Alkyl halides

Chemistry Notes

Alkyl Halides

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  • Definition:

    • Alkyl halides, or haloalkanes, introduce alkyl groups into molecules.

    • Organic compounds where hydrogen atoms in an alkane are replaced by halogen atoms.

    • General formula: R-X (R = alkyl group, X = Cl, Br, I, or F).

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  • Classification:

    • Primary alkyl halides (1°): Carbon bonded to halogen attached to one alkyl group.

    • Secondary alkyl halides (2°): Carbon bonded to halogen attached to two alkyl groups.

    • Tertiary alkyl halides (3°): Carbon holding halogen attached to three alkyl groups.

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  • Nomenclature:

    • Common names are two-word names, IUPAC names are one-word names.

    • Steps for IUPAC naming: select longest carbon chain, number chain for lowest halogen position, indicate halogen position, name other constituents.

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  • Example Nomenclature:

    • Illustration of naming alkyl halides with bromine atoms on an 8-carbon chain.

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  • Common / IUPAC Nomenclature:

    • Examples of common and IUPAC names for alkyl halides with different alkyl groups and halogens.

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  • IUPAC Nomenclature:

    • More examples of IUPAC names for alkyl halides with various alkyl and halogen groups.

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  • Structure:

    • Orbital makeup of alkyl halides like methyl chloride with sp3 hybridized carbon and half-filled p orbital in chlorine.

Isomerism and Preparation

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  • Isomerism:

    • Chain, position, and optical isomers in alkyl halides.

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  • Preparation - Halogenation of Alkanes:

    • Method using chlorine at high temperatures, also possible with bromine.

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  • Preparation - Addition of Halogen Acids to Alkenes:

    • Markovnikov rule for addition of protic acids to asymmetric alkenes.

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  • Preparation - Action of Halogen Acids on Alcohols:

    • Method using halogen acids, thionyl chloride, or phosphorous halides on alcohols.

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  • Preparation - Halogen Exchange Reaction:

    • Method suitable for preparing alkyl iodides using halogen exchange reactions.

Physical Properties of Alkyl Halides

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  • Alkyl halides have varying physical states: gases, liquids, or solids.

    • Methyl chloride, methyl bromide, methyl fluoride, ethyl chloride are gases.

    • Alkyl halides up to (C18) are colorless liquids.

    • Higher alkyl halides are colorless solids.

  • Solubility characteristics:

    • Insoluble in water due to the inability to form hydrogen bonds.

    • Soluble in organic solvents due to London Dispersion forces.

  • Density differences:

    • Alkyl bromides and iodides are denser than water.

    • Alkyl chlorides and fluorides are lighter than water.

  • Electronegativity and bond characteristics:

    • Halogens are more electronegative than carbons.

    • Carbon-halogen bond is polarized with partial charges.

    • Electronegativity increases from iodine to fluorine.

  • Molecular size and bond strength:

    • Molecular size increases down the periodic table.

    • Increase in size leads to longer bonds and decreased bond strength.

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  • Boiling point differences between alkanes and haloalkanes:

    • Haloalkanes have higher boiling points.

    • Contributing factors: London dispersion forces and dipole-dipole interactions.

  • Influence of molecular size on boiling point:

    • Increase in size leads to higher boiling points.

    • Substitution of halogen for hydrogen increases surface area.

  • IR spectrum absorption:

    • Strong absorption from vibrations of C-X bond.

    • Different absorption ranges for C-F, C-Cl, C-Br, C-I bonds.

Chemical Properties of Alkyl Halides

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  • Substitution reactions:

    • Carbon-halogen bond polarity makes carbon a target for nucleophiles.

    • Nucleophilic substitution reactions are common.

  • Representation and products of substitution reactions.

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  • Mechanism of substitution reactions:

    • SN2 mechanism involves simultaneous attack and halide ion ejection.

    • Transition state in hydrolysis of methyl bromide example.

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  • Further details on the mechanism of substitution reactions.

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  • Substitution mechanisms based on alkyl halide types:

    • Primary alkyl halides undergo SN2 mechanism.

    • Tertiary alkyl halides undergo SN1 mechanism.

    • Solvent influence on secondary alkyl halides.

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  • Differences between SN1 and SN2 reactions.

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  • Various nucleophilic substitution reactions:

    • Reaction with aqueous KOH.

    • Reaction with sodium alkoxides (Williamson Ether Synthesis).

    • Reaction with ammonia.

    • Reaction with sodium cyanide.

    • Reaction with moist silver oxide.

    • Reactions with KSH, K2S, and AgNO2.

Chemical Properties of Alkyl Halides

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  • Elimination Reactions

    • Alkyl halides react with alcoholic KOH to form alkenes.

    • Dehydrohalogenation reaction occurs, eliminating HX from alkyl halides.

    • Examples: Ethyl bromide forms ethylene, 1-Bromopropane forms propene.

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  • Mechanism of Elimination Reactions

    • Equilibrium between solvent and KOH in ethanol produces potassium ethoxide, a strong base favoring elimination.

    • Competition between elimination and substitution reactions.

    • Example: Ethyl bromide with alcoholic KOH can yield ethylene or diethyl ether.

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  • Mechanisms of Elimination Reactions

    • E1 Reaction

      • Unimolecular elimination involving ionization and deprotonation.

      • Formation of carbocation intermediate and pi-bond.

      • First-order kinetics, rate proportional to substance concentration.

    • E2 Reaction

      • Bimolecular elimination in a one-step mechanism.

      • Reaction rate proportional to concentrations of eliminating agent and substrate.

      • Second-order kinetics, major product is the most stable alkene.

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  • Mechanisms of Elimination Reactions

    • Reaction Parameters

      • Comparison between E2 and E1 mechanisms based on alkyl halide structure, nucleophile, mechanism, rate law, stereochemistry, and solvent.

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  • Elimination Reactions

    • Saytzeff’s Rule

      • Predicts regioselectivity of the olefin formed by the elimination reaction of secondary or tertiary alkyl halides.

      • Proton removed from the carbon atom with fewer substituents.

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  • Miscellaneous Reactions

    • Reduction

      • Alkyl halides reduced to alkanes with reducing agents.

    • Reactions with Metals

      • Alkyl halides react with magnesium or lithium to form Grignard reagents or alkyllithiums.

      • Alkyllithiums behave like Grignard reagents but with increased reactivity.

    • Wurtz Reaction

      • Alkyl halides react with metallic sodium to form alkanes with double the carbon atoms.

    • Halogenation

      • Alkyl halides react with Cl2 or Br2 to form polyhalogenation derivatives.

    • Friedel-Crafts Alkylation

      • Alkyl halides react with benzene in the presence of AICI3 to form alkylbenzenes.

Alkyl Dihalides

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  • Dihalogen Derivatives

    • Compounds obtained by replacing two hydrogen atoms with halogen atoms.

    • Vicinal and geminal dihalides based on halogen atom positions.

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  • Methods of Preparation

    • gem-Dihalides

      • Prepared by phosphorus pentahalides on aldehydes/ketones or addition of hydrogen halides to alkynes.

    • vic-Dihalides

      • Prepared by addition of halogens to alkenes or action of phosphorus halides on glycols.

Page 39: Alkyl Dihalides Chemical Properties

  • Hydrolysis with Aqueous NaOH or KOH:

    • vic-Dihalides on heating with aqueous NaOH or KOH give glycols.

    • gem-Dihalides on treatment with alcoholic KOH give alkynes.

  • Hydrolysis Reactions:

    • vic-Dihalides on hydrolysis with aqueous KOH give glycols.

    • gem-Dihalides on hydrolysis with aqueous KOH give aldehydes or ketones.

  • Examples:

    • 1,2-Dichloroethane hydrolysis gives glycols.

    • 1,1-Dichloroethane (gem-dihalide) hydrolysis gives aldehydes or ketones.

Page 40: Alkyl Trihalides

  • Preparation Methods:

    • Chloroform is prepared from ethanol or acetone and bleaching powder.

    • Iodoform is prepared from ethanol or acetone by the action of iodine and alkali.

  • Reaction Steps for Chloroform Preparation:

    • Oxidation of ethanol to acetaldehyde.

    • Chlorination of acetaldehyde to chloral.

    • Hydrolysis of chloral to chloroform.

  • Other Preparation Method:

    • Chloroform can also be prepared from methane by chlorination at 400°C.

  • Chemical Properties:

    • Chloroform undergoes oxidation, reduction, hydrolysis, chlorination, and nitration reactions.

Page 41: Alkyl Trihalides - Chemical Properties

  • Chemical Reactions:

    • Oxidation of chloroform to form phosgene.

    • Reduction of chloroform to dichloromethane.

    • Hydrolysis of chloroform to sodium formate.

    • Chlorination of chloroform to form carbon tetrachloride.

    • Nitration of chloroform to form chloropicrin.

  • Storage:

    • Chloroform is stored in dark brown bottles to prevent the formation of phosgene.

Page 42: Alkyl Trihalides - Chemical Properties

  • Chloretone:

    • Used as an hypnotic and nervous sedative.

    • IUPAC name: 1,1,1-trichloro-2-methyl-2-propanol.

  • Iodoform:

    • Undergoes hydrolysis with hot aqueous NaOH.

Page 43: Alkyl Tetrahalides - Carbon Tetrachloride

  • Carbon Tetrachloride:

    • Used as a fire-extinguisher under the name Pyrene.

    • Excellent solvent due to being non-inflammable