15. Haloalkanes
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Define a nucleophile.
an electron pair donor
What do all nucleophiles possess?
at least one lone pair
What are the 4 common nucleophiles?
OH- , CN- , NH3, H2O
Define hydrolysis.
the breaking of a chemical compound into 2 products, using H2O or OH-
What governs the rate of hydrolysis? Therefore do weaker or stronger bonds hydrolyse faster?
bond strength weaker bonds: e.g. C-I
Why is C-I the weakest C-halogen bond?
I = larger atom = more shielding = less nuclear attraction to bonding pair = weaker bond
Does the rate of hydrolysis increase or decrease from C-I to C-F ?
decreases up the group C-I hydrolyses faster than C-F
Which is the most polar C-halogen? What does this mean for it?
C-F greatest difference in electronegativity it attracts nucleophiles better an has the highest bond enthalpy
Describe an experiment to measure RoH.
• add equal vol. 1-iodopropane, 1-bromopropane and 1-chloropropane to equal vol. of ethanol in separate test tubes • warm mixture to 50 degrees in a water bath and add equal vol. of AgNO3 • record time taken for precipitate to form • -white (Cl) appears last, yellow (I) appears first
Why do you add ethanol to the haloalkane when measuring RoH?
ethanol = v good solvent has H bonding so dissolves aqueous/ionic and LDFs so dissolves organic
What does ozone do for us?
prevents genetic damage and cancer by absorbing UVB rays from the sun
What is the steady state of ozone? (reversible reaction)
O2 + O. <> O3
What happens to CFCs when they reach the ozone layer?
UV rays break C-Cl bond, as it has the lowest enthalpy, into Cl. radicals.
Write out the set of equations for ozone depletion by chlorine radicals.
go find it somewhere in ur little book
What is the overall equation for ozone depletion?
O3 + O => 2O2
What other radical catalyses ozone depletion? How is it formed?
NO. formed in lightning strikes and by air travel
Write out the set of equations for ozone depletion by NO. radicals.
go find it somewhere in ur little book
What kind of mechanism do haloalkanes undergo?
nucleophilic substitution
Draw out the nucleophilic substitution of chloroethane and NaOH
go find ur book katherine
What are the conditions for the nucleophilic substitution of a haloalkane?
sodium or potassium hydroxide solution and reflux
Why does the AgNO3 have to be aqueous when precipitating with haloalkanes?
the H2O in the solution acts as a nucleophile to cleave the halogen from the haloalkane creates a halide ion that can bond to the Ag+ ions to form the precipitate
Which stage of branching of haloalkanes hydrolyse the fastest? Why?
tertiary haloalkanes more stable
What are nucleophiles attracted to in a haloalkane?
the slightly +ve (e- deficient) Carbon
What is the initiation step for the ozone depletion with CFCs?
CF2Cl2 => CF2Cl. + Cl.