15 - Haloalkanes Flashcards
Define nucleophile
electron pair donor
Examples of nucleophiles
Hydroxide ions / water molecules / ammonia molecules
How are haloalkanes classifies
Same as alcohols = primary/secondary/tertiary
How do haloalkanes react
Nucleophilic Substitution
Why are haloalkanes reactive
• Halogens more reactive than carbons = polar bond
• Carbon = slightly positive so can attract species with lone pairs
• When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction = NUCLEOPHILLIC SUBSTITUTION
Define hydrolysis
involves water or aqueous solution of a hydroxide.
Product of hydrolysis of haloalkanes
Alcohols + sodium halide
Conditions for hydrolysis of haloalkanes
aqueous sodium hydroxide, heated under reflux (
Why do we heat under reflux
Reaction slow at room temp
Equation for 1-bromobutane reacting with sodium hydroxide
.1-bromobutane + NaOH -> butan-1-ol + NaBr
What type of haloalkane does nucleophilic substitution work on
Primary
How to draw nucleophilic substitution mechanism
• Only need to show the OH ion
• Draw dipole between C-X (halogens are more electronegative than C)
• Curly arrow from lone pair on OH- to C𝛅+
• Curly arrow from C-X bond to X
• Draw final product and the X- ion
Draw the mechanism for 1-bromoethane reacting with sodium hydroxide
What does the rate of hydrolysis depend on
C-X bond strength
Strengths of all relative bonds
• C-F = very strong bond = unreactive
• C-Cl = strong bond = relatively unreactive
• C-Br = weaker bond = reacts faster
• C-I = weakest bond = reacts even faster
Does the rate of hydrolysis increase or decrease as c-x bind strength increases
Decreases
Why does the rate of hydrolysis increases as the C-X bond strength decrease
as less energy is required to break the C-X bond.
How to measure the rate of hydrolysis in primary haloalkanes
• Undergo the reaction with aqueous silver nitrate
- add ethanol
What acts as the nucleophile in the reaction with aqueous silver nitrate
o water (from the nitrate) acts as a nucleophile
Why do we add ethanol
• Haloalkanes are insoluble in water = Need ethanol (solvent) to allow the water and haloalkane to mix (prevents the mixture from separating into two layers)
What would we see in the reaction with silver nitrate
silver halide precipitate forms
• Ag+(aq) + X-(aq) = > AgX(s)
Colours if precipitates
• Chlorine = white
• Bromine = cream
• Iodine = yellow
Which precipitate forms more quickly
The yellow precipitate will form the fastest - Idiome
Why would the yellow precipitate form the fastest
because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I- ions to form the fastest
Strength of c-x bond not only factor effecting rate of hydrolysis?
Structural isomers
Do Tertiary haloalkanes more readily accept OH - ion
Yes = because more stable than primary
How does the c-x bond break
heterolytic fission
Draw the mechanism for 2-chloro-2-methyl propane
Define organohalogen
at least one halogen atom joined to carbon chain
Uses of organohalogen
General solvents, dry cleaning solvents, making polymers, flame retardants, refrigerants, pesticides
Why have they become a focus of concern
• Unnatural and not broken down by environment
Where is the ozone layer found
in the outer edge of the stratosphere
• Only a tiny fraction of gases making up the ozone layer is
ozone (O3)
Function of ozone gas
absorbs most of the UV-B from sun
What is UV-B
biologically damaging, causes sunburn
Therefore, why is the depletion of the ozone layer dangerous
More UV-B = could lead to increased genetic damage and/or an increased risk of skin cancer.
R/s between ozone + UV
• Ozone is continually formed and broken down by UV
Equation for ozone continually being formed and broken down by UV
• O = RADICAL
THIS IS NORMAL
Initially, what breaks down to get the O radical
high energy UV O2 -> 2O
THIS IS NORMAL
What are CFC’s
are very stable as C-X bond is strong
CFC’s remain stable until..
they reach the stratosphere
What happens when the CFC come into con tact with UV
• UV radiation provides enough energy to break C-X bond by homolytic fission
What forms after the C-X bond is broken by homolytic fission
Chlorine radicals
Effect of chlorine radicals = catalyse break down of ozone layer
Initial step
• Photodissociation – triggered by UV radiation
• CF2Cl2 -> CF2Cl∙ + Cl∙
• C-Cl bond has lowest enthalpy thus is the bond that breaks
Propagation step
• Very reactive Cl∙ reacts with ozone in 2 step process:
• Cl∙ + O3 -> ClO∙ + O2
• ClO∙ + O -> Cl∙ + O2
• OVERALL: O3 + O ->2O2
• This repeats in a cycle = chain reaction.
Effect of nitrogen oxide
oxide radicals (NO∙) are formed naturally during lightning strikes and from aircraft travel in stratosphere
• Breakdown of ozone is similar reaction to chlorine radicals
Probation step with nitrogen oxide
- NO∙ + O3 -> NO2∙ + O2
- NO2∙ + O -> NO∙ + O2
- OVERALL: O3 + O -> 2O2
why are oxygen radicals called diradicals
each atom has 2 unpaired electrons
