Halogenoalkanes and Reaction Mechanisms

Vocabulary and key concepts regarding the properties, naming, and reaction mechanisms of halogenoalkanes, including nucleophilic substitution and elimination.

Halogenoalkanes

Organic compounds consisting of an alkane skeleton with one or more halogen atoms (fluorine, chlorine, bromine, or iodine) in place of hydrogen atoms.

General formula of a single-halogen halogenoalkane

$C_nH_{2n+1}X$ where $X$ is the halogen, often shortened to $R-X$.

Bond polarity in halogenoalkanes

The $C-X$ bond is polar ($C^{\delta+}-X^{\delta-}$) because halogens are more electronegative than carbon.

Van der Waals forces trend

Force that increases with larger molecules and greater numbers of electrons, causing boiling points to increase with chain length and going down the halogen group.

Nucleophile

An electron pair donor that attacks electron-deficient carbon atoms (e.g., $:OH^-$, $:NH_3$, and $:CN^-$).

Bond enthalpy trend in $C-X$ bonds

Bond strength decreases going down the group: $C-F$ ($467\,kJ\,mol^{-1}$) is the strongest, and $C-I$ ($228\,kJ\,mol^{-1}$) is the weakest.

Nucleophilic substitution

A reaction mechanism where a nucleophile replaces a halogen atom in a halogenoalkane.

Curly arrows

Symbols used in organic reaction mechanisms to show the movement of an electron pair, starting at a lone pair or bond and moving towards an electron-deficient area.

Leaving group

The halide ion ($X^-$) that detaches from the carbon atom during a substitution or elimination reaction.

Hydrolysis of halogenoalkanes

The reaction of a halogenoalkane with aqueous sodium or potassium hydroxide in ethanol to produce an alcohol ($ROH$).

Nitriles

Compounds with the functional group $-C\equiv N$, formed by reacting halogenoalkanes with cyanide ions ($:CN^-$), which increases the carbon chain length by one.

Amines ($RNH_2$)

Products formed when halogenoalkanes react with an excess concentrated solution of ammonia ($:NH_3$) in ethanol under pressure.

Elimination reaction

A reaction where a hydroxide ion acts as a base to remove an $H^+$ ion and a halide ion from a halogenoalkane, resulting in an alkene.

Conditions for substitution vs. elimination

Substitution is favored by hydroxide ions in water (aqueous) at room temperature; elimination is favored by hydroxide ions in ethanol (ethanolic) at high temperatures.

Chlorofluorocarbons (CFCs)

Halogenoalkanes containing both chlorine and fluorine but no hydrogen; they are responsible for decomposing ozone ($O_3$) in the stratosphere.

HCFCs and HFCs

Safer replacements for CFCs; HCFCs contain hydrogen and decompose more easily, while HFCs (hydrofluorocarbons) contain no chlorine and do not damage the ozone layer.