Nucleophilic Substitution: SN1 vs SN2 Reactions

This page compares and contrasts SN1 and SN2 nucleophilic substitution reactions:

SN1 (Substitution Nucleophilic Unimolecular):

• Occurs with tertiary alkyl halides or alcohols
• Forms a carbocation intermediate
• Proceeds in two steps
• Rate depends only on substrate concentration

SN2 (Substitution Nucleophilic Bimolecular):

• Favored for primary and some secondary substrates
• Single-step mechanism with backside attack
• Rate depends on both substrate and nucleophile concentrations

Definition: SN1 - "Substitution Nucleophilic Unimolecular" Definition: SN2 - "Substitution Nucleophilic Bimolecular" Example: SN1: (CH3)3C-Br + OH- → (CH3)3C-OH + Br- Example: SN2: CH3CH2Br + I- → CH3CH2I + Br-

Understanding the differences between SN1 and SN2 mechanisms is crucial for predicting organic reaction outcomes and planning synthetic routes.

Radical Substitution of Halogens with Alkanes

This page explains the radical substitution reaction between alkanes and halogens, specifically chloroethane and bromine. The reaction proceeds through three main stages:

1. Initiation: Homolytic cleavage of Br2 by light or heat to form bromine radicals.

2. Propagation:

   • Bromine radical abstracts a hydrogen from chloroethane.
   • Resulting alkyl radical reacts with Br2 to form the product and regenerate a bromine radical.

3. Termination: Radicals combine to form stable molecules, ending the chain reaction.

Vocabulary: Homolytic cleavage - Bond breaking where each atom keeps one electron Example: CH3CH2Cl + Br2 → CH3CHClBr + HBr (main product) Highlight: Multiple substitution products are possible, leading to a mixture

The radical substitution mechanism is important for understanding halogenation reactions and the reactivity of alkanes under certain conditions.

Electrophilic Addition of Bromine to Alkenes

This page details the mechanism of electrophilic addition of bromine to alkenes. The process occurs in several key steps:

1. The electron-rich alkene double bond attracts the bromine molecule, inducing partial charges.

2. Heterolytic cleavage of the Br-Br bond occurs as one bromine is pulled towards the alkene.

3. A cyclic bromonium ion intermediate forms as the alkene's pi bond breaks.

4. The remaining bromide ion attacks from the backside, opening the bromonium ring.

5. The final product is a vicinal dibromide (halogenoalkane).

Vocabulary: Heterolytic cleavage - Bond breaking where both electrons go to one atom Example: The reaction of bromine with ethene to form 1,2-dibromoethane Highlight: The backside attack in step 4 results in anti addition across the original double bond

This electrophilic addition mechanism is crucial for understanding how alkenes are converted to more functionalized products in organic synthesis.