3.3.3 - Halogenoalkanes Flashcards
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 attraction and van der Waals forces
- Halogenoalkanes mix with hydrocarbons
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
Bromoethane + aqueous NaOH
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 → C<span>2</span>H5CN + 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?
- The amine group in the product = nucleophile ∴ could react further & make secondary/tertiary amines OR
- Ammonia acts both initially as a nucleophile and then as a base
What 2 factors affect the carbon-halogen bond’s reactivity?
- Cδ+—Xδ- bond polarity
- C—X bond enthalpy (i.e. strength)
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?
- C—F most reactive
- ∵ its most polar - Cδ+ has most positive charge ∴ more easily attacked by nucleophile
- C—I = least reactive
- ∵ it’s least polar
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
2-bromopropane + ethanolic KOH
Mechanism
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)
- Use water as a solvent (aq conditions)
- Lower temperature
- Dilute KOH
Suggest one change to the reaction conditions that would increase the yield of an alkene
- Ethanolic solvent
- High temperature
- High concentration of KOH
