HS Chemistry - Essential Functional Groups

Halogenoalkanes

Overview of The Page

This page will cover:

• What are halogenoalkenes?
• How are halogenoalkenes named? What are the different types of halogenoalkanes?
• What reactions do halogenoalkanes participate in?

Halogenoalkanes $aka haloalkanes or alkyl halides$, as we have seen previously, can be formed when halogens or Hydrogen halides are added to alkenes, or when alkanes undergo free-radical substitution with halogens. Halogenoalkanes can also undergo other reactions. However, these reactions often take an extremely long time. They are therefore done using the technique of refluxing.

Since many organic reactions often require boiling a non-volatile solute inside a relatively volatile solvent, scientists have to ensure they don't lose large volumes of solvent. They do this through refluxing. After setting up a reflux apparatus, with the reaction flask connected to a tube surrounded by another tube of cold water, they start heating the flask. When the solvent boils, it rises into the tube, at which point it cools down because of the cold water in the surrounding tube and condenses back into the reaction flask. This prevents too much solvent from being lost while supplying heat for the reaction.

Halogenoalkanes can be classified into three different categories:

• Primary halogenoalkanes are halogenoalkanes where the Carbon atom to which the halogen is connected is also connected to two Hydrogen atoms and one other group. An example is shown below:

  

  The skeletal formula of this halogenoalkane, 1-bromoethane $or simply bromoethane, as we don't need to add the "1\-" since there are only two Carbon atoms in the main chain$, would be:

  

  
  

• Secondary halogenoalkanes are halogenoalkanes where the Carbon atom to which the halogen is connected is also connected to one Hydrogen atom and two other groups. An example is shown below:

  

  The skeletal formula of this halogenoalkane, 2-bromopropane, would be:

  

  
  

• Tertiary halogenoalkanes are halogenoalkanes where the Carbon atom to which the halogen is bonded is also bonded to three other groups. An example is shown below:

  

  The skeletal formula of this halogenoalkane, 2-bromo-2-methylpropane, would be:

  

Halogenoalkane Reactions

Apart from the halogenoalkane reactions already covered, there are three other main types of reactions that halogenoalkanes can undergo. All of them are nucleophilic substitution reactions:

• Substitution Reaction With Aqueous Hydroxide:

  • When a halogenoalkane is dissolved in an aqueous solution of a hydroxide compound, a nucleophilic substitution occurs where the hydroxide ion, which has an excess electron and is therefore negatively charged, is attracted to the Carbon atom that is bonded to the halogen. This happens because the Carbon atom has a lower electronegativity than the halogen, which leaves that Carbon atom with a slightly positive charge that attracts the hydroxide ion. The hydroxide ion then bonds with the Carbon, forming an alcohol, and the halogen is removed from the organic compound as a halide ion. The negatively charged halide ion then bonds with the metal ion displaced in the solution $as a hydroxide compound is made of a metal cation and a hydroxide anion$.
  • The hydroxide must be dissolved in water. It cannot be dissolved in a solvent like ethanol, or otherwise it will bond with the ethanol and the reaction will not occur.

• Substitution Reaction With Ethanolic Cyanide $CN^\-^$:

  • When a halogenoalkane is dissolved in an ethanolic solution $a solution with a solvent of ethanol rather than water$ of cyanide ions $CN^\-^$, a nucleophilic substitution occurs where the cyanide ion, being negatively charged, is attracted to the Carbon atom that is bonded to the halogen. This happens because the Carbon atom has a lower electronegativity than the halogen, which leaves that Carbon atom with a slightly positive charge that attracts the cyanide ion. The cyanide then bonds with the Carbon, forming a nitrile, and the halogen is removed from the organic compound.
  • The cyanide must be dissolved in ethanol. It cannot be dissolved in a solvent like water, or otherwise it will react with the water, and the substitution reaction will not occur.

• Substitution Reaction With Excess Ammonia $NH~3~$ Dissolved in Ethanol:

  • When a halogen is dissolved in an ethanolic solution of excess ammonia, a nucleophilic substitution occurs where NH₂ substitutes for the halogen, forming an amine. There are two conditions for this reaction:

    1. The ammonia must be present in excess, or else a mixture of amines will be formed rather than one large amine (and we only want one large amine), as the amines will themselves act as nucleophiles and attack halogenoalkanes, unless ammonia is present in excess.

    2. The ammonia must be dissolved in ethanol. It cannot be dissolved in a solvent like water, or otherwise it will react with the water, and the substitution reaction will not occur.

Reaction Mechanisms

There are two different mechanisms by which all three of the above reactions occur:

• S_N2 reactions are nucleophilic substitution reactions where the substituting atom/molecule $let's call it molecule A$ bonds with the organic molecule BEFORE the substituted atom/molecule $let's call it molecule B$ leaves the organic molecule. First molecule A bonds with the organic molecule, then molecule B is forced out.

• S_N1 reactions are nucleophilic substitution reactions where the substituting atom/molecule $let's call it molecule A$ bonds with the organic molecule AFTER the substituted atom/molecule $let's call it molecule B$ leaves the organic molecule. First molecule B leaves the organic molecule, then molecule A bonds with the organic molecule.

Both S_N1 and S_N2 reactions are nucleophilic substitution reactions $that's what the S~N~ stands for$, but they describe different reaction mechanisms. They do not affect the end product, but they help us understand how we got there, and what the end product looks like, as they occur with different types of halogenoalkanes:

• Reactions with primary halogenoalkanes will always have the S_N2 reaction mechanism.

• Reactions with tertiary halogenoalkanes will always have the S_N1 reaction mechanism.

• Reactions with secondary halogenoalkanes can have either the S_N2 or S_N1 reaction mechanism.

Other Reactions

Halogenoalkanes can also undergo elimination reactions to form alkenes. In this reaction, a halogenoalkane reacts with a hydroxide compound dissolved in ethanol to form an alkene, H₂O and a halide compound. The hydroxide ion "steals" a Hydrogen atom bonded to one of the Carbon atoms in the halogenoalkane. This causes the halogen to be removed from the organic molecule as a Carbon-Carbon double bond is formed. The Bromine $which now has a negative charge$, bonds with the metal atom in the original hydroxide compound.

There is one final thing to mention in regard to halogenoalkanes. The presence of a halogenoalkane functional group in a compound can be tested by acidifying it with dilute nitric acid, then adding silver nitrate solution. The Silver will form precipitates with the halide ions if there are any halogens present:

• If Fluorine is present, no precipitate will be formed.
• If Chlorine is present, a white precipitate will be formed.
• If Bromine is present, a pale cream-colored precipitate will be formed.
• If Iodine is present, a pale yellow precipitate will be formed.

If a precipitate is seen $i.e. if the solution contains Chlorine, Bromine, or Iodine$, the results can be verified by adding ammonia:

• If Chlorine was present, the white precipitate will dissolve into a colorless solution.

• If Bromine was present, then nothing will happen at dilute concentrations of ammonia solution, but at high concentrations, the cream-colored precipitate will dissolve into a colorless solution.

• If Iodine was present, then nothing will happen, as the yellow Iodine precipitate is insoluble.