates an electron pair to the + carbocation, forming a C-Br σbond and yielding the neutral addition product. ch5 Page 9 ch5 Page 10

6 5.6 Using curved arrows in polar reaction mechanisms Using curved arrows in polar reaction mechanisms Practice and the knowledge of a few rules will help you draw correct curved arrows for reaction mechanisms. 1. Electrons always move froma nucleophile (Nu: or Nu: - ) toan electrophile (E or E + ). Always start with an electron pair! Electrons usually flow from one of these nucleophiles: 3. The electrophile can be either positively charged or neutral. i. Positive electrophiles become neutral products. ii. Neutral electrophiles become negative products. Electrons usually flow to one of these electrophiles: 4. Never exceed a first-or second-row atom's octet. Carbon never makes more than four bonds! Hydrogen never makes more than one bond! 2. The nucleophile can be either negatively charged or neutral. i. Negative nucleophiles become neutral products. ii. Neutral nucleophiles become positive products. ch5 Page 11 ch5 Page 12

7 5.7 Equilibria, rates, and energy changes Reactions can move either forward or reverse to reach equilibrium. Recall from general chemistry that the equilibrium constant, K eq, is the ratio of product concentrations over reactant concentrations (each raised to the power of the balancing coefficient) For aa + bb cc + dd, For the reaction we just studied, there is a very large equilibrium constant. K eq > 1000 means the reaction goes to completion -the amount of unreacted starting material will usually be undetectable. (The product concentration is 1000x the reactant concentration at equilibrium) Free energy and equilibrium In order for a reaction to proceed to completion, the products must be lower in potential energy than the reactants. (Reactive high-energy molecules lose energy until they become more stable low-energy molecules.) This change of potential energy during a reaction is called the Gibbs free energy change (ΔG). For K eq > 1, ΔG o must be negative (the system releases energy to become more stable). This is a spontaneous process. For K eq < 1, ΔG o must be positive (the system would have to gain potential energy and become less stable in order to proceed as written) ΔGis influenced by two energetic properties: enthalpy and entropy. ΔG = ΔH -TΔS Enthalpy change (ΔH)is related to the strength of bonds that are broken and formed. A favorable enthalpy change will have stronger (more stable) bonds in the product. Entropy change (ΔS)is related to freedom of motion and dispersion of energy. Elimination: A B + C: favorable entropy change Addition: A + B C: unfavorable entropy change ch5 Page 13 ch5 Page 14

8 5.8 Bond dissociation energies Whenever a bond is formed, energy is released (like the sound made when two magnets hit each other). 5.9 Energy diagrams and transition states In the reaction of HBr and ethylene, the first step is formation of the carbocation. Whenever a bond is broken, energy is absorbed (like the force required for you to pull two magnets apart). Bond dissociation energy (D): amount of energy required to break a bond to produce two radical fragments: This energy is mostly dependent of the type of bond, not the molecule that the bond is in. The highest energy, most unstable point in one step of a reaction is called the transition state. The transition state occurs when the C-H bond is partially formed and the H-Br bond is partially broken. Among the weakest bonds are those that can readily produce radicals: Cl-Cl 242 kj/mol Br-Br 194 HO-OH 211 σbonds become stronger the more scharacter they have CH 3 -H 439 kj/mol H 2 C=CH-H 464 HC C-H 558 ch5 Page 15 ch5 Page 16

9 Activation energy and free energy The energy needed to go from reactant to transition state is the activation energy (ΔG ). Kinetics vs. equilibrium Activation energy determines kinetics. Gibbs free energy change determines equilibrium. The size of the activation energydetermines the rate of the reaction (whether it will occur quickly or slowly). A high ΔG means very few of the molecules in the sample will have enough energy to reach the transition state and the reaction will be slow. A low ΔG means most of the molecules will already have enough energy to reach the transition state and the reaction will be fastat room temperature. The energy difference between the reactant and the product is the standard free energy difference, ΔG o -as we saw before, this determines what the equilibrium constant will be. A reaction with a very high activation energy may never reach its equilibrium mixture of products because it is too slow! (The combustion of gasoline has a large equilibrium constant -the products are favored energetically -but at room temperature, the reaction is too slow to observe.) Solution: heat the reaction so more molecules have enough energy to reach the transition state Solution: use a catalyst so there is a series of different, lower-energy transition states. ch5 Page 17 ch5 Page 18

10 5.10 Intermediates An intermediate is the product of one step of the reaction, and reactant of the next step. In the complete energy diagram, intermediates are minima in the curve, while transition states are maxima in the curve. Catalysts and enzymes make reactions faster by changing the mechanism to have several lower-energy transition states ch5 Page 19