Halogenoalkanes and their reactions

In a nutshell

Halogenoalkanes are alkanes with at least one halogen atom to the chain. They can be classed as primary, secondary or tertiary halogenoalkanes, which have different reactivity rates. They can undergo nucleophilic substitution or elimination reactions.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

​$$RX \ + \ H_2O \rightarrow ROH \ + \ H^+ \ +\ X^- \\ \ \\Ag^+\ + \ X^- \rightarrow AgX \\ \ \\R-X \ + \ KOH \xrightarrow{reflux} ROH \ + \ K^+X^- \\ \ \\R-X \ + \ KCN \xrightarrow[ethanol]{reflux} RCN \ + K^+X^- \\ \ \\R-X \ + \ NH_3 \xrightarrow[ethanol]{warm} R-NH_2 \ + \ NH_4^+ \\ \ \\R-X \ + \ KOH \xrightarrow[ethanol]{reflux} R_1C=CR_2 \ + \ H_2O \ + \ K^+X^-$$​​

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Classifying halogenoalkanes

Halogenoalkanes also commonly known as haloalkanes and alkyl halides are alkanes with at least one halogen atom. They are classed as primary, secondary or tertiary halogenoalkanes.

Primary halogenoalkanes have one alkyl group attached to the carbon attached to a halogen. Secondary halogenoalkanes have two alkyl groups attached to the carbon attached to a halogen. Tertiary halogenoalkanes have three alkyl groups attached to the carbon attached to a halogen.

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Reactivity rate of the types of halogenoalkanes

By carrying out experiments you can determine the reactivity of the types of halogens. 

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Example

The results table below is an example of what you may expect when comparing reactivities for between primary, secondary and tertiary chloroalkanes. 

Tertiary halogenoalkanes have the fastest reaction rate which means they are the most reactive.

Reactivity of halogenoalkanes

Halogenoalkanes react with water to produce an alcohol, via nucleophilic substitution reaction.

​$$RX \ + \ H_2O \rightarrow ROH \ + \ H^+ \ +\ X^-$$​​

The general equation may also be written as 

​$$RX \ + \ H_2O \rightarrow ROH \ + \ HX$$​​

Example

​$$chloromethane \ + \ water \rightarrow methanol \ +\ hydrogen \ ion \ + \ chloride \ ion \\CH_3Cl \ + \ H_2O \rightarrow CH_3OH \ + \ H^+ \ +\ Cl^-$$​​

Reactivity of halogenoalkanes can be determined by adding silver nitrate solution to the reaction mixture of the halogenoalkane and water. The silver ions react with the halides to form a silver halide precipitate.

​$$Ag^+ (aq)\ + \ X^- (aq) \rightarrow AgX (s)$$

Example

​$$silver \ ions \ + \ chloride \ ions \rightarrow silver \ chloride \\ \ \\Ag^+ (aq)\ + \ Cl^- (aq) \rightarrow AgCl (s)$$​​

procedure

The faster the precipitate forms, the faster the rate of alcohol formation (or rate of hydrolysis of halogenoalkane).

The rate of hydrolysis of halogenoalkanes is determined by the bond enthalpy of the $$C-X$$ bond. The weaker the $$C-X$$ bond, the easier it is to break the bond therefore the faster the rate of reaction.

Bond enthalpies are dependant on the halogen's size. The larger the size of the halogen, the longer the $$C-X$$ bond so a lower bond enthalpy. This means iodoalkanes have the weakest $$C-X$$ bonds so they hydrolyse the fastest. This means the formation of a precipitate when an iodoalkane is hydrolysed will be much faster compared to when a chloroalkane is hydrolysed.

Nucleophilic substitution reactions

Halogenoalkanes can undergo nucleophilic substitution. This is because the shared pair of electrons in the $$C-X$$ bond are closer to the halogen, making the halogen partially negative and the carbon partially positive. This makes the carbon the subject of a nucleophilic attack. 

Forming alcohols

Halogenoalkanes react with warm aqueous potassium hydroxide, to form alcohols. They react via a nucleophilic substitution mechanism. The reaction is done under reflux.

​$$R-X \ + \ KOH \xrightarrow{reflux} ROH \ + \ K^+X^-$$​​

Example

​$$chloromethane \ + \ potassium \ hydroxide \xrightarrow{reflux} methanol \ + \ potassium \ chloride \\ \ \\CH_3Cl \ + \ KOH \xrightarrow{reflux} CH_3OH \ + \ KCl$$​​

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Mechanism

Forming nitriles

A nitrile can be produced by reacting potassium cyanide in ethanol with a halogenoalkane.  Cyanides react in the same way hydroxides react with halogenoalkanes. The reaction mechanism is the same.

​$$R-X \ + \ KCN \xrightarrow[ethanol]{reflux} RCN \ + K^+X^-$$​​

Example

​$$chloromethane \ + \ potassium \ cyanide \xrightarrow[ethanol]{reflux} acetonitrile \ + \ potassium \ chloride \\ \ \\CH_3Cl \ + \ KCN \xrightarrow[ethanol]{reflux} CH_3CN \ + K^+Cl^-$$​​

Forming amines

A halogenoalkane can be warmed with excess ethanolic ammonia to form amines. The reaction mechanism is the same however the final product is in equilibrium with its conjugate acid.

$$R-X \ + \ NH_3 \xrightarrow[ethanol]{warm} R-NH_2 \ + \ NH_4^+$$​​

Example 

​$$chloromethane \ +\ ammonia \xrightarrow[ethanol]{warm} methylamine \ +\ ammonium \ ion \\ \ \\CH_3Cl \ + \ NH_3 \xrightarrow[ethanol]{warm} CH_3-NH_2 \ + \ NH_4^+$$​​

Mechanism

Elimination reactions

​Halogenoalkanes react with warm ethanolic potassium hydroxide under reflux, to form alkenes.

​$$R-X \ + \ KOH \xrightarrow[ethanol]{reflux} R_1C=CR_2 \ + \ H_2O \ + \ K^+X^-$$

There is a possibility of more than one isomer forming.

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Example

​$$chloropropane + potassium \ hydroxide \xrightarrow[ethanol]{reflux} propene \ + \ water \ + \ potassium \ chloride \\ \ \\CH_3CH_2CH_2Cl \ + \ KOH \xrightarrow[ethanol]{reflux} CH_3HC=CH_2 \ + \ H_2O \ + \ K^+Cl^-$$​​