Chapter 15 Haloalkanes

define haloalkanes

compounds containing carbon, hydrogen, and at least one halogen

why is the C-Halogen bond polar

1) halogen atoms are more electronegative that carbon atoms

2) the electron pair in the c-halogen bond is closer to the halogen atom than the c atom

define nucleophile

electron pair donor

name three common nucleophiles

hydroxide ions, water molecules, ammonia molecules

define hydrolysis

a chemical reaction involving water or a an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule resulting in the molecule being split into two products

steps of nucleophilic addition of a haloalkane

1) the nucleophile approaches the c atom attached to the halogen atom on the opposite side of the molecule from the halogen atom

2) this direction of attack by the nucleophile minimises repulsion between the nucleophile and the delta negative halogen atom

3) a lone pair of electrons on the nucleophile is attracted and donated to the delta positive c atom

4) the c-halogen bond breaks by heterolytic fission

6) an alcohol and a halide ion are formed

diagram of nucleophilic substitution mechanism for the hydrolysis of chloroethane

describe how haloalkanes be converted to alcohols

aqueous sodium hydroxide

heated under reflux to obtain a good yield of product

equation for the hydrolysis of 1-bromobutane

CH₃CH₂CH₂CH₂Br + NaOH → CH₃CH₂CH₂CH₂OH + NaBr

3 predictions that can be made from the bond enthalpy of c-halogen bonds

1) iodoalkanes react faster than bromoalkanes

2) bromoalkanes react faster than chloroalkanes

3) fluoroalkanes are unreactive

general equation for the hydrolysis of haloalkanes in water

CH₃CH₂CH₂CH₂X +H₂O = CH₃CH₂CH₂CH₂OH + X^-

how can the rate of hydrolysis reactions of haloalkanes be measured

carrying out the reaction in the presence of aqueous silver nitrate

general equation for the reaction of aqueous silver nitrate and haloalkanes

Ag⁺ (aq) + X^- (aq) = AgX (s)

why is the hydrolysis of haloalkanes reaction done in the presence of an ethanol solvent

ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers

steps of hydrolysis of haloalkanes

1) set up three test tubes and add 1cm³ of ethanol and 2 drops of 1-chloro/bromo/iodobutane in each separate test tube

2) stand the tubes in a water bath at 60°c

3) place a test tube containing 0.1moldm^-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature

4) add 1cm³ of the silver nitrate quickly to each test tube. start stopwatch

5) observe test tubes for 5 minutes and record the time taken for precipitate to form

observation of hydrolysis of 1-chlorobutane

white precipitate forms very slowly

observation of hydrolysis of 1-bromobutane

cream precipitate forms slower that 1-iodobutane but faster that 1-chlorobutane

observation of hydrolysis of 1-iodobutane

yellow precipitate forms rapidly

define organohalogen compound

molecules containing at least one halogen atom joined to a carbon chain

uses of organohalogen compounds

pesticides, dry cleaning solvents, flame retardants, general solvents, making polymers, refrigerants

equations for ozone formation

O₂ → 2O

O₂ + O → O₃

why are CFCs very stable

strength of the carbon-halogen bonds within their molecules

how does the stability of CFCs affect the ozone layer

long residence time

takes them years to reach the stratosphere which has UV radiation to break down the ozone layer

equation for the photodissociation of CF₂Cl₂

CF₂Cl₃ = CF₂Cl radical + Cl radical

propagation steps of ozone depletion

Cl radical + O₃ = ClO radical + O₂

ClO radical + O = Cl radical + O₂

overall equation for the propagation steps of ozone depletion

O₃ + O = 2O₂

how are nitrogen oxide radicals formed

lightning strikes and aircraft travel in stratosphere

propagation steps of ozone depletion involving NO

NO radical + O₃ = NO₂ radical + O₂

NO₂ radical + O = NO radical + O₂

overall equation for propagation steps of ozone depletion involving NO

O₃ + O = 2O₂