Haloalkanes and Haloarenes Comprehensive Study Notes

General Course Information and Resources

• Subject: Organic Chemistry.

• Topic: Halo-alkanes and Halo-arenes.

• Instructors: Lakshmi Narasimhan and Shravan Kumar.

• Academic Levels: Suitable for PU (Pre-University), NEET, and IIT-JEE preparation.

• Reference Materials:

  • NCERT (Standard University/Board textbook).

  • Cengage Advanced Organic Chemistry (Competitive reference).

  • Stereochemistry by K.S. Kalsi.

• Guiding Mantra: "Practice without Seeing" — Repeated multiple times as a core method for mastering organic structures and reactions.

Standard Symbols and Abbreviations

• Alkyl and Phenyl Groups:

  • Me: $-CH_3$ (Methyl group).

  • Et: $-C_2H_5$ (Ethyl group).

  • n-Pr: $-CH_2-CH_2-CH_3$ (n-propyl group).

  • i-Pr: $-CH(CH_3)_2$ (Isopropyl group).

  • R: Alkyl group.

  • Me (Repeated): Methyl group.

  • n-Bu: n-butyl group ($-CH_2-CH_2-CH_2-CH_3$ connected to a functional unit).

  • $C_6H_5$ / Ar / Ph: Phenyl group.

  • s-Bu: Sec-butyl group ($CH_3-CH(F.G)-CH_2-CH_3$).

  • i-Bu: Iso-butyl group ($(CH_3)_2CH-CH_2-(F.G)$).

  • t-Bu: Tert-butyl group ($(CH_3)_3C-(F.G)$).

• Reaction Types and Mechanisms:

  • F.G: Functional Group.

  • SN1: Unimolecular Nucleophilic Substitution.

  • SN2: Bimolecular Nucleophilic Substitution.

  • E1: Unimolecular Elimination.

  • E2: Bimolecular Elimination.

  • E1CB: Unimolecular Elimination via Conjugate Base.

• Solvents and Reagents:

  • DMF: Dimethyl Formamide.

  • DMSO: Dimethyl Sulphoxide (Structure: $(CH_3)_2S=O$).

  • LA: Lewis Acid.

  • LB: Lewis Base.

Carbon Classification and Structural Positions

• Carbon Atom Types:

  • $\alpha$-carbon (alpha): The carbon atom immediately connected to the functional group (e.g., the halogen atom).

  • $\beta$-carbon (beta): The carbon atom connected directly to the $\alpha$-carbon.

• Phenyl Ring Positions:

  • o- (ortho): 1,2 position in the phenyl ring.

  • m- (meta): 1,3 position in the phenyl ring.

  • p- (para): 1,4 position in the phenyl ring (used when similar groups are in both positions).

IUPAC Nomenclature and Priority Rules

• Instruction: Students are strictly advised to memorize the Priority Table for IUPAC naming from their 1st PU textbook.

• Prefix Substituents: The following groups are always treated as prefixes in nomenclature:

  • Alkyl groups ($-R$).

  • Phenyl groups ($-Ph$, $-C_6H_5$, or $-Ar$).

  • Halogen groups ($-X$, where $X = F, Cl, Br, I$).

  • Nitro group ($-NO_2$).

  • Alkoxy group ($-O-R$).

• Suffix Priority Table (Functional Groups in Decreasing Order):

  1. $-COOH$: Carboxylic Acid.

  2. $-SO_3H$: Sulphonic Acid.

  3. $-COOR$: Ester.

  4. $-COX$: Acyl Halide / Acid Chloride.

  5. $-CONH_2$: Amide.

  6. $-C \equiv N$: Nitrile (Cyanide).

  7. $-CH=O$: Aldehyde.

  8. $-C=O$: Ketone.

  9. $-OH$: Alcohol.

  10. $-NH_2$: Amine.

  11. $-C=C-$: Alkene.

  12. $-C \equiv C-$: Alkyne.

Classification of Haloalkanes and Haloarenes

• General Formation: Replacing a Hydrogen ($-H$) in an Alkane ($R-H$) with a Halogen ($X$) results in a Haloalkane ($R-X$) or Alkyl Halide.

• Monohalogen Compounds: These contain a single halogen atom.

  • Primary Alkyl Halide ($1^\circ$ R-X): The halogen atom is connected to an $\alpha$-carbon that is further connected to only one other carbon atom (or no other carbons in the case of Methyl Halide). The $\alpha$-carbon contains 2 hydrogen atoms and is $sp^3$ hybridized.

    • Examples:

      • Methyl chloride: $CH_3-Cl$ (Chloro-methane).

      • Ethyl chloride: $CH_3-CH_2-Cl$ (Chloro-ethane).

      • n-propyl chloride: $CH_3-CH_2-CH_2-Cl$ (1-chloro-propane).

      • n-butyl chloride: $CH_3-CH_2-CH_2-CH_2-Cl$ (1-chloro-butane).

      • Iso-butyl chloride: $(CH_3)_2CH-CH_2-Cl$ (1-chloro-2-methyl propane).

      • n-pentyl/n-amyl chloride: $CH_3-(CH_2)_4-Cl$ (1-chloro-pentane).

  • Secondary Alkyl Halide ($2^\circ$ R-X): The halogen is connected to an $\alpha$-carbon that is connected to two other carbon atoms. The $\alpha$-carbon contains only one hydrogen atom and is $sp^3$ hybridized.

    • Examples:

      • Isopropyl bromide: $(CH_3)_2CH-Br$ (2-bromo-propane).

      • Sec-butyl bromide: $CH_3-CH(Br)-CH_2-CH_3$ (2-bromo-butane).

      • Sec-pentyl bromide: $CH_3-CH(Br)-CH_2-CH_2-CH_3$ (2-bromo-pentane).

  • Tertiary Alkyl Halide ($3^\circ$ R-X): The halogen is connected to an $\alpha$-carbon that is connected to three other carbon atoms. The $\alpha$-carbon contains zero hydrogen atoms and is $sp^3$ hybridized.

    • Example:

      • Tertiary butyl chloride: $(CH_3)_3C-Cl$ (2-chloro-2-methyl propane).

• Allylic Halides: The halogen is connected to an $sp^3$ hybridized carbon that is adjacent to a carbon-carbon double bond ($C=C$).

  • Primary allylic: Allyl chloride ($CH_2=CH-CH_2-Cl$), also named 3-chloro-prop-1-ene. The $\alpha$-carbon is a $1^\circ$ carbon.

  • Secondary allylic: $CH_2=CH-CH(Cl)-CH_3$ (3-chloro-but-1-ene). The $\alpha$-carbon is a $2^\circ$ carbon.

  • Tertiary allylic: $CH_2=CH-C(Cl)(CH_3)_2$. The $\alpha$-carbon is a $3^\circ$ carbon.

  • Cyclic Allyl Halide: 3-chloro-cyclohexene (Halo on the carbon next to the double bond in a ring).

• Benzylic Halides: The halogen atom is bonded to an $sp^3$ carbon atom that is attached to an aromatic ring.

  • Primary benzylic: Benzyl halide ($C_6H_5-CH_2-X$). (Halo-phenyl methane).

  • Secondary benzylic: $C_6H_5-CH(Cl)-CH_3$ (1-chloro-1-phenyl ethane).

  • Tertiary benzylic: $C_6H_5-C(Br)(CH_3)_2$ (2-bromo-2-phenyl propane).

• Vinylic Halides: The halogen atom is bonded to an $sp^2$ hybridized carbon atom of a carbon-carbon double bond ($C=C$).

  • Vinyl Chloride: $CH_2=CH-Cl$ (1-chloro-ethene).

  • Cyclic Vinyl Halide: 1-chloro-cyclohexene (Halogen directly on the double-bonded carbon in a ring).

• Aryl Halides: The halogen atom is directly bonded to an $sp^2$ hybridized carbon atom of a phenyl ring ($X = F, Cl, Br, I$).

• Dihalo Compounds:

  • Aliphatic:

    • Vicinal Dihalides: Two similar halogen atoms are bonded to adjacent $sp^3$ carbon atoms. Example: 1,2-dichloropropane ($CH_3-CH(Cl)-CH_2-Cl$).

    • Geminal Dihalides: Two similar halogen atoms are bonded to the same carbon atom. Example: 2,2-dichloropropane ($CH_3-C(Cl)_2-CH_3$).

  • Aromatic (Aryl Dihalides):

    • o-dichlorobenzene (1,2-dichlorobenzene).

    • m-dichlorobenzene (1,3-dichlorobenzene).

    • p-dichlorobenzene (1,4-dichlorobenzene).

    • Heterogeneous example: 1-bromo-3-chlorobenzene.

Preparation Chart of Haloalkanes

1. From Alcohols

• Groove's Process: Alcohol + conc. $HCl$ in the presence of anhydrous $ZnCl_2$ (Lucas Reagent). Produces alkyl chlorides.

• Reaction with HBr: Alcohol + $48\%$ conc. $H_2SO_4$ (catalyst) + $HBr$ reagent. Primarily produces $1^\circ$ alkyl bromides.

• Reaction with Phosphorus Halides:

  • $3R-OH + PCl_3 \rightarrow 3R-Cl + H_3PO_3$ (Phosphorous acid).

  • $R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl$.

• Reaction with NaI or KI: Alcohol + $95\%$ $H_3PO_4$ yields $R-I$ (via $HI$ generated in situ).

• Reaction with KBr: Alcohol + conc. $H_2SO_4 + KBr \rightarrow R-Br + KHSO_4 + H_2O$.

• Darzens Process: Alcohol + $SOCl_2$ (Thionyl chloride) + Pyridine (reflow) $\rightarrow R-Cl + SO_2 \uparrow + HCl \uparrow$. This is the preferred method for producing pure alkyl chlorides as the byproducts are gases.

2. From Other Alkyl Halides (Halogen Exchange)

• Finkelstein Reaction: $R-X + NaI \xrightarrow{\text{Dry Acetone}} R-I + NaX \downarrow$. Note: $X$ is typically $Cl$ or $Br$.

• Swarts Reaction: $R-X + \text{Metallic Fluorides}$ ($AgF$, $Hg_2F_2$, $CoF_2$, or $SbF_3$) $\rightarrow R-F$.

3. From Alkanes

• Photochemical Free Radical Halogenation: $R-H + X_2 \xrightarrow{h\nu} \text{Mixture of alkyl chlorides}$.

4. From Alkenes (Addition Reactions)

• Markovnikov Addition: Addition of $HX$ ($X = Cl, Br, I$) to an alkene.

• Anti-Markovnikov / Kharasch Effect: Addition of $HBr$ in the presence of peroxides ($R_2O_2$ / organic oxide) to an alkene.

Detailed Mechanisms and Specific Reactions

Lucas Reagent and Groove's Process

• Reagent: Anhydrous $ZnCl_2 + \text{conc. } HCl$.

• Reactivity Order of Alcohols: 3^\circ > 2^\circ > 1^\circ.

  • Tertiary ($3^\circ$): Immediate turbidity/cloudiness from the formation of alkyl halide.

  • Secondary ($2^\circ$): Turbidity appears in 5–10 minutes.

  • Primary ($1^\circ$): Turbidity appears only upon heating.

• Reasoning: The reactivity is determined by the stability of the carbocation intermediate: 3^\circ \text{ C.C.} > 2^\circ \text{ C.C.} > 1^\circ \text{ C.C.}.

Carbocation Rearrangements

• Carbocations can rearrange to form a more stable species via:

  • 1,2-Hydride shift.

  • 1,2-Methyl (or Alkyl) shift.

  • 1,2-Phenyl shift.

• Example: Neo-pentyl Alcohol reaction with Lucas Reagent

  1. $Neo-pentyl alcohol$ ($(CH_3)_3C-CH_2-OH$) reacts with the Lewis Acid ($LA$) to form an Oxonium ion intermediate.

  2. Loss of $H_2O$ results in a Primary Carbocation ($(CH_3)_3C-CH_2^+$), which is least stable.

  3. A 1,2-Methyl shift occur: one methyl group migrates with its bond to the positive carbon.

  4. This forms a Tertiary Carbocation, which is highly stable.

  5. The final product is 2-chloro-2-methyl butane, a tertiary alkyl chloride.

Important Instructions for Examinations

• Explain Type Questions: If a question asks to "Explain," only completing the equation is insufficient for full marks.

• Writing Requirements: Documentation must include symbols, products, solvents, catalysts, temperature, and pressure conditions.

• Structure: Explanations should be followed by equations containing the full structures of reactants and products.

• Notation: In the provided notes, sections are labeled according to relevance for PU-part or IIT-part to help students prioritize according to their exam goals.