Halogenoalkanes Flashcards
Physical properties
Slightly soluble - generally polar but attractive forces aren’t as strong as hydrogen bonding in water
Trend in boiling temperatures of primary, secondary, tertiary healogenoalkanes
Relatively low as no permanent dipole forces act. From primary to secondary to tertiary halogenoalkanes, the boiling temperature decreases as the London forces are weaker. This is because it is easier to stack a straight chain molecule without branching. On secondary, the halogen sticks out, acting as a branch.
What type of reactions do halogenoalkanes undergo?
Nucleophilic substitution - attracted to partially positive C - replacement of halogen
C-X bond is polarised due to the electronegativity of the halogen
Polarity of different halogens
Down the group, the halogens becomes less electronegative. This means the C-X bond becomes less polar and the C is less partially positive (difference in electronegativity is less - bond is less polar) so it is less likely to be attack by a nucleophile, making it less reactive
Reactivity of different halogens
Down the group, the bond length increases between the carbon and halogen. This is because the halogens becomes larger as they contain more electrons and have increased shielding. This makes the bonding pair of electrons further away from the nuclei and hence the electrostatic force of attraction between the bonding pair of electrons and carbon and halogen nuclei weaker. This is easier to overcome, making the bond enthalpy value lower and the halogenoalkane more reactive.
Trend in reactivity of primary, secondary, tertiary halogenoalkanes
As you go from primary to secondary to tertiary halogenoalkanes, the positive inductive effect increases, making the carbocations formed more stable and hence making it persist for longer, making it more likely for products to be formed and a reaction to be completed
halogenoalkanes –> alkenes
elimination
heat under reflux
ethanolic KOH/NaOH
halogenoalkane –> alcohol
nucleophilic substitution
heat under reflux
aqueous KOH/NaOH (alkali)
halogenoalkane –> nitrile
nucleophilic substitution
heat under reflux
KCN in ethanol
halogenoalkane –> amine
nucleophilic substitution
heat under pressure in a sealed container
concentrated NH3 dissolves in ethanol
Why is ethanol often used as a condition
It promotes elimination
Why are aqueous conditions used
They promote nucleophilic substitution reactions
