[15.1] chemistry of haloalkanes

Question 1

what are haloalkanes synthesised to form?

Answer 1

• PVC
• teflon
• anaesthetics
• solvents

Question 2

what is the general formula for haloalkanes?

Answer 2

CₙH₍₂ₙ₊₁₎X (X = F, Cl, Br, I)

Question 3

what are haloalkanes also called?

Answer 3

halogenalkanes or alkyl halides

Question 4

how are haloalkanes classified?

Answer 4

• 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

Question 5

why are haloalkanes more reactive than alkanes?

Answer 5

• 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

Question 6

what is a nucleophile and what will they do?

Answer 6

• 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

Question 7

what are nucleophiles attracted to?

Answer 7

low electron density

Question 8

what are 3 examples of nucleophiles?

Answer 8

• hydroxide ions, :OH-
• water molecules, H₂O:
• ammonia molecules, :NH₃

Question 9

what is nucleophilic substitution?

Answer 9

• when nucleophiles replace halogens in substitution reactions
• only primary haloalkanes undergo nucleophilic substitution

Question 10

draw and describe the nucleophilic substitution mechanism

Answer 10

1. nucleophile attacks the partially-positive carbon with its lone pair of electrons
2. C-X bond breaks: both electrons in the bond move onto the leaving group - the halide ion
3. the halogen has been substituted for the nucleophile on the alkyl chain

Question 11

1. hydrolysis of haloalkanes by aqueous alkali

Answer 11

• 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!

Question 12

what are the 4 kinds of nucleophilic substitution of haloalkanes?

Answer 12

1. hydrolysis of haloalkanes to produce alcohols by aqueous alkali
2. hydrolysis of haloalkanes to produce alcohols by water
3. reaction of haloalkanes with NH₃ to produce amines
4. reaction of haloalkanes with KCN to produce nitriles

Question 13

2. hydrolysis of haloalkanes by water

Answer 13

• 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)

Question 14

3. reaction of haloalkanes with NH₃ to produce amines

Answer 14

• 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

Question 15

4. reaction of haloalkanes with KCN to produce nitriles

Answer 15

• 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-

Question 16

describe the boiling points of haloalkanes

Answer 16

• increases with molecular size due to increased london forces
• increases for straight chain isomers; greater branching = lower intermolecular forces

Question 17

describe the solubility of haloalkanes

Answer 17

soluble in organic solvents but insoluble in water

Question 18

how does the rate of hydrolysis change with different haloalkanes?

Answer 18

• 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