Alkenes: Structure and Preparation via Elimination Reactions

Alkenes: Structure and Preparation via Elimination Reactions
Organic Chemistry Second Edition David Klein Chapter 8 Alkenes: Structure and Preparation via Elimination Reactions Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.1 Introduction to Elimination
Elimination reactions often compete with substitution reactions. What are the two main ingredients for a substitution? A nucleophile and an electrophile with a leaving group What are the two main ingredients for an elimination? A base and an electrophile with a leaving group How is a base both similar and different from a nucleophile? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.1 Introduction to Elimination
Consider –OH, which can act as a base or a nucleophile Attack at the α Carbon ALKENE β or 1,2 Reaction at the β Hydrogen Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.2 Alkenes Important alkenes Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.2 Alkenes C=C double bonds are found in a variety of compounds including pheromones and many other classes of compounds Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.2 Alkenes Alkenes are also important compounds in the chemical industry 70 billion pounds of propylene (propene) and 200 billion pounds of ethylene (ethene) are both made from cracking petroleum each year Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.2 Alkenes Klein, Organic Chemistry 2e

8.3 Alkene Nomenclature Alkenes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications Identify the parent chain, which should include the C=C double bond Identify and Name the substituents Assign a locant (and prefix if necessary) to each substituent. Give the C=C double bond the lowest number possible List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically The C=C double bond locant is placed either just before the parent name or just before the -ene suffix Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.3 Alkene Nomenclature Identify the parent chain, which should include the C=C double bond The name of the parent chain should end in -ene rather than –ane The parent chain should include the C=C double bond Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.3 Alkene Nomenclature Identify and Name the substituents
Assign a locant (and prefix if necessary) to each substituent. Give the C=C double bond the lowest number possible The locant is ONE number, NOT two. Although the double bond bridges carbons 2 and 3, the locant is the lower of those two numbers Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.3 Alkene Nomenclature List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically The C=C double bond locant is placed either just before the parent name or just before the -ene suffix Practice with SkillBuilder 8.1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.3 Alkene Nomenclature Name the following molecule

8.4 Alkene Isomerism For the pi bond to remain intact, rotation around a double bond is prohibited As a result, cis and trans structures are not identical Is there a difference between cis-butane and trans-butane? What specific type of isomers are cis and trans butene? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.4 Alkene Isomerism In cyclic alkenes with less than 8 atoms in the ring, only cis alkenes are stable. WHY? Draw the structure for trans-cyclooctene When applied to bicycloakenes, this rule is called Bredt’s rule Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Apply Bredt’s rule to the compounds below
The carbons of the C=C double bond and the atoms that are directly attached to them must be planar to maintain the pi bond overlap. A handheld model can be used to help visualize the p orbital overlap and resulting geometry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Cis and trans modifiers are strictly used to describe C=C double bonds with identical groups on each carbon. Where are the identical groups in trans-2-pentene? For molecules with different groups attached to the C=C double bond, the E/Z notation is used instead of cis/trans notation Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Assigning E or Z to a stereoisomers
prioritize the groups attached to the C=C double bond based on atomic number Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Assigning E or Z to a stereoisomers
prioritize the groups attached to the C=C double bond based on atomic number If the top priority groups are on the same side of the C=C double bond, it is Z (for zussamen, which means together) If the top priority groups are on opposite sides of the C=C double bond, it is E (for entgegen, which means opposite) Practice with SkillBuilder 8.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.5 Alkene Stability The difference in stability can be quantified by comparing the heats of combustion How does heat of combustion relate to stability? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.5 Alkene Stability Klein, Organic Chemistry 2e

8.5 Alkene Stability Consider the following stability trend
What pattern do you see? Practice with SkillBuilder 8.3 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.5 Alkene Stability List the following molecules in order of increasing heat of combustion 2,3,4-trimethyl-1,3-pentadiene 2-isopropyl-1,4-pentadiene 3,3-dimethyl-1,5-hexadiene 4,5-dimethylcyclohexene Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.6 Elimination Reactions in Detail
In general, a H atom and a leaving group are eliminated To understand the mechanism of elimination, first recall the 4 mechanistic steps we learned in chapter 7 Nucleophilic attack Loss of a leaving group Proton transfer Rearrangement Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The 4 mechanistic steps we learned in chapter 7 Which of the 4 steps MUST take place in every elimination mechanism? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

All elimination reactions involve both loss of a leaving group and proton transfer The mechanism may be a concerted (one step) process or a step-wise process. Which process is shown below? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

All elimination reactions involve both Loss of a leaving group and proton transfer The mechanism of the step-wise process: Could the steps happen in the reverse order? Practice with SkillBuilder 8.4 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 Elimination by E2 The E2 mechanism below matches the observed rate law. Write a reasonable rate law for the mechanism How will a change in [base] or [substrate] affect the reaction rate? What do the E and the 2 of the E2 notation represent? Practice with conceptual checkpoint 8.13 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 The Effect of Substrate on E2
The kinetics of E2 and SN2 are quite similar. WHY? However, tertiary substrates are unreactive toward SN2 while they react readily by E2. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

3° substrates are more reactive toward E2 than are 1° substrates even though 1° substrates are less hindered The 3° substrate should proceed through a more stable transition state (kinetically favored) and a more stable product (thermodynamically favored). Let’s take a look at the energy diagram – see next slide Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 Regioselectivity of E2 If there are multiple reactive sites or regions on a molecule, multiple products are possible In elimination reactions, there are often different β sites that could be deprotonated to yield different alkenes What is the relationship between the alkene products? Regioselectivity occurs when one constitutional product is formed predominantly over the other Zaitsev product Hofmann product Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 Regioselectivity of E2 Why does the Zaitsev product predominate when a base that is NOT sterically hindered is used? Is the Zaitsev product kinetically favored, thermodynamically favored, or both? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 Regioselectivity of E2 Why does a sterically hindered base favor the Hofmann product? Sterically hindered bases (sometimes called non-nucleophilic) are useful in many reactions Practice with SkillBuilder 8.5 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 Stereoselectivity of E2
Consider the dehydrohalogenation (elimination of a hydrogen and a halogen) of 3-bromopentane Why are both the transition state and product more stable for the trans product? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.7 Stereospecificity of E2
What is the difference between stereoselective and stereospecific? Consider dehydrohalogenation for the molecule below There is only one available β Hydrogen to be eliminated You might imagine that it would be possible to form both the E and Z alkene products from this reaction. Draw both the E and the Z products Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

When the reaction is actually performed, only the E product is observed Is the E formed exclusively because it is formed through a slightly lower transition state? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

To rationalize the stereospecificty of the reaction, consider the transition state for the reaction In the transition state, the C-H and C-Br bonds that are breaking must be rotated into the same plane as the pi bond that is forming Draw the transition state structure illustrating the coplanar geometry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

There are two coplanar options for the molecule Why does the reaction proceed exclusively through the Anti coplanar structure? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Evidence suggests that a strict 180° angle is not necessary for E2 mechanisms. Similar angles (175–179°) are sufficient The term, anti-periplanar is generally used instead of anti-coplanar to account for slight deviations from coplanarity Although the E isomer is usually more stable because it is less sterically hindered, the requirement for an anti-periplanar transition state can often lead to the less stable Z isomer Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Assuming they proceed through an anti-periplanar transition state, predict the products for the following reactions, and label them as cis or trans Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Assuming they proceed through an anti-periplanar transition state, predict all of the products for the following reaction Will the reaction be stereospecific or stereoselective, and what factors most affect the product distribution? Practice with SkillBuilder 8.6 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Consider the dehydrohalogenation of a cyclohexane Given the anti-periplanar requirement, which of the two possible chair conformations will allow for the elimination to occur? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Which of the two molecules below will NOT be able to undergo an E2 elimination reaction? WHY? It might be helpful to draw their chair structures and make a handheld model Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Draw all of the possible products if each of the molecules below were to undergo dehydrohalogenation Practice with conceptual checkpoint 8.20 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.8 Predicting Products for E2
Consider both regioselestivity and stereoselectivity to predict the products for the eliminations below, and draw complete mechanisms Practice with SkillBuilder 8.7 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.9 The E1 Mechanism The E1 mechanism is a 2-step process
Similar to SN1 (chapter 7), the reaction rate for E1 is not affected by the concentration of the base What does the E and the 1 stand for in the E1 notation? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.9 The E1 Mechanism Given the rate law for E1, which step in the mechanism is the rate-determining slow step? If the second step were the slow step, how would you write the rate law? Practice with conceptual checkpoint 8.26 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

How does the substrate reactivity trend for E1 compare to the trend we discussed in chapter 7 for SN1? WHY? Just like we did for SN1 in chapter 7, to explain the reactivity trend above, we must compare the energy diagrams for each substrate Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Because E1 and SN1 proceed by the same first step, their competition will generally result in a mixture of products How might you promote one reaction over the other? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Alcohols can also undergo elimination or dehydration by E1, but the –OH group is not a stable leaving group In the E1 reaction below, once the water leaving group leaves the carbocation, what base should be used to complete the elimination? Practice with conceptual checkpoint 8.27 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.9 Regioselectivity for E1
The final step of the E1 mechanism determines the regioselectivity E1 reactions generally produce the Zaitsev product predominantly. WHY? Why can’t we control the regioselectivity in this reaction like we can in and E2 reaction? Practice with SkillBuilder 8.8 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.9 Stereoselectivity for E1
In the last step of the mechanism, a proton is removed from a β carbon adjacent to the sp2 hybridized carbocation Draw the appropriate carbocation that forms in the reaction below, and rationalize the product distribution Practice with conceptual checkpoint 8.31 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.9 Stereoselectivity for E1
Considering stereochemistry and regiochemistry, predict the products if the molecule below was treated with concentrated sulfuric acid Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.10 Complete E1 Mechanisms Recall the similarities between SN1 and E1
After the carbocation is formed and possibly rearranged, the E1 proton transfer neutralizes the charge Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.10 Complete E1 Mechanisms Why is the first proton transfer necessary? Practice with conceptual checkpoint 8.32 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.10 Complete E1 Mechanisms Explain why the carbocation rearranges
Practice with conceptual checkpoint 8.33 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.10 Complete E1 Mechanisms Consider the energy diagram for the mechanism on the previous slide Assess each free energy change Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.10 Complete E1 Mechanisms The mechanism shows the formation of the major products Predict the minor elimination products as well Practice with SkillBuilder 8.9 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.11 Complete E2 Mechanisms In E2, the base removes the β proton as the LG leaves Will such a reaction require a relatively strong base? Will E2 dehydrations likely involve a proton transfer prior to the elimination? Practice with conceptual checkpoint 8.37 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.12 Substitution vs. Elimination
Substitution and Elimination are always in competition Sometimes products are only observed from S or E Sometimes a mixture of products is observed Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.12 Substitution vs. Elimination
To predict whether substitution or elimination will predominate, consider the factors below Determine the function of the reagent. Is it more likely to act as a base, a nucleophile, or both? Kinetics control nucleophilicity. WHY? HOW? Thermodynamics control basicity. WHY? HOW? Analyze the substrate and predict the expected mechanism (SN1, SN2, E1, or E2) Consider relevant regiochemical and stereochemical requirements Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.12 Reagent Function: Nucleophilicy
Assessing the strength of a nucleophile The greater the negative charge, the more nucleophilic it is likely to be The more polarizable it is, the more nucleophilic it should be The less sterically hindered it is, the more nucleophilic it should be. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.12 Reagent Function: Basicity
Assessing the strength of a base Assess the strength of its conjugate acid quantitatively using the pKa of its conjugate acid Which is a stronger base Cl- or HSO4-? As bases, are Cl- and HSO4- relatively strong or weak? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.12 Reagent Function: Basicity
Assessing the strength of a base If the base is neutral, assess the strength of its conjugate acid qualitatively using ARIO (atom, resonance, induction, orbital) Compare CH3OH and CH3NH2 If the base carries a negative formal charge, qualitatively assess the strength of the base using ARIO Compare Cl- and HSO4- Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.12 Basicity vs. Nucleophilicity
Consider each of the reagent categories Reagents that act as nucleophiles only are either highly polarizable and/or they have very strong conjugate acids Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The stronger the reagent, the more likely it is to promote SN2 or E2. WHY? The more sterically hindered reagents are more likely to promote elimination than substitution. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.13 Predicting Subst. vs. Elim.
Analyze the function of the reagent (nucleophile and/ or base) Analyze the substrate (1°, 2°, or 3°) Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.13 Predicting Subst. vs. Elim.
Analyze the function of the reagent (nucleophile and/ or base) Analyze the substrate (1°, 2°, or 3°) Practice with SkillBuilder 8.11 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

8.14 Predicting Products Analyze the function of the reagent (nucleophile and/ or base) Analyze the substrate (1°, 2°, or 3°) Consider regiochemistry and stereochemistry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

For the substrate, give both the kinetically favored E2 product and the thermodynamically favored E2 product. Explain what conditions can be used to favor each. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Since tertiary substrates react more readily than secondary or primary in both E1 and E2 mechanisms, what factor(s) usually controls which mechanism will dominate and why? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Predict the major product if the alcohol below were treated with concentrated sulfuric acid. Be aware of the possible rearrangements. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Predict the major product for the following reactions considering competing substitution and elimination pathways. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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Alkenes: Structure and Preparation via Elimination Reactions

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