C15- Haloalkanes Flashcards
Why add conc HCl slowly (dropwise) to a reaction
Addition of concentrated acid to liquids causes heating.
If the all of the acid was added at once, the temperature of the mixture may become hazardously high.
At the end of the reaction (synthesis of a haloalkane), what impurities contaminate the organic product? How are each of these impurities removed?
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Any unreacted starting product will remain in the aqueous layer, which is run off after the reaction.
Addition of the hydrogencarbonate neutralises any remaining hydrochloric acid, which results in release of carbon dioxide - the gas given off in this washing step. When no more gas is given off, this shows that all hydrochloric acid has reacted.
Anhydrous sodium sulfate removes any traces of water from the organic product.
Finally, the distillation separates the desired product from any organic by-products.
How does the final distillation step allow a check to be made on the identity and purity of the organic product?
The boiling point measured during the distillation gives information about both the identity and the purity of the product.
If the expected boiling point is observed this is an indication that the correct product has been obtained.
If the boiling point is a range, as opposed to a well-defined temperature, this indicates the presence of impurities.
Discuss the factors that can lead to a percentage yield less than 100%.
synthesis of a haloalkane
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a. production of side-products in the reaction
b. incomplete reaction
c. loss of some of the product in the washing steps
d. loss of some of the product in running the liquid off from the separating funnel into the flask, and in decanting the liquid from the flask into the pear-shaped flask
e. loss of product in the distillation.
Why is the carbon- halogen bond polar
halogens are more electronegative than carbon atoms
The electrons in the covalent bond are more attracted to the halogen, making the bond polar
Three sources of CFCs in the atmosphere
Aerosols
Air conditioning
Refrigerants
Name the layer of the atmosphere above the troposphere
stratosphere
Why is ozone depletion a major concern to humans
Allows more harmful UVB rays to pass through the troposphere
Increases incidence of skin cancer and genetic damage
Type of mechanism that represents the hydrolysis of a haloalkane
Nucleophilic substitution
Products of Hydrolysis of a haloalkane
Alcohol and halide ion
Type of bond breaking that forms ions
Homolytic fission
Type of bond breaking that forms radicals
Heterolytic fission
5 key steps to the mechanism of hydrolysis of a haloalkane
1- OH- nucleophile approaches c-cl on opposite side of molecule to the halogen
2- Lone pair of electrons on OH- attracted to the partially positive carbon
3- New covalent bond forms between carbon and oxygen from OH
4- Carbon- halogen bond breaks by heterolytic fission
5- Forms alcohol and halide ion
What is the nucleophile in hydrolysis of a haloalkane mechanism
OH-
Why does the OH- nucleophile approach C-Cl bond on opposite side to the halogen
Minimises repulsion between the partially negative halogen and the negative pair of electrons
Reaction between haloalkanes and NaOH
Very slow at RTP so done at reflux
Forms alcohol and sodium halide
Which carbon- halogen bond is the strongest
C - F
makes fluoroalkanes pretty unreactive due to large quantities of energy needed to overcome the bond
How does bond strength affect rate of reaction with haloalkanes
Affects energy need to overcome bond
Iodoalkanes will react faster than bromoalkanes
How to measure the rate of hydrolysis of haloalkanes
Carry out reaction in prescence of AgBr
As reaction takes place, the halide ions will form a precipitate with the Ag+ ions
Carry out in ethanol
Why measure the rate if hydrolysis of haloalkanes in ethanol not water
haloalkanes are insoluble in water- reaction would form 2 layers
layers can mix in ethanol
Why are tertiary haloalkanes hydrolysed the fastest
Carbon halogen bond is weaker
What is UVB
Biologically damaging radiation
Ozone in the stratosphere
equilibrium
Continually broken down and reformed by UV radiation
equilibrium of formation and breakdown
Why are CFCs so stable
Many carbon- halogen bonds
CFCs
general
Very stabvle so reach the stratosphere
break down to form CL radicals that catalyse the breakdown of ozone
How are nitrogen oxide radicals formed in the atmosphere
Lightning strikes
Aircraft in the stratosphere
Why do CFCs produce cl radicals not F radicals
C-cl bond is the weakest so overcome first
Propagation steps Clorine radical and ozone
CL. + O3 –> ClO. + O2
ClO. + O –> Cl. +O2
Overall reaction chlorine radical and ozone
O3 + O –> 2 O2
Propagation and overall reaction
Nitrogen oxide radical and ozone
NO. + O3 –> NO2. + O2
NO2. + O –> NO. + O2
Overall= O3 +O –> 2 O2
Significance of second propagation step in reaction between chlorine radical and ozone
Regenerates Cl radical
Chain reaction repeats reaction in a cycle
Define nucleophilic substitution
An atom or group with an electron pair is exchanged for another atom or group
The atom or group causing the substitution is a nucleophile, donates a pair of electrons to form the new covalent bond
What properties make CFCs suitable for usage as an aerosol?
low reactivity
non toxic
volatile / low boiling point
Name two sources of NO in stratosphere
Lightning strikes
Aircraft
Over all equation chlorine radicals breaking sown CFC
O3 + O –> 2O2
Equation for equilibrium of ozone
O3 <–> O2 + O
