Module 4: Chapter 15 - Haloalkanes Flashcards
What is a haloalkane?
Compounds containing carbon, hydrogen, and at least one halogen
What is a haloarene?
A halogen attached directly to a benzene ring
What is the solubility of haloalkanes?
Soluble in organic solvents but insoluble in water
Describe how nucleophilic substitution occurs in haloalkanes:
The nucleophile has a lone pair of electrons and the carbon-halogen bond is polar. The electrons in the lone pair move to form a bond with the slightly positive carbon atom. However, carbon is restricted to 8 electrons in its outer shell so the polar carbon halogen bond breaks heterolytically forming a negative halide ion (the leaving group).
What 2 types of reaction mechanisms can occur when a haloalkane reacts with an OH- ion?
- Nucleophilic substitution
- Elimination
What is the organic product of nucleophilic substitution between haloalkanes and OH-?
Alcohol
What is the organic product of elmination between haloalkanes and OH-?
Alkene
Why does 1-iodopropane react faster with nucleophiles than 1-chloropropane?
THe C-I bond is longer and therefore weaker than the C-Cl bond. Therefore, 1-iodopropane reacts faster than 1-chloropropane.
What is the reaction between a haloalkane and warm aqueous NaOH?
What is the mechanism between a haloalkane and warm aqueous NaOH?
What is the type of mechanism is the reaction between a haloalkane and warm aqueous NaOH?
Nucleophilic substitution
What are the conditions for the reaction between a haloalkane and NaOH?
- Warm
- aqueous
What is the reaction between a haloalkane and warm ethanolic KCN?
What is the mechanism between a haloalkane and warm ethanolic KCN?
What is the type of mechanism in the reaction between a haloalkane and warm ethanolic KCN?
Nucleophilic substitution
What are the conditions in the reaction between a haloalkane and KCN?
- Ethanolic
- Warm
What is the reaction between a haloalkane and ammonia?
What is the mechanism between a haloalkane and ammonia?
What is the type of mechanism in the reaction between a haloalkane and ammonia?
nucleophilic substitution
What are the conditions in the reaction between a haloalkane and ammonia?
Excess concentrated ammonia dissolved in ethanol at pressure in a sealed container
What is the reaction between a haloalkane and hot ethanolic KOH (or NaOH)?
What is the mechanism between a haloalkane and hot ethanolic KOH (or NaOH)?
What is the type of mechanism in the reaction between a haloalkane and hot ethanolic KOH (or NaOH)?
Elimination
What are the conditions in the reaction between a haloalkane and KOH (or NaOH)?
- Hot
- Ethanolic
Why do haloalkanes have higher boiling points than equivalent alkanes?
Alkanes are non-polar molecules but haloalkanes contain a polar bond and are polar molecules. Therefore, although both contain London forces, only haloalkanes contain permanent dipole-dipole interactions. Therefore haloalkanes have a higher boiling popint than equivalent alkanes
Why, during nucleophilic substitution in the hydrolysis of haloalkanes, does the OH- approach the carbon from the opposite side than the halogen atom?
This minimises the repulsion between the negative hydroxide ion and negative halogen atom
What is a haloalkane hydrolysis reaction?
A nucleophilic substitution reaction in which the halogen atom is substituted for a hydroxide group.
What is a hydrolysis reaction?
A reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule causing the molecule to be split into 2 products
What is the trend in the strength of carbon-halogen bonds?
The strongest is C-F and it decreases down the group
How would you compare the rates of hydrolysis of different carbon-halogen bonds
Carry out the reaction in the presense of silver nitrate. As the reaction takes palce, halide ions produced will react with silver ions to form a silver halide precipitate. Observe the time taken for the precipitate to form and compare with different haloalkanes
What colour is the precipitate of silver chloride?
White precipitate (forms slowly)
What colour is the precipitate of silver bromide?
Cream precipitate (forms slower than iodide but faster than chloride)
What colour is the precipitate of silver iodide?
Yellow precipitate (forms rapidly)
How does the rate of hydrolysis change depending on the halide?
The rate of hydrolysis increases as the strength of the carbon-halogen bond decreases (rate increases down group)
What are organohalogen compounds?
Molecules that containa t elast one halogena tom joined to a carbon chain
What are the uses of organohalogen compounds?
- Pesticides
- General solvents
- Making polymers
- Flame Retardants
- Refrigerants
What is the ozone layer and why is it important?
The ozone layer is found at the outer edge of the stratosphere. Only a small amount of the gases that make up this layer are actually ozone, however, this is enough to absorb most of the biologically damaging ultraviolet radiation (UV-B) from the suns rays, only allowing a small amount to reach the earths surface
Why is UV-B damaging?
UV-B is damaging as it is linked to sun burn, increased genetic damage, and a greater risk of skin cancer in humans
What is ozone?
O₃
Describe what happens to ozone naturally in the stratosphere
Ozone is continually being formed and broken down by the action of UV light. Initially the very high energy UV breaks oxygen molecules down into oxygen radicals “O₂ -> 2O^ “. A steady state is then set up involving O₂ and the oxygen radicals in which ozone forms and then breaks down. This is in equilibrium “O₂ + O^ ⇌ O₃”
LET ^ = BULLET POINT (FOR RADICALS)
What are CFC’s?
Chlorofluorocarbons
How do CFC’s deplete the ozone layer?
Use CF₂Cl₂ as an example CFC
They deplete the ozone layer through a series of free radical substitution reactions:
Initiation: CF₂Cl₂ -> CF₂Cl^ + Cl^
Propogation Step 1: Cl^ + O₃ -> ClO^ + O₂
Propogation Step 2: ClO^ + O -> Cl^ + O₂
Overall equation: O₃ + O -> 2O₂
LET ^ = BULLET POINT FOR RADICALS
What is the overall equation for the breakdown of ozone by CFC’s?
O₃ + O -> 2O₂
Are CFC’s responsible for all ozone depleting reactions?
No, there are other harmful radicals which can also catalyse the breakdown of ozone, such as nitrogen oxide radicals
What are the 2 propogation steps for when nitrogen oxide radicals deplete the ozone in the atmosphere?
How nitrogen oxide radicals found in the environment?
- formed naturally during lightning strikes
- as a result of aircraft travel
What is photodissociation?
The breakdown of a molecule by radiation
What 2 types of radicals act as catalysts for the breakdown of ozone?
- Chlorine radicals (formed from homolytic fission of CFC’s)
- Nitrogen Oxide radicals
What are the steps for EVERY hydrolysis reaction?
all previous examples follow this
VERY IMPORTANT
- The nucleophile OH⁻, approaches the carbon atom attached to the ahlogen on the opposite side of the molecule from the halogen atom (this direction minimises the respulsion between the nucleophile and the δ- halogen atom)
- A lone pair of electrons on the hydroxide ion is attracted and donated to the δ+ carbon atom
- A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
- The carbon-halogen bond breaks by heterloytic fission
- The new product formed is an alcohol and a halide ion is also formed
