Addition Reactions and Alkenes

Addition Reactions and Alkenes
Organic Chemistry Second Edition David Klein Chapter 9 Addition Reactions and Alkenes Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.1 Addition Reactions Addition is the opposite of elimination
A pi bond is converted to a sigma bond Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.1 Addition Reactions A pi bond will often act as a Lewis base (as a nucleophile or as a Brønsted-Lowry base) Why are pi bonds more reactive in this sense than sigma bonds? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.2 Addition / Elimination Equilibria
Because an addition is the reverse of an elimination, often the processes are at equilibrium An equilibrium is a thermodynamic expression We assess ΔG (the free energy) to determine which side the equilibrium will favor Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Bonds broken – bonds formed = 166 kcal/mol – 185 kcal/mol = –19 kcal/mol Typical addition reactions have a –ΔH. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Typical addition reactions have a –ΔH Will heat be absorbed by or released into the surroundings? What will the sign (+/-) be for ΔSsurr? Will the enthalpy term favor the reactants or products? The heat change (ΔH) will remain roughly constant regardless of temperature Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Having a –ΔH (or a +ΔSsurr) favors the addition reaction rather than the elimination reaction To get ΔG (or ΔStot) and make a complete assessment, we must also consider the entropy of the system (ΔSsys) What will the sign (+/-) be for ΔSsys? WHY? What will the sign (+/-) be for -TΔSsys? Will the enthalpy term favor the reactants or products? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Plugging into the formula gives… To favor addition, a –ΔG (or a +ΔStot) is needed How can the temperature be adjusted to favor addition? To favor elimination (the reverse reaction in this example), a +ΔG (or a –ΔStot) is needed How can the temperature be adjusted to favor elimination? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Hydrohalogenation Note the temperature used in this addition reaction Does it matter whether the Br adds to the right side of the C=C double bond or whether it adds to the left? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Hydrohalogenation Regiochemistry becomes important for asymmetrical alkenes In 1869, Markovnikov showed that in general, H atoms tend to add to the carbon already bearing more H atoms Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Hydrohalogenation Markovnikov’s rule could also be stated by saying that in general, halogen atoms tend to add to the carbon that is more substituted with other carbon groups This is a regioselective reaction, because one constitutional isomer is formed in greater quantity than another Draw the structure of the minor product Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Hydrohalogenation Anti-Markovnikov products are observed when reactions are performed in the presence of peroxides such as H2O2 Why would some reactions follow Markovnikov’s rule, while other reactions give Anti-Markovnikov products? The answer must be found in the mechanism Practice with conceptual checkpoint 9.1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Hydrohalogenation Mechanism
The mechanism is a two step process Which step do you think is rate determining? Write a rate law for the reaction Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Recall that there are two possible products, Markovnikov and anti-Markovnikiv Which process looks more favorable? WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Stereochemical Aspects
In many addition reactions, chirality centers are formed There are two possible Markovnikov products Which step in the mechanism determines the stereochemistry of the product? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Stereochemical Aspects
Recall the geometry of the carbocation Practice with conceptual checkpoint 9.6 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Rearrangements Rearrangements (hydride or methyl shifts) occur for the carbocation if the shift makes it more stable Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Rearrangements A mixture of products limits synthetic utility
With an INTRAmolecular rearrangement, WHY isn’t the rearrangement product an even greater percentage? How might [Cl-] be used to alter the ratio of products? Practice with SkillBuilder 9.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.3 Hydrohalogenation Example
Predict the major product(s) for the reaction below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.4 Hydration The components of water (-H and –OH) are added across a C=C double bond The acid catalyst is often shown over the arrow, because it is regenerated rather than being a reactant Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.4 Hydration Given the data below, do you think the acid catalyzed hydration goes through a mechanism that involves a carbocation? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.4 Hydration Mechanism Why does the hydrogen add to this carbon of the alkene? Mechanism continues on next slide Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.4 Hydration Mechanism Could a stronger base help promote the last step? Practice with conceptual checkpoint 9.10 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.4 Hydration Thermodynamics
Similar to Hydrohalogenation, hydration reactions are also at equilibrium Explain HOW and WHY temperature could be used to shift the equilibrium to the right or left Addition Elimination Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

How could Le Châtelier’s principle be used to shift the equilibrium to the right or left? Practice with conceptual checkpoint 9.11 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Similar to Hydrohalogenation, the stereochemistry of hydration reactions is controlled by the geometry of the carbocation Draw the complete mechanism for the reaction above to show WHY a racemic mixture is formed Practice with SkillBuilder 9.3 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.4 Hydrations Ethanol is mostly produced from fermentation of sugar using yeast, but industrial synthesis is also used to produce ethanol through a hydration reaction Predict the major product(s) for the reaction below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.5 Oxymercuration-Demercuration
Because rearrangements often produce a mixture of products, the synthetic utility of Markovnikov hydration reactions is somewhat limited Oxymercuration-demercuration is an alternative process that can yeild Markovnikov products without the possibility of rearrangement Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Oxymercuration begins with mercuric acetate How would you classify the mercuric cation? As a nucleophile or an electrophile? As a Lewis acid or Lewis base? How might an alkene react with the mercuric cation? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Similar to how we saw the alkene attack a proton previously, it can also attack the mercuric cation Resonance stabilizes the mercurinium ion and the carbocation. Draw a reasonable resonance hybrid Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The mercurinium ion is also a good electrophile, and it can easily be attacked by a nucleophile, even a weak nucleophile such as water NaBH4 is generally used to replace the –HgOAc group with a –H group via a free radical mechanism Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.6 Hydroboration-Oxidation
To achieve anti-Markovnikov hydration, Hydroboration-Oxidation is often used Note that the process occurs in two steps Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Hydroboration-Oxidation reactions achieve syn addition Anti addition is NOT observed To answer WHY, we must investigate the mechanism Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Let’s examine how this new set of reagents might react The BH3 molecule is similar to a carbocation but not as reactive, because it does not carry a formal charge Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Because of their broken octet, BH3 molecules undergo intermolecular resonance to help fulfill their octets The hybrid that results from the resonance (diborane) involves a new type of bonding called banana bonds Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

In the hydroboration reaction, BH3•THF is used. BH3•THF is formed when borane is stabilized with THF (tetrahydrofuran) What general role do you think BH3•THF is likely to play in a reaction? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

What evidence is there for a concerted addition of the B-H bond across the C=C double bond? Use sterics and electronics to explain the regioselectivity of the reaction Practice with conceptual checkpoint 9.17 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

When ONE chirality center is formed, a racemic mixture results WHY? What is the geometry of the alkene as the borane attacks? The squiggle bond above shows two products, a 50/50 mixture of the R and the S enantiomer Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

When TWO chirality centers are formed, a racemic mixture results Why aren’t the other stereoisomers formed? Practice with SkillBuilder 9.4 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.7 Catalytic Hydrogenation
The addition of H2 across a C=C double bond If a chirality center is formed, syn addition is observed Draw the stereoisomers that are produced Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Analyze the energy diagram below Why is a catalyst necessary? Does the catalyst affect the spontaneity of the process? Typical catalysts include Pt, Pd, and Ni Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

The metal catalyst is believed to both adsorb the H atoms and coordinate the alkene The H atoms add to the same side of the alkene pi system Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Draw product(s) for the reaction below. Pay close attention to stereochemistry How many chirality centers are there in the alkene reactant above? How does the term, mesocompound, describe the product(s) of the reaction? Practice with SkillBuilder 9.5 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is heterogeneous. WHY? Homogeneous catalysts also exist What advantage might a homogeneous catalyst have? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.7 Asymmetric Hydrogenation
In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand A chiral catalyst can produce one desired enantiomer over another. HOW? Why would someone want to synthesize one enantiomer rather than a racemic mixture? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

A chiral catalyst allows one enantiomer to be formed more frequently in the reaction mixture Some chiral catalysts give better enantioselectivity than others. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

BINAP is a chiral ligand that gives pronounced enantioselectivity For any reaction, stereoselectivity can only be achieved if at least one reagent (reactant or catalyst) is chiral Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation Halogenation involves adding two halogen atoms across a C=C double bond Halogenation is a key step in the production of PVC Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation Halogenation with Cl2 and Br2 is generally effective, but halogenation with I2 is too slow and halogenation with F2 is too violent Halogenation occurs with anti addition Given the stereospecificity, is it likely to be a concerted or a multi-step process? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation Let’s look at the reactivity of Br2. Cl2 is similar
It is nonpolar, but it is polarizable. WHY? What type of attraction exists between the Nuc:1- and Br2? Does the Br2 molecule have a good leaving group attached to it? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation We know alkenes can act as nucleophiles
Imagine an alkene attacking Br2. You might imagine the formation of a carbocation However, this mechanism DOES NOT match the stereospecificity of the reaction. HOW? WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation Mechanism continued on next slide

9.8 Halogenation Only anti addition is observed. WHY?
Prove to yourself that the products are enantiomers rather than identical Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation Only anti addition is observed
Can you design a synthesis for ? Practice with conceptual checkpoint 9.26 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halogenation Predict the major product(s) for the reactions below

9.8 Halohydrin Formation Halohydrins are formed when halogens (Cl2 or Br2) are added to an alkene with WATER as the solvent The bromonium ion forms from Br2 + alkene, and then it is attacked by water Why is the bromonium attacked by water rather than a Br1- ion? Is water a better nucleophile? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halohydrin Formation A proton transfer completes the mechanism producing a neutral halohydrin product The net reaction is the addition of –X and –OH across a C=C double bond Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.8 Halohydrin Regioselectivity
The –OH group adds to the more substituted carbon The key step that determines regioselectivity is the attack of water on the bromonium ion Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

When water attacks the bromonium ion, it will attack the side that goes through the lower energy transition state Water is a small molecule that can easily access the more sterically hindered site Practice with SkillBuilder 9.6 Transition state Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.9 Anti Dihydroxylation Dihydroxylation occurs when two –OH groups are added across a C=C double bond Anti dihydroxylation is achieved through a multi-step process Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.9 Anti Dihydroxylation First, an epoxide is formed
Replacing the relatively unstable O-O single bond is the thermodynamic driving force for this process Is there anything unstable about an epoxide? Is an epoxide likely to react as a nucleophile (Lewis base) or as an electrophile (Lewis acid)? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.9 Anti Dihydroxylation Note the similarities between three key intermediates Ring strain and a +1 formal charge makes these structures GREAT electrophiles They also each yield anti products, because the nucleophile must attack from the side opposite the leaving group Practice with SkillBuilder 9.7 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.10 Syn Dihydroxylation Like other syn additions, syn dihydroxylation adds across the C=C double bond in ONE step Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.10 Syn Dihydroxylation Because OsO4 is expensive and toxic, conditions have been developed where the OsO4 is regenerated after reacting, so only catalytic amounts are needed Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.10 Syn Dihydroxylation MnO41- is similar to OsO4 but more reactive
Syn dihydroxylation can be achieved with KMnO4 but only under mild conditions (cold temperatures) Diols are often further oxidized by MnO41-, and MnO41- is reactive toward many other functional groups as well The synthetic utility of MnO41- is limited Practice with conceptual checkpoint 9.33 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.11 Oxidative Cleavage with O3
C=C double bonds are also reactive toward oxidative cleavage Ozonolysis is one such process Ozone exists as a resonance hybrid of two contributors Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.12 Predicting Addition Products
Analyze the reagents used to determine what groups will be added across the C=C double bond Determine the regioselectivity (Markovnikov or anti-Markovnikov) Determine the stereospecificity (syn or anti addition) Each step can be achieved with minor reagent memorization and a firm grasp of the mechanistic rational The more familiar you are with the mechanisms, the easier predicting products will be Practice with SkillBuilder 9.9 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 One Step Syntheses To set up a synthesis, assess the reactants and products to see what changes need to be made Label each of the processes below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 One Step Syntheses To set up a synthesis, assess the reactants and products to see what changes need to be made Give reagents and conditions for the following Practice with SkillBuilder 9.10 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses Multistep syntheses are more challenging, but the same strategy applies This is not a simple substitution, addition or elimination, so two processes must be combined Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses For the strategy to work, the regioselectivty must be correct A smaller base should be used to produce the more stable Zaitsev product Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses For the strategy to work, the regioselectivty must be correct Will the regioselectivity for the HBr reaction give the desired product? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses Multistep syntheses are more challenging, but the same strategy applies This is not a simple substitution, addition or elimination, so two processes must be combined Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses How can the alcohol be eliminated to give the less stable Hoffmann product? H3O+ will give the Zaitsev product OH- is too poor of a leaving group to use the bulky base, t-BuOK The OH must first be converted to a better leaving group, and then t-BuOK can be used Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses In the last step, –H and –OH must be added across the C=C double bond Is the desired addition Markovnikov or anti-Markovnikov? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses Use reagents that give anti-Markovnikov products Is stereochemistry an issue in this specific reaction? Practice with SkillBuilder 9.11 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

9.13 Multi-Step Syntheses Solve the multistep syntheses below
Again, two processes must be combined What reagents should be used? Practice with SkillBuilder 9.12 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Additional Practice Problems
If you want to favor addition rather than elimination, do you generally want a high or low temperature, and why? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Predict the major product for the addition reaction below. Be aware of possible rearrangements and stereochemical concerns. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

How and why will the concentration of acid affect whether an acid catalyzed hydration will favor products or reactants at equilibrium? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Give an example reaction for Markovnikov hydration without the possibility of rearrangement. Give an example reaction for syn antiMarkovnikov hydration. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

Should a halogenation reaction be overall first or second order kinetics? Also, Explain why it gives anti addition rather than syn. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e

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