Introduction to Halogenoalkanes

Welcome to the world of halogenoalkanes! In your previous study of alkanes (Topic 6B), you saw that they are generally quite "boring" and unreactive. Halogenoalkanes are the exciting cousins. By replacing a hydrogen atom in an alkane with a halogen (like Chlorine, Bromine, or Iodine), we create a molecule that is ready to react. This makes them incredibly useful in the chemical industry for making everything from plastics to medicines.

In this chapter, we will explore why they are so reactive, how they transform into other molecules, and why some react much faster than others.

The Polar Bond: Why They React

The secret to a halogenoalkane's reactivity lies in the C-X bond (where \(X\) is the halogen). Halogens are more electronegative than carbon. This means they pull the shared pair of electrons in the covalent bond towards themselves.

  • The carbon atom becomes slightly positive (\(\delta+\)).
  • The halogen atom becomes slightly negative (\(\delta-\)).

Because the carbon is \(\delta+\), it is attractive to nucleophiles. A nucleophile is a "nucleus-lover"—a species that has a lone pair of electrons it wants to donate to an electron-deficient carbon atom.

1. Nucleophilic Substitution Reactions

In a substitution reaction, one group is swapped for another. For halogenoalkanes, a nucleophile attacks the \(\delta+\) carbon and kicks out the halogen (the "leaving group").

A. Reaction with Aqueous Potassium Hydroxide (\(KOH\))

Reagent: Aqueous \(KOH\) (or \(NaOH\)).
Conditions: Heat under reflux.
Nucleophile: Hydroxide ion, \(OH^-\).
Product: An alcohol.
Equation: \(CH_3CH_2Br + KOH \rightarrow CH_3CH_2OH + KBr\)

B. Reaction with Potassium Cyanide (\(KCN\))

This is a very special reaction because it adds an extra carbon atom to the chain!
Reagent: \(KCN\) dissolved in ethanol/water mixture.
Conditions: Heat under reflux.
Nucleophile: Cyanide ion, \(CN^-\).
Product: A nitrile.
Example: Bromoethane (\(C_2\)) becomes Propanenitrile (\(C_3\)).

C. Reaction with Ammonia (\(NH_3\))

Reagent: Excess ethanolic ammonia.
Conditions: Heat in a sealed tube (to prevent the ammonia gas from escaping).
Nucleophile: Ammonia, \(:NH_3\).
Product: A primary amine.
Equation: \(CH_3CH_2I + 2NH_3 \rightarrow CH_3CH_2NH_2 + NH_4I\)

2. The Mechanism: How it Happens (\(S_N2\))

For primary halogenoalkanes (where the carbon attached to the halogen is only attached to one other carbon), the reaction follows a "one-step" process. Don't worry if this seems tricky; just follow the "curly arrows"!

  1. The nucleophile (e.g., \(OH^-\)) uses its lone pair to form a bond with the \(\delta+\) carbon.
  2. At the same time, the \(C-X\) bond breaks, and the electrons go to the halogen.
  3. Crucial Rule: Curly arrows must start from a lone pair or a bond.

Note: You are expected to draw this mechanism showing the \(\delta+\) and \(\delta-\) charges and the movement of electron pairs.

Key Takeaway: Nucleophilic substitution swaps a halogen for a nucleophile like \(OH^-\), \(CN^-\), or \(NH_3\).

Hydrolysis is a reaction where a molecule is broken down by water or hydroxide ions. If we compare fluoro-, chloro-, bromo-, and iodoalkanes, which one reacts the fastest?

Bond Enthalpy vs. Polarity

You might think chloroalkanes react fastest because the \(C-Cl\) bond is the most polar. However, bond enthalpy (bond strength) is the deciding factor.

  • The \(C-F\) bond is very strong (high enthalpy), making it hard to break.
  • The \(C-I\) bond is much weaker (low enthalpy).

Because the \(C-I\) bond is the easiest to break, iodoalkanes react the fastest. Chloroalkanes react the slowest.

Core Practical 4: Testing the Rate

To prove this in a lab, we react halogenoalkanes with aqueous silver nitrate in ethanol.

  • The ethanol acts as a solvent so the halogenoalkane and water can mix.
  • As the halogenoalkane hydrolyses, it releases halide ions (\(Cl^-\), \(Br^-\), or \(I^-\)).
  • These react with silver ions (\(Ag^+\)) to form colored precipitates:
    • Chlorine: White precipitate (forms slowly).
    • Bromine: Cream precipitate (forms at a medium rate).
    • Iodine: Yellow precipitate (forms very quickly).

4. Elimination Reaction

Sometimes, the hydroxide ion (\(OH^-\)) changes its mind. Instead of acting as a nucleophile, it acts as a base. This happens when we change the conditions.

Reagent: Potassium hydroxide (\(KOH\)) dissolved in ethanol.
Conditions: Heat under reflux (no water present).
Process: The \(OH^-\) removes a hydrogen atom from a carbon neighbor to the halogen. The halogen also leaves, and a double bond forms.
Product: An alkene.
Equation: \(CH_3CH_2Br + KOH(ethanol) \rightarrow CH_2=CH_2 + H_2O + KBr\)

Memory Trick:
Aqueous = Alcohol (Substitution)
Ethanolic = Elimination (Alkene)

Quick Review Table

Reagent Solvent/Conditions Type of Reaction Main Product
\(KOH\) Aqueous / Heat Substitution Alcohol
\(KCN\) Ethanol + Water / Heat Substitution Nitrile
\(NH_3\) Ethanol / Sealed Tube Substitution Amine
\(KOH\) Ethanol / Heat Elimination Alkene
\(AgNO_3\) Ethanol + Water Hydrolysis Precipitate

Common Mistakes to Avoid

  • Wrong solvent: Using ethanol for substitution with \(KOH\) will give you an alkene instead of an alcohol!
  • Arrow direction: Always start curly arrows from the lone pair of the nucleophile, pointing towards the \(\delta+\) Carbon.
  • Missing charges: Don't forget the \(\delta+\) on the Carbon and \(\delta-\) on the Halogen in mechanisms.
  • Bond Enthalpy confusion: Remember, a weaker bond means a faster reaction. \(C-I\) is the weakest and fastest.

For more context on naming these molecules, see Topic 6A. For how they compare to alcohols, see Topic 6E.