3.3- halogenoalkanes

Question 1

why are halogenoalkanes able to be attacked by nucleophiles (3)

Answer 1

-covalent bond between carbon and an a halogen is polar
-halogen atoms are more EN than carbon atoms so will attract the pair of electrons in the covalent bond more strongly than carbon
-this produces a permanent dipole-dipole with the C atoms being slightly positively charged, which leaves it open to attack by nucleophiles

Question 2

how to determine the reactivity of a covalent bond in halogenoalkanes

Answer 2

look at both bond polarity and bond enthalpy

Question 3

reactivity of halogenoalkanes
trend in halogenoalkane bond polarity- halogen size and strength of the polarity of C-halogen bond (2)

Answer 3

-halogen size increases- more shells, more shielding
-greater distance between nucleus of the halogen and shared pair of electrons in the covalent bond decreases their attraction

Question 4

reactivity of halogenoalkanes
trend in halogenoalkane bond enthalpy- halogen size and bond enthalpy (2)

Answer 4

-size of the halogens increases down the group. Distance between nucleus and the shared pair of electrons in the carbon-halogen bond increases. Bond length increases.
-Bond enthalpy decreases down the group. Nucleus of the halogen is further away from the pair of electrons down the group, so electrostatic attraction is weaker, so there is a weaker covalent bond

Question 5

in terms of bond enthalpy, which bond would be most reactive, which would be least reactive (2)

Answer 5

C-F bond= least reactive
C-I bond= most reactive
-C-F bond has a higher bond enthalpy than the C-I bond, so it would take the longest time to be broken and the rate of reaction to be the slowest

Question 6

in terms of bond polarity, which bond would be most reactive, which would be least reactive (2)

Answer 6

C-F bond= most reactive
C-I bond= least reactive
-fluorine is more EN than iodine so the C-F bond is more polar than the C-I bond

Question 7

what is hydrolysis

Answer 7

using water to break a covalent bond+split the molecule

Question 8

how does hydrolysis take place (4)

Answer 8

-takes place via nucleophilic substitution with water as the nucleophile
-introduce water by adding AgNO3(aq) so water acts as a nucleophile
-the halogen is covalently bonded so it cant react with silver ions in solution
Ag+(aq) + Br-(aq) –> AgBr(s)

Question 9

how is rate of hydrolysis measured (3)

Answer 9

-measured by the rate at which the ppt appears
-rate depends on C-X bond enthalpy
-bond enthalpy decreases down group (C-F has highest bond enthalpy)

Question 10

which haloalkane would have the fastest rate of hydrolysis and why

Answer 10

-1-iodobutane because it has a weaker C-X bond enthalpy so its easier to break as it needs less energy so its ppt would appear fastest

Question 11

why cant rate of hydrolysis for 1-fluorobutane be measured

Answer 11

AgF(aq) is formed which isnt a ppt- its colourless and hard to detect