Chapter 5. An Overview of Organic Reactions

John E. McMurry www.cengage.com/chemistry/mcmurry Chapter 6 An Overview of Organic Reactions 2016 Cengage Learning. All Rights Reserved. Learning Objectives (6.1) Kinds of organic reactions

(6.2) How organic reactions occur: Mechanisms (6.3) Radical reactions (6.4) Polar reactions 2016 Cengage Learning. All Rights Reserved. Learning Objectives (6.5) An example of a polar reaction: Addition of HBr to ethylene

(6.6) Using curved arrows in polar reaction mechanisms (6.7) Describing a reaction: Equilibria, rates, and energy changes 2016 Cengage Learning. All Rights Reserved. Learning Objectives (6.8) Describing a reaction: Bond dissociation energies

(6.9) Describing a reaction: Energy diagrams and transition states (6.10) Describing a reaction: Intermediates (6.11) A comparison between biological reactions and laboratory reactions 2016 Cengage Learning. All Rights Reserved. Kinds of Organic Reactions Addition reactions: Two reactants combining to

form a single product Elimination reactions: Single reactant splitting into two products 2016 Cengage Learning. All Rights Reserved. Kinds of Organic Reactions Substitution reactions: Two reactants exchanging parts to form two new products

Rearrangement reactions: Single reactant yielding an isomeric product through reorganization of its bonds and atoms 2016 Cengage Learning. All Rights Reserved. Worked Example Classify each of the following reactions as an addition, elimination, substitution, or rearrangement

a)CH3Br + KOH CH3OH + KBr b) CH3CH2Br H2CCH2 + HBr c) H2CCH2 + H2 CH3CH3 Solution: a)CH Br + KOH CH OH + KBr (Substitution)

3 3 b) CH3CH2Br H2CCH2 + HBr (Elimination) c) H2CCH2 + H2 CH3CH3 (Addition) 2016 Cengage Learning. All Rights Reserved. How Organic Reactions Occur: Mechanisms

Reaction mechanism: Overall description of a reaction process Reactions occur in defined steps that lead from reactant to product Must account for all reactants and products Chemical reactions involve bond-breaking and bond-making

2016 Cengage Learning. All Rights Reserved. How Organic Reactions Occur: Mechanisms Bond formation or breakage can be symmetrical or unsymmetrical Symmetrical cleavage - Homolytic Unsymmetrical cleavage - Heterolytic

2016 Cengage Learning. All Rights Reserved. How Organic Reactions Occur: Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads with a half head (fish-hook) indicate movement of one electron in the symmetrical process Arrowheads with a complete head indicate movement of two electrons in the unsymmetrical process

2016 Cengage Learning. All Rights Reserved. How Organic Reactions Occur: Mechanisms Radical reactions: Processes that involve symmetrical bond-breaking and bond-making Radical: Neutral chemical species containing odd number of electrons Has a single, unpaired electron in one of its

orbitals Polar reactions: Processes involving unsymmetrical bond-breaking and bond-making 2016 Cengage Learning. All Rights Reserved. Radical Reactions Not as common in comparison to polar reactions Radicals react to complete electron octet of valence shell

A radical can add to an alkene to give a new radical, causing an addition reaction 2016 Cengage Learning. All Rights Reserved. Radical Reactions Steps required for methyl chlorination Initiation

Propagation Reaction with molecule to generate radical 2016 Cengage Learning. All Rights Reserved. Radical Reactions Termination

Combination of two radicals to form a stable product 2016 Cengage Learning. All Rights Reserved. Worked Example Using curved fishhook arrows, propose a mechanism for the formation of the cyclopentane ring of prostaglandin H2 2016 Cengage Learning. All Rights Reserved. Worked Example

Solution: The tails of the arrows show the location of the bond to be broken The heads show where the electrons are moving The reaction is a radical addition to a double bond 2016 Cengage Learning. All Rights Reserved.

Polar Reactions Molecules can contain local unsymmetrical electron distributions Causes bond polarity Bonds in functional groups are polar Carbon atoms bonded to electronegative atoms have a partial positive charge

Carbon atoms bonded to metals have a partial negative charge 2016 Cengage Learning. All Rights Reserved. Table 6.1 - Polarity Patterns in Some Common Functional Groups 2016 Cengage Learning. All Rights Reserved. Polar Reactions Polarizability: Tendency of atoms in a molecule

to undergo polarization Larger atoms, easily polarizable Smaller atoms, less polarizable 2016 Cengage Learning. All Rights Reserved. Polar Reactions Nucleophile: Electron-rich species that donate electron pairs to electrophile in a polar bondforming reaction Is a Lewis base

Electrophile: Substances that accept electron pairs from a nucleophile Is a Lewis acid 2016 Cengage Learning. All Rights Reserved. Figure 6.1 - Some Nucleophiles and Electrophiles

2016 Cengage Learning. All Rights Reserved. Worked Example An electrostatic potential map of boron trifluoride is shown Is BF3 likely to be a nucleophile or an electrophile? Draw a Lewis structure for BF3

2016 Cengage Learning. All Rights Reserved. Worked Example Solution: The electrostatic potential map indicates that it is electron-poor (blue) BF3 is likely to be an electrophile

Lewis structure 2016 Cengage Learning. All Rights Reserved. An Example of a Polar Reaction: Addition of HBr to Ethylene Electrophilic addition reaction part results from pp overlap results from sp2sp2 overlap Double bond is more accessible to approaching reactants than a single bond

More electron-rich 2016 Cengage Learning. All Rights Reserved. Figure 6.2 - A Comparison of Carbon Carbon Single and Double Bonds 2016 Cengage Learning. All Rights Reserved. Figure 6.3 - Mechanism 2016 Cengage Learning. All Rights Reserved.

Mechanism of Addition of HBr to Ethylene Polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile Carbocation: Substance that contains a trivalent, positively charged carbon atom having six electrons in its outer shell 2016 Cengage Learning. All Rights Reserved.

Worked Example What product would you expect from reaction of cyclohexene with HBr and HCl? Solution: Reaction is an electrophilic addition reaction Halogen acid adds to a double bond to produce a haloalkane 2016 Cengage Learning. All Rights Reserved. Worked Example

2016 Cengage Learning. All Rights Reserved. Rules for Using Curved Arrows Electrons move from a nucleophilic source to an electrophilic sink The nucleophilic site can be neutral or negatively charged 2016 Cengage Learning. All Rights Reserved.

Rules for Using Curved Arrows The electrophilic site can be neutral or positively charged The octet rule should be followed 2016 Cengage Learning. All Rights Reserved. Worked Example Add curved arrows to the following polar

reaction to indicate the flow of electrons 2016 Cengage Learning. All Rights Reserved. Worked Example Solution: A double bond forms between oxygen and

carbon CCl bond is broken Electrons move from oxygen to form the double bond and from carbon to chlorine 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Equilibria, Rates, and Energy Changes To reach equilibrium, reactions go in either forward or reverse directions The multiplied concentrations of the products divided by the multiplied concentrations of the

reactant is the equilibrium constant, Keq Each concentration is raised to the power of its coefficient in the balanced equation a A b B cC d D c d C D K eq

a b A B 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Equilibria, Rates, and Energy Changes If value of Keq is > 1 Indicates that product concentration term is

larger than the reactant concentration term Reaction takes place from left to right If Keq is 1 Quantity of reactant and product present at equilibrium is large If value of Keq is < 1 Reaction takes place in the reverse direction

2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Equilibria, Rates, and Energy Changes Gibbs free energy (G)G)G): Change in energy that occurs during a chemical reaction G = GG = Gproducts - Greactants

Energy is released on the favored side of an equilibrium reaction Exergonic: Reaction that has negative free energy change and is therefore spontaneous Energy is lost Endergonic: Reaction that has positive free

energy change Energy is absorbed 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Equilibria, Rates, and Energy Changes Standard free energy change at 1 atm pressure and 298 K is denoted as G = GG Relationship between free energy change and an equilibrium constant is:

G = - RT ln KG = - RT ln Keq Where, R = 1.987 cal/(K mol) T = Temperature in Kelvin ln Keq = Natural logarithm of Keq 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Equilibria,

Rates, and Energy Changes Enthalpy change (G)G)H): Measure of change in total bonding energy during a reaction Called heat of reaction Exothermic: Reaction that releases heat Endothermic: Reaction that absorbs heat Entropy change (G)G)S): Measure of the change in the amount of molecular randomness

Keq tells the position of the equilibrium Amount of product that is theoretically possible 2016 Cengage Learning. All Rights Reserved. Table 6.2 - Explanation of Thermodynamic Quantities: G = - RT ln KG= G = - RT ln K H- T G = - RT ln K S 2016 Cengage Learning. All Rights Reserved. Worked Example

Which reaction is likely to be more exergonic, one with Keq = 1000 or one with Keq = 0.001? Solution: G = - RT ln KG = -RT ln K eq Large Keq is related to a large negative G = GG A favorable reaction

Therefore, a reaction with Keq = 1000 is more exergonic than a reaction with Keq = 0.001 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Bond Dissociation Energies Bond dissociation energy (G)D): Amount of energy required to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25 C

Energy is determined by the type of bond Changes in bonds can be used to calculate net changes in heat 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Energy Diagrams and Transition States Transition state: Highest energy point in a reaction step

Activation energy, G)G: Energy required to go from reactant to transition state 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Energy Diagrams and Transition States In the addition of HBr Transition-state structure for the first step bond between the carbons begins to break

CH bond is partially formed HBr bond begins to break 2016 Cengage Learning. All Rights Reserved. Describing a Reaction: Energy Diagrams and Transition States Once the transition state is reached the reaction may:

Continue to give the carbocation product Revert back to reactants Every reaction has its own energy profile 2016 Cengage Learning. All Rights Reserved. Figure 6.6 - Some Hypothetical Energy Diagrams 2016 Cengage Learning. All Rights Reserved. Worked Example

Which reaction is faster, one with G = GG = +45 kJ/ mol or one with G = GG = +70 kJ/mol? Solution: Larger value for G = GG indicates a slower reaction Therefore, reaction with G = GG = +45 kJ/mol is faster than a reaction with G = GG = +70 kJ/mol 2016 Cengage Learning. All Rights Reserved.

Describing a Reaction: Intermediates If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the final product Called reaction intermediate Each step has its own free energy of activation 2016 Cengage Learning. All Rights Reserved.

Figure 6.7 - An Energy Diagram for the Reaction of Ethylene with HBr 2016 Cengage Learning. All Rights Reserved. Worked Example Sketch an energy diagram for a two-step reaction in which both steps are exergonic

Second step has a higher-energy transition state than the first Label the parts of the diagram corresponding to reactant, product, intermediate, overall G = GG, and overall G = GG 2016 Cengage Learning. All Rights Reserved. Worked Example Solution: 2016 Cengage Learning. All Rights Reserved.

Comparison Between Biological Reactions and Laboratory Reactions Laboratory reactions are often carried out in an organic solvent Biological reactions occur in an aqueous medium inside cells Active site: Pocket in an enzyme where a substrate is bound and undergoes reaction 2016 Cengage Learning. All Rights Reserved.

Comparison Between Biological Reactions and Laboratory Reactions Laboratory reactions Often done using relatively simple reagents Catalyst might be used Biological reactions Involve relatively complex reagents called coenzymes Enzymes provide an alternative mechanism that

makes life possible 2016 Cengage Learning. All Rights Reserved. Figure 6.9 - Models of Hexokinase in Space-filling and Wire-frame Formats 2016 Cengage Learning. All Rights Reserved. Summary Common kinds of reactions: Addition reactions, elimination reactions, substitution reactions, and

rearrangement reactions Mechanism describes of how a reaction occurs Mechanisms by which most reactions take place: radical and polar Polar reactions occur due to an attractive interaction between a nucleophilic site in one molecule and an electrophilic site in another 2016 Cengage Learning. All Rights Reserved. Summary Position of a chemical equilibrium is determined

by the value of the free-energy change (G = GG) Enthalpy (G = GH) corresponds to the net change in strength of chemical bonds broken and formed during the reaction Entropy (G = GS) corresponds to the change in the amount of molecular randomness during the reaction Exergonic reactions are that have negative values of G = GG, release energy 2016 Cengage Learning. All Rights Reserved. Summary Reactions that have positive values of G = GG and

absorb energy are endergonic Transition state is an activated complex occurring at the highest-energy point of a reaction Amount of energy needed by reactants to reach this high point is the activation energy, G = GG Reactions take place in more than one step and involve the formation of a reaction intermediate 2016 Cengage Learning. All Rights Reserved.

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