[15.1] chemistry of haloalkanes Flashcards
chemical and physical properties of haloalkanes
1
Q
what are haloalkanes synthesised to form?
A
- PVC
- teflon
- anaesthetics
- solvents
2
Q
what is the general formula for haloalkanes?
A
CₙH₍₂ₙ₊₁₎X (X = F, Cl, Br, I)
3
Q
what are haloalkanes also called?
A
halogenalkanes or alkyl halides
4
Q
how are haloalkanes classified?
A
- primary haloalkane: halogen bonded to C which is bonded to 1 C
- secondary haloalkane: halogen bonded to C which is bonded to 2 C
- tertiary haloalkane: halogen bonded to C which is bonded to 3 C
5
Q
why are haloalkanes more reactive than alkanes?
A
- they have a polar C-X bond because the greater electronegativity of the halogen creates a dipole making the carbon atom electron deficient
- this makes it susceptible to nucleophilic attack
6
Q
what is a nucleophile and what will they do?
A
- an electron pair donor
- they will attack positively changes / partially-positively charged atoms and donate a lone pair of electrons to create a new covalent bond
7
Q
what are nucleophiles attracted to?
A
low electron density
8
Q
what are 3 examples of nucleophiles?
A
- hydroxide ions, :OH-
- water molecules, H₂O:
- ammonia molecules, :NH₃
9
Q
what is nucleophilic substitution?
A
- when nucleophiles replace halogens in substitution reactions
- only primary haloalkanes undergo nucleophilic substitution
10
Q
draw and describe the nucleophilic substitution mechanism
A
- nucleophile attacks the partially-positive carbon with its lone pair of electrons
- C-X bond breaks: both electrons in the bond move onto the leaving group - the halide ion
- the halogen has been substituted for the nucleophile on the alkyl chain
11
Q
- hydrolysis of haloalkanes by aqueous alkali
A
- reagent: NaOH
- overall equation: R-CH₂X + NaOH —> R-CH₂OH + NaX
- conditions: aqueous, warm NaOH
- what happens: halogen atoms is replaced by an OH group
- reflux
- nucleophile is :OH-
- draw the mechanism!
12
Q
what are the 4 kinds of nucleophilic substitution of haloalkanes?
A
- hydrolysis of haloalkanes to produce alcohols by aqueous alkali
- hydrolysis of haloalkanes to produce alcohols by water
- reaction of haloalkanes with NH₃ to produce amines
- reaction of haloalkanes with KCN to produce nitriles
13
Q
- hydrolysis of haloalkanes by water
A
- overall equation: R-CH₂X + H₂O —> R-CH₂OH + H⁺ + X⁻
- the X⁻ ions produced then react with Ag⁺ ions to form silver halide precipitate
> Ag⁺ (aq) + X- (aq) —> AgX (s) [colours!] - nucleophile is H₂O:, which is in AgNO₃ (aq)
14
Q
- reaction of haloalkanes with NH₃ to produce amines
A
- reagent: NH₃
- conditions: excess concentrated ammonia dissolved in ethanol at pressure in a sealed container
- what happens: first molecule of NH₃ - halogen atom is replaced by NH₂ group, second molecule of NH₃ - leads to formation of NH₄X
- overall equation: R-X + 2NH₃ —> R-NH₂ + NH₄X
- draw the mechanism! (nucleophile is :NH₃
15
Q
- reaction of haloalkanes with KCN to produce nitriles
A
- reagent: KCN
- conditions: ethanolic, warm
- what happens: halogen atom is replaced by CN group
- overall equation: R-X + KCN —> R-CN + KX
- draw the mechanism! (nucleophile is :CN-
16
Q
describe the boiling points of haloalkanes
A
- increases with molecular size due to increased london forces
- increases for straight chain isomers; greater branching = lower intermolecular forces
17
Q
describe the solubility of haloalkanes
A
soluble in organic solvents but insoluble in water
18
Q
how does the rate of hydrolysis change with different haloalkanes?
A
- trend rate: (slow) chloroalkanes, bromoalkanes, iodoalkanes (fast)
- C-I bonds are weaker because there is a smaller difference in electronegativity between the carbon and the iodine so the bonds require less energy to be broken
- C-F bond is the strongest C-halogen bond because F is more electronegative than Cl
- fluoroalkanes are unreactive as a large amount of energy is needed to break the C-F bond
