Reactions of Alkenes | Organic Chemistry 2
Reactivity of alkenes is 'all about the π bond'
The π bond of alkenes is electron-rich ⇒ it behaves as a nucleophile and will attack an electrophile
Typical reactions of alkenes are addition reactions
Syn and Anti additions:
- Syn addition: both X and Y are added from the same side
- Anti addition: X and Y are added frpm opposite sides
Addition of H2:
A catalyst (Pd or Pt) is needed to lower the activation energy of the reaction.
Syn addition: the 2 hydrogens of H2 are added on the same side (the least hindered) ⇒ hydrogenation of alkenes is stereospecific.
Hydrohalogenation of Alkenes
Addition of Hydrogen Halides (HCl, HBr, HI):
The electrophilic addition of hydrogen halides follows the Markovnikov rule: formation of the most stable carbocation (most substituted carbocation).
Hydration of Alkenes
Addition of H2O:
Acid catalyzed addition (H2SO4) that follows the Markovnikov rule: the hydrogen atom from the acid (H2SO4) attaches to the less substituted carbon of the alkene, while the hydroxyl group (OH) attaches to the more substituted carbon. This addition is reversible.
Carbocation rearrangements can occur.
Halogenation of Alkenes
Addition of dihalogen X2 (X = Cl, Br, I):
Halogenation of alkenes is a stereospecific reaction that occurs in two steps:
- Addition of the electrophile (X+) to the π bond, forming an unstable bridged halonium ion.
- Nucleophilic attack of X- occuring from the back side to form a trans product.
The overall result is an anti addition of X2 to the alkene.
Bromonium ion can also be trapped by other nucleophiles (e.g. H2O)
Hydroboration - Oxidation
Hydroboration (addition of BH3):
- Addition of BH3: one-step reaction during which the π bond and a H-BH2 bond break as the C-H and C-B bonds form. Regioselectivity: boron atom binds to the less substituted carbon.
- Oxidation: the C-B bond is replaced by a C-O bond to form an OH group while maintaining the configuration (retention of configuration).
The overall result of this two-step sequence is the formation of Anti-Markovnikov alcohols (least substituted alcohol as the major product).
syn stereospecificity ⇒ trans alkene gives trans epoxide
Addition of two OH groups across an alkene. Some reagents provide anti dihydroxylation, while others provide syn dihydroxylation.
- Formation of an epoxide.
- Acid-catalyzed opening of the epoxide.
Addition of two hydroxyl groups to an alkene, occurring simultaneously and resulting in the placement of the hydroxyl groups on the same side of the double bond. 2 reagents are typically used:
- Osmium tetroxide OsO4:
- Potassium permanganate KMnO4:
Oxidative Cleavage: Ozonolysis
Oxidative cleavage of an alkene using ozone (O3), resulting in the formation of 2 carbonyl compounds. Ozonolysis proceeds in 2 steps:
- Formation of an ozonide in the presence of O3.
- Reduction of the ozonide with a mild reducing agent such as dimethyl sulfide (Me2S) or Zn/H2O.
Addition of HBr:
Anti-Markovnikov addition of HBr through a radical process
- Initiation steps:
- Propagation steps:
- Alkenes are reactive primarily due to the presence of the high electron density in the π bond, which is subject to attack by electrophiles (electrophilic addition).
- The steric hindrance and electronic effects of substituents on the alkene can affect its reactivity; for example, electron-donating groups can increase reactivity by making the double bond more electron-rich, while electron-withdrawing groups can decrease reactivity.
- The position of the double bond within the carbon chain can also play a role, with terminal alkenes generally being more reactive than internal ones.
The mechanism of hydrogenation involves the addition of hydrogen across the double bond of an alkene. This typically occurs via a syn-addition mechanism, resulting in the formation of a saturated alkane.
Catalysts, commonly metal catalysts like platinum or palladium, play a significant role in alkene hydrogenation by providing an alternate reaction pathway with lower activation energy, thus facilitating the reaction under milder conditions.
The electrophilic addition of hydrogen halides (HX) to alkenes involves two key steps:
- The π bond reacts with the electrophilic hydrogen atom of HX to form a carbocation on the more substituted carbon (following Markovnikov's rule).
- The halide ion (X-) acts as a nucleophile, attacking the positively charged carbon to form the final alkyl halide product.
This process exemplifies Markovnikov addition, where the more electronegative atom (halogen) attaches to the more substituted carbon.
Markovnikov's rule predicts the regiochemistry of electrophilic addition to alkenes. In the addition of a protic acid (HX) to an asymmetric alkene, the hydrogen (H) attaches to the less substituted carbon, and the halide (X) attaches to the more substituted carbon. This is determined by the stability of the intermediate carbocation, with a more substituted carbocation being more stable due to hyperconjugation and inductive effects.
Electrophilic hydration of alkenes involves the addition of water (H2O) to the double bond to form alcohols. The process typically requires an acid catalyst, such as sulfuric acid (H2SO4), to initiate the reaction. Key steps are:
- Protonation of the alkene to form the more stable carbocation intermediate (the more substituted carbocation).
- Nucleophilic attack of water on the carbocation.
- Deprotonation to give the alcohol product.
Acid-catalyzed hydration proceeds through a Markovnikov addition.
The halogenation of alkenes involves an electrophilic addition mechanism. The π electrons of the alkene act as nucleophilesand attack a halogen molecule (X2), forming a cyclic intermediate. The halogen (X-) then attacks the positively charged carbon, resulting in the addition of the halogen to the alkene.
Anti-addition in halogenation means that the halogen atoms are added to opposite sides of the carbon-carbon double bond. This is a stereospecific reaction, and the term anti refers to the opposite orientation of the halogen atoms.
Hydroboration-oxidation is a two-step reaction process where an alkene is converted into an alcohol:
- First step = hydroboration: a borane (BH3 or a BHR2) adds to the double bond of an alkene in a syn-addition manner. This initial addition is regioselective for the less substituted carbon due to steric and electronic reasons.
- Second step = oxidation: the boron atom is replaced by a hydroxyl group using hydrogen peroxide (H2O2) in a basic aqueous solution.
The overall result is the formation of an alcohol with anti-Markovnikov regiochemistry, i.e., the hydroxyl group attaches to the less substituted carbon atom, which is the opposite outcome of what is typically seen in acid-catalyzed hydrations of alkenes.
An epoxide is a cyclic ether consisting of a three-membered ring with one oxygen and two carbon atoms. It is characterized by its high reactivity due to the strain of the three-membered ring.
Alkenes can be transformed into epoxides through a reaction with a peroxy acid, such as m-chloroperoxybenzoic acid (mCPBA), in a process known as epoxidation. The oxygen from the peroxyacid is inserted into the carbon-carbon double bond of the alkene, resulting in the formation of the epoxide.
Anti dihydroxylation refers to the addition of two hydroxyl groups to an alkene in a way that they end up on opposite sides of the double bond. This is in contrast to syn dihydroxylation, where the hydroxyl groups add to the same side.
Vicinal syn dihydroxylation of alkenes is achieved by treating alkenes with potassium permanganate (KMnO₄) in an alkaline medium or by using osmium tetroxide (OsO₄) as a reagent, followed by a reducing agent such as H2S or NaHSO₃. Both methods add a hydroxyl group (OH) to each carbon of the double bond, yielding a glycol in which the two OH groups are added to the same side (syn addition) of the former alkene.
Osmium tetroxide (OsO4) acts as an oxidizing agent in the dihydroxylation of alkenes, a reaction that adds two hydroxyl groups (OH) across the double bond of an alkene to form a vicinal diol. The reaction with OsO4 is stereospecific, resulting in the syn addition of OH groups.
Alkenes can be converted into carbonyl compoundsby a process known as ozonolysis. This reaction involves the cleavage of the carbon-carbon double bond using ozone (O3), followed by a reductive workup, typically using a reducing agent like zinc in acetic acid or dimethyl sulfide. The result is the formation of aldehydes or ketones, depending on the substitution pattern of the original alkene.
Ozonolysis is a reaction in which an alkene reacts with ozone (O3) to form an ozonide intermediate, which then decomposes to form carbonyles. The reaction involves the addition of ozone across the double bond of the alkene, resulting in the cleavage of the carbon-carbon double bond. For symmetrical alkenes, two identical carbonyl compounds are formed, while for asymmetrical alkenes, a mixture of different carbonyl compounds is obtained.
Radical bromination of alkenes involves the addition of bromine radicals to the double bond. This reaction is regioselective, favoring the addition of the bromine atom to the less substituted carbon atom due to stability considerations of the resulting radical intermediates (tertiary radicals are more stable than secondary or primary radicals).