Alkyl & Aryl Halides (NEET)

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### Introduction to Halogen Derivatives Halogen derivatives are organic compounds containing carbon-halogen bonds. They are broadly classified into alkyl halides (haloalkanes) and aryl halides (haloarenes). These compounds are important in organic chemistry due to their reactivity and applications. ### Classification of Alkyl Halides (Haloalkanes) Alkyl halides are classified based on the number of halogen atoms and the hybridization of the carbon atom bearing the halogen. #### Based on Number of Halogen Atoms - **Monohaloalkanes:** Contain one halogen atom ($\text{R--X}$). Example: $\text{CH}_3\text{Cl}$ (Chloromethane). - **Dihaloalkanes:** Contain two halogen atoms. - **Geminal dihalides:** Both halogens on the same carbon. Example: $\text{CH}_3\text{CH(Cl)}_2$ (1,1-Dichloropropane). - **Vicinal dihalides:** Halogens on adjacent carbons. Example: $\text{CH}_2\text{(Cl)CH}_2\text{Cl}$ (1,2-Dichloroethane). - **Polyhaloalkanes:** Contain three or more halogen atoms. Example: $\text{CHCl}_3$ (Chloroform), $\text{CCl}_4$ (Carbon tetrachloride). #### Based on Hybridization of Carbon Bearing Halogen - **$\text{sp}^3$ Hybridized Carbon:** - **Primary (1°):** Halogen attached to a primary carbon. Example: $\text{CH}_3\text{CH}_2\text{Cl}$ (Chloroethane). - **Secondary (2°):** Halogen attached to a secondary carbon. Example: $\text{CH}_3\text{CH(Cl)CH}_3$ (2-Chloropropane). - **Tertiary (3°):** Halogen attached to a tertiary carbon. Example: $\text{(CH}_3)_3\text{CCl}$ (2-Chloro-2-methylpropane). - **Allylic halides:** Halogen attached to an $\text{sp}^3$ carbon next to a $\text{C=C}$ double bond. Example: $\text{CH}_2\text{=CH-CH}_2\text{Br}$ (3-Bromopropene). - **Benzylic halides:** Halogen attached to an $\text{sp}^3$ carbon next to an aromatic ring. Example: $\text{C}_6\text{H}_5\text{CH}_2\text{Cl}$ (Benzyl chloride). - **$\text{sp}^2$ Hybridized Carbon:** - **Vinylic halides:** Halogen attached directly to an $\text{sp}^2$ carbon of a $\text{C=C}$ double bond. Example: $\text{CH}_2\text{=CHCl}$ (Chloroethene). - **Aryl halides:** Halogen attached directly to an $\text{sp}^2$ carbon of an aromatic ring. Example: $\text{C}_6\text{H}_5\text{Cl}$ (Chlorobenzene). ### Nomenclature (IUPAC and Common Names) - **IUPAC:** Halogen is treated as a substituent. Prefix 'halo-' is used (e.g., chloro, bromo, iodo, fluoro). Numbering starts from the end giving the lowest number to the halogen. - **Common Names:** Alkyl group name followed by halide (e.g., methyl chloride, ethyl bromide). For complex alkyl groups, prefixes like *iso*, *sec*, *tert* are used. - **Aryl halides:** Common names are often accepted IUPAC names (e.g., bromobenzene). For disubstituted compounds, *o-, m-, p-* are used. ### Physical Properties of Alkyl Halides - **Boiling Points:** Increase with increasing molecular mass (I > Br > Cl > F). For isomeric haloalkanes, boiling point decreases with branching. - **Density:** Bromo-, iodo-, and polychloroalkanes are denser than water. Density increases with increasing atomic mass of halogen and number of halogen atoms. - **Solubility:** Sparingly soluble in water but soluble in organic solvents. This is because they cannot form hydrogen bonds with water molecules, and the energy required to break existing H-bonds in water and form new attractions is higher than the energy released. - **Polarity:** They are polar molecules due to the polar C-X bond. ### Preparation of Alkyl Halides #### From Alcohols - **Reaction with HX:** $\text{R-OH} + \text{HX} \rightarrow \text{R-X} + \text{H}_2\text{O}$. Reactivity of HX: HI > HBr > HCl. Reactivity of alcohols: 3° > 2° > 1°. Lucas Test uses $\text{HCl} + \text{ZnCl}_2$. - **Reaction with $\text{PCl}_3$, $\text{PCl}_5$, $\text{PBr}_3$, $\text{PI}_3$:** - $\text{3R-OH} + \text{PCl}_3 \rightarrow \text{3R-Cl} + \text{H}_3\text{PO}_3$ - $\text{R-OH} + \text{PCl}_5 \rightarrow \text{R-Cl} + \text{POCl}_3 + \text{HCl}$ - $\text{3R-OH} + \text{PBr}_3 \rightarrow \text{3R-Br} + \text{H}_3\text{PO}_3$ (PBr3/PI3 are generated *in situ* from red P and Br2/I2) - **Reaction with Thionyl Chloride ($\text{SOCl}_2$):** $\text{R-OH} + \text{SOCl}_2 \xrightarrow{\text{Pyridine}} \text{R-Cl} + \text{SO}_2 + \text{HCl}$. This is a preferred method as the byproducts ($\text{SO}_2$ and $\text{HCl}$) are gaseous and escape, leaving pure alkyl chloride. (Darzen's process) #### From Hydrocarbons - **Free Radical Halogenation of Alkanes:** $\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light or heat}} \text{CH}_3\text{Cl} + \text{HCl}$. This is not a good method for preparation of single haloalkane due to formation of polyhalogenated compounds and mixtures of isomers. - **Electrophilic Addition to Alkenes (Markovnikov's Rule):** $\text{CH}_3\text{CH=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH(Br)CH}_3$. Anti-Markovnikov's addition occurs in presence of peroxides (for HBr only). - **Halogen Addition to Alkenes:** $\text{CH}_2\text{=CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{(Br)CH}_2\text{Br}$ (Vicinal dihalide formation, test for unsaturation). #### Halogen Exchange Reactions - **Finkelstein Reaction:** $\text{R-X} + \text{NaI} \xrightarrow{\text{Acetone}} \text{R-I} + \text{NaX}$. Used for preparing alkyl iodides. NaX (NaCl or NaBr) precipitates in acetone, driving the reaction forward. - **Swarts Reaction:** $\text{R-Br} + \text{AgF} \rightarrow \text{R-F} + \text{AgBr}$. Used for preparing alkyl fluorides. Other metallic fluorides like $\text{Hg}_2\text{F}_2$, $\text{CoF}_2$, $\text{SbF}_3$ can also be used. ### Chemical Reactions of Alkyl Halides #### Nucleophilic Substitution Reactions ($\text{SN}_1$ and $\text{SN}_2$) - **$\text{SN}_2$ (Bimolecular Nucleophilic Substitution):** - **Mechanism:** Concerted, one-step reaction. Nucleophile attacks from the back-side, leading to inversion of configuration (Walden inversion). - **Rate:** Rate = $\text{k[R-X][Nu}^-]$. Second order. - **Stereochemistry:** Inversion of configuration. - **Reactivity:** $\text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ$. Steric hindrance around the carbon bearing the halogen decreases reactivity. - **Leaving Group:** Good leaving groups are weak bases (I > Br > Cl > F). - **Nucleophile:** Strong nucleophiles favor $\text{SN}_2$. - **Solvent:** Polar aprotic solvents (e.g., acetone, DMSO, DMF) favor $\text{SN}_2$. - **$\text{SN}_1$ (Unimolecular Nucleophilic Substitution):** - **Mechanism:** Two-step reaction. Step 1: Slow ionization to form carbocation (rate-determining step). Step 2: Fast attack of nucleophile on carbocation. - **Rate:** Rate = $\text{k[R-X]}$. First order. - **Stereochemistry:** Racemization (formation of equal amounts of enantiomers) if the carbon is chiral, due to planar carbocation intermediate. - **Reactivity:** $3^\circ > 2^\circ > 1^\circ > \text{CH}_3\text{X}$. Carbocation stability (3° > 2° > 1°) determines reactivity. Allylic and benzylic halides are highly reactive due to resonance stabilization of carbocations. - **Leaving Group:** Good leaving groups are weak bases (I > Br > Cl > F). - **Nucleophile:** Weak nucleophiles can participate. - **Solvent:** Polar protic solvents (e.g., water, ethanol) favor $\text{SN}_1$ by stabilizing the carbocation. #### Elimination Reactions (Dehydrohalogenation) - **$\text{R-CH}_2\text{-CH}_2\text{-X} + \text{Alc. KOH} \rightarrow \text{R-CH=CH}_2 + \text{KX} + \text{H}_2\text{O}$.** - **Saytzeff's Rule:** In dehydrohalogenation, the preferred product is the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms (more substituted alkene). - **Competition with $\text{SN}_2$:** Strong, bulky bases favor elimination, while strong, less hindered nucleophiles favor substitution. High temperature favors elimination. #### Reaction with Metals - **Wurtz Reaction:** $\text{2R-X} + \text{2Na} \xrightarrow{\text{Dry Ether}} \text{R-R} + \text{2NaX}$. Used for synthesizing symmetrical alkanes. - **Wurtz-Fittig Reaction:** $\text{R-X} + \text{Ar-X} + \text{2Na} \xrightarrow{\text{Dry Ether}} \text{R-Ar} + \text{2NaX}$. Used for synthesizing alkylarenes. - **Grignard Reagents:** $\text{R-X} + \text{Mg} \xrightarrow{\text{Dry Ether}} \text{R-Mg-X}$. Organometallic compounds, highly reactive nucleophiles. React with compounds containing active hydrogen (e.g., water, alcohol, amines) to give alkanes. - $\text{R-MgX} + \text{H}_2\text{O} \rightarrow \text{R-H} + \text{Mg(OH)X}$ #### Reduction - Alkyl halides can be reduced to alkanes using reducing agents like $\text{Zn/HCl}$, $\text{LiAlH}_4$, or by catalytic hydrogenation. - $\text{R-X} + \text{H}_2 \xrightarrow{\text{Pd/C}} \text{R-H} + \text{HX}$ ### Polyhalogen Compounds - **Dichloromethane ($\text{CH}_2\text{Cl}_2$):** Solvent, paint remover. - **Chloroform ($\text{CHCl}_3$):** Solvent, anesthetic (now limited use due to toxicity). Stored in dark bottles to prevent oxidation by air and light to phosgene ($\text{COCl}_2$), a poisonous gas. - **Iodoform ($\text{CHI}_3$):** Antiseptic. Yellow solid with characteristic smell. - **Carbon Tetrachloride ($\text{CCl}_4$):** Solvent, fire extinguisher (pyrene), refrigerant. Causes liver damage. - **Freons (Chlorofluorocarbons, CFCs):** Used as refrigerants, propellants. Deplete ozone layer. Example: $\text{CCl}_2\text{F}_2$ (Freon-12). - **DDT (Dichlorodiphenyltrichloroethane):** Insecticide. Non-biodegradable, causes environmental pollution. ### Aryl Halides (Haloarenes) #### Preparation of Aryl Halides - **Direct Halogenation (Electrophilic Substitution):** - $\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{FeCl}_3, \text{dark, cold}} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}$. Requires Lewis acid catalyst ($\text{FeCl}_3$, $\text{FeBr}_3$). Fails for fluorine (too reactive) and iodine (reversible, requires oxidizing agent like $\text{HNO}_3$). - **From Diazonium Salts:** - **Sandmeyer Reaction:** $\text{Ar-N}_2^+\text{X}^- \xrightarrow{\text{CuX/HX}} \text{Ar-X} + \text{N}_2$. Used for preparing aryl chlorides and bromides. - **Gattermann Reaction:** $\text{Ar-N}_2^+\text{X}^- \xrightarrow{\text{Cu/HX}} \text{Ar-X} + \text{N}_2$. Similar to Sandmeyer but uses copper powder instead of cuprous halide. - **Balz-Schiemann Reaction:** $\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{HBF}_4} \text{Ar-N}_2^+\text{BF}_4^- \xrightarrow{\text{heat}} \text{Ar-F} + \text{BF}_3 + \text{N}_2$. Used for preparing aryl fluorides. - **From Phenols (limited industrial use):** - Phenols can be converted to aryl halides by reaction with $\text{PCl}_5$ but the yield is generally poor and side reactions occur. #### Physical Properties of Aryl Halides - **Boiling Points:** Generally increase with molecular mass (I > Br > Cl > F). Isomeric dihalobenzenes have nearly same boiling points but para-isomers have higher melting points due to symmetry and better packing in crystal lattice. - **Density:** Denser than water. - **Solubility:** Insoluble in water, soluble in organic solvents. - **Dipole Moment:** Chlorobenzene has a lower dipole moment than cyclohexyl chloride. This is due to the $\text{sp}^2$ hybridized carbon in chlorobenzene being more electronegative than the $\text{sp}^3$ carbon in cyclohexyl chloride. The greater electronegativity of $\text{sp}^2$ carbon reduces the electron density on the carbon, and thus reduces the net dipole moment. Also, resonance effects in aryl halides can reduce the dipole moment. #### Chemical Reactions of Aryl Halides - **Lower Reactivity towards Nucleophilic Substitution:** - **Reason 1:** Resonance stabilization of C-X bond: Partial double bond character of C-X bond makes it stronger and shorter, harder to break. - **Reason 2:** Difference in hybridization: Carbon atom of C-X bond in aryl halides is $\text{sp}^2$ hybridized, which is more electronegative than $\text{sp}^3$ carbon in alkyl halides. This makes the C-X bond shorter and stronger. - **Reason 3:** Instability of phenyl carbocation: Phenyl cation formed by self-ionization would be unstable because of the positive charge on an $\text{sp}^2$ carbon. - **Reason 4:** Repulsion between nucleophile and electron-rich aromatic ring. - **Nucleophilic Substitution Reactions (require harsh conditions):** - **Replacement by hydroxyl group (Dow's Process):** $\text{C}_6\text{H}_5\text{Cl} + \text{NaOH} \xrightarrow{\text{623K, 300 atm}} \text{C}_6\text{H}_5\text{ONa} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH}$ (Phenol). - **Effect of Electron-Withdrawing Groups (EWG):** Presence of electron-withdrawing groups (e.g., $-\text{NO}_2$) at *ortho* and *para* positions *increases* the reactivity towards nucleophilic substitution. This is because EWGs stabilize the intermediate carbanion formed during the reaction. Example: 2,4,6-Trinitrochlorobenzene reacts readily with warm water. - **Electrophilic Substitution Reactions:** Halogen is a deactivating but *o, p*-directing group. - **Halogenation:** $\text{C}_6\text{H}_5\text{Cl} + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} \text{o- and p-Dichlorobenzene}$. - **Nitration:** $\text{C}_6\text{H}_5\text{Cl} + \text{Conc. HNO}_3 \xrightarrow{\text{Conc. H}_2\text{SO}_4} \text{o- and p-Chloronitrobenzene}$. - **Sulfonation:** $\text{C}_6\text{H}_5\text{Cl} + \text{Conc. H}_2\text{SO}_4 \rightarrow \text{o- and p-Chlorobenzenesulphonic acid}$. - **Friedel-Crafts Alkylation:** $\text{C}_6\text{H}_5\text{Cl} + \text{CH}_3\text{Cl} \xrightarrow{\text{Anhy. AlCl}_3} \text{o- and p-Chlorotoluene}$. - **Friedel-Crafts Acylation:** $\text{C}_6\text{H}_5\text{Cl} + \text{CH}_3\text{COCl} \xrightarrow{\text{Anhy. AlCl}_3} \text{o- and p-Chloroacetophenone}$. - **Reaction with Metals:** - **Wurtz-Fittig Reaction:** $\text{R-X} + \text{Ar-X} + \text{2Na} \xrightarrow{\text{Dry Ether}} \text{R-Ar} + \text{2NaX}$. - **Fittig Reaction:** $\text{2Ar-X} + \text{2Na} \xrightarrow{\text{Dry Ether}} \text{Ar-Ar} + \text{2NaX}$. Used for synthesizing symmetrical diaryls. - **Grignard Reagents:** $\text{Ar-X} + \text{Mg} \xrightarrow{\text{Dry Ether}} \text{Ar-Mg-X}$ (Arylmagnesium halides).