Organic 3: Halogenoalkanes Flashcards
Why do halogenoalkanes have polar bonds?
Because halogens are more electronegative than carbon
So the electrons are drawn towards the halogen
Making the carbon delta positive (electron deficient)
And the halogen delta negative
Describe the solubility of halogenoalkanes.
Halogenoalkanes are insoluble in water because the polar C-X bonds are not polar enough to interact with polar water molecules
Describe the boiling points of halogenoalkanes.
Boiling points increase with increased carbon chain length
Also increase going down the halogen group
Both are because of increased van der Waal forces
Why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths?
1) Increased molecular mass due to halogen - more van der Waals, higher boiling point
2) Polarity increases strength of IMFs (i.e. dipole-dipole bonding) so more energy needed to overcome IMFs
What are the two factors that determine how readily the C-X bond reacts?
Bond polarity
Bond enthalpy (more important)
How does bond polarity affect reactivity of halogenoalkanes?
More polar bonds are more reactive
More electronegative halogens attract more electrons in the C-X bond
Making the carbon atom more delta positive (electron-deficient)
So more attractive to nucleophiles (electron pair donors)
What is a nucleophile?
A donor of a lone pair of electrons
How does bond enthalpy affect halogenoalkane reactivity?
Bonds get weaker down the group
Because atomic radius increases
So force of attraction between the valence electron involved in the C-X bond and nucleus is weaker
So the C-X bond is weaker
If the bond is weaker, it is easier to break,
So a lower bond enthalpy makes a halogenoalkane more reactive
What are the three products of a nucleophilic substitution reaction with a halogenoalkane?
1) Amine
2) Alcohol
3) Nitrile
Describe the mechanism of producing an alcohol from a halogenoalkane.
Nucleophilic substitution
Reagents = Ethanolic aqueous NaOH or KOH
Conditions = Warm
:OH- acts as nucleophile
Na/K from NaOH/KOH react with halide nucleophile produced
Describe the mechanism of producing a nitrile from a halogenoalkane.
Mechanism = Nucleophilic substitution
Reagents = Ethanolic aqueous KCN
Conditions = Warm
:CN- acts as nucleophile
K from KCN reacts with halide nucleophile produced
Describe the mechanism of producing an amine from a halogenoalkane.
Nucleophliic substitution
Reagents = Excess ethanolic concentrated ammonia
Conditions = Under pressure
:NH3- acts as nucleophile
NH4+ reacts with halide nucleophile to produce ammonium halide
Explain the mechanism for nucleophilic substitution.
1) X is more electronegative than carbon, so attracts the electrons in the C-X bond, making the carbon partially positive so attractive to nucleophiles
2) Nucleophiles donate their lone pair to the delta positive carbon atom
3) The C-X bond breaks, and a bond forms between the nucleophile and carbon atom
4) The halogen is removed as the leaving group with its pair of electrons, forming a halide ion
Explain is the second step of the nucleophilic substitution reaction with ammonia.
1) A proton must be lost from the NH3 group to form the neutral amine NH2
2) The lone pair from a second ammonia molecule is attracted to the delta positive hydrogen atom, removing the H+ and forming a bond to produce an ammonium ion
Describe the mechanism of producing an alkene from a halogenoalkane.
Eliminiation
Reagent: Hot ethanolic KOH or NaOH
Conditions: Heat
OH- ion acts as a base
Produces water and a halide ion which reacts with K or Na to produce KX or NaX
