Understanding the basics Mechanisms Some Basic rgo I Reactions Mechanisms are the most mind-boggling part of organic chemistry. Students, generally speaking, have spent their time memorizing their way through science courses. Mechanisms require a student to UNDERSTAND the fundamentals of electron flow Everyone knows that electrons are negatively charged. Everyone knows that electrons are attracted to things with positive charges. Yet, the understanding of a mechanism remains elusive to many students Let s review the basics Electron flow is always from the electron-rich to the electron-poor species. The electron-rich species is a Lewis Base (must have a lone pair) and is called the nucleophile. The electron-poor species is a Lewis Acid (must have empty orbital) and is called the electrophile. Examples of Nucleophiles: S electron flow is always from nucleophile to electrophile, electronrich to electron-poor Examples of Electrophiles: - + Watch the direction of your arrows from lone pairs to carbocation 1
from anion to cation from anion to partial positive charge from alkene pi bond to cation or partial positive charge + + - When working through a mechanism, the goal is NT to memorize the steps of a mechanism of a SPECIFIC molecule. When you do that, typically you become too focused on the structures provided in a single example. If that happens, you will get confused when the next mechanism problem has a DIFFERENT structure. What you want to do is make a game plan - break down the steps of the mechanism, into little parts or steps. The basic little steps are easier to remember. By knowing the steps, you know how the mechanism progresses, regardless of the structure you are given to work with. S break them down Dehydration of Alcohols Identify this mechanism Starts with alcohol, ends with alkene losing water Dehydration, Acid-catalyzed sulfuric or phosphoric acid + + 2 + from Dehydration of Alcohols Convert to 2 (use that acid!) Loss of 2 and carbocation formation Removal of +, resulting in formation of pi bond to complete the conversion to alkene E1 mechanism think Zaitsev and Trans! 2
Dehydration of Alcohols Dehydration of Alcohols Step 1: Convert to 2 + Step 2: Loss of 2 ( spontaneous dissociation ) to form carbocation + -- Dehydration of Alcohols Dehydration of Alcohols - Again Step 3: Removal of +, resulting in formation of pi bond to complete conversion to alkene Try another example: 2 S 4 heat + 3 + + -- + -- Alcohol to alkene (using acid) = dehydration (make water, lose water, form alkene) Dehydration of Alcohols - Again Step 1: Convert to 2 + Remember the soul purpose for the acid is to turn the into a water molecule. Now it wants to leave Dehydration of Alcohols - Again Step 2: Make the leaving group leave spontaneous dissociation occurs: + 3
Dehydration of Alcohols - Again Step 3: Form the pi bond, make an alkene Dehydration of Alcohols And Again Try one more example: + 3 P 4 heat + 3 + Find the beta- on the more substituted side (Zaitsev!) use your water as a Lewis Base and pull that + off, forming a pi bond Done! Do the steps and check that your arrows and intermediates look like those you re about to see Dehydration of Alcohols And Again Step 1: Convert to 2 Dehydration of Alcohols And Again Step 2: Spontaneous Dissociation + + Always draw the arrow from electron rich (lone pairs!) to electron poor (positive charge!) Next step? Form the pi bond Dehydration of Alcohols And Again Step 3: Form the pi bond, make an alkene Find the beta- on the more substituted side that has an use your water and pull it off, forming a pi bond Done! + Acid-Catalyzed ydration Identify this mechanism Starts with alkene, ends with alcohol +, 2 4
Acid-Catalyzed ydration Reaction of pi bond with + (acid catalyst!) resulting in Markovnikov carbocation formation Addition of 2 (this is where the is coming from) Removal of extra proton ( + ) to finish formation of. Acid-Catalyzed ydration Step 1: Reaction of pi bond with + (acid cat.) resulting in Carbocation formation + Acid-Catalyzed ydration Acid-Catalyzed ydration Step 2: Addition of 2 Step 3: Removal of extra proton ( + ) to finish formation of. + -- + -- + -- Acid-Catalyzed ydration - Again Try Again Identify the mechanism alkene to alcohol, using acid and water Acid-Catalyzed ydration - Again Step 1: React the pi bond with + and form that carbocation: 2, 2 S 4 + We don t have to show this new but make sure you are drawing the Markovnikov carbocation! 5
Acid-Catalyzed ydration - Again Step 2: Add the 2 (the green is still there, just didn t show it in the second structure) Now - Finish it off pull the extra proton Acid-Catalyzed ydration - Again Step 3: Removal of extra proton ( + ) to finish formation of. + Acid-Catalyzed ydration And Again Third time s the charm Try one more example: Acid-Catalyzed ydration And Again Step 1: React that nucleophilic pi bond with the proton +: 2 2 S 4 + 3 + + Do the steps and check that your arrows and intermediates look like those you re about to see We don t have to show this new but make sure you are drawing the Markovnikov carbocation! Acid-Catalyzed ydration And Again Step 2: Add the 2 Finish it off pull the extra proton Acid-Catalyzed ydration And Again Step 3: Removal the extra proton ( + ) to finish formation of. + 6
Addition of - Identify this mechanism Starts with alkene, ends with single halide - Addition of - Reaction of pi bond with + (of -), concurrent separation of -, and formation of Markovnikov carbocation intermediate. Attack on carbocation by - to finish formation of product Addition of - Step 1: Reaction of pi bond with + (of - ), concurrent separation of -, and formation of carbocation intermediate. Addition of - Step 2: Attack of - to finish formation of product. + Addition of - - Again Try it again Identify the mechanism Adding an and an to an alkene Addition of - - Again Step 1: Reaction of pi bond with + (of - ), concurrent separation of -, and formation of carbocation intermediate. - + 7
Addition of - - Again Step 2: Attack of - to finish formation of product. Addition of nce Again Try it again Identify the mechanism Adding a chloride (and an, not drawn in) to an alkene - Addition of nce Again Step 1: React the pi bond with + (of - ), separate off the -, form the more substituted carbocation intermediate. + Addition of nce Again Step 2: Attack of - to finish formation of product. Addition of 2 Identify this mechanism Starts with alkene, ends with two halides 2 Addition of 2 Attack by pi bond on polarized - with alonium Ion formation Attack of - to pop open three-membered ring and finish formation of product. 8
Addition of 2 Step 1: Attack by pi bond on polarized - with alonium Ion formation (and loss of - ) + Addition of 2 Step 2: Attack of - to pop open threemembered ring and finish formation of product. Addition of 2 f course, you can attack the other end of the halonium and open in the other direction + Regardless, electrons must always flow towards the positive charge Addition of 2 - Again Identify this mechanism: Starts with alkene, ends with two halides Yes, this reaction occurs Anti - Addition of 2 - Again Step 1: Attack by pi bond on polarized - with alonium Ion formation - + The halonium ion has to form on one face or the other whichever you want Addition of 2 - Again Step 2: Attack of - to pop open threemembered ring and finish formation of product. + As with the last example, remember the flow of electrons is towards the positive charge 9
Addition of 2 And Again Draw the mechanism steps for the example below and then check to see if you drew the arrows flowing correctly: - Addition of 2 And Again Step 1: Attack by pi bond on - followed by alonium Ion formation - The halonium ion has to form on one face or the other random choice for up on this problem Addition of 2 And Again Step 2: Attack of - to pop open threemembered ring and finish formation of product. + Addition of 2 and 2 Identify this mechanism Starts with alkene, ends with one alcohol and one halide 2 2 + Addition of 2 and 2 Attack by pi bond on polarized - with alonium Ion formation Attack by 2 on more substituted side to pop open three-membered ring. Removal of extra proton ( + ) by - to complete the formation of. Addition of 2 and 2 Step 1: Attack by pi bond on polarized - with alonium formation + 10
Addition of 2 and 2 Step 2: Attack by 2 to pop open threemembered ring. + -- + Markovnikov addition means the water is attacking the more substituted side Addition of 2 and 2 Step 3: Removal of extra proton ( + ) by - to complete the formation of. + Again, electron flow towards the positive charge + - Addition of 2 and 2 - Again Identify the mechanism Starts with alkene, adding an and an Markovnikov 2, 2 Addition of 2 and 2 - Again Step 1: Attack by pi bond on polarized - with alonium formation - + alonium ion forms on one face of the alkene Addition of 2 and 2 - Again Step 2: Attack by 2 to pop open threemembered ring. Note the inversion on the methyl group from the backside attack of the water Addition of 2 and 2 - Again Step 3: Removal of extra proton ( + ) by - to complete the formation of. + Again, electron flow towards the positive charge + - 11
Formation of 3º halides from alcohols Identify this mechanism Starts with alcohol, ends with a halide - Formation of 3º halides: Tertiary must turn into a better leaving group by picking up a + to form water Loss of 2 and Carbocation formation Attack of halide to form tertiary halide Formation of 3º halides: Formation of 3º halides Step 1: Convert to 2 - + Step 2: Loss of 2 ( spontaneous dissociation ) to form carbocation + Good leaving group can now leave Formation of 3º halides Step 3: alide attacks the carbocation + Formation of 3º Alcohols Identify the mechanism Starts with 3º halide, ends with a 3º alcohol 2 f course, the E1 elimination happens simultaneously but we ll deal with that separately. 12
Formation of 3º alcohols: Loss of Leaving group and carbocation formation Attack by water Removal of extra proton to complete formation of group Formation of 3º alcohols: Step 1: Spontaneous dissociation of to form carbocation + Formation of 3º alcohols Step 2: Addition of 2 Formation of 3º alcohols Step 3: Loss of extra proton to finish formation of group: + + Again, flow of electrons towards positive charge, not a direct attack of it E1 elimination of 3º alkyl halide Identify the mechanism Starts with 3º halide, ends with an alkene, weak base 2 E1 Elimination of 3º halide Loss of Leaving group and carbocation formation Attack by water on beta- to form alkene appening at the same time as the S N 1 we just looked at 13
E1 Elimination of 3º halides Step 1: Spontaneous dissociation of to form carbocation E1 Elimination of 3º halides Step 2: Addition of 2 to remove beta- and form pi bond + AND REMEMBER Finally If you UNDERSTAND the basic steps to these mechanisms, it won t matter what the double bond in the molecule looks like... Every alkene reacts the same way, every time, regardless of what s attached Track the pieces you need to add or subtract overall See where you are starting and where you are ending Don t memorize a specific molecule but go ahead and memorize the sequence of steps involved get a feel for how to approach the problems develop the instinct Thanks for walking through yet another really long power point hope it helped let me know what you think if there s something else that you think should be added in, I ll be happy to try to fill in any other missing blanks Dr.Discordia jdiscord@towson.edu 14