Chapter 8: Addition Reactions of Alkenes

Introduction to Addition Reactions

• the addition of two groups across a double bond, breaking the pi bond

• specially named addition reactions indicate the types of groups that are added

• the different addition reactions serve as synthetic precursors for a variety of functional groups→ due to the reactivity of pi bonds (functioning as weak bases and nucleophiles)

8.2 Alkenes in Nature and in Industry

• alkenes are naturally abundant, in acylic, cyclic, bicyclic, and polycyclic forms

• frequently found in pheromones→ chemicals that trigger behavioral responses within species

• ethylene and propylene are the most important industrial alkenes and are formed from cracked petroleum→ the starting materials of many compounds

8.3 Nomenclature of Alkenes

• steps for naming alkanes/alkyl halides → used for alkanes as well

  • identify the parent

  • identify substituents

  • assign a locant to each substituent

  • arrange the substituents alphabetically

• additionally for alkanes:

  • same naming conventions as alkanes with -ene replacing -ane

  • name the parent with the longest carbon chain that contains the pi bond/s

  • the pi bond/s should receive the lowest possible number/s

  • position of the pi bond should be indicated by a number either before the parent name or within it just prior to -ene.

  • E/Z configuration should be designated at the beginning of the name (similar to R/S configuration)

• Some common names for simple alkenes are: ethylene, propylene, and styrene

• common names for groups appearing as substituents: methylene, vinyl, allyl

• alkenes are also classified by degree of substitution (how many carbon groups are bonded to the carbons at either end of the pi bond)→ mono-, di-, tri-, and tetrasubstituted.

8.4: Addition vs. Elimination: a Thermodynamic Perspective

• an addition is frequently a reverse of an elimination→ additions favor low temperatures, eliminations favor high temperatures

• in addition: a pi bond and a sigma bond are broken. in elimination: 2 sigma bonds are formed

• when bonds break, energy is released, when bonds formed, energy is absorbed. This explains why elimination reactions need higher energy then addition reactions.

8.4: Hydrohalogenation

• Regioselectivity of hydrohalogenation

  • hydrohalogenation is an addition reaction involving an alkene and a halogen hydride (HX)→ adds H and X (=Cl, I, Br) across the pi bond

  • Markovnikov’s Rule/Addition: H generally adds to the side with the larger number of Hs already. → hydrohalogenation follows this rule

  • in other words, the halogen adds to the more substituted position

  • However, for Br in the presence of alkyl peroxides: anti-Markovnikov’s addition→ Br adds to the less substituted side.

• A mechanism for hydrohalogenation

  • The H from HX protonates the pi bond which generates a carbocation intermediate and a halogen ion

  • the intermediate is then attacked by the halogen ion

  • the rate determining step is the protonation

  • regioselective protonation explains Markovnikov’s rule: more substituted carbocations are more stable so protonation will favor the less substituted side (the side containing more hydrogens)

• Stereochemistry of hydrohalogenation

  • in cases where a chiral center is formed (most of the time), there are two possible products→ a pair of enantiomers, generally in a racemic mixture

  • explanation: since the carbocation is trigonal planar, it leaves either side open for nucleophilic attack of the halogen ion

• Hydrohalogenation with carbocation rearrangements

  • when it is possible for a primary or secondary carbocation to change to a tertiary one through a methyl or hydride shift, it typically will do so

  • generally, a mixture of the products will form since the carbocation has to have enough time between encountering a halogen ion and undergoing the shift

  • changing the concentration of the halogen hydride can shift the concentration→ generally the product resulting from the more stable carbocation is the major product

8.6: Acid-Catalyzed Hydration

• adding elements of water (H and OH) across a double bond, facilitated by the presence of water

• Experimental observations

  • acid catalyzed hydration is a Markovnikov addition for most simple alkenes, where OH adds to the more substituted carbon

  • Possible reagents: H3O+, H2O+H2SO4 (equivalent statements). Must indicate that H2SO4 is catalytic

• Mechanism and source of regioselectivity

  • similar to hydrohalogenation: first the double bond is protonated to form a carbocation→ following Markovnikov’s addition

  • However, the nucleophile in this case (water) is neutral which produces a second charged intermediate (positive this time)

  • the second intermediate must undergo deprotonation by water

  • water acts as both solvent and nucleophile in this reaction

  • more substituted alkenes are more reactive and the reactions occur more quickly

• Controlling the position of equilibrium

  • acid-catalyzed hydration is an equilibrium reaction→ the reverse elimination reaction to produce an alkene from an alcohol→ acid-catalyzed dehydration

  • this is why diluted H2SO4 is used in addition reactions, and concentrated H2SO4 is used in elimination reactions.

• Stereochemistry of acid-catalyzed hydration

  • similar to hydrohalogenation: intermediate carbocation is attacked from either face with equal probability

  • results in a racemic mix of enantiomers.

8.8: Hydroboration-Oxidation

• An introduction to hydroboration-oxidation

  • a method for anti-Markovnikov’s addition of water (alcohol synthesis)→ OH adds to the less substituted position

  • a syn addition: H and OH add to the same face of the pi bond→ only two of the possible stereoisomers are formed

• Reagents for hydroboration-oxidation

  • BH3 is very reactive→ one of the reactants in hydroboration-oxidation addition

• A mechanism for hydroboration-oxydation

  • the first step is where the pi bond attacks borane

  • at the same time there is a hydride shift

  • Both C-BH2 and C-H bonds form at the same time which explains the regioselectivity and stereospecificity

• Regioselectivity of hydroboration-oxidation

  • BH2 is added to the less substituted position and is replaced by an OH group

  • electronic considerations: the first step causes a partial positive charge on the opposite carbon which triggers the hydride shift from B to C which is why BH2 must be added to the less substituted group

  • steric considerations: since BH2 is bigger than H, and they are added simultaneously, BH2 must be added to the position with less steric hindrance

• Stereospecificity of hydroboration-oxidation

  • since the H and BH2 that are added in the same step are initially connected, they must add to the same face→ syn addition

  • stereochemistry is only a relevant consideration if chiral centers are formed

  • typically enantiomers form a racemic mixture, but in enantioselective additive reactions, one might be the major product over the other

8.10: Halogenation and Halohydrin Formation

• Experimental observations

  • halogenation→ addition of Br2 or Cl2 across an alkene (F is too reactive and I isn’t reactive enough)

  • anti addition meaning the halogen ions add on opposite faces of the pi bond

• A mechanism for halogenation

  • X2 induces a temporary dipole despite being overall nonpolar→ allows one atom to act as an electrophile while the pi bond attacks as a nucleophile

  • the halogen ion forms a bridge intermediate called bromonium or chloronium

  • the remaining halogen attacks from the reverse side as the bridged halogen leading to the observed anti-addition

  • configuration of the starting alkene determines configuration for the product of the halogenation (trans→ meso, cis→ enantiomers)

• Halohydrin formation

  • addition of X2 in the presence of a solvent like CHCl3 results in the above halogenation

  • using water as a solvent: addition of the halogen and an OH across the bond (a bromohydrin or chlorohydrin)

• Regiochemistry of halohydrin formation

  • halohydrin is regioselective where the OH is added to the more substituted position

  • the transition state is slightly carbocationic, which is more stable on the more substituted carbon and also the site where water binds

8.13: Oxidative Cleavage

• Ozonolysis: breaks the C-C completely and adds a double bonded O at the cleaved ends

• done using O3 and a reducing agent (DMS or Zn/H2O)

8.14: Predicting the Products of an Addition Reaction

• factors to consider:

  • identities of groups being added across the double bond

  • Markovnikov vs anti-Markovnikov addition (regioselectivity)

  • syn vs anti addition (stereospecificity)

• understanding reactions vs just memorizing them

8.15: Synthesis Strategies

• One-step syntheses

  • Substitution reactions: convert one group to another (i.e. halogen to an alcohol)

  • elimination reactions: convert alkyl halides into alkenes

  • addition reactions: adding to groups across a double bond

• Changing the position of a halogen or OH group

  • typically achieved through combination of one step reactions (i.e. moving the position of a bromo substituent is to eliminate the original, then add a new Br across the pi bond)

  • retrosynthetic analysis: given a start and target molecule and determining the intermediate/s that will result in the synthesis

• Changing the position of a pi bond

  • another retrosynthetic strategy

  • add and then eliminate

Additional 13.8: Preparation of Epoxides

• Preparation with peroxy acids

  • common peroxy acids are MCPBA and peroxyacetic acid

  • stereospecific reaction that adds substituents cis to each other

  • Addition reaction to form an epoxide

• Preparation from halohydrins

  • halohydrins can be converted to an epoxide with a strong base like NaOH

  • an intramolecular Williamson ether synthesis

  • same stereochemical outcome as direct synthesis from a peroxy acid