Chemistry Class XII Vocabulary Review

Classification and Nomenclature of Haloalkanes and Haloarenes

• Definitions and Basic Concepts

  • The replacement of hydrogen atom(s) in a hydrocarbon (aliphatic or aromatic) by halogen atom(s) results in the formation of alkyl halides ($haloalkanes$) and aryl halides ($haloarenes$) respectively.
  • Haloalkanes: Contain halogen atom(s) attached to the $sp^3$ hybridised carbon atom of an alkyl group.
  • Haloarenes: Contain halogen atom(s) attached to the $sp^2$ hybridised carbon atom(s) of an aryl group.
  • Clinical and Industrial Importance:
    • Chloramphenicol: A chlorine-containing antibiotic produced by soil microorganisms; used for typhoid fever.
    • Thyroxine: An iodine-containing hormone produced by the human body; deficiency causes goiter.
    • Chloroquine: A synthetic halogen compound used for malaria treatment.
    • Halothane: Used as an anaesthetic during surgery.
    • Fluorinated Compounds: Considered potential blood substitutes in surgery.

• Classification Schemes:

  • On the Basis of Number of Halogen Atoms: Classified as mono, di, or polyhalogen (tri-, tetra-, etc.) compounds.
  • Compounds Containing $sp^3$ C–X Bond:
    • Alkyl Halides (R—X): Halogen is bonded to an alkyl group. General formula: $C_nH_{2n+1}X$. Classified as primary ($1^{\circ}$), secondary ($2^{\circ}$), or tertiary ($3^{\circ}$) depending on the nature of the carbon to which the halogen is attached.
    • Allylic Halides: Halogen is bonded to an $sp^3$ hybridised carbon atom adjacent to a carbon-carbon double bond ($C=C$).
    • Benzylic Halides: Halogen is bonded to an $sp^3$ hybridised carbon atom next to an aromatic ring.
  • Compounds Containing $sp^2$ C–X Bond:
    • Vinylic Halides: Halogen is bonded to an $sp^2$ hybridised carbon atom of a $C=C$ bond.
    • Aryl Halides: Halogen is bonded directly to the $sp^2$ hybridised carbon atom of an aromatic ring.

• Nomenclature and Isomerism:

  • IUPAC System: Alkyl halides are named as halosubstituted hydrocarbons. For dihalogen derivatives, numerals ($1$,$2$; $1$,$3$; $1$,$4$) are used.
  • Common Names: For dihalogen derivatives, prefixes $o-$, $m-$, $p-$ are used.
  • Geminal vs. Vicinal:
    • Gem-dihalides: Both halogen atoms on the same carbon (alkylidene halides).
    • Vic-dihalides: Halogens on adjacent carbon atoms (alkylene dihalides).

Nature of Carbon-Halogen (C–X) Bond

• Polarity: Halogen atoms are more electronegative than carbon, leading to a polarized C–X bond where the carbon bears a partial positive charge ($\delta+$) and the halogen bears a partial negative charge ($\delta-$).
• Bond Trends down the Group:
  • As we move from fluorine to iodine, the size of the halogen atom increases.
  • Bond Length: Increases from C–F to C–I.
  • Bond Enthalpy: Decreases from C–F to C–I.
• Typical Data (Methyl Halides):
  • $CH_3-F$: Bond length $139\,pm$; Enthalpy $452\,kJ\,mol^{-1}$; Dipole moment $1.847\,Debye$.
  • $CH_3-Cl$: Bond length $178\,pm$; Enthalpy $351\,kJ\,mol^{-1}$; Dipole moment $1.860\,Debye$.
  • $CH_3-Br$: Bond length $193\,pm$; Enthalpy $293\,kJ\,mol^{-1}$; Dipole moment $1.830\,Debye$.
  • $CH_3-I$: Bond length $214\,pm$; Enthalpy $234\,kJ\,mol^{-1}$; Dipole moment $1.636\,Debye$.

Methods of Preparation of Haloalkanes and Haloarenes

• From Alcohols:

  • Replacement of hydroxyl group ($-OH$) by reaction with $HX$ (conc. halogen acids), $PX_3$, $PX_5$, or $SOCl_2$ (thionyl chloride).
  • Thionyl Chloride Advantage: Preferred because byproducts ($SO_2$ and $HCl$) are escapable gases, leaving pure alkyl halides.
  • Catalysts: Reaction with primary and secondary alcohols requires $ZnCl_2$.
  • Reactivity Order of Alcohols: $tertiary > secondary > primary$.

• From Hydrocarbons:

  • Free Radical Halogenation: Gives a mixture of mono- and polyhaloalkanes which are difficult to separate.
  • Electrophilic Substitution: Preparation of aryl chlorides/bromides using arenes and $Cl_2/Br_2$ in the presence of Lewis acids ($Fe$ or $FeCl_3$).
  • Sandmeyer’s Reaction: Primary aromatic amine treated with $NaNO_2 + HX$ ($273-278\,K$) forms a diazonium salt. Mixing with cuprous chloride ($Cu_2Cl_2$) or cuprous bromide ($Cu_2Br_2$) replaces the diazonium group with –Cl or –Br.

• From Alkenes:

  • Addition of HX: Follows Markovnikov’s rule.
  • Addition of Halogens: Br2 in $CCl_4$ added to alkene results in discharge of reddish-brown color, forming vic-dibromides (a test for unsaturation).

• Halogen Exchange Reactions:

  • Finkelstein Reaction: Alkyl chlorides/bromides react with $NaI$ in dry acetone to prepare alkyl iodides.
  • Swarts Reaction: Synthesis of alkyl fluorides by heating alkyl chlorides/bromides with metallic fluorides ($AgF$, $Hg_2F_2$, $CoF_2$, or $SbF_3$).

Physical Properties of Haloalkanes

• Melting and Boiling Points:

  • Boiling points are higher than parent hydrocarbons due to stronger dipole-dipole and van der Waals forces.
  • Order for same alkyl group: $RI > RBr > RCl > RF$.
  • Branching Effects: Boiling points decrease as branching increases because of decreased surface area.
  • Isomeric Dihalobenzenes: $Para$-isomers have higher melting points due to symmetry and better packing in the crystal lattice compared to $ortho$ and $meta$ isomers.

• Density and Solubility:

  • Density: Bromo, iodo, and polychloro derivatives are heavier than water. Density increases with more halogens and higher atomic mass of halogen.
  • Solubility: Only very slightly soluble in water because they cannot form hydrogen bonds with water. Highly soluble in organic solvents.

Chemical Reactions: Nucleophilic Substitution ($S_N1$ and $S_N2$)

• Substitution Nucleophilic Bimolecular ($S_N2$):

  • Kinetics: Second order (Rate depends on concentration of both nucleophile and substrate).
  • Mechanism: Single-step transition state. Incoming nucleophile attacks from the side opposite to the leaving group.
  • Stereochemistry: Complete inversion of configuration (Walden inversion).
  • Reactivity Order: $Primary > Secondary > Tertiary$ (due to steric hindrance).

• Substitution Nucleophilic Unimolecular ($S_N1$):

  • Kinetics: First order (Rate depends only on substrate concentration).
  • Mechanism: Two steps. Step 1 (Slow): Formation of a planar carbocation. Step 2 (Fast): Nucleophile attack.
  • Stereochemistry: Racemisation (formation of both $d$ and $l$ forms).
  • Reactivity Order: $Tertiary > Secondary > Primary$ (stability of carbocation).

• Ambident Nucleophiles: Groups with two nucleophilic centers ($CN^-$, $NO_2^-$).

  • $KCN$ + $RX \rightarrow$ Alkyl cyanide ($RCN$) [C-C bond is more stable].
  • $AgCN$ + $RX \rightarrow$ Alkyl isocyanide ($RNC$) [Ag-C bond is covalent; N is free to donate].

Elimination and Reaction with Metals

• Elimination Reaction (\beta-elimination):

  • Reaction with alcoholic $KOH$ leads to the removal of hydrogen from the $\beta$-carbon and halogen from the $\alpha$-carbon, forming an alkene.
  • Zaitsev Rule: The preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms.

• Reaction with Metals:

  • Grignard Reagents: Organo-metallic compounds ($RMgX$) formed by reacting haloalkanes with magnesium in dry ether.
  • Wurtz Reaction: Two molecules of alkyl halide react with sodium in dry ether to form a hydrocarbon with double the carbon atoms.
  • Fittig Reaction: Aryl halides react with sodium in dry ether to form biphenyl (joining of two aryl groups).

Chemical Reactions of Haloarenes

• Low Reactivity: Aryl halides are less reactive toward nucleophilic substitution due to:
  1. Resonance Effect: Partial double bond character of the C–X bond.
  2. Hybridisation: $sp^2$ carbon is more electronegative than $sp^3$, holding the bond tighter.
  3. Instability of Phenyl Cation: Prevents $S_N1$ mechanism.
• Effect of Substituents: Presence of electron-withdrawing groups ($-NO_2$) at $ortho$ and $para$ positions increases reactivity.
• Electrophilic Substitution: Halogens are $ortho-$ and $para-$ directing but deactivating. Reactions include halogenation, nitration, sulphonation, and Friedel-Crafts alkylation/acylation.

Polyhalogen Compounds and Environmental Impact

• Dichloromethane ($CH_2Cl_2$): Solvent, paint remover, propellant; harms central nervous system.
• Trichloromethane ($CHCl_3$ - Chloroform): Production of Freon $R-22$. Oxidises to poisonous $Phosgene$ ($COCl_2$) in light.
• Triiodomethane ($CHI_3$ - Iodoform): Formerly used as an antiseptic; properties due to liberation of free iodine.
• Tetrachloromethane ($CCl_4$): Feedstock for refrigerants; depletes ozone layer.
• Freons: Chlorofluorocarbons ($CFCs$). $Freon\,12$ ($CCl_2F_2$) is common. Extremely stable but initiate radical chain reactions that upset the ozone balance in the stratosphere.
• DDT ($p,p'-Dichlorodiphenyltrichloroethane$): First chlorinated organic insecticide. Highly toxic to fish and chemically stable (bioaccumulate in fatty tissues). Banned in the US in $1973$.

Alcohols, Phenols, and Ethers: Preparation and Properties

• Classification:

  • Alcohols: $sp^3\,C-OH$. Classified as primary, secondary, and tertiary.
  • Phenols: $-OH$ attached to aromatic ring.
  • Ethers: Symmetrical ($R-O-R$) or unsymmetrical ($R-O-R'$).

• Preparation of Alcohols:

  • Hydration of Alkenes: Acid catalysed ($H^+/H_2O$). Follows Markovnikov’s rule.
  • Hydroboration-Oxidation: Yields alcohols in an anti-Markovnikov manner. Reagents: $B_2H_6$ followed by $H_2O_2/OH^-$.
  • Reduction: Carbonyls ($H_2/Pd$, $NaBH_4$, or $LiAlH_4$).
  • Grignard Synthesis: Methanal yields primary alcohol; other aldehydes yield secondary; ketones yield tertiary.

• Acidity of Alcohols and Phenols:

  • Acidity order for alcohols: $primary > secondary > tertiary$.
  • Phenols are more acidic than alcohols: Due to resonance stabilization of the phenoxide ion ($C_6H_5O^-$). Electron-withdrawing groups (like $-NO_2$) increase acidity.

• Named Reactions (Unit 11):

  • Kolbe’s Reaction: Phenols treated with $NaOH$ then $CO_2$ (acidified) to form salicylic acid.
  • Reimer-Tiemann Reaction: Phenol treated with $CHCl_3/NaOH$ to form salicylaldehyde.

Aldehydes, Ketones, and Carboxylic Acids

• Nomenclature and Structure: Carbonyl group ($>C=O$) is $sp^2$ hybridised; planar with bond angles of approx $120^{\circ}$.

• Preparation:

  • Rosenmund Reduction: Acyl chloride hydrogenated over $Pd/BaSO_4$ to form aldehydes.
  • Etard Reaction: Toluene treated with $CrO_2Cl_2$ to give benzaldehyde.
  • Gatterman-Koch: Benzene + $CO/HCl$ in presence of $AlCl_3 \rightarrow$ Benzaldehyde.

• Chemical Reactions:

  • Nucleophilic Addition: Addition of $HCN$, $NaHSO_3$, $Grignard$, $alcohols$ (acetal/ketal formation).
  • Reduction: Clemmensen ($Zn-Hg/HCl$) and Wolff-Kishner ($NH_2NH_2/KOH$) reduce carbonyls to hydrocarbons ($-CH_2-$).
  • Aldol Condensation: Aldehydes/ketones with at least one $\alpha$-hydrogen react with dilute alkali to form $\beta$-hydroxy carbonyls.
  • Cannizzaro Reaction: Aldehydes with no $\alpha$-hydrogen (e.g., formaldehyde, benzaldehyde) undergo self-oxidation and reduction with concentrated alkali.

• Acidity of Carboxylic Acids:

  • Stronger than phenols. Stabilized by two equivalent resonating structures of the carboxylate ion ($RCOO^-$).
  • EWG Effect: $-CF_3 > -NO_2 > -CN > -F$ increases acidity.

Amines and Biomolecules

• Basicity of Amines:

  • Aliphatic amines are stronger bases than $NH_3$ in gas phase ($3^{\circ} > 2^{\circ} > 1^{\circ} > NH_3$).
  • In aqueous phase, a combination of inductive effect, solvation, and steric hindrance affects basicity ($2^{\circ} > 1^{\circ} > 3^{\circ}$ for methyl amines; $2^{\circ} > 3^{\circ} > 1^{\circ}$ for ethyl amines).

• Preparation of Amines:

  • Gabriel Phthalimide Synthesis: For pure primary aliphatic amines.
  • Hoffmann Bromamide Degradation: Amide + $Br_2/NaOH \rightarrow$ Amine with one less carbon atom.

• Carbohydrates:

  • Glucose: Aldohexose. Cyclic structures (Pyranose). Invert sugar is a mixture of glucose and fructose from sucrose hydrolysis.
  • Starch: Two components - Amylose (linear, water soluble) and Amylopectin (branched, insoluble).

• Proteins and Nucleic Acids:

  • Amino Acids: Zwitterions (electrically neutral total but contain charges). Essential amino acids must be obtained through diet.
  • DNA/RNA: DNA has deoxyribose sugar and bases A, G, C, T. RNA has ribose and bases A, G, C, U.

Polymers and Chemistry in Everyday Life

• Polymers:

  • Addition (Chain Growth): Polythene, Teflon, Polyacrylonitrile.
  • Condensation (Step Growth): Nylon 6,6, Terylene, Bakelite.
  • Biodegradable Polymers: $PHBV$, Nylon 2-nylon 6.

• Drugs:

  • Analgesics: Non-narcotic (Aspirin) and Narcotic (Morphine).
  • Antiseptics/Disinfectants: Dettol (chloroxylenol + terpineol). $Iodine$ is a strong antiseptic.
  • Antimalarials: Chloroquine.
  • Antacids: Cimetidine, Ranitidine (block histamine receptors in stomach).

Questions & Discussion

• Question: Why is sulphuric acid not used during the reaction of alcohols with $KI$?

• Response: $H_2SO_4$ is an oxidising agent. It converts $KI$ to $HI$ and then oxidises $HI$ to iodine ($I_2$), preventing the reaction with the alcohol.

• Question: Why is use of aspartame limited to cold foods and drinks?

• Response: Aspartame is unstable at cooking temperatures; it decomposes upon heating.

• Question: What are ambident nucleophiles?

• Response: Nucleophiles having two nucleophilic centers through which they can attack, such as nitrate ($NO_2^-$) and cyanide ($CN^-$).