Addition eactions of arbon-arbon Pi Bonds - Part 1 3 δ+ 2 δ 3 3 3 + 2 3 2 3 What Is an Addition eaction? Addition reaction: Atoms or groups are added to opposite ends of a pi bond. X Y Why should I study this topic? Addition reactions common in organic, industrial, and biological chemistry. Examples: O O 3PO4 Industrial production of ethanol Pt Partial hydrogenation of polyunsaturated fat Addition not limited to just alkenes. Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 1
Alkene Pi Bonds Details of addition controlled by structure of functional groups Alkene: Two sp2 carbons Side view One pi bond (two lobes) Top view End view Explore alkene pi bond shape with molecular models ow does an alkene appear to the outside world? Pi electron cloud ow does an alkene react? Pi electrons leophilic eacts with electrophiles Alkyne Pi Bonds Details of addition controlled by structure of functional groups Alkyne: Two sp carbons pz py Two perpendicular pi bonds Side view Top view End view Explore alkyne pi bond shape with molecular models ow does an alkyne appear to the outside world? Pi electron cloud ow does an alkyne react? Similar to alkene... Pi electrons leophilic eacts with electrophiles Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 2
Generic Pi Bond Addition Mechanism has +,, or open octet rds? Lost: Pi bond Gained: - bond arbocation fates! apture a nucleophile addition rds? Lost: No bonds Gained: - bond Similar process for alkynes Overall process: trophilic addition Not every pi bond addition reaction follows this mechanism arbocation aptures Not Always S N 1 An important caveat. Avoid this common misconception: arbocation captures a nucleophile is always S N 1. Generic S N 1 mechanism: LG apture a nuc leads to substitution. Generic electrophilic addition to an alkene mechanism: apture a nuc leads to addition. onclusion: Not all reactions in which a carbocation captures a nucleophile are S N 1. Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 3
ydrogen alide Addition to an Alkene δ X X = F, l,, I Overall reaction: X X Why is -X electrophilic? Mechanism: 3 2 δ igher ΔG δ 3 2 3 3-2 3 3 2 3 3 Lower ΔG 3 3 δ 2 3-2 3 3 2 3 Markovnikov's ule 3 3 δ 2 3 3 2 3 o 1 o 3 + 3 2 Major Minor Vladimir Markovnikov PhD thesis 1870 Markovnikov's ule: In the addition of a protic acid X to an alkene, the acid hydrogen () becomes attached to the carbon with fewer alkyl substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents. "Them that has, gets" "The (hydrogen) rich get (hydrogen) richer" Orientation due to stability of carbocation intermediate Similar for F, l,, I, -O 2+, etc. Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 4
ydrogen alide Addition to an Alkyne Observation: 3 3 δ 2 3 3 2 Alkene has one pi bond Alkene adds one molecule Questions: Alkyne has two pi bonds. Does alkyne add two molecules? 3 δ??? Does alkyne + follow Markovnikov's rule? Explore via mechanism... Mechanism... ydrogen alide Addition to an Alkyne 3 δ Alkyne is nucleophile (just like alkene) Protonate pi bond to form more stable carbocation (just like alkene) 3 2 or 3 Vinyl carbocation: as positive charge on = carbon arbocation fates be deprotonated? earrange? Vinyl carbocations do not rearrange apture a nucleophile? - 3 2 Are we done yet? Product = alkene can be protonated... Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 5
ydrogen alide Addition to an Alkyne 3 2 δ Alkene is nucleophile Protonate pi bond to form more stable carbocation Which carbocation is favored? 3 2 or 3 3 arbocation fates be deprotonated? earrange? apture a nucleophile? - 3 Product is a dibromide ydrogen alide Addition to an Alkyne Overall mechanism: Addition follows Markovnikov's rule? Yes No δ 3 3 2 δ 3 3 3 3 2 Alkyne has two pi bonds Markovnikov addition of two molecules Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 6
Addition of Water to a Pi Bond (ydration) 2 3 δ Observation: 2 Why? 3 3 δ - 3 Question: Do other δ - X molecules add to carbon-carbon pi bonds? Example: δ O 3 δ 2 O 3 O 2 3 3 Prediction: Markovnikov addition of water But... 2 O (pk a 15.7) is not sufficiently acidic to protonate pi bond Strong acid + 2 O 3 O + (pk a -1.8) Strong acid = 2 SO 4, I,, l, 3 PO 4 Addition of Water to a Pi Bond (ydration) Does alkene + 3 O + mechanism parallel alkene + mechanism? 3 O 2 3 2 Protonate pi bond; form more stable carbocation 3 3 3 O 2 or 3 3 2 arbocation fates: Be deprotonated? earrange? apture nucleophile? Most prevelant nuc = 2 O 3 O 3 3 O 2 Oxonium ion: Oxygen has three bonds and +1 formal charge Deprotonation driven by Le hatelier's principle Use strongest base present = 2 O 3 O 3 3 + 3 O + The product is an alcohol Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 7
Addition of Water to a Pi Bond (ydration) Overall mechanism: 3 O 2 3 O 2 3 2 3 3 3 O 3 3 O 2 3 O + 3O + 3 3 Net reaction: Markovnikov addition of water eaction is reversible: 3 3 2 + 2 O 3 O + ydration 3 O + E1 dehydration 3 O 3 3 Equilibrium position controlled by Le hatelier's principle... Excess water favors alcohol emoving water favors alkene 3 O + is a Alkyne ydration Observation: 3 2 O 2 3 O 2 Alkene has one pi bond. Alkene adds one molecule of 2 O. 3 3 Addition obeys Markovnikov's rule. Questions: Alkyne has two pi bonds. Does alkyne add 2 O? Does this addition follow Markovnikov's rule? Does alkyne add two molecules 2 O? O Ph O 2 O 2 Ph 2 O 2 O?? Explore via mechanism... Ph O O Addition eactions of arbon-arbon Pi Bonds Lecture Supplement Part 1 -- Page 8