HALOGENOALKANES

• Halogenoalkanes contain polar bonds as halogens are more electronegative than carbon atoms, leading to electron density being drawn towards the halogen forming ∂+ and ∂- regions
• Nucleophiles are species with a lone electron pair attracted to ∂+ regions of molecules; common nucleophiles include CN​:​-, :​NH​3, and -​:​OH
• Nucleophilic substitution is a reaction mechanism where nucleophiles attack halogenoalkanes, producing alcohols or amines
• The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy, making it easier to break and resulting in faster reaction rates for halogenoalkanes
• Nucleophilic substitution reactions can only occur for 1​o (primary) and 2​o (secondary) halogenoalkanes
• Elimination occurs when a halogenoalkane is heated to high temperatures under alcoholic conditions, resulting in the removal of a hydrogen atom and the halide, producing a carbon-carbon double bond (alkene)
• Elimination reactions can only occur from 2​o and 3​o (tertiary) halogenoalkanes
• Ozone in the atmosphere absorbs UV radiation, while CFCs break down carbon-halogen bonds to form free radicals that catalyze ozone depletion
• CFC-free solvents are now being produced to prevent ozone depletion and minimize global warming
• Haloalkanes contain the functional group C-X where X is a halogen (F, Cl, Br, or I)
• Haloalkanes are classified according to what is attached to the functional group
• Names of haloalkanes are based on the original alkane with a prefix indicating halogens and their position
• Physical properties of haloalkanes:

  • Boiling point increases with mass
  • For isomeric compounds, the greater the branching, the lower the boiling point
  • Solubility: haloalkanes are soluble in organic solvents but insoluble in water due to not being polar enough to exhibit hydrogen bonding
• Nucleophilic substitution reactions theory:

  • Halogens have greater electronegativity than carbon
  • A dipole is induced in the C-X bond, making it polar
  • Nucleophiles like OH¯, CN¯, NH3, and H2O possess at least one lone pair of electrons and are attracted to the slightly positive carbon
• Basic mechanism of nucleophilic substitution:

  • The nucleophile uses its lone pair to provide electrons for a new bond
  • Carbon can only have 8 electrons in its outer shell, so a halide ion is displaced
  • The mechanism is known as Nucleophilic Substitution
• Rate of reaction in nucleophilic substitution depends on the strength, not the polarity, of the C-X bond
• Practical investigation for nucleophilic substitution reactions:

  • Measure the time taken for a precipitate of silver halide to form
  • Faster precipitate formation indicates faster hydrolysis and a weaker C-X bond
  • Procedure: warm equal amounts of each haloalkane in a water bath, add a solution of ethanol, water, and aqueous silver nitrate, record the time for a precipitate to appear (AgCl - white, AgBr - cream, AgI - yellow)
• Reagent for nucleophilic substitution with NaOH:

  • Aqueous sodium (or potassium) hydroxide
  • Conditions: reflux in aqueous solution
  • Product: Alcohol
  • Nucleophile: hydroxide ion (OH¯)
  • Equation: e.g., C2H5Br(l) + NaOH(aq) → C2H5OH(l) + NaBr(aq)
• In the reaction between C2H5Br and NH3(aq/alc), the products formed are C2H5NH2 and HBr
• Excess ammonia is used to ensure the removal of HBr, preventing further substitution reactions and leading to the formation of a salt
• The amine produced in the reaction, C2H5NH2, can act as a nucleophile and attack another molecule of haloalkane to produce a 2° amine
• Further reactions can occur where the 2° amine produced can react to form a 3° amine and eventually an ionic quarternary ammonium salt
• In the Friedel Crafts alkylation reaction, an alkyl group is substituted onto a benzene ring using a haloalkane and anhydrous aluminium chloride AlCl3 as reagents
• Haloalkanes play a crucial role in synthetic organic chemistry due to the reactivity of the C-X bond, allowing for substitution by various groups via a nucleophilic substitution mechanism
• Elimination reactions of haloalkanes with alcoholic sodium hydroxide under reflux conditions in an alcoholic solution result in the formation of an alkene
• In elimination reactions, the OH¯ ion acts as a base, picking up a proton from a carbon atom adjacent to the one bonded to the halogen, leading to the elimination of HBr
• Halogenoalkanes are synthetic compounds used to make refrigerants, solvents, pharmaceuticals, PVC, Teflon, anesthetics, and more
• Halogenoalkanes have the same carbon skeleton as alkanes but are more reactive due to the presence of a halogen, making the carbon-halogen bond polar
• Naming halogenoalkanes:

  • Find the longest unbranched carbon chain
  • Use prefixes like fluoro-, chloro-, bromo-, iodo- depending on the halogen present
  • If there are multiple halogens, use prefixes like di-, tri-, tetra-
  • Number the carbons to indicate the position of the halogens
• Bond polarity in halogenoalkanes is due to the difference in electronegativity between carbon and the halogen, with fluorine being the most electronegative
• Solubility of halogenoalkanes:

  • Insoluble in water due to nonpolar R groups
  • Soluble in hydrocarbons due to nonpolar nature
• Boiling point of halogenoalkanes:

  • Increases with chain length
  • Decreases with branching
  • Increases down the group of halogens due to stronger van der Waals forces
• In chemistry, the reactivity of halogenoalkanes depends on the ease with which the C-X bond breaks, with a more easily broken bond indicating a more reactive halogenoalkane
• Reactivity increases as we go down the halogen group, with iodine-containing halogenoalkanes reacting faster than those with chlorine
• Nucleophilic substitution reactions involve a nucleophile replacing an atom or group of atoms in a molecule, with the rate of substitution depending on the halogen present
• Nucleophiles, like hydroxide ions, cyanide ions, and ammonia, use their lone pair of electrons to form a new bond with an electron-deficient atom in halogenoalkanes
• In nucleophilic substitution mechanisms, curly arrows show the movement of electrons, with the halogen being replaced by the nucleophile to form new compounds like alcohols, nitriles, and amines
• The reactivity of halogenoalkanes in nucleophilic substitution reactions follows the trend: iodo- > bromo- > chloro- > fluoro-, with iodine being more reactive than chlorine
• In nucleophilic substitution reactions, the leaving group is the halide ion, and the nucleophile replaces it to form new compounds like alcohols, nitriles, and amines
• The ammonia nucleophile, being uncharged, requires two ammonia molecules in a reaction, resulting in the formation of amines and ammonium chloride