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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.2 Alkenes Important alkenes Copyright 2012 John Wiley & Sons, Inc.

8.2 Alkenes C=C double bonds are found in a variety of compounds including pheromones and many other classes of compounds Copyright 2012 John Wiley & Sons, Inc.

8.2 Alkenes Why might it be helpful to know the chemical structure of pheromones such as those below? Copyright 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.2 Alkenes Copyright 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.3 Alkene Nomenclature Name the following molecule Copyright 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.5 Alkene Stability Because of steric strain, cis isomers are generally less stable than trans Copyright 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.5 Alkene Stability Copyright 2012 John Wiley & Sons, Inc.

8.5 Alkene Stability Consider the following stability trend What pattern do you see? Practice with SkillBuilder 8.3 Copyright 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.6 Elimination Reactions in Detail The 4 mechanistic steps we learned in chapter 7 Which of the 4 steps MUST take place in every elimination mechanism? Copyright 2012 John Wiley & Sons, Inc.

8.6 Elimination Reactions in Detail 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 2012 John Wiley & Sons, Inc.

8.6 Elimination Reactions in Detail 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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

8.7 The Effect of Substrate on E2 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 2012 John Wiley & Sons, Inc.

8.7 The Effect of Substrate on E2 How would both the transition state energy and the product energy be different if the substrate were 1°? Copyright 2012 John Wiley & Sons, Inc.

8.7 The Effect of Substrate on E2 Notice the differences in transition state and in product energies Practice with conceptual checkpoint 8.14 Copyright 2012 John Wiley & Sons, Inc.

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 product is formed predominantly over the other Copyright 2012 John Wiley & Sons, Inc.

8.7 Regioselectivity of E2 The identity of the base can affect the regioselesctivity Copyright 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

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 2012 John Wiley & Sons, Inc.

Study Guide for sections 8.1-8.7regio DAY 17, Terms to know: Sections 8.1-8.7regio Elimination reaction, alkene, Bredt’s rule, E/Z notation, E1, E2, Zaitsev product, Hofmann product, regioselectivity DAY 17, Specific outcomes and skills that may be tested on exam 3: Sections 8.1-8.7regio Be able to compare and contrast bases and nucleophiles. Be able to predict products given reactants and reagents for an elimination reaction including regioselectivity concerns. Be able to name a given alkene or draw an alkene given its name. Be able to differentiate between cis and trans isomers in alkenes. Be able to apply and explain Bredt’s rule for syclic and bicyclic alkenes. Be able to use proper E/Z notation with alkenes, and be able to determine where cis and trans notation is not appropriate and E/Z is. Be able to assess the stability of alkenes based on degree of substitution, sterics, and number of pi vs sigma bonds. Be able to relate stability and heat of combuistion. Be able to draw both E1 and E2 mechanisms with proper arrow pushing. Be able to explain how the identity of the base/nucleophile used either promotes E1 or E2. Be able to explain how the identity of the electrophile (sterics and quality of leaving group) used either promotes E1 or E2. Be able to explain how the identity of the base/nucleophile used either promotes elimination or substitution. Be able to explain how the identity of the electrophile (sterics and quality of leaving group) used either promotes elimination or subsitution. Given the mechanism, be able to give the correct rate law for an elimination and explain why. Given information about the rate law for a reaction, be able to distinguish between E1 and E2 and explain why. Be able to draw transition states for each step in an E1 or E2 reaction and explain how the properties of the reactant and substrate affect the stability of the transition state and thus the rate of one mechanism vs. another. Be able to choose an appropriate base that will give desired regioselectivity for an elimination reaction. Be able to make a kinetic vs. thermodynamic argument to explain regioselectivity in elimination reactions. Klein, Organic Chemistry 2e

Practice Problems for sections 8.1-8.7regio Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.50 8.51 8.53 8.54 8.55 Klein, Organic Chemistry 2e

Prep for Day 18 Must Watch videos: Other helpful videos: https://www.youtube.com/watch?v=_oodhLhuD5U (E2 stereoselectivity) https://www.youtube.com/watch?v=SkSgbH5kAD4 (E2 stereospecificity) https://www.youtube.com/watch?v=j3kporh4foo (E1 reactions) https://www.youtube.com/watch?v=Gna9dmf_UZ0 (E1 regioselectivity) https://www.youtube.com/watch?v=12Rvts2NR7M (substitution competing with elimination) Other helpful videos: https://www.youtube.com/watch?v=JrI3ZuLsKHI (E2 stereospecificity with cyclohexane) https://www.youtube.com/watch?v=dB8JrzVJqDA (E1 regioselectivity with rearrangement) http://ps.uci.edu/content/chem-51a-organic-chemistry (lecture 26) Read sections 8.7stereochemistry-8.12 Klein, Organic Chemistry 2e