Addition Reactions and Alkenes
introduction to Addition Reactions
- can prepare alkenes using elimination reactions - addition reactions:common reactions of alkenes and are characterized by addition of two groups across a double bond (pi bond is broken) - hydrohalogenation: addition of H and a halogen which includes CL, Br, and I - hydration: addition of H and OH - hydrogenation: addition of H and H - Halogenation: addition of X and X (Cl or Br) -Halohydrin formation: addition of OH and X(Cl, Br, or I) - dihyroxylation: addition of OH and OH - versatility of alkenes: attributed to reactivity of pi bonds which can function as weak bases or weak nucleophiles - pi bonds: easily protonate and an attack electrophilic centers
hydrohalogenation with Carbocation Rearrangements
- carbocations; ability to rearrange via methyl shift or hydride shift - since HX addition involves formation of carbocation intermediate: addition is subject to carbocation rearrangements - when carbocation rearrangements occur: will result in mixture of products -when carbocation rearrangements occur: they do occur - therefore: HX addition is only synthetically useful in situations where carbocation rearrangements are not possible
mechanism for hydrohalogenation
- first: proton transfer in which alkene is protonated and forms a carbocation intermediate and bromide ion - second; nucleophilic attack: bromide ion functions as nucleophile and attacks carbocation intermediate - regioselectivity for process: attributed to first step of mechanism which is the rate-determining step because it exhibits a higher transition state energy - protonation: occur with two regiochemical outcomes: form less subsituted, secondary carbocation or more substituted tertiary carbocation - tertiary carbocations: more stable than secondary due to hyper conjugation - tertiary carbocation's energy diagram is lower in energy than that of the secondary carbocation - Hammond postulate: transition states have significant carbocationic character - transition states for formation of tertiary carbocation will be signficantly lower in energy than transition state for formation of secondary carbocation - reaction: more favorable and will proceed faster with more stable carbocation intermediate - regioselectivity of ionic addition reaction: determined by preference for reaction to proceed through more stable carbocation intermediate
Addition vs Elimination: A Thermo Perspective
- many cases: addition is reverse of elimination - both: exhibit temp dependence - addition: low temp - elimination: high temp - reasoning: determined by delta G as sign and magnitude of delta H can be mainly attributed to bond strength - usually: one pi and one sigma are broken while two sigma are formed (exothermic) - entropy term: always positive for addition because two molecules join together to form one product (entropy lowered/negative) -since delta H is negative and delta S is positive, enthalpy and entropy are competing terms and reaction is only favored at low temps - reverse process: favored at high temperatures (delta S positive and delta H positive) for elimination - addition reactions: performed at low temps
stereochemistry of hydrohalgenation
- many cases: hydrohalogenation involves formation of chirality center - two possible products: R and S enantiomers - stereochemical outcome for reaction: explained by proposed mechanism which identifies intermediate as a carbocation - since carbocation is trigonal planar with empty p orbital orthogonal to plane and p orbital is subject to attack by nucleophile: both faces of plane are attacked with equal likelihood (will produce racemic mixture)
Hydrohalogenation
- treatment of alkenes with HX (X=Cl,Br, or I) results in addition reaction called hydrohalogenation in which H and X are added across the pi bond - when alkene is symmetrical: don't need to worry about placement - cases where alkene is unsymmetrical placement of H and X must be considered - above: issue of regiochemistry - 1869 Valdimir Markovnikov (Russian chemist): investigated addition of HBr across different alkenes -noticed that H is generally placed at vinyl position already bearing larger number of hydrogen atoms - described regiochemical preference in terms of where hydrogen is ultimately positioned/where halogen is ultimately positioned - halogen: generally placed at more subsituted position -vinylic position bearing more alkyl groups: more substituted where halogen is ultimately placed - above reaction: Markovnikov addition for addition reactions involving HCl and HI -reactions that proceed with chemical preference are said to be regioselective -anti-Markovnikov addition: when regioselectivity is opposite of what is expected (addition of Br at less substituted vinylic position) - purity of reagents: critical factor -Markovnikov addition: purified reagents -anti-Markovnikov: impure reagents - identity can affect regioselectivity of reaction - peroxides: cause HBr to add across an alkene in anti-Markovnikov addition - can propose mechanism for Maokovnikov addition that involves ionic intermediates - anti-Markovnikov: proceed with entirely different mechanism in which radical intermediates are used - radical pathway: efficient for addition of HBr but not for HCl or HI - regiochemical outcome of HBr addition can be controlled by choosing whether or not to use peroxides
