Halogen Derivatives - Key Concepts and Nomenclature (Flashcards)

10.1 Introduction to Halogen Derivatives

• Halogen derivatives are organic compounds where hydrogen atoms in hydrocarbons are replaced by halogen atoms (F, Cl, Br, I).

• The functional group involves a halogen attached to a carbon atom (e.g., sp3 carbon in haloalkanes).

• Classification by Number of Halogen Atoms:

  • Mono-halogen compounds: One halogen atom.

  • Di-, Tri-, or Poly-halogen compounds: Two, three, or more halogen atoms (e.g., Hexachlorobenzene, Hexachlorocyclohexane).

10.1.1 Classification of Monohalogen Compounds

Monohalogen compounds are classified based on the carbon atom to which the halogen is bonded and its hybridization.

• a. Alkyl halides (Haloalkanes):

  • Halogen is bonded to an sp^3-hybridized carbon atom in a saturated carbon chain.

  • Types:

    • Primary (1º): Halogen attached to a carbon bonded to only one other carbon (R–CH2–X).

    • Secondary (2º): Halogen attached to a carbon bonded to two other carbons (R–CH(R)–X).

    • Tertiary (3º): Halogen attached to a carbon bonded to three other carbons (R–C(R)2–X).

  • Formulas: R{-}CH_2{-}X, R{-}CH{-}R{-}X, R{-}C{-}X

• b. Allylic halides:

  • Halogen attached to an sp^3-hybridized carbon next to a carbon-carbon double bond (C=C).

  • Example: CH2=CH{-}CH2{-}X

• c. Benzylic halides:

  • Halogen attached to an sp^3-hybridized carbon that is next to an aromatic (benzene) ring.

  • Example: C6H5{-}CH_2{-}X

• d. Vinylic halides:

  • Halogen bonded directly to an sp^2-hybridized carbon of an aliphatic (open chain) double bond.

  • Example: CH_2=CH{-}X

• e. Haloalkynes:

  • Halogen bonded directly to an sp-hybridized carbon of a carbon-carbon triple bond.

  • Example: CH{ ext{≡}}C{-}X

• f. Aryl halides (Haloarenes):

  • Halogen atom bonded directly to an sp^2 carbon of an aromatic ring.

  • Example: Chlorobenzene (C6H5{-}Cl)

Table: Naming Examples (Common vs. IUPAC)

10.2 Nomenclature of Halogen Derivatives

• Common Names: Alkyl halides are named by combining the alkyl group name and the halide (e.g., methyl iodide).

• IUPAC System: Alkyl halides are named as haloalkanes (halogen is a prefix). Aryl halides are named as haloarenes.

• For dihalogen derivatives of benzene, common names use o-, m-, p- (ortho, meta, para) prefixes, while IUPAC uses numbers (1,2-; 1,3-; 1,4-).

10.3 Methods of Preparation of Alkyl Halides

10.3.1 From Alcohols

• Most Common Method: Replacing the hydroxyl group (- ext{OH}) with a halogen atom.

• Reagents Used:

  • (a) Halogen Acids (HX): ROH + HX ightarrow RX + H_2O

    • Reactivity order of alcohols: 3^ ext{°} > 2^ ext{°} > 1^ ext{°} (tertiary alcohols react fastest).

    • Lucas Reagent: Concentrated HCl + ZnCl2 (ZnCl2 is a Lewis acid that helps break the C–O bond). Used for primary/secondary alcohols; tertiary alcohols react easily without ZnCl2.

  • (b) Hydrogen Halides (generated in situ):

    • For primary alkyl bromides: ROH + NaBr + H2SO4
      ightarrow RBr (HBr is formed first).

    • For alkyl iodides: Heat alcohols with NaI in 95\% phosphoric acid (H3PO4). Phosphoric acid is preferred over sulfuric acid for HI generation because it's cleaner.

  • (c) Phosphorous Halides (PX3): 3ROH + PX3 ightarrow 3RX + H3PO_3 (X can be Cl, Br, I).

  • (d) Thionyl Chloride (SOCl2): ROH + SOCl2 ightarrow RCl + SO_2 ( ext{gas}) + HCl ( ext{gas})

    • This is excellent for preparing alkyl chlorides as gaseous byproducts make separation easy and it avoids rearrangements.

10.3.2 From Hydrocarbons

• Direct Halogenation of Alkanes: Not ideal, as it gives a mixture of products (mono-, di- substituted, etc.).

• From Alkenes (Addition Reaction):

  • R{-}CH=CH2 + HX ightarrow R{-}CHX{-}CH3 (Major Product by Markovnikov's Rule).

  • Markovnikov's Rule: The negative part of the adding molecule (X from HX) attaches to the carbon with fewer hydrogen atoms.

  • Peroxide Effect (Anti-Markovnikov): In the presence of peroxides, addition of HBr to unsymmetrical alkenes gives the anti-Markovnikov product (H adds to the carbon with fewer hydrogens, Br to the carbon with more). This occurs via a radical mechanism.

10.3.3 Halogen Exchange Reactions

• Finkelstein Reaction (for Iodides): Converts alkyl chlorides or bromides to iodides.

  • R{-}Cl + NaI
    ightleftharpoons R{-}I + NaCl( ext{s})

  • Sodium iodide is used in acetone. NaCl precipitates out, driving the reaction forward.

• Swartz Reaction (for Fluorides): Converts alkyl chlorides or bromides to fluorides.

  • R{-}Cl + AgF
    ightarrow R{-}F + AgCl (Other metal fluorides like HgF2, CoF2 can also be used).

10.3.4 Electrophilic Substitution (for Aryl Halides)

• Halogenation of aromatic rings (like benzene) occurs via electrophilic substitution.

• Typically requires a Lewis acid catalyst (e.g., Fe, FeCl3, AlCl3) and conducted in the dark to prevent radical reactions.

• Example: Bromination of toluene with Fe produces o- and p-bromotoluene.

• Iodination: Is reversible. To get good yield, an oxidizing agent like HNO3 or HIO4 is used to remove the HI formed, shifting the equilibrium.

• Fluorination: F_2 is too reactive for routine lab synthesis.

10.3.5 Sandmeyer’s Reaction (for Aryl Halides)

• Aryl halides (like chlorobenzene or bromobenzene) can be prepared from diazonium salts by reacting with copper salts (e.g., CuCl/HCl or CuBr/HBr). This reaction specifically replaces a nitrogen (-N_2^+) group with a halogen.

10.4 Physical Properties

10.4.1 Intermolecular Forces and Polarity

• Polarity: The C–X bond (where X is a halogen) is polar because halogens are more electronegative than carbon, creating partial positive ( ext{C}^ ext{δ+}) and partial negative ( ext{X}^ ext{δ-}) charges.

• Bond Length & Strength: As halogen size increases (F < Cl < Br < I), bond length increases, and bond strength (enthalpy) decreases due to poorer orbital overlap.

  • Example: C–Cl bond length ~178 pm, C–I bond length ~214 pm.

10.4.2 Boiling Points

• Higher than Alkanes: Alkyl halides have higher boiling points than corresponding alkanes due to their increased polarity (dipole-dipole interactions) and higher molecular weight (stronger van der Waals forces).

• Trend with Halogen Mass: For a given alkyl group, boiling points increase with the mass of the halogen: RI > RBr > RCl > RF.

• Branching Effect: Branching in isomers lowers boiling points because it reduces the surface area, leading to weaker van der Waals forces.

• Haloarenes: Halogenated benzenes often show unique melting point trends for isomers (para isomer often has a higher melting point due to better crystal packing).

10.4.3 Solubility

• Insoluble in Water: Alkyl halides are only slightly soluble or insoluble in water because they cannot form strong hydrogen bonds with water molecules.

• Soluble in Organic Solvents: They are soluble in non-polar organic solvents due to similar intermolecular forces.

10.5 Optical Isomerism in Halogen Derivatives

10.5.1 Chiral Atom and Molecular Chirality

• Chiral Carbon: A carbon atom bonded to four different groups.

• Chiral Molecule: A molecule that is non-superimposable on its mirror image. Such molecules are called optically active.

• Enantiomers: Non-superimposable mirror images of chiral molecules. They have identical physical properties but rotate plane-polarized light in opposite directions.

10.5.2 Plane Polarized Light and Optical Activity

• Plane-Polarized Light: Ordinary light vibrates in all directions. A polarizer (like a Nicol prism) filters it so it vibrates in only one plane.

• Optical Activity: Chiral compounds can rotate the plane of plane-polarized light.

  • Dextrorotatory (d or +): Rotates light to the right (clockwise).

  • Levorotatory (l or –): Rotates light to the left (anticlockwise).

• Racemic Mixture (dl or ext{±}): Contains equal amounts of both enantiomers. It is optically inactive because the rotations cancel each other out.

10.5.5 Representation of Configuration

• Fischer Projection: A 2D representation where horizontal lines project out of the plane (towards you) and vertical lines go into the plane (away from you).

• Wedge Formula: Uses solid wedges for bonds coming out of the plane and dashed wedges for bonds going behind the plane.

10.6 Chemical Properties: Nucleophilic Substitution (S_N) Reactions

10.6.1 Laboratory Test of Haloalkanes

• Haloalkanes are neutral. To confirm halogen presence:

  1. Hydrolysis: Heat with aqueous KOH or NaOH to perform nucleophilic substitution (R{-}X + OH^-
     ightarrow R{-}OH + X^-).

  2. Silver Nitrate Test: Acidify the solution (with HNO3) and add AgNO3. A precipitate of silver halide (AgX) confirms the presence of halogen ion (Ag^+ + X^-
     ightarrow AgX( ext{s})).

10.6.2 Nucleophilic Substitution Reactions

• In these reactions, a nucleophile (Nu^-) replaces the halogen (X^-) from the alkyl halide (R{-}X).

• General reaction: R{-}X + Nu^-
  ightarrow R{-}Nu + X^-

• The carbon connected to the halogen is electrophilic due to the polar C–X bond.

10.6.3 Mechanisms of S_N Reactions

Two main mechanisms:

• S_N2 (Substitution Nucleophilic Bimolecular):

  • One-step Process: Bond breaking and bond forming occur simultaneously.

  • Transition State: A highly unstable state where the nucleophile and leaving group are partially bonded to the carbon.

  • Backside Attack: The nucleophile attacks from the side opposite to the leaving group.

  • Walden Inversion: If the reaction occurs at a chiral center, the configuration is inverted (like an umbrella turning inside out).

  • Rate: Depends on both alkyl halide and nucleophile concentration ( ext{Rate} = k [R{-}X] [Nu^-]).

  • Favored by: Primary alkyl halides (less steric hindrance around the carbon).

• S_N1 (Substitution Nucleophilic Unimolecular):

  • Two-step Process:

    1. Slow Step: The leaving group detaches, forming a carbocation intermediate ( ext{R}{-} ext{X}
       ightarrow ext{R}^+ + ext{X}^-).

    2. Fast Step: The nucleophile attacks the carbocation.

  • Carbocation Intermediate: Planar (sp^2 hybridized) and highly reactive.

  • Racemization: If the reaction occurs at a chiral center, the carbocation is planar, allowing attack from either side, leading to a racemic mixture (equal amounts of both enantiomers).

  • Rate: Depends only on the alkyl halide concentration ( ext{Rate} = k [R{-}X]).

  • Favored by: Tertiary alkyl halides and resonance-stabilized carbocations (e.g., allylic, benzylic) because they form more stable carbocations.

10.6.4 Factors Influencing SN1 and SN2 Mechanisms

• Substrate Structure:

  • S_N2: Favored for less hindered centers (e.g., Primary > Secondary > Tertiary).

  • SN1: Favored when a stable carbocation can be formed (e.g., Tertiary > Secondary > Primary). Allylic and benzylic halides also favor SN1 unusually well due to resonance stabilization of their carbocations.

• Leaving Group: A good leaving group (weak base like I^-, Br^-, Cl^-) is essential for both reactions.

• Nucleophile Strength: Strong nucleophiles favor SN2. Weak nucleophiles often favor SN1 (where carbocation formation is rate-determining).

• Solvent:

  • S_N2: Favored by polar aprotic solvents (e.g., acetone, DMSO) that don't solvate the nucleophile strongly.

  • S_N1: Favored by polar protic solvents (e.g., water, alcohol) that can stabilize the carbocation and leaving group through hydrogen bonding.

10.7.1 Exam Preparation: Important Questions & Answers

Here are some common questions asked in exams related to this chapter:

1. Q: Explain Markovnikov's rule with an example.
   A: Markovnikov's rule states that when a protic acid (like HX) adds to an unsymmetrical alkene, the hydrogen atom (H) adds to the carbon atom of the double bond that already has more hydrogen atoms, and the halogen (X) adds to the carbon atom with fewer hydrogen atoms. This is because the reaction proceeds via the formation of the more stable carbocation intermediate.
   Example: Addition of HBr to Propene (CH3{-}CH=CH2 + HBr
   ightarrow CH3{-}CHBr{-}CH3). The major product is 2-Bromopropane.

2. Q: What is the peroxide effect (anti-Markovnikov addition)? Give an example.
   A: The peroxide effect is observed when HBr (and only HBr) is added to an unsymmetrical alkene in the presence of peroxides. In this case, the addition occurs in an anti-Markovnikov manner, meaning the hydrogen atom adds to the carbon with fewer hydrogens, and the bromine atom adds to the carbon with more hydrogens. This is a free radical mechanism.
   Example: CH3{-}CH=CH2 + HBr ext{ (in presence of peroxide)}
   ightarrow CH3{-}CH2{-}CH_2Br. The product is 1-Bromopropane.

3. Q: Distinguish between SN1 and SN2 reaction mechanisms.
   A:

   Feature	S_N1 (Substitution Nucleophilic Unimolecular)	S_N2 (Substitution Nucleophilic Bimolecular)
   Number of Steps	Two steps	One step
   Rate Dependence	Depends only on substrate (alkyl halide)	Depends on both substrate and nucleophile
   Intermediate	Carbocation	No intermediate (has a transition state)
   Stereochemistry	Racemization (loss of optical activity)	Inversion of configuration (Walden inversion)
   Substrate Favored	Tertiary > Secondary > Primary alkyl halides	Primary > Secondary > Tertiary alkyl halides
   Solvent Favored	Polar protic solvents (e.g., water, alcohol)	Polar aprotic solvents (e.g., acetone, DMSO)

4. Q: How will you prepare 1-bromobutane from butan-1-ol?
   A: 1-Bromobutane can be prepared by reacting butan-1-ol with sodium bromide (NaBr) in the presence of concentrated sulfuric acid (H2SO4).
   CH3CH2CH2CH2OH + NaBr + H2SO4
   ightarrow CH3CH2CH2CH2Br + NaHSO4 + H2O
   Alternatively, reacting butan-1-ol with phosphorus tribromide (PBr3) can also give 1-bromobutane. 3CH3CH2CH2CH2OH + PBr3
   ightarrow 3CH3CH2CH2CH2Br + H3PO3

5. Q: Define chiral carbon and enantiomers.
   A:

   • Chiral Carbon: A carbon atom that is bonded to four different atoms or groups of atoms. It is also known as a stereocenter.

   • Enantiomers: Stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties (like melting point, boiling point) but rotate the plane of plane-polarized light in equal but opposite directions.

6. Q: Give the IUPAC name for CH2=CH{-}CH2{-}Br and C6H5{-}CH_2{-}Cl.
   A:

   • For CH2=CH{-}CH2{-}Br: 3-Bromopropene (Common name: Allyl bromide).

   • For C6H5{-}CH_2{-}Cl: Chlorophenylmethane or (Chloromethyl)benzene (Common name: Benzyl chloride).

7. Q: Explain why haloalkanes are insoluble in water but soluble in organic solvents.
   A: Haloalkanes are moderately polar molecules, but they are insoluble in water because they cannot form strong hydrogen bonds with water molecules. To dissolve in water, they would need to break the existing hydrogen bonds between water molecules, which requires a significant amount of energy. The new forces formed between haloalkane and water molecules are weaker than the hydrogen bonds. However, they are soluble in non-polar organic solvents because the intermolecular forces in both (van der Waals forces, dipole-dipole interactions) are similar and can easily interact.

Connections to Foundational Principles & Real-World Relevance

• Halogen derivatives demonstrate fundamental concepts like reaction mechanisms (e.g., SN1 vs SN2), stereochemistry (optical isomerism, enantiomers, Walden inversion), and how molecular structure influences physical properties (e.g., boiling point, solubility).

• They are important in the synthesis of many other organic compounds, acting as intermediates in pharmaceuticals, polymers, and as common solvents.