Comprehensive Notes on Halogenoalkanes: Properties, Reactions, and Environmental Chemistry

Naming and Classification of Halogenoalkanes

• Halogenoalkanes are named based on the original alkane chain from which they are derived, with a specific prefix indicating the halogen atom substitute:
  • Fluoro for $F$
  • Chloro for $Cl$
  • Bromo for $Br$
  • Iodo for $I$
• When naming halogenoalkanes with multiple substituents, the substituents are listed in alphabetical order. For example, in $2-chloro-2-methylbutane$, the chloro substituent is listed before the methyl substituent.
• Specific examples of halogenoalkane nomenclature include:
  • $1-bromopropane$
  • $2-chloro-2-methylbutane$
  • $3-methylbutanenitrile$ (for nitriles)
• Halogenoalkanes are classified as primary, secondary, or tertiary based on the number of carbon atoms attached to the carbon atom adjoining the halogen ($C-X$) functional group:
  • Primary ($1^{\circ}$) halogenoalkane: One carbon is attached to the carbon atom adjoining the halogen.
  • Secondary ($2^{\circ}$) halogenoalkane: Two carbons are attached to the carbon atom adjoining the halogen.
  • Tertiary ($3^{\circ}$) halogenoalkane: Three carbons are attached to the carbon atom adjoining the halogen.

Nucleophilic Substitution Reactions

• Halogenoalkanes primarily undergo either substitution or elimination reactions.
• Substitution is defined as the swapping of a halogen atom for another atom or groups of atoms.
• The central mechanism for these substitutions is nucleophilic attack. A nucleophile is defined as an electron pair donator. Common nucleophiles include:
  • $:OH^-$
  • $:NH_3$
  • $:CN^-$
• Nucleophiles attack the positive carbon atom in the halogenoalkane structure. This carbon possesses a partial positive charge ($\delta+$) due to the electronegativity difference between the carbon and the halogen ($\delta-$).
• Mechanism Representation:
  • Organic reactions are classified by their mechanisms, which show in detail how a reaction proceeds.
  • The symbol $:Nu$ represents any nucleophile; they always possess a lone pair of electrons.
  • Curly arrows with two-line heads are used to signify the movement of two electrons.
  • A curly arrow must always start from either a lone pair of electrons or the center of a chemical bond.

Bond Enthalpy and Reactivity Trends

• The rate of nucleophilic substitution reactions is determined by the strength of the carbon-halogen ($C-X$) bond.
• The weaker the bond, the easier it is to break, leading to a faster reaction rate.
• Bond Enthalpy values (measured in $kJ\,mol^{-1}$) for common $C-X$ bonds are as follows:
  • $C-I = 238\,kJ\,mol^{-1}$
  • $C-Br = 276\,kJ\,mol^{-1}$
  • $C-Cl = 338\,kJ\,mol^{-1}$
  • $C-F = 484\,kJ\,mol^{-1}$
• Based on these enthalpies, iodoalkanes demonstrate the fastest substitution rates as they have the weakest bonds. Conversely, fluoroalkanes are the slowest due to the extreme strength of the $C-F$ bond, which renders them largely unreactive.

Comparing the Rate of Hydrolysis Reactions

• Hydrolysis is defined as the splitting of a molecule (specifically a halogenoalkane) through a reaction with water.
• Although water is a poor nucleophile, it can react slowly with halogenoalkanes:
  • $CH_3CH_2X + H_2O \rightarrow CH_3CH_2OH + X^- + H^+$
• The rate of hydrolysis can be compared by adding aqueous silver nitrate ($AgNO_3$) to the halogenoalkane. As the halide leaving group ($X^-$) is released, it combines with a silver ion ($Ag^+$) to form a silver halide precipitate.
• The rate of precipitate formation measures the reactivity of the halogenoalkane. The chemical equation for this formation is:
  • $Ag^+(aq) + X^-(aq) \rightarrow AgX(s)$
• Observed precipitates and their rates:
  • Silver Iodide ($AgI$): Yellow precipitate; forms fastest due to the weakest bond.
  • Silver Bromide ($AgBr$): Cream precipitate; forms at a moderate rate.
  • Silver Chloride ($AgCl$): White precipitate; forms slowest because the $C-Cl$ bond is stronger than $C-Br$ or $C-I$.

Specific Nucleophilic Substitution Pathways

• Substitution with Aqueous Hydroxide Ions:
  • Change in functional group: Halogenoalkane to alcohol.
  • Reagent: Potassium hydroxide ($KOH$) or Sodium hydroxide ($NaOH$).
  • Conditions: Aqueous solution, warm conditions.
  • Type of Reagent: Nucleophile ($OH^-$).
  • Example: $1-bromopropane + KOH \rightarrow propan-1-ol + KBr$.
  • Note: Using aqueous conditions is critical; if the solvent is changed to ethanol, an elimination reaction occurs instead.
• Substitution with Cyanide Ions:
  • Change in functional group: Halogenoalkane to nitrile.
  • Reagent: Potassium cyanide ($KCN$) dissolved in an ethanol/water mixture.
  • Conditions: Heating under reflux.
  • Type of Reagent: Nucleophile ($:CN^-$).
  • Reaction Significance: This reaction increases the length of the carbon chain. For example, $1-bromopropane$ becomes $butanenitrile$.
  • Naming Nitriles: The chain numbering must start from the carbon in the nitrile group ($CN$). Example: $CH_3CH_2CN$ is $propanenitrile$. Note the inclusion of the 'e' in spelling: $butanenitrile$, not butannitrile.
• Substitution with Ammonia:
  • Change in functional group: Halogenoalkane to amine.
  • Reagent: Ammonia ($NH_3$) dissolved in ethanol.
  • Conditions: Heating under pressure in a sealed tube.
  • Type of Reagent: Nucleophile ($:NH_3$).
  • Example: $CH_3CH_2CH_2Br + 2NH_3 \rightarrow CH_3CH_2CH_2NH_2 + NH_4Br$.
  • Naming Amines: $CH_3CH_2CH_2NH_2$ is $propylamine$ (or IUPAC $propan-1-amine$).
  • Secondary Reaction Mitigation: Further substitution can occur between the halogenoalkane and the newly formed amine, lowering the yield. Using an excess of ammonia helps minimize this further substitution.

Nucleophilic Substitution in Tertiary Halogenoalkanes

• Tertiary halogenoalkanes undergo nucleophilic substitution via an alternative mechanism because the bulky methyl groups prevent the standard nucleophilic attack.
• Process:
  1. The halogen atom (e.g., $Br$) first breaks away from the halogenoalkane to form a carbocation intermediate.
  2. The hydroxide nucleophile then attacks the positive carbon of the carbocation.
• This mechanism is possible because tertiary carbocations are stabilized by the electron-releasing effect of the surrounding methyl groups.

Elimination Reactions

• Elimination is defined as the removal of a small molecule (often water or a hydrogen halide) from an organic molecule.
• Elimination with Alcoholic Hydroxide Ions:
  • Change in functional group: Halogenoalkane to alkene.
  • Reagents: Potassium or Sodium hydroxide ($KOH$ or $NaOH$).
  • Conditions: Dissolved in ethanol and heated under reflux.
  • Type of Reagent: Base ($OH^-$).
  • Example: $1-bromopropane + KOH \rightarrow propene + KBr + H_2O$.
• Solvent Influence: The choice of solvent determines the reaction pathway. Aqueous solutions favor substitution, while alcoholic/ethanolic solutions favor elimination.
• Structural Effects on Products:
  • Unsymmetrical secondary and tertiary halogenoalkanes can produce two or three different structural isomers. For instance, $2-methyl-2-chlorobutane$ can yield $2-methylbut-1-ene$ and $2-methylbut-2-ene$.
  • Primary halogenoalkanes tend to favor substitution.
  • Tertiary halogenoalkanes tend to favor elimination.
  • Often, a mixture of both substitution and elimination products is formed.

Ozone Chemistry and Environmental Impact

• The Ozone Layer: Naturally occurring ozone ($O_3$) in the upper atmosphere is beneficial because it filters out harmful UV radiation from the sun. In the lower atmosphere, however, ozone is a pollutant that contributes to smog formation.
• CFCs and Ozone Depletion: Man-made chlorofluorocarbons (CFCs) caused the formation of a hole in the ozone layer. UV radiation in the upper atmosphere provides enough energy to break the $C-Cl$ bonds in CFCs, creating chlorine radicals ($Cl\cdot$).
  • Initiation: $CF_2Cl_2 \rightarrow CF_2Cl\cdot + Cl\cdot$
  • Propagation Step 1: $Cl\cdot + O_3 \rightarrow ClO\cdot + O_2$
  • Propagation Step 2: $ClO\cdot + O_3 \rightarrow 2O_2 + Cl\cdot$
  • Overall Equation: $2O_3 \rightarrow 3O_2$
• Chlorine radicals act as catalysts because they are regenerated in the second propagation step and provide an alternative reaction route with lower activation energy. A single $Cl\cdot$ radical can destroy thousands of ozone molecules.
• Alternatives and Legislation: Chemists supported legislation to ban CFCs and developed chlorine-free alternatives:
  • HFCs (Hydrofluorocarbons): Compounds like $CH_2FCF_3$ are used in refrigerators and air conditioners. They are safer because they do not contain $C-Cl$ bonds.
  • The $C-F$ bond is significantly stronger than the $C-Cl$ bond and remains unaffected by UV radiation.

Industrial Applications of Halogenoalkanes

• Halogenoalkanes have diverse industrial uses, although many are being phased out due to toxicity and environmental concerns:
  • Solvents: Chloroalkanes and chlorofluoroalkanes are used as solvents. $CH_3CCl_3$ was historically used for dry cleaning.
  • Refrigerants: Used in cooling systems.
  • Pesticides: Used in agriculture.
  • Aerosol Propellants: Used in pressurized spray cans.