A-level Chemistry: 3.3.3 Halogenoalkanes
These flashcards describe halogenoalkanes as alkanes with halogen substitutions, highlighting their physical properties such as low water solubility, intermolecular forces, and factors affecting boiling points, including chain length, halogen type, and polarity.
What is a halogenoalkane?
Alkanes with at least one halogen atom in place of hydrogen atom
Key Terms
What is a halogenoalkane?
Alkanes with at least one halogen atom in place of hydrogen atom
Describe 3 physical properties of halogenoalkanes
Insoluble in water ∵ polar Cδ+—Xδ- bonds are not polar enough
Main intermolecular forces of attraction are dipole—dipole att...
Boiling points increase with … & increases when …
Boiling points increase with increased chain length & increases when going down the halogen group
Why do boiling points increase with increased chain length or when going down the halogen group?
∵ larger the molecule = greater the number of electrons ∴ greater the van der Waals forces
Why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths?
∵ have higher relative molecular masses & more polar
Why are carbon-halogen bonds polar?
Halogens are much more electronegative than carbon
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| Term | Definition |
|---|---|
What is a halogenoalkane? | Alkanes with at least one halogen atom in place of hydrogen atom |
Describe 3 physical properties of halogenoalkanes |
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Boiling points increase with … & increases when … | Boiling points increase with increased chain length & increases when going down the halogen group |
Why do boiling points increase with increased chain length or when going down the halogen group? | ∵ larger the molecule = greater the number of electrons ∴ greater the van der Waals forces |
Why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths? | ∵ have higher relative molecular masses & more polar |
Why are carbon-halogen bonds polar? | Halogens are much more electronegative than carbon |
What makes halogenoalkanes prone to attacks from nucleophiles? | δ+ charge on carbon/polar carbon-halogen bond |
What is a nucleophile? | Electron-pair donor (negatively charged ion or an atom with δ- charge) |
What do nucleophiles have? | Has lone (unshared) pair of electrons |
As you go down the halogen group, the bonds (C-X) get __ ___ | less polar |
What is a nucleophilic substitution reaction? | When a nucleophile reacts with polar molecules by replacing the functional group |
Why are nucleophilic substitution reactions used? | To introduce new functional groups into organic compounds |
Nucleophilic Substitution Name the reactant | Halogenoalkane |
Nucleophilic Substitution Name the nucleophiles | :OH-, :CN- & :NH3 |
Nucleophilic Substitution Name the condition & product for the nucleophile :OH- | Warm, aqueous NaOH or KOH → Alcohol |
Nucleophilic Substitution Name the condition & product for the nucleophile :CN- | Warm halogenoalkane with ethanolic KCN(aq) → Nitrile |
Nucleophilic Substitution Name the condition & product for the nucleophile :NH3 | Warm halogenoalkane with excess ethanolic ammonia → Amine |
Why is the reaction between hydroxide ions & halogenoalkanes sometimes called hydrolysis? | ∵ in the mechanism, the exact same reaction will happen with water |
Bromoethane + aqueous NaOH Name the mechanism | Nucleophilic Substitution |
Bromoethane + aqueous NaOH Balanced Equation | C2H5Br + NaOH → C2H5OH + NaBr |
Bromoethane + aqueous NaOH Name of Organic Product | Ethanol |
Mechanism | |
Bromoethane + NH3 Name of Mechanism | Nucleophilic Substitution |
Bromoethane + NH3 Balanced Equation | |
Bromoethane + NH3 Mechanism | |
Bromoethane + NH3 Name of Organic Compound | Ethylamine |
Bromoethane + KCN Name of Mechanism | Nucleophilic Substitution |
Bromoethane + KCN Mechanism | |
Bromoethane + KCN Balanced Equation | C2H5Br + KCN → C2H5CN + KBr |
Bromoethane + KCN Name of Organic Compound | Propanenitrile |
When halogenoalkanes react with ammonia, what can the amine group in the product act as and why & what can this lead to? | Can acts as nucleophile∵ it has lone pair of electrons ∴ may react with halogenoalkane molecules = giving mixture of products |
Why is an excess of ammonia needed to react with a halogenoalkane to produce a high yield? |
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What 2 factors affect the carbon-halogen bond's reactivity? |
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Bond Enthalpies Bonds get ____ going down the group | Bonds get weaker going down the group |
Explain why fluorine has the strongest bond | It's the smallest atom of halogens & shared electrons in C—F bond = strongly attracted to nucleus |
Explain why bonds get weaker going down the group | ∵ shared electrons in C—X bond gets further away from halogen nucleus = bond becomes weaker |
What does the polarity of C—X predict & why? |
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What is the main factor that decides reactivity? | Carbon-halogen bond enthalpy (Experiments show that reactivity increases down the group ∵ bond enthalpy is more important factor than bond polarity) |
Why do fluoroalkanes undergo nucleophilic substitution reactions more slowly than other halogenoalkanes? | C-F bond is strongest = highest bond enthalpy |
Why are iodoalkanes substituted more quickly than say fluoroalkanes? | C-I bond has lowest bond enthalpy = easier to break |
When do you get an elimination reaction? | If you warm a halogenoalkane with hydroxide ions dissolved in ethanol instead of water = elimination reactions happens |
Elimination Reaction Name the condition and product when a halogenoalkane is used as a rectant | Ethanolic KOH - Alkene |
2-bromopropane + ethanolic KOH Name of Mechanism | Elimination Reaction |
2-bromopropane + ethanolic KOH Balanced Equation | C3H7Br + KOH → C3H6 + H2O + KBr |
2-bromopropane + ethanolic KOH Name of Organic Compound | Propene |
In an elimination reaction, why do you have to heat mixture under reflux? | ∵ you'll otherwise lose the volatile stuff |
In elimination reactions: small group of atoms…. | break away from molecule Group is NOT replaced |
When halogenoalkanes are reacted with hydroxides, they can undergo… | either nucleophilic substitution or elimination → 2 reactions are said to be competing |
Identify & draw an isomer of 2-bromopentane that does not react with hot, ethanolic sodium hydroxide to produce an alkene | 1-bromo-2,2-dimethylpropane |
Suggest one change to the reaction conditions that would increase the yield of an alcohol (Hot concentrated ethanolic potassium hydroxide reacts with halogenoalkane) |
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Suggest one change to the reaction conditions that would increase the yield of an alkene |
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