# Study Notes: Reactivity and Electron Sharing Reactions --- ## Key Concepts in Electron Sharing Reactions When molecules react, **electrons move** from one species to another. This movement forms new bonds and drives chemical reactions. Understanding the behavior of **nucleophiles**, **electrophiles**, and **radicals** is essential to mastering these reactions. --- ## Nucleophiles and Electrophiles ### **Nucleophiles**: Electron Donors - **Definition**: Nucleophiles are electron-rich species that seek positively charged or electron-deficient centers (nucleus-seeking). - **Characteristics**: - Contain a **lone pair of electrons** or a **negative charge**. - Examples: - **Lone-pair nucleophiles**: $Cl^-$, $OH^-$, $NH_3$ - **Pi-bond nucleophiles**: Alkenes and arenes (weakly nucleophilic due to delocalized pi electrons). - **Sigma-bond nucleophiles**: Electrons come from a sigma bond, e.g., $NaBH_4$. **Example of a Nucleophile**: Hydroxide ion ($OH^-$) attacking a carbon atom in a halogenoalkane. ### **Electrophiles**: Electron Acceptors - **Definition**: Electrophiles are electron-deficient species that accept a pair of electrons. - **Characteristics**: - Often **positively charged** or have a **partial positive charge**. - Examples: - $NO_2^+$, $H^+$, $Br\delta^+$ Electrophiles and nucleophiles are complementary: nucleophiles donate electrons, while electrophiles accept them. --- ## Bond Breaking (Bond Fission) When a covalent bond breaks, the electrons in the bond can be distributed in two ways: ### **1. Homolytic Fission** - **Definition**: Each atom retains one electron from the bond, forming two **free radicals**. - **Example**: $$Cl_2 \xrightarrow{\text{UV light}} 2Cl\cdot$$ - **Key Feature**: Produces species with unpaired electrons. Homolytic fission is common in reactions involving UV light or heat, such as the formation of chlorine radicals. ### **2. Heterolytic Fission** - **Definition**: Both electrons in the bond move to one atom, forming two **ions**. - **Example**: $$HCl \rightarrow H^+ + Cl^-$$ - **Key Feature**: Produces a cation and an anion. Students often confuse homolytic and heterolytic fission. Remember: homolytic produces radicals, while heterolytic produces ions. --- ## Curly Arrow Notation Curly arrows are used to show the movement of electrons in organic reactions: - **Full-headed arrow**: Movement of a **pair of electrons**. - **Half-headed arrow (fish hook)**: Movement of a **single electron**. Always start the arrow at the source of the electrons (e.g., a lone pair or bond) and point it toward the destination (e.g., an atom or bond). --- ## Radical Reactions Radicals are species with unpaired electrons. They are highly reactive and play a key role in certain reactions. ### **Formation of Radicals** 1. **Homolytic Fission**: UV light or heat breaks weak sigma bonds (e.g., $Cl_2$). 2. **Radical Abstraction**: A radical reacts with a molecule to form a new radical. **Example**: Chlorine radicals ($Cl\cdot$) formed from $CFCs$ in the stratosphere contribute to ozone depletion. ### **Radical Substitution in Alkanes** - **Mechanism**: 1. **Initiation**: Radicals are generated (e.g., $Cl_2 \xrightarrow{\text{UV light}} 2Cl\cdot$). 2. **Propagation**: Radicals react with molecules to form new radicals. 3. **Termination**: Radicals combine to form stable molecules. Radical substitution is used to distinguish between alkanes and alkenes. Alkanes react with bromine water under UV light, causing a color change from brown to colorless. --- ## Nucleophilic Substitution Reactions ### **Definition** A nucleophile replaces a leaving group on an electrophile. ### **Mechanisms** 1. **$S_N2$ Mechanism** (Primary Halogenoalkanes): - **One-step process**: Nucleophile attacks the carbon atom while the leaving group departs. - **Key Features**: - Bimolecular (rate depends on both reactants): $$\text{Rate} = k[\text{halogenoalkane}][\text{nucleophile}]$$ - **Back-side attack**: Causes inversion of stereochemistry. - **Stereospecific**: Only one optical isomer is formed. **Example**: $CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-$ 2. **$S_N1$ Mechanism** (Tertiary Halogenoalkanes): - **Two-step process**: 1. Formation of a **carbocation intermediate**. 2. Nucleophile attacks the carbocation. - **Key Features**: - Unimolecular (rate depends only on halogenoalkane): $$\text{Rate} = k[\text{halogenoalkane}]$$ - Produces a **racemic mixture** due to the planar nature of the carbocation. In $S_N1$ reactions, students often forget that the intermediate carbocation is planar, leading to a racemic mixture. --- ## Electrophilic Addition Reactions ### **Definition** An electrophile adds to a molecule, forming a single product. ### **Reactions with Alkenes** - **Why Alkenes React**: - The **pi bond** in alkenes is electron-rich, making it attractive to electrophiles. - The pi bond is weaker than sigma bonds, making it more reactive. ### **Examples** 1. **Addition of Hydrogen Halides ($H-X$)**: - The $H^+$ acts as an electrophile. - Reactivity order: $HI > HBr > HCl$ (weaker bonds react faster). - **Markovnikov's Rule**: The hydrogen adds to the carbon with more hydrogens already attached. **Example**: $$CH_3CH=CH_2 + HBr \rightarrow CH_3CHBrCH_3$$ (2-bromopropane is the major product due to the stability of the secondary carbocation). 2. **Hydration of Alkenes**: - Converts alkenes to alcohols. - Requires a strong acid catalyst (e.g., $H_3PO_4$). 3. **Addition of Halogens ($Br_2$, $Cl_2$)**: - Alkenes decolorize bromine water, forming dihalogenoalkanes. Use bromine water to test for alkenes. A color change from brown to colorless indicates the presence of a double bond. --- ## Electrophilic Substitution in Benzene ### **Why Benzene Reacts Differently** - Benzene's delocalized pi electrons make it **less nucleophilic** than alkenes. - Substitution reactions preserve the stability of the aromatic ring. ### **Mechanism** 1. **Electrophile Formation**: Often generated in situ (e.g., $NO_2^+$ from $HNO_3$ and $H_2SO_4$). 2. **Reaction with Benzene**: Forms a carbocation intermediate stabilized by resonance. 3. **Restoration of Aromaticity**: A proton is lost, restoring the delocalized pi system. **Example**: Nitration of benzene: $$C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O$$ --- ## Key Takeaways - **Nucleophiles** donate electrons; **electrophiles** accept electrons. - Bond fission can be **homolytic** (radicals) or **heterolytic** (ions). - **Radical reactions** involve initiation, propagation, and termination steps. - **Nucleophilic substitution** occurs via $S_N2$ (primary) or $S_N1$ (tertiary) mechanisms. - **Electrophilic addition** is common in alkenes, while **electrophilic substitution** occurs in benzene. **Practice Question**: Write the mechanism for the reaction of ethene with $HBr$. Use curly arrows to show electron movement.