≫3.3 - Halogenoalkanes ✔ Flashcards
Why do halogenoalkanes contain polar bonds?
- The halogens are more electronegative than carbon atoms, this means electron density is drawn towards the halogen forming 𝛿+ and 𝛿- regions.
What does the ability for the bond to react in a halogenoalkane depend on?
- Depends on the bond enthalpy, not the difference in electronegativity.
- C-F has the highest bond enthalpy value and therefore is the least likely to react. The C-I bond has the lowest bond enthalpy so it will break the easiest and react the fastest.
- If more than one halogen is present in a molecule the bond with the lowest bond enthalpy will break first.
- Bond enthalpy has a greater overriding effect than bond polarity.
Classification of Halogenoalkanes:
What is a primary halogenoalkane?
- The halogen is attached to 1 carbon connected to 1 carbon and 2 hydrogen, exception is CH₃Br as it is attached to only 1 carbon.
*Most likely to undergo nucleophilic substitution.
Classification of Halogenoalkanes:
What is a secondary halogenoalkane?
- The halogen is attached to a carbon connected to 2 carbons with 1 hydrogen.
*Likely to undergo nucleophilic substitution or elimination reaction.
Classification of Halogenoalkanes:
What is a tertiary halogenoalkane?
- The halogen is attached to a carbon connected to 3 other carbons with no hydrogen.
*Most likely to undergo elimination reaction.
What is a nucleophile?
- A nucleophile is an electron pair donor, they contain a lone electron pair that is attracted to 𝛿+ regions of molecules.
e.g: :OH⁻, :NH₃, CN:⁻
What does a curly arrow show?
- Shows where a pair of electrons go during a reaction.
What is substitution?
(in terms of reactions)
- A reaction in which an atom or group of atoms is replaced by another atom or group of atoms.
What is reflux?
- The continuous boiling and condensing of a reaction mixture.
Reaction of Halogenoalkanes to form alcohols:
What is the name of the mechanism, the reagents, the nucleophile, the conditions and the overall equation for this reaction?
(C₂H₅Br)
- Nucleophilic substitution.
- NaOH or KOH
- :OH⁻
- Aqueous reagent, dissolve halogenoalkane in a little ethanol, reflux.
- C₂H₅Br + KOH → C₂H₅OH + KBr
Reaction of Halogenoalkanes to form alcohols:
Draw out and outline the nucleophilic substitution mechanism for the production of an alcohol:
(Starting reagent: C₂H₅Br)
- The C-Br bond is polar, the C𝛿+ attracts a lone pair of electrons from the OH⁻ ion.
- The OH⁻ ion acts as a nucleophile attacking the slightly positive carbon atom.
- A new bond forms between the C and OH⁻ ion making an alcohol.
- And the C-Br bond breaks, both the electrons from the bond are taken by the Br.
Reaction of Halogenoalkanes to form alcohols:
What are the disadvantages of making alcohol this way?
- Low yield.
- Slow rate.
- Bromoethane has to be made first.
- Bromoethane is expensive.
Reaction of Halogenoalkanes to form nitriles:
What is the name of the mechanism, the reagents, the nucleophile, the conditions and the overall equation for this reaction?
(C₂H₅Br)
- Nucleophilic substitution.
- Potassium cyanide, KCN (white solid.)
- :CN⁻
- Aqueous reagent, dissolve halogenoalkane in a little ethanol, reflux.
- C₂H₅Br + KCN → C₃H₅N + KBr
Reaction of Halogenoalkanes to form nitriles:
Draw out the nucleophilic substitution mechanism for the production of a nitrile:
(Starting reagent: C₂H₅Br)
Reaction of Halogenoalkanes to form amines:
What is the name of the mechanism, the reagents, the conditions and the overall equation for this reaction?
(C₂H₅Br)
- Nucleophilic substitution.
- NH₃ (ammonia.)
- Concentrated excess of ammonia under pressure.
- C₂H₅Br + 2NH₃ → C₂H₅NH₂ + NH₄Br
Reaction of Halogenoalkanes to form amines:
Draw out the nucleophilic substitution mechanism for the production of an amine for the first step:
(Starting reagent: C₂H₅Br)
Reaction of Halogenoalkanes to form amines:
Draw out and outline the nucleophilic substitution mechanism for the production of an amine for the second step:
(Starting reagent: C₂H₅NH₃)
- In the second step a second ammonia molecule removes a hydrogen from the NH₃ group on the amine to form an ammonium ion (NH₄⁺) and an amine.
- The ammonia molecule donates its lone pair of electrons to the hydrogen to form a bond, so the nitrogen atom in the ammonium ion now has a positive charge and the amine has no charge.
- If you have excess halogenoalkane you can continue this reaction to form primary, secondary, tertiary or quaternary amines.
- However if you only want a primary amine, then you must have excess of the ammonia.
Reaction of Halogenoalkanes to form alkenes:
Draw out and outline the elimination mechanism for the production of an alkene:
(Starting reagent: C₃H₇Br)
- OH⁻ acts as a base and takes a proton, H⁺, from the carbon on the left (making water.)
- The left carbon now has a spare pair of electrons so it forms a double bond with the middle carbon.
- To form the double bond the middle carbon has to let go of the Br, which it drops off as a Br⁻ ion.
