# 15.6 Types of Organic Chemical Reactions Organic compounds undergo a variety of chemical reactions based on reaction conditions and the types of reagents used. This section covers some of the most important types of organic reactions. ### i. Free Radical Substitution Reactions of Alkanes In this reaction type, a hydrogen atom in an alkane is successively replaced by a halogen free radical. This process yields a mixture of halogenoalkanes and typically occurs in the presence of UV light. The reaction proceeds in three steps: initiation, propagation, and termination. **Example Reaction: Chlorination of Methane** $$CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$$ ### ii. Electrophilic Addition Reactions These reactions involve the attack of an electrophile (an electron-deficient species) on a multiple bond (double or triple). The multiple bond breaks, and the electrophile is added to the unsaturated compound, converting it to a saturated or less-unsaturated compound. This is characteristic of alkenes and alkynes. **Example Reaction: Addition of HBr to Propene** $$CH_3CH=CH_2 + HBr \rightarrow CH_3CH(Br)CH_3$$ ### iii. Elimination Reactions Elimination reactions involve the removal of atoms or groups from adjacent carbon atoms in an organic molecule. This process forms a new multiple bond (for example, a double bond) and a small molecule like $H_2O$ or $HCl$. **Example Reaction: Dehydration of Ethanol** $$CH_3CH_2OH \xrightarrow{H_2SO_4} CH_2=CH_2 + H_2O$$ ### iv. Nucleophilic Substitution Reactions In these reactions, a nucleophile (an electron-rich species) attacks an electron-deficient carbon atom and replaces another atom or group of atoms (the leaving group). **Example Reaction: Hydrolysis of Bromoethane** $$CH_3CH_2Br + NaOH \rightarrow CH_3CH_2OH + NaBr$$ ### v. Nucleophilic Addition Reactions A nucleophile adds to a polar double bond, typically the carbonyl group ($C=O$) in aldehydes and ketones. The double bond is converted into a single covalent bond. **Example Reaction: Addition of HCN to Acetaldehyde** $$CH_3CHO + HCN \xrightarrow{NaCN/HCl} CH_3CH(CN)OH$$ ### vi. Hydrolysis Hydrolysis is a chemical reaction where a water molecule breaks one or more chemical bonds. This process often splits a larger organic compound into smaller substances and is frequently catalyzed by acids or bases. **Example Reaction: Acid-catalyzed Hydrolysis of an Ester** $$CH_3COOCH_2CH_3 + H_2O \xrightarrow{H_2SO_4} CH_3COOH + CH_3CH_2OH$$ ### vii. Condensation Condensation reactions involve the joining of two organic molecules to form a larger molecule, often with the elimination of a small molecule like water ($H_2O$) or hydrogen chloride ($HCl$). This is the reverse of hydrolysis. **Example Reaction: Esterification (a type of condensation)** $$CH_3COOH + CH_3CH_2OH \xrightarrow{H_2SO_4} CH_3COOCH_2CH_3 + H_2O$$ ### viii. Oxidation Reaction In organic chemistry, oxidation often involves: - Gain of oxygen atoms - Loss of hydrogen atoms - An increase in the number of bonds between carbon and oxygen **Example Reaction: Oxidation of Methanol** $$CH_3OH + [O] \xrightarrow{K_2Cr_2O_7/H_2SO_4} HCOOH \text{ (Formic Acid)}$$ ### ix. Reduction Reaction Reduction in organic chemistry often involves: - Loss of oxygen atoms - Gain of hydrogen atoms - A decrease in the number of bonds between carbon and oxygen **Example Reaction: Reduction of Acetaldehyde** $$CH_3CHO + 2[H] \xrightarrow{\text{Reducing Agent}} CH_3CH_2OH$$ ## Reaction Mechanisms ### 15.6.1 Free Radical Substitution Reaction of Alkanes This mechanism details how halogens substitute hydrogen atoms on alkanes in the presence of sunlight (UV light). #### Initiation Sunlight provides the energy for the homolytic fission of a halogen molecule, creating two highly reactive free radicals. $$Cl-Cl \xrightarrow{h\nu} Cl^{\bullet} + Cl^{\bullet}$$ #### Propagation This step consists of a chain reaction where radicals are consumed and regenerated. 1. A chlorine radical attacks a methane molecule, abstracting a hydrogen atom to form $HCl$ and a methyl free radical ($CH_3^{\bullet}$). 2. The methyl radical then attacks a chlorine molecule, forming chloromethane ($CH_3Cl$) and regenerating a chlorine radical, which can continue the chain. If excess chlorine is present, further substitution occurs, producing dichloromethane ($CH_2Cl_2$), trichloromethane ($CHCl_3$), and finally tetrachloromethane ($CCl_4$). #### Termination The reaction stops when two free radicals combine to form a stable, neutral molecule, ending the chain reaction. - $Cl^{\bullet} + Cl^{\bullet} \rightarrow Cl_2$ - $CH_3^{\bullet} + Cl^{\bullet} \rightarrow CH_3Cl$ - $CH_3^{\bullet} + CH_3^{\bullet} \rightarrow CH_3CH_3$ ### 15.6.2 Elimination Reactions In this mechanism, atoms are removed from adjacent carbons to form a double bond. For bromoethane, a base ($OH^-$) removes an acidic hydrogen. Simultaneously, the bromine atom leaves with the bonding pair of electrons, forming a bromide ion ($Br^-$) and ethene. ### 15.6.3 Nucleophilic Substitution Reactions A nucleophile attacks the partially positive (electrophilic) carbon atom of a halogenoalkane. This displaces the halogen atom, which leaves as a halide ion (the leaving group). ### 15.6.4 Electrophilic Addition Reactions The electron-rich pi ($\pi$) bond of an alkene attacks an electrophile (like the $H^+$ from $HBr$). This breaks the pi bond and forms a carbocation intermediate. The remaining nucleophile ($Br^-$) then attacks the carbocation to form the final product. ## Possible Questions and Answers **Q:** What is the key difference between an addition and an elimination reaction? **A:** An addition reaction breaks a multiple bond to add atoms to a molecule, converting an unsaturated compound to a more saturated one. An elimination reaction removes atoms from adjacent carbons to form a multiple bond, converting a saturated compound to an unsaturated one. **Q:** What initiates a free radical substitution reaction? **A:** UV light ($h\nu$), which causes the homolytic fission of a halogen molecule to produce free radicals. **Q:** What is the difference between an electrophile and a nucleophile? **A:** An electrophile is an electron-lover, a species that is electron-deficient and seeks electrons (for example, $H^+$). A nucleophile is a nucleus-lover, a species that is electron-rich and donates an electron pair (for example, $OH^-$, $Br^-$). ## Summary - Organic reactions are classified based on the changes in chemical structure and the type of reagent involved. - **Substitution** reactions involve replacing one atom or group with another. They can be initiated by free radicals or involve nucleophiles. - **Addition** reactions occur at multiple bonds (double/triple), breaking them to form single bonds. They can be initiated by electrophiles or nucleophiles. - **Elimination** reactions form multiple bonds by removing atoms from adjacent carbons. - **Redox** reactions involve the gain/loss of oxygen or hydrogen. Oxidation increases C-O bonds; reduction decreases them. - **Hydrolysis and Condensation** are opposing reactions involving water. Hydrolysis uses water to break a bond, while condensation joins molecules by removing water. | Reaction Type | Key Feature | Common Substrate | |---------------|-------------|------------------| | **Free Radical Substitution** | Substitution by a free radical | Alkanes | | **Electrophilic Addition** | Addition of an electrophile across a $\pi$ bond | Alkenes, Alkynes | | **Elimination** | Removal of atoms to form a $\pi$ bond | Alcohols, Haloalkanes | | **Nucleophilic Substitution** | Replacement of a leaving group by a nucleophile | Haloalkanes | | **Nucleophilic Addition** | Addition of a nucleophile to a C=O bond | Aldehydes, Ketones |