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Elimination Reactions

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Elimination Reactions

Elimination reactions are reactions in which two atoms, or groups of atoms, are removed from a molecule to form a new product.

If you think about it, this makes sense. When you eliminate something, you are getting rid of it. In elimination reactions you get rid of parts of a molecule.

Elimination Reactions, elimination halogenoalkanes, StudySmarterAn example of an elimination reaction. Notice how the atoms shown in red are lost from the molecule. Anna Brewer, StudySmarter Originals

Elimination reactions with halogenoalkanes

An elimination reaction can occur between the hydroxide ion, , and a halogenoalkane. This reaction produces water, a halide ion, and an alkene.

Remember that a halogenoalkane is a hydrocarbon containing one or more halogen atoms. An example is chloromethane, , shown below.

Elimination Reactions, chloromethane halogenoalkane, StudySmarterChloromethane, an example of a halogenoalkane. Anna Brewer, StudySmarter Originals

The hydroxide ion acts as a base by accepting a proton, , from the halogenoalkane to form water.

A base is simply a proton acceptor.

One of the adjacent carbon atoms then loses a halide ion in order to achieve a stable structure. This is also known as dehydrohalogenation. You’ll find the general mechanism below, using to represent the halogen.

Elimination, general mechanism elimination reaction, StudySmarterThe general mechanism for elimination reactions involving halogenoalkanes. Anna Brewer, StudySmarter Originals

We’ve included a labelled diagram to help you understand the mechanism.

Elimination Reactions Mechanism for Elimination StudySmarter

The mechanism for elimination. Anna Brewer, StudySmarter Originals
  1. The hydroxide ion attacks a hydrogen atom with its lone pair of electrons. Notice which carbon atom this hydrogen atom is attached to - it is always a carbon atom adjacent to the C-X bond.
  2. The bonded pair of electrons from the C-H bond now becomes part of a C=C double bond.
  3. The halogen takes the pair of electrons from the C-X bond and is expelled as a leaving group.

A leaving group is a fragment of a molecule that leaves the parent molecule in a chemical reaction. When the chemical bond joining the leaving group and parent molecule together is broken, the bonding pair of electrons moves over to the leaving group.

The reactant is either potassium hydroxide, , or sodium hydroxide, . The reaction is carried out under reflux in hot, ethanolic conditions, and the sodium or potassium ion reacts with the halide ion to form either a sodium or potassium halide.

Study tip: Make sure that you remember that elimination occurs in ethanolic, NOT aqueous conditions.

This type of elimination has what is known as an E2 mechanism, because there are two species involved in the initial slow part of the reaction. An E1 mechanism is possible, but this is a more complicated process.

Suitable halogenoalkanes

For elimination to occur, there must be a hydrogen on an adjacent carbon to the carbon bonded to the halogen. This sounds more complicated than it is! The easiest way to see if a halogenoalkane is suitable is to draw the molecule, then follow these simple steps:

  1. Circle the C-X bond.
  2. Identify the adjacent carbons.
  3. See if any of these adjacent carbons contain a C-H bond.

If they do, your halogenoalkane can react!

Elimination Reactions Identifying suitable halogenoalkanes StudySmarterIdentifying suitable halogenoalkanes. Notice that the carbon containing the C-X bond is also bonded to an R group. This may also be suitable for elimination. Anna Brewer, StudySmarter Originals

For example, an elimination reaction could occur between a hydroxide ion and 2-bromobutane, but not with 1-bromo-2,2-dimethylpropane. We’ve drawn these molecules out below, labelling the steps we described above to make the explanation clearer:

Elimination reactions suitable halogenoalkanes StudySmarterFinding suitable halogenoalkanes for elimination. Anna Brewer, StudySmarter Originals

Products

Depending on the halogenoalkane used, we can form multiple different alkenes in an elimination reaction. This is because there could be multiple carbons with a C-H bond adjacent to the C-X bonded carbon, and so various hydrogen atoms can be attacked. Some of these alkenes may be stereoisomers.

Stereoisomers are molecules that have the same structural formula but different spatial arrangements of atoms.

For example, the reaction between 2-chlorobutane and potassium hydroxide can produce three different alkenes:

Elimination with 2-chlorobutane. Anna Brewer, StudySmarter Originals

For more information about stereoisomers, see Isomerism.

Reactivity of the halogenoalkane

Some halogenoalkanes are a lot more reactive than others. Iodopropane, for example, will react faster with hydroxide ions than chloropropane. This is because the C-I bond has a lower bond enthalpy than the C-Cl bond. Iodine is a larger atom than chlorine and thus the bonded pair of electrons involved in the C-X bond are further from the nucleus. This means that there is weaker attraction between the nucleus and the electrons and the bond is easier to break.

To explore the reactivity of halogenoalkanes further, see Halogenoalkanes.

Elimination reactions vs nucleophilic substitution

When carrying out an elimination reaction, some substitution will always occur, and vice versa. However, the reaction conditions can be controlled to favour one or the other of the reactions. The following table summarises the similarities and differences between the elimination and nucleophilic substitutions of halogenoalkanes. Favoured conditions, favoured halogenoalkane, and the role of the hydroxide ion will be explored further below.

Elimination reactions, elimination vs nucleophilic substitution table comparison,  StudySmarterA table comparing nucleophilic substitution with elimination. Anna Brewer, StudySmarter Originals

Favoured conditions

We stated above that elimination requires ethanolic potassium hydroxide. In actual fact, the solvent used is roughly a 50:50 split of ethanol and water. You might remember that halogenoalkanes are insoluble in water (see Nucleophilic Substitution Reactions), so a small amount of ethanol is needed to dissolve them. This is the case for both elimination and nucleophilic substitution reactions. However, the exact temperature and proportion of ethanol to water in the solvent influences the dominant reaction:

  • Warm conditions favour nucleophilic substitution, whereas hot conditions favour elimination.
  • A more aqueous, i.e., diluted solution favours nucleophilic substitution, whereas a more ethanolic solution favours elimination.
  • Using more concentrated sodium or potassium hydroxide also favours elimination.

Study tip: Learn what your exam board wants you to know concerning reaction conditions. Some exam boards are happy with aqueous solution for nucleophilic substitution reactions and ethanolic solution for elimination, whereas some want you to explore the conditions in more depth, as given above.

Favoured halogenoalkane

The type of halogenoalkane used as a reactant affects the dominant reaction. Tertiary halogenoalkanes contain three alkyl groups attached to the carbon with the C-X bond, whereas primary halogenoalkanes have at most one. This means that tertiary halogenoalkanes have more opportunities for elimination, simply because they have more carbons with hydrogens adjacent to the C-X bonded carbon. Therefore, using tertiary halogenoalkanes favours elimination, whereas using primary halogenoalkanes favours nucleophilic substitution.

The type of halogenoalkane used has an effect on which reaction is favoured. Anna Brewer, StudySmarter Originals

The role of the hydroxide ion

The hydroxide ion takes different roles in the elimination and nucleophilic substitution, although this does not affect the dominant reaction type. In elimination, the hydroxide ion acts as a base whereas in nucleophilic substitution it acts as a nucleophile.

  • A base is a proton acceptor. In elimination, the hydroxide ion takes a proton from one of the carbons adjacent to the carbon with the C-X bond.
  • A nucleophile is an electron pair donor. In nucleophilic substitution, the hydroxide ion donates its lone pair of electrons to the partially charged carbon atom, causing the halogen to be substituted out of the molecule.

Examples of elimination reactions with halogenoalkanes

2-chloropropane undergoes elimination in hot ethanolic sodium hydroxide as shown below. Although hydrogens on both carbons 1 and 3 can be attacked, the alkenes produced are identical, so we don’t produce any isomers. In both cases, the products are propene, water, and the chloride ion, which reacts with sodium to form sodium chloride:

The elimination of 2-chloropropane to form propene, water, and the chloride ion. Anna Brewer, StudySmarter Originals

Another example of an elimination reaction with a halogenoalkane is the elimination of 2-bromo-2-methylbutane using potassium hydroxide. This produces 2 different alkenes:

Elimination reactions, elimination 2-bromo-2-methylbutane,  StudySmarterThe elimination of 2-bromo-2-methylbutane. Anna Brewer, StudySmarter Originals

Other common elimination reactions

Another type of elimination reaction is the dehydration of alcohols to form alkenes. (See Elimination Reactions of Alcohols.)

Elimination Reactions - Key takeaways

  • Elimination reactions are reactions in which two atoms, or groups of atoms, are removed from a molecule to form a new product.
  • Elimination reactions with hydroxide ions and halogenoalkanes take place in hot, ethanolic conditions and produce water, a halide ion, and an alkene. This is also known as dehydrohalogenation and has an E2 mechanism.
  • In elimination reactions, the hydroxide ion acts as a base whereas in nucleophilic substitution, it acts as a nucleophile.
  • Reaction conditions can be altered to favour either elimination or nucleophilic substitution.

Frequently Asked Questions about Elimination Reactions

An elimination reaction is a reaction in which two atoms, or groups of atoms, are removed from a molecule to form a new product.

Elimination reactions are important because they transform saturated compounds into unsaturated ones by forming a C=C double bond.

Condensation reactions are elimination reactions, in which the two atoms removed react to form water. Condensation reactions are also known as dehydration reactions, and include the dehydration of alcohols to form an alkene and water.

In substitution reactions, an atom or group of atoms on a molecule is swapped for another atom or group of atoms. In elimination reactions, two atoms or groups of atoms are removed from a molecule.

Final Elimination Reactions Quiz

Question

 What is an elimination reaction?

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Answer

A reaction in which two atoms, or groups of atoms, are removed from a molecule to form a new product.


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Question

What are the conditions for the elimination of halogenoalkanes?

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Answer

  • Hot, ethanolic potassium or sodium hydroxide.
  • Reflux.

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Question

 What are the products of the elimination of halogenoalkanes?

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Answer

Water, an alkene, and a halide ion.

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Question

Why is the elimination of halogenoalkanes known as dehydrohalogenation?


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Answer

Both a hydrogen and halide ion are lost from the molecule.

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Question

Which of the following halogenoalkanes will produce a mixture of isomers when reacting in an elimination reaction?

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Answer

2-chloropentane

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Question

Why can the elimination of 2-bromobutane using a hydroxide ion produce two different isomeric alkenes?


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Answer

  • There are two carbons with attached hydrogens adjacent to the carbon with the C-Br bond, carbons 1 and 3.
  • Any of the hydrogens on either of these carbons can be attacked by the hydroxide ion.
  • This forms but-1-ene and but-2-ene respectively.

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Question

Which condition favours elimination over nucleophilic substitution?

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Answer

Concentrated sodium hydroxide solution

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Question

Which condition favours elimination over nucleophilic substitution?


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Answer

Mixture heated vigorously.

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Question

Which condition favours elimination over nucleophilic substitution?


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Answer

Tertiary halogenoalkane used.

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Question

​Which condition favours elimination over nucleophilic substitution?

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Answer

Solution is more ethanolic.

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Question

Why does using a tertiary halogenoalkane result in elimination being favoured over nucleophilic substitution?


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Answer

  • Tertiary halogenoalkanes have more carbons with attached hydrogens adjacent to the carbon containing the C-X bond than primary halogenoalkanes. 
  • There are more hydrogens available to be attacked by the hydroxide ion and so more opportunities for elimination.

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