6D.30-6D.32 - Halogenoalkane Reactions

Classifying The Carbon Attached To X

A halogenoalkane contains a carbon-halogen bond, written generally as $\ce{R-X}$, where $\ce{X}$ is usually $\ce{Cl}$, $\ce{Br}$ or $\ce{I}$. The classification does not depend on the whole molecule; it depends on the carbon atom directly bonded to the halogen.

The common error is to classify by the halogen itself or by the longest chain. Instead, circle the carbon attached to $\ce{X}$, then count how many carbon atoms are directly bonded to that carbon.

Worked classification:

$\ce{CH3CH2CHBrCH3}$ is a secondary halogenoalkane. The carbon bearing $\ce{Br}$ is bonded to the $\ce{CH3}$ group on one side and the $\ce{CH3CH2}$ group on the other side, so it is bonded directly to two carbon atoms.

[DIAGRAM: asset_name: Halogenoalkane Carbon Classes; asset_slug: organic_halogen_classes; recommended_method: deterministic_chart; description: Primary, secondary and tertiary bromoalkanes: count the carbon groups directly attached to the carbon bearing bromine.]

Nucleophiles And The Polar C-X Bond

The carbon–halogen bond creates an electron-deficient carbon that can accept a lone pair from a reagent.

A nucleophile is attracted to an electron-deficient atom and uses an available electron pair, usually a lone pair, to form a new covalent bond.

Halogenoalkanes are vulnerable to nucleophiles because the halogen is more electronegative than carbon. The $\ce{C-X}$ bond is polar: the carbon is $\delta^{+}$ and the halogen is $\delta^{-}$. That makes the carbon atom attached to the halogen an electrophilic site, so electron-rich species can attack it.

Useful nucleophiles in this lesson:

Nucleophile is a role, not a permanent label. $\ce{OH-}$ acts as a nucleophile in aqueous potassium hydroxide, but as a base in ethanolic potassium hydroxide. The condition changes the main reaction pathway.

Substitution To Alcohols

Warm or heat the halogenoalkane with aqueous potassium hydroxide to form an alcohol; reflux allows longer heating without losing volatile reactants. The hydroxide ion acts as a nucleophile and replaces the halide ion.

General ionic equation:

Example with bromoethane:

The condition aqueous matters. In water, hydroxide ions are well placed to donate a lone pair to the $\delta^{+}$ carbon and substitute the halogen. In explanations, do not just name the reagent; state the role of $\ce{OH-}$ and the product functional group.

Common-error contrast:

Water As Nucleophile With Silver Nitrate

Aqueous silver nitrate in ethanol gives another hydrolysis route. In this reaction mixture, water acts as the nucleophile and substitutes the halogen to form an alcohol. The released halide ion then reacts with silver ions to form a silver halide precipitate.

The organic change can be represented as:

The observation-forming step is:

For bromoethane:

Ethanol helps the organic halogenoalkane mix with the aqueous reagent. Silver ions remove halide ions as a precipitate, so the appearance of $\ce{AgX(s)}$ is evidence that $\ce{X-}$ has been released.

The silver halide is white for chloride, cream for bromide and yellow for iodide. This observation follows release of the halide; silver nitrate does not replace the halogen in the organic molecule.

New Functional Groups From Cyanide And Ammonia

Potassium cyanide and ammonia both substitute the halogen, but the synthetic consequence is different.