Alkene Addition Reactions Flashcards

Hydrohalogenation of Alkenes

Overview: Hydrohalogenation results in the addition of a hydrogen atom and a halogen (typically chlorine, bromine, or iodine) across a carbon-carbon double bond (alkene) to form an alkyl halide.
Reaction Components: - Reagents Added: $HX$ (specifically $HCl$ , $HBr$ , or $HI$ ). - Groups Added: $H$ and $X$ ( $Cl$ , $Br$ , or $I$ ).
Regioselectivity: This reaction follows Markovnikov regioselectivity. - The hydrogen ( $H$ ) is added to the less substituted side of the alkene. - The halogen ( $X$ ) is added to the more substituted side of the alkene.
Stereospecificity: There is none; the reaction is not stereospecific.
Other Characteristics: - The reaction proceeds through a carbocation intermediate. - Because of the carbocation intermediate, rearrangements are possible if a more stable carbocation can be formed (e.g., through hydride or methyl shifts).
Mechanism: - Step 1 (Rate Determining Step/Slow Step): The pi electrons of the alkene act as a nucleophile and attack the hydrogen of the $HX$ molecule. This results in the formation of a carbocation intermediate and a halide ion ( $X^-$ ). The carbocation is a high-energy intermediate, defining this as the RDS. - Step 2: If no favorable rearrangement occurs, the halide ion ( $X^-$ ) performs a nucleophilic attack on the carbocation, forming the final alkyl halide. - Geometry and Stereochemistry: The carbocation intermediate has a trigonal planar geometry. The halide ion can attack from either side of this plane. If the attack occurs at a chiral center, both the $R$ and $S$ stereoisomers are produced, resulting in a racemic mixture.

Acid-Catalyzed Hydration of Alkenes

Overview: This reaction transforms an alkene into an alcohol through the addition of water in the presence of an acid catalyst.
Reaction Components: - Reagents Added: $H_2SO_4$ and $H_2O$ (often represented collectively as $H_3O^+$ ). - Groups Added: $H$ and $OH$ .
Regioselectivity: Markovnikov addition. - The hydrogen ( $H$ ) adds to the less substituted carbon. - The hydroxyl group ( $OH$ ) adds to the more substituted carbon.
Stereospecificity: There is none.
Other Characteristics: - The intermediate is a carbocation, which makes rearrangements possible. - The acid (most commonly sulfuric acid, $H_2SO_4$ ) is used in catalytic amounts.
Mechanism: - Step 1 (Rate Determining Step/Slow Step): The pi electrons of the alkene attack a hydrogen from the hydronium ion ( $H_3O^+$ ), resulting in the formation of a carbocation. - Step 2: In the absence of rearrangements, a water molecule performs a nucleophilic attack on the carbocation to form an oxonium ion intermediate. - Step 3: A second water molecule deprotonates the oxonium ion, yielding the final alcohol product and regenerating the acid catalyst.

Oxymercuration-Demercuration

Overview: This is an alternative pathway to create an alcohol from an alkene. It is also referred to as oxymercuration-reduction.
Reaction Components: - Step 1 Reagents: $Hg(OAc)_2$ , $H_2O$ (Mercuric acetate and water). - Step 2 Reagents: $NaBH_4$ (Sodium borohydride).
Regioselectivity: Markovnikov addition. - $H$ adds to the less substituted carbon; $OH$ adds to the more substituted carbon.
Stereospecificity: Anti addition.
Other Characteristics: - The intermediate is a three-membered ring called a mercurinium ion. - Unlike acid-catalyzed hydration, no rearrangements occur because a free carbocation is not formed.
Mechanism: - Step 1 (Oxymercuration - RDS): Mercuric acetate ( $Hg(OAc)_2$ ) ionizes. The pi electrons of the alkene attack the mercury cation, which simultaneously attacks one of the alkene carbons back, forming the three-membered mercurinium ion ring. This is the slow step. - Step 2 (Ring-Opening): Water performs a back-side attack (similar to an $S_N2$ mechanism) on the more substituted carbon of the mercurinium ion. The bond between mercury and that carbon breaks, opening the ring. The back-side attack leads to inversion of configuration, explaining the anti stereospecificity. - Step 3 (Proton Transfer): Either acetate ( $OAc^-$ ) or water deprotonates the resulting oxonium ion to produce the alcohol-mercury species. - Step 4 (Demercuration/Reduction): Treatment with $NaBH_4$ replaces the mercury group with a hydrogen atom. While the exact consensus for this mechanism is not fully known in undergraduate curricula, it completes the formation of the alcohol.

Hydroboration-Oxidation

Overview: This reaction converts an alkene into an alcohol with anti-Markovnikov regioselectivity.
Reaction Components: - Step 1 Reagents: $BH_3 \cdot THF$ (Borane complexed with tetrahydrofuran) or $B_2H_6$ (Diborane). - Step 2 Reagents: $H_2O_2$ (Hydrogen peroxide) and $NaOH$ (Sodium hydroxide) in water.
Regioselectivity: Anti-Markovnikov. - The hydroxyl group ( $OH$ ) adds to the less substituted side. - The hydrogen ( $H$ ) adds to the more substituted side.
Stereospecificity: Syn addition (both groups add to the same face of the alkene).
Other Characteristics: - No carbocation intermediate; therefore, no rearrangements occur. - The transition state involves a concerted addition.
Mechanism (Step 1: Hydroboration): - Boron is electron-deficient (lacks an octet) and acts as a reactive electrophile. - The pi electrons attack the boron, while one $B-H$ bond simultaneously breaks to attach the hydrogen to the more substituted carbon. - This step repeats until a trialkylborane intermediate is formed. - Because the boron and hydrogen originate from the same molecule and attach simultaneously, they must add to the same face (Syn).
Mechanism (Step 2: Oxidation): - Step 1: Hydroxide ( $OH^-$ ) deprotonates $H_2O_2$ to form a hydroperoxide ion ( $HO_2^-$ ). - Step 2: The hydroperoxide ion attacks the boron atom. - Step 3: An anionic rearrangement occurs where an alkyl group migrates from boron to oxygen, displacing a hydroxide ion. - Steps 1-3 repeat twice more for the other alkyl groups. - Step 4: Hydroxide attacks the boron of the resulting borate ester. - Step 5: The bond between boron and oxygen breaks, with an alkoxide ion ( $RO^-$ ) acting as the leaving group. - Step 6: The alkoxide is protonated by water to yield the alcohol. These steps repeat to yield three equivalents of alcohol per trialkylborane.

Catalytic Hydrogenation

Overview: The addition of molecular hydrogen across a double bond to form an alkane, often called catalytic reduction.
Reaction Components: - Reagents: $H_2$ and a metal catalyst ( $Pd$ , $Pt$ , or $Ni$ ).
Regioselectivity: N/A (symmetric addition of $H$ and $H$ ).
Stereospecificity: Syn addition.
Other Characteristics: The precise mechanism is not fully known but occurs on the surface of the metal catalyst, where the alkene and hydrogen are adsorbed, leading to addition on the same face.

Halogenation of Alkenes

Overview: The addition of two halogen atoms across an alkene to form a vicinal dihalide.
Reaction Components: - Reagents: $X_2$ ( $Cl_2$ or $Br_2$ ). - Solvent: Inert solvents such as $CH_2Cl_2$ (dichloromethane) or $CCl_4$ (carbon tetrachloride).
Stereospecificity: Anti addition.
Regioselectivity: N/A.
Mechanism: - Step 1 (RDS): The pi electrons attack a halogen atom, which simultaneously attacks back to form a three-membered halonium ion (specifically a bromonium or chloronium ion). The bond between the halogens breaks, releasing a halide ion ( $X^-$ ). - Step 2: The halide ion performs a back-side attack on one of the carbons of the ring (the more substituted one if unequal, though the ring opens in both cases), leading to the anti-dihalide product. No rearrangements are possible.

Halohydrin Formation

Overview: The formation of a compound containing both a halogen and a hydroxyl group on adjacent carbons.
Reaction Components: - Reagents: $X_2$ ( $Cl_2$ or $Br_2$ ) and $H_2O$ .
Regioselectivity: Markovnikov (in this context, the halogen acts as the electrophile and adds to the less substituted side; the $OH$ group adds to the more substituted side).
Stereospecificity: Anti addition.
Mechanism: - Step 1: Formation of a halonium ion intermediate (same as halogenation). - Step 2: Because water is the solvent and present in much higher concentration than the halide ion, water acts as the nucleophile. It performs a back-side attack on the more substituted carbon of the halonium ion. - Step 3: A second water molecule deprotonates the resulting oxonium ion to yield the final halohydrin.
Alternative: If an alcohol ( $ROH$ ) is used as the solvent instead of water, the product is a halo-ether (addition of $X$ and an alkoxy group $OR$ ).

Epoxidation and Dihydroxylation

Epoxidation: - Reagents: Peroxyacid ( $RCO_3H$ ), most commonly MCPBA. - Stereospecificity: Syn addition. - Product: An epoxide (three-membered ether ring) and a carboxylic acid.
Anti Dihydroxylation: - Reagents: 1. $RCO_3H$ (MCPBA); 2. $H_3O^+$ (acid-catalyzed ring opening). - Intermediate: Epoxide. - Stereospecificity: Anti (resulting in trans-diols on rings). - Logic: The first step creates a syn-epoxide; the second step involves a back-side attack by water on the protonated epoxide, inverting one configuration and resulting in anti-addition.
Syn Dihydroxylation: - Reagents Option A: 1. $OsO_4$ (Osmium tetroxide); 2. $NaHSO_3$ or $Na_2SO_3$ . - Reagents Option B: 1. $KMnO_4$ (cold, dilute); 2. $NaOH$ . - Intermediate: Cyclic osmate or manganate ester. - Stereospecificity: Syn addition (cis-diols).

Oxidative Cleavage

Ozonolysis: - Reagents: 1. $O_3$ ; 2. $DMS$ (Dimethyl sulfide) or $(CH_3)_2S$ . - Effect: Cleaves the $C=C$ bond completely, forming two carbonyl groups (aldehydes or ketones).
Permanganate Cleavage: - Reagents: $KMnO_4$ (warm/concentrated). - Effect: Cleaves the bond; aldehydes are further oxidized to carboxylic acids, while terminal alkenes produce $CO_2$ .