3.3.3- Halogenalkanes (PAPER 2) Flashcards
What are haloalkanes?
Homologous series of compounds where one or more H atoms has been replaced by halogen atoms (F, Cl, Br, I)
How do you classify alkanes- primary/secondary/tertiary?
Based on the number of C atoms attached to the halogen containing atom.
Primary= one C attached to halogen containing atom.
Secondary= 2 C attached.
Tertiary= 3 C attached.
Why do haloalkanes have higher melting and boiling points than alkanes?
The addition of the halogen atom creates permanent dipole-dipole forces in addition to the Vdw’s forces which require more energy to overcome.
Extra molecule weigh added by halogen.
Effect of reactivity on haloalkanes? In reactions + solubility.
Insoluble in water as dipole not strong enough for H bonding between water and haloalkane.
Soluble in organic solvents.
d+ C is attractive to molecules with a lone pair of electrons (nucleophiles), the carbon-halogen bond is broken, this is the weakest bond!!!!!!!!
Which nucleophiles do haloalkanes undergo nucleophilic substitution with?
OH–, CN– and NH3
What is a nucleophile?
Have a lone pair of electrons, attract electron deficient areas such as d+ C.
In nucleophilic substitution, halogen atom is substituted by nucleophile.
What is the ease of halogen-carbon bond breaking?
C-I > C-Br > C-Cl > C-F
iodoalkanes react faster than other haloalkanes as the C-I bond is the weakest: larger atom so weaker bond
iodoalkanes most reactive
Which conditions favour nucleophilic substitution?
warm, aqueous conditions.
Draw the reaction between chloromethane and sodium hydroxide solution
do not need to include Na
lone pair from O donated to d+ C in middle of haloalkane, 2 bonding electrons from carbon-halogen bond move to the halogen and a halide ion forms.
Draw the reaction between iodomethane and cyanide ions.
same as reaction between haloalkane and OH-
C chain is lengthened by one C, add cyanide to end of name. C TRIPLE BOND N AT END OF MOLECULE, HALOGEN AT END OF MOLECULE.
eg methylcyanide for this reaction.
Why are cyanide ions provided using KCN not HCN?
HCN is a very toxic gas and also a weak acid so slightly ionised, leading to a low conc of :CN- ions= slow reaction.
Draw the reaction between bromoethane and ethanolic ammonia.
First step same as usual, bromine displaced.
2nd step: +N bonded to 3 H on end of molecule, 2nd ammonia molecule with lone pair attracted to N. 2e- from bond move to N
curly arrow from Nh3 to H, curly arrow from bond between N-H to N.
Molecule at end has N with lone pair attached to 2H, Br radical and NH4+ released.
ethylamine
Which conditions favour elimination?
hot ethanol solvent
What happens in an elimination reaction?
Hydroxide ion acts as a base- a PROTON ACCEPTOR
:OH- donates lone pair to H on carbon adjacent to halogen carrying carbon, alkene formed.
C-H bond breaks to form C=C
water formed, halide formed.
Draw the reaction between 1-bromopropane and hydroxide ions in ethanolic conditions.
curly arrow from OH to H next to halogen carrying C
curly arrow from halogen-carbon bond to carbon-carbon bond
curly arrow from carbon in bond to halogen
forms an alkene and water and the halide ion
