Orbitals - Chemeketa Community College

Orbitals - Chemeketa Community College

6. An Overview of Organic Reactions Why this chapter? To understand organic and/or biochemistry, it is necessary to know: -What occurs -Why and how chemical reactions take place We will see how a reaction can be 6.1 Kinds of Organic Reactions In general, we look at what occurs and try to learn how it happens Common patterns describe the changes

Addition reactions two molecules combine Elimination reactions one molecule splits into two 2 Substitution parts from two molecules exchange Rearrangement reactions a molecule undergoes changes in the way its atoms are connected 3 Learning Check: What kind of reaction is the transformation shown

below? + 1. 2. 3. 4. 5. HCl an elimination reaction a rearrangement reaction a substitution

reaction an addition reaction none of these Cl Solution: What kind of reaction is the transformation shown below? + 1. 2. 3. 4. 5.

HCl an elimination reaction a rearrangement reaction a substitution reaction an addition reaction none of these Cl 6.2 How Organic Reactions Occur: Mechanisms

In a clock 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 mechanism describes the steps behind the changes that we can observe Reactions occur in defined steps that lead from reactant to product 6 Steps in Mechanisms We

classify the types of steps in a sequence A step involves either the formation or breaking of a covalent bond Steps can occur in individually or in combination with other steps When several steps occur at the same time they are said to be concerted 7 Types of Steps in Reaction Mechanisms Bond formation or breakage can be symmetrical or unsymetrical

Symmetrical- homolytic Unsymmetrical- heterolytic Bond Breaking Bond Making 8 Indicating Steps in Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads

with a half head (fish-hook) indicate homolytic and homogenic steps (called radical processes) Arrowheads with a complete head indicate heterolytic and heterogenic steps (called polar processes) 9 6.3 Radical Reactions Not

as common as polar reactions Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule A radical can add to an alkene to give a new radical, causing an addition reaction 10 Steps in Radical Substitution Three types of steps

Initiation homolytic formation of two reactive species with unpaired electrons Example formation of Cl atoms form Cl2 and light Propagation reaction with molecule to generate radical Example - reaction of chlorine atom with methane to give HCl and CH3. Termination combination of two radicals to form a stable product: CH3. + CH3. CH3CH3 11 Steps in Radical Substitution: Monochlorination of Methane

Initiation Propagation 12 Steps in Radical Substitution Termination With excess concentration of Cl2 present continued reaction is probable with formation of dichloro, trichloro, and tetrachloro methanes. 13 Radical Substitution Example: Biosynthesis of Prostaglandins

p. 141 Radical Example: p. 142 Learning Check: Propose a mechanism. Add curved arrows to indicate the flow of electrons: p. 142 Solution: Propose a mechanism. Add curved arrows to indicate the flow of electrons:

p. 142 Learning Check: In a radical chain reaction, what would be the best description of the following reaction? H3C + Cl CH3Cl 1. 2. 3. 4. 5. propagation

elimination initiation termination substitution Solution: In a radical chain reaction, what would be the best description of the following reaction? H3C + Cl CH3Cl 1. 2. 3. 4. 5.

propagation elimination initiation termination substitution Radical Substitution: With >1 kind of H H H Cl H H H H C C C H + H H H Cl2

hv H C C C H H Cl H + H H H 1-chloropropane H C C C H H H H 2-chloropropane When there is >1 type of H then there is

>1 option for radical formation and therefore >1 option for a monohalogenation product. 20 Learning Check: In the reaction of Cl2 with 2-methylbutane, how many monochlorinated isomers are produced? H H H H C H H

H C C C C H + H H H H 1. 2. 3. 4. 5. 2 3 4 5 6 Cl2

hv Solution: In the reaction of Cl2 with 2-methylbutane, how many monochlorinated isomers are produced? H H H H C H H H C C C C H + H H H H

1. 2. 3. 4. 5. 2 3 4 5 6 Cl2 hv

6.4 Polar Reactions Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom The

more electronegative atom has the greater electron density Elements such as O, F, N, Cl more electronegative than carbon 23 p. 143 25 Polarizability Polarization is a change in electron distribution as a response to change in electronic nature of the surroundings

Polarizability is the tendency to undergo polarization Polar reactions occur between regions of high electron density and regions of low electron density 26 Polarizability Bonds inherently polar already can be made more

polar by reactions with acids or bases. p. 144 Polarizability Bonds not inherently polar can be polarizable as interactions with solvent or other polar molecules effect the electron distribution. 2.58 2.66 2.55 2.55

Large atoms with loosely held electrons are more polarizable than small atoms with few tightly held electrons. So: S is more polarizable than O I is more polarizable than Cl p. 144 Generalized Polar Reactions electrophile, an electron-poor species, combines with a nucleophile, an electron-rich An species An electrophile is a Lewis acid A nucleophile is a Lewis base

The combination is indicate with a curved arrow from nucleophile to electrophile 29 Fig. 5-1a, p. 145 Learning Check: Which of the following is likely to be a nucleophile and which an electrophile? p. 146 Solution: Which of the following is likely to be a nucleophile and which an electrophile?

E N N E p. 146 Learning Check: Is BF3 is likely to be a nucleophile or an electrophile? p. 146

Solution: Is BF3 is likely to be a nucleophile or an electrophile? E p. 146 Learning Check: Which of the following is expected to be the worst nucleophile? 1. 2. 3. 4. 5.

NH3 H 2O BH3 ethylene (CH3) 3P Solution: Which of the following is expected to be the worst nucleophile? 1. 2. 3. 4. 5.

NH3 H 2O BH3 ethylene (CH3) 3P 6.5 An Example of a Polar Reaction: Addition of HBr to Ethylene HBr adds to the part of C-C double bond The bond is e- rich, allowing it to function as a nucleophile H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile

37 Mechanism of Addition of HBr to Ethylene HBr electrophile is attacked by electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br bond The result is that ethylene and HBr combine to form bromoethane

All polar reactions occur by combination of an electronrich site of a nucleophile and an electron-deficient site of an electrophile 38 Fig. 5-2a, p. 148 Learning Check: What product would you expect? p. 142 Solution:

What product would you expect? Br p. 142 6.6 Using Curved Arrows in Polar Reaction Mechanisms Curved arrows are a way to keep track of changes in bonding in polar reaction The arrows track electron movement Electrons always move in pairs Charges change during the reaction One curved arrow corresponds to one

step in a reaction mechanism 42 Rules for Using Curved Arrows The arrow (electrons) goes from the nucleophilic reaction site (Nu: or Nu:- ) to the electrophilic reaction site (sink, E or E+) The nucleophilic site can be neutral or negative

43 Rules for Using Curved Arrows The nucleophilic site can be negative or neutral 44 The electrophilic site can be positive or neutral 45 The

octet rule must be followed The hydrogen already has two e-s so when another pair moves in the 2 already owned have to leave. 46 Learning Check: + H 3O + H 2O What is the role of the alkene in the reaction above?

1. 2. 3. 4. 5. electrophile nucleophile free radical catalyst Lewis acid Solution: + H 3O + H 2O

What is the role of the alkene in the reaction above? 1. 2. 3. 4. 5. electrophile nucleophile free radical catalyst Lewis acid Learning Check:

Add curved arrows to indicate the flow of electrons: p. 151 Solution: Add curved arrows to indicate the flow of electrons: p. 151 Learning Check: Add curved arrows to indicate the flow of electrons: p. 152 Solution: Add curved arrows to indicate the flow of electrons:

52 Learning Check: Predict the products of the following reaction: p. 152 Solution: Predict the products of the following reaction: H H O

- H C CO2O2C CH2 C CO 2 H + H O H p. 152

Learning Check: Br 1. 2. 3. 4. 5. Br x Br x

xBr Br x Br x + OH2 O H H Br +

OH2 O H Br + OH2 O H H Br +

OH2 O H H Br + OH2 O H Which of the following represents a correctly drawn reaction mechanism?

Which of the following represents a correctly drawn reaction mechanism? Solution: Br 1. 2. 3. 4. 5. Br

x Br x xBr Br x Br x + OH2

O H H Br + OH2 O H Br + OH2 O H

H Br + OH2 O H H Br + OH2 O H

Learning Check: What carbocation intermediate is consistent with the product formed? Propose a mechanism. (Add curved arrows to indicate the flow of electrons.) p. 142 Solution: What carbocation intermediate is consistent with the product formed? Propose a mechanism. (Add curved arrows to indicate the flow of electrons.) H3C H3C

CH3 C CH2 + H3C C Br H Br CH3 Br H3 C H C CH2 H3 C 1o carbocation

or H3C H3C H C CH2 3o carbocation Not formed p. 142 6.7 Describing a Reaction: Equilibria, Rates, and Energy Changes Reactions

can go either forward or backward to reach equilibrium 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. aA + bB cC + dD 59 p. 153

Magnitudes of Equilibrium Constants aA + bB cC + dD If the value of Keq >1, this indicates that at equilibrium most material is present as products If Keq = 10, then the concentration of the product is ten times that of the reactant value of Keq < 1 indicates that at equilibrium most material is present as the reactant A

If Keq = 0.10, then the concentration of the reactant is ten times that of the product 61 Free Energy and Equilibrium The ratio of products to reactants is controlled by their relative Gibbs free energy This energy is released on the favored side of an equilibrium reaction The change in Gibbs free energy between products and reacts is written as G

If Keq > 1, energy is released to the surroundings (exergonic reaction) If Keq < 1, 1 energy is absorbed from the surroundings (endergonic reaction) 62 Numeric Relationship of Keq and Free Energy Change The standard free energy change at 1 atm pressure and 298 K is G The relationship between free energy

change and an equilibrium constant is: G = - RT ln Keq where R = 1.987 cal/(K x mol) T = temperature in Kelvin ln Keq = natural logarithm of Keq 63 HHo is the heat, or enthalpy, of reaction. HHo < 0 ; The reaction is exothermic. HHo > 0 ; The reaction is endothermic. This reaction is exothermic HG, the Gibbs free energy change, determines Keq

and whether the reaction is spontaneous or This reaction nonspontaneous. spontaneous HGo < 0 ; The reaction is spontaneous. HGo > 0 ; The reaction is nonspontaneous. HGo < 0 ; Keq > 1 HGo = 0 ; Keq = 1 HGo > 0 ; Keq < 1 is The equilibrium lies toward product p. 154 6.2

65 Learning Check: Which of the following equations is correct? 1. 2. 3. 4. 5. G = H TS G = s + TH H = TG TS G = H + TS

S = TH G Solution: Which of the following equations is correct? 1. 2. 3. 4. 5. G = H TS G = s + TH H = TG TS G = H + TS S = TH G

6.8 Describing a Reaction: Bond Dissociation Energies Bond dissociation energy (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 The energy is mostly determined by the type of bond, independent of the molecule The C-H bond in methane requires a net heat input of 105 kcal/mol (438 kJ/mol) to be broken at 25 C.

Table 5.3 lists energies for many bond types Changes in bonds can be used to calculate net changes in heat 68 Bond Dissociation Energies Table 5-3a, p. 156 Bond Dissociation Energies Table 5-3b, p. 156 Bondo Dissociation Energies bond

Calculate the H for the chlorination of methane : Cl2 + CH4 CH3Cl + HCl CH3H CH3Cl HCl HH ClCl 438

351 432 436 243 BDE (kJ/mol) Bonds Broken Bonds Made Cl-Cl

+243 kJ/mol H3C-Cl -351 kJ/mol H3C-H +438 kJ/mol H-Cl -432 kJ/mol +681 kJ/mol

-783 kJ/mol Ho = + 681 - 783 = -102 kJ/mol p. 157 p. 157 Learning Check: Consider the following reaction: CH3I + H2O CH3OH + HI If BDEs are used to estimate the enthalpy of this reaction, which bonds BDE is not needed? 1. 2.

3. 4. 5. CH3I HOH HI CH3OH CH3OH Solution: Consider the following reaction: CH3I + H2O CH3OH + HI If BDEs are used to estimate the enthalpy of this reaction, which bonds BDE is not needed? 1.

2. 3. 4. 5. CH3I HOH HI CH3OH CH3OH Learning Check: The BDEs for some bonds are collected in the table below: bond

BDE (kJ/mol) CH3H CH3Cl HCl HH ClCl 438 351

432 436 243 An alternative step in chlorination of methane might be as follows: Cl + CH4 CH3Cl + H What is the reason that this step is not observed? 1. 2. 3.

4. Cl cannot approach the carbon atom close enough to form a bond because of steric interference from hydrogens H is not among the products of methane chlorination Cl can only bond to hydrogen atoms this step is too endothermic as compared to Solution: The BDEs for some bonds are collected in the table below: bond BDE (kJ/mol)

CH3H CH3Cl HCl HH ClCl 438 351 432

436 243 An alternative step in chlorination of methane might be as follows: Cl + CH4 CH3Cl + H What is the reason that this step is not observed? 1. 2. 3. 4.

Cl cannot approach the carbon atom close enough to form a bond because of steric interference from hydrogens H is not among the products of methane chlorination Cl can only bond to hydrogen atoms this step is too endothermic as compared to Solution: The following reaction is an example of: H3C+ + Cl CH3Cl 1. 2. 3.

4. a termination step a heterolytic process an initiation step a homogenic process 6.9 Describing a Reaction: Energy Diagrams and Transition States The highest energy point in a reaction step is called the

transition state The energy needed to go from reactant to transition state is the activation energy (G) 78 First Step in Addition In

the addition of HBr the (conceptual) transition-state structure for the first step The bond between carbons begins to break The CH bond begins to form The HBr bond begins to break 79 Fast exergonic

Fast endergonic Slow exergonic Slow endergonic Fig. 5-6a, p. 159 Learning Check: Which part of the following energy diagram indicates whether the overall reaction is exergonic or endergonic? 1. 2.

3. 4. 5. I II III IV V Solution: Which part of the following energy diagram indicates whether the overall reaction is exergonic or endergonic? 1.

2. 3. 4. 5. I II III IV V Learning Check: Which statement about free energy is correct? 1. 2.

3. 4. 5. The G of a reaction depends on the reaction mechanism (path). The G of a reaction depends on the mechanism (path). The G of a reaction may be changed by adding a catalyst. The bigger the G value, the faster the reaction. For exergonic reactions G >0. Solution: Which statement about free energy is

correct? 1. 2. 3. 4. 5. The G of a reaction depends on the reaction mechanism (path). The G of a reaction depends on the mechanism (path). The G of a reaction may be changed by adding a catalyst. The bigger the G value, the faster the reaction. For exergonic reactions G >0.

6.10 Describing a Reaction: Intermediates If a reaction occurs in >1step, it must involve species that are neither the reactant nor the final product These are called reaction intermediates or simply intermediates Each step has its own free energy of activation

The complete diagram for the reaction shows the free energy changes associated with an intermediate 85 Learning Check: On the reaction energy profile shown below, which of the species is the highest energy intermediate? 1. 2. 3. 4.

5. I II III IV V Solution: On the reaction energy profile shown below, which of the species is the highest energy intermediate? 1. 2. 3.

4. 5. I II III IV V Learning Check: Which of the following statements is true? 1. 2. 3. 4.

5. Transition states can be observed and sometimes isolated. Transition states can not be observed and never isolated. Intermediates can be observed and sometimes isolated. Intermediates can be observed but never isolated. Intermediates can never be observed. Solution: Which of the following statements is true? 1.

2. 3. 4. 5. Transition states can be observed and sometimes isolated. Transition states can not be observed and never isolated. Intermediates can be observed and sometimes isolated. Intermediates can be observed but never isolated. Intermediates can never be observed. Learning Check:

Consider the reaction profile shown below. If there is a sufficient amount of thermal energy available to overcome the highest activation energy and sufficient time for all molecules to do that multiple times, what is the main structure present under such conditions? 1. 2. 3. 4. 5. A B C

D E Solution: Consider the reaction profile shown below. If there is a sufficient amount of thermal energy available to overcome the highest activation energy and sufficient time for all molecules to do that multiple times, what is the main structure present under such conditions? 1. 2. 3. 4. 5.

A B C D E Learning Check: Which energy difference corresponds to the activation energy of the ratelimiting step for A to E transformation? 1. 2. 3. 4. 5.

BA CA DA EA DC Solution: Which energy difference corresponds to the activation energy of the ratelimiting step for A to E transformation? 1. 2. 3. 4. 5.

BA CA DA EA DC Learning Check: Which of the following statements is incorrect? 1. 2. 3. 4. 5.

Polar reactions must have ionic intermediates. Initiation in radical-chain reactions must generate species with unpaired electrons. Polar reactions occur between electrophiles and nucleophiles. Lewis bases generally behave as nucleophiles. Electrophiles can be neutral as well as Solution: Which of the following statements is incorrect? 1. 2. 3.

4. 5. Polar reactions must have ionic intermediates. Initiation in radical-chain reactions must generate species with unpaired electrons. Polar reactions occur between electrophiles and nucleophiles. Lewis bases generally behave as nucleophiles. Electrophiles can be neutral as well as 6.11 A Comparison between Biological Reactions and Laboratory Reactions Laboratory

reactions usually carried out in organic solvent Biological reactions in aqueous medium inside cells They are promoted by catalysts that lower the activation barrier The catalysts are usually proteins, called enzymes Enzymes provide an alternative mechanism that is compatible with the conditions of life 97 Fig. 5-8, p. 161 6.4

Table 5-4, p. 164 Learning Check: Which of the following does not describe the role that can be played by an enzyme in a biological reaction? 1. 2. 3. 4. 5. It changes the mechanism of the reaction. It lowers the energy of a transition state.

It changes the free-energy of the reaction. It changes the rate of the reaction. It increases the specificity of the Solution: Which of the following does not describe the role that can be played by an enzyme in a biological reaction? 1. 2. 3. 4. 5. It changes the mechanism of the

reaction. It lowers the energy of a transition state. It changes the free-energy of the reaction. It changes the rate of the reaction. It increases the specificity of the

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