Organic Chemistry: Electrophilic Additions and Hammond's Postulate Notes
Learning Goals: September 26, 2025
Explain why alkenes react with strong acids to produce alkyl halides.
Explain the regiochemical outcome of the reaction of asymmetric alkenes with strong acids.
Problem Set 4 Prioritization:
Prioritize the following questions for Problem Set 4:
III
VI
VII
IX
X
XIII
XIV
XV
Hammond's Postulate
Hammond's Postulate states that the structure and energy of the Transition State (TS) can be approximated by the structure and energy of the nearby high-energy intermediate.
Application of Hammond's Postulate to Potential Energy Surfaces:
Question 1: Which potential energy surface corresponds to the formation of the secondary carbocation?
Answer: A. Potential energy surface A shows a lower energy transition state (\ddagger) and a lower energy intermediate (B compared to A) pathway. The formation of a more stable carbocation (like a secondary one) typically has a lower activation energy and a more stable intermediate, making pathway A more corresponding.
Question 2: Which potential energy surface corresponds to the slowest carbocation formation?
Answer: B. The slowest carbocation formation corresponds to the reaction pathway with the highest activation energy (the highest energy transition state, denoted by \ddagger). Potential energy surface B shows a higher energy transition state compared to A.
Question 3: Which potential energy surface corresponds to the reaction path that produces the major product?
Answer: A. The major product is typically formed via the reaction pathway with the lowest activation energy and the most stable intermediate. This corresponds to the faster reaction rate. Potential energy surface A represents a lower energy transition state and a more stable intermediate, thus leading to the major product.
Electrophilic Addition to Alkynes
Inference from Major Product Formation:
When considering the formation of a major product from the electrophilic addition to an alkyne, the following inference can be made:
B. The major product may have formed from the more stable of the two possible carbocations generated by protonating the alkyne. This suggests that alkynes, like alkenes, follow Markovnikov's rule, where the proton adds to the carbon that results in the more stable carbocation intermediate (e.g., secondary over primary).
Electrophilic Addition with H-Br (1 equivalent):
Reaction: Alkyne reacts with H-Br (1 equiv.) in CH2Cl2.
Mechanism:
Protonation of the alkyne by H-Br leads to a vinyl carbocation. Markonikov's rule dictates the hydrogen adds to the less substituted carbon, placing the positive charge on the more substituted carbon, forming the more stable vinyl carbocation.
The bromide ion (Br^-) then attacks the carbocation.
Product: A vinyl halide is formed.
Example: R-C ext{≡}C-H
ightarrow R-C(Br)=CH_2
Electrophilic Addition with H-Br (excess):
Reaction: Alkyne reacts with H-Br (excess) in CH2Cl2.
Predict the Major Product:
Answer: C. When an alkyne reacts with excess H-Br, two equivalents of H-Br can add across the triple bond. The first addition follows Markovnikov's rule, forming a geminal vinyl halide. The second addition also follows Markovnikov's rule, leading to the formation of a geminal dihalide, where both bromine atoms are attached to the same carbon atom.
Example: R-C ext{≡}C-H ext{ + excess } H-Br
ightarrow R-C(Br2)-CH3
Hydration Reaction (Addition of H_2O) (Chapter 4.10)
Reaction with dilute H2SO4:
Reaction: Alkynes undergo hydration (addition of H2O) in the presence of dilute sulfuric acid (H2SO_4).
Mechanism:
Similar to electrophilic addition, the alkyne is protonated. For terminal alkynes, Markovnikov's rule applies, placing the H on the less substituted carbon and the resulting positive charge on the more substituted carbon.
Water (H_2O) acts as a nucleophile, attacking the carbocation.
Deprotonation of the oxygen yields an enol (a molecule with a hydroxyl group attached to a carbon of a carbon-carbon double bond).
The enol then rapidly tautomerizes to its more stable keto form (a ketone or aldehyde).
Predict the Major Product for Alkyne Hydration:
Reaction: Alkyne reacts with dilute H2SO4
Predict the Major Product:
Answer: B. For a terminal alkyne subjected to acid-catalyzed hydration, the reaction proceeds via a Markovnikov addition of water. The hydroxyl group (-OH) initially attaches to the more substituted carbon, forming an enol. This enol immediately tautomerizes to the more stable ketone. Therefore, the major product is a ketone, with the carbonyl group at the more substituted position.
Example: For R-C ext{≡}C-H, the major product will be a methyl ketone, R-C(=O)-CH_3.
General Electrophilic Addition Reactions
Mechanism:
Cation Formation (Carbocations): The alkene or alkyne acts as a nucleophile, attacking an electrophile (often a proton from a strong acid) to form a carbocation intermediate.
Carbocation Rearrangement?: If a more stable carbocation can be formed (e.g., secondary to tertiary, or primary to secondary), 1,2-hydride or 1,2-methyl shifts can occur. This is a crucial step that can lead to unexpected products.
Nucleophile Attaches?: A nucleophile (e.g., a halide ion, water) then attacks the carbocation (or rearranged carbocation) to form the final product.
Scope:
Reagents: Electrophile-Nucleophile pairs (Elec-Nuc).
Examples: HX (as seen in Chapter 4), and other reagents in Chapter 5.
Substrates: Normal alkenes and alkynes. These reactions typically do not apply to arenes (aromatic compounds) under these conditions, as aromaticity provides significant stability.
Carbocation Rearrangement
Process: Carbocation rearrangements occur to form a more stable carbocation. This typically involves a 1,2-hydride shift (movement of a hydrogen atom with its bonding electrons) or a 1,2-methyl shift (movement of a methyl group with its bonding electrons).
Driving Force: The stability order of carbocations is tertiary (3^ ext{o}) > secondary (2^ ext{o}) > primary (1^ ext{o}). Rearrangements occur if they can convert a less stable carbocation into a more stable one.
Example (1,2-Hydride Shift):
A secondary carbocation (2^ ext{o}) can undergo a 1,2-hydride shift if an adjacent carbon has a hydrogen and would form a more stable tertiary carbocation (3^ ext{o}).
The hydrogen atom from an adjacent carbon moves to the positively charged carbon, and the positive charge shifts to the carbon that lost the hydrogen, resulting in a more stable carbocation.
(CH3)2CH-CH2^+ ightarrow (CH3)2C^+-CH3 (a 1^ ext{o} carbocation becoming a 3^ ext{o} carbocation via a 1,2-hydride shift if it was ext{CH}3- ext{CH}2- ext{CH}2^+ changing to ext{CH}3- ext{CH}^+- ext{CH}_3 is 1^ ext{o} to 2^ ext{o}) The image provided on page 12 shows a visual representation of a transition from a 2^ ext{o} carbocation to a 3^ ext{o} carbocation via a hydride shift to increase stability.