Haloalkanes Flashcards
What are the three types of haloalkanes? Define each.
Primary haloalkanes - the halo group is attached to a carbon atom with at least two H atoms attached.
Secondary haloalkanes - the halo group is attached to a carbon atom with only one H atom attached.
Tertiary haloalkane - the halo group is attached to a carbon atom with no H atoms attached .
How would you describe the trend in reactivity of the haloalkanes? Why?
Fluoroalkanes are Unreactive due to a the high strength Carbon-Fluorine bond. Iodoalkanes are the most reactive of the haloalkanes because the Carbon-Iodine bond is weak and easily broken.
What are the different types of haloalkanes? Define each.
Primary haloalkane - the halo group is attached to a carbon atom with at least two H atoms attached (end of a molecule)
Secondary haloalkane - the halo group is attached to a carbon atom with only one H atom attached (middle of a molecule with no branching)
Tertiary haloalkane - the halo group is attached to a carbon atom with no H atoms attached.
What are the differences between alkanes and haloalkanes in physical properties and reactivity?
The halogen has a higher electro negativity than the carbon to which it is attached. Increased intermolecular forces between molecules. It is these permanent dipole-dipole forces. As a result there will be an increase in boiling points, melting points and density in comparison to the corresponding alkanes. Because the haloalkanes do not possess hydrogen bonding they are often insoluble in water, although haloalkanes are more soluble in water than the corresponding alkanes and the alkenes.
Why are haloalkanes more soluble in water then comparative alkanes and alkenes?
Due to permanent dipole-dipole interactions with the water molecules.
Draw chloromethane include relevant dipoles and explain why it is polar?
- Carbon is delta positive
- Cl Is delta negative
Chlorine is more electronegative than carbon. The positive carbon atom can be attacked by negatively charged species known as nucleophiles.
Define a nucleophile
A nucleophile is a species which is an electron pair donor
What is the name of the mechanism that we learn about where haloalkanes are involved?
Nucleophilic Substitution Reaction
Give an example of a nucleophile, why can both molecules and ions be nucleophiles?
OH-
CN-
NH3
H2O
They only need to have a lone pair of electrons and are sometimes negatively charged.
Draw a general mechanism for nucleophilic substitution
Look in photos
What are the reagents and conditions for the alkaline hydrolysis of a Haloalkane reaction?
Reagents: Sodium Hydroxide (aq)
Conditions: Warm (or reflux)
Why do you never include the Na ion in the mechanism?
The sodium hydroxide dissociates in aqueous solution, so the attacking species is the hydroxide ion (whether NaOH or KOH is used). The sodium ion is a spectator therefore never include the metal ion.
What is the equation for the reaction of bro mortgage with OH-?
C2H5Br + OH- ➡️ C2H5OH + Br-
Draw the mechanism for bromoethane? Include relevant dipoles.
See picture
What bond is present in cyanide?
Triple bond
Why is the addition of cyanide to a haloalkane in a mechanism useful in the industry?
It extends the length of the carbon chain.
What conditions and products are made when an alcoholic solution ammonia reacts with bromoethane?
When refluxed under pressure in aqueous, alcoholic solution ammonia reacts with bromoethane to produce the amine, aminoethane
Write an equation for the reaction of bromoethane and alcoholic solution ammonia.
C2H5Br + 2NH3 ➡️ C2H5NH2 + NH4BR
Why are two moles of ammonia needed? (Apart from the fact that the equation needed to be balanced)
The second ammonia molecule ensures the removal of HBr which leads to the formation of a salt.
Using these bond enthalpies explain why reactivity is the way it is for haloalkanes: C-F = 467 C-Cl = 340 C-Br = 280 C-I = 240
Bond enthalpies decrease down group 7 so these bonds require less energy to break and are therefore easier to break making them more reactive.
Give the order of ease of hydrolysis of the haloalkanes in order from most reactive to least reactive
Iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes
Explain a practical that you would use to show the reactivity of the haloalkanes
- The haloalkane is mixed with water and aqueous than pic silver nitrate. This is placed in a water bath at 60•c. (Ethanol is used as it dissolves the haloalkanes better)
- The water acts as a nucleophile. As the hydrolysis reaction occurs the halide ion, X is displaced.
- The halide ions, X produced react with the Ag+ ion in the silver nitrate to form a precipitate of the silver halide.
- By timing the first appearance of a silver halide precipitation you can deduce the rate of hydrolysis.
What would be formed for the following haloalkanes when the aqueous ethanolic silver nitrate was added?
1-Chlorobutane
1-Bromobutane
1-Iodobutane
1-Chlorobutane - White precipitate
1-Bromobutane - cream precipitate
1-Iodobutane - yellow precipitate
State the role of the following reagents or conditions in the practical experiment: Ethanol Water Heated in a water bath at 60•c Silver Nitrate
A - is a better solvent for the haloalkane
B - acts as a nucleophile
C - increase the reaction rate
D - produces a precipitate of the silver halide
Give to uses of halogen compounds
- Polymers - PVC, Teflon
* Chlorofluorocarbons are used as refrigerants and aerosol propellants
What are the main environmental problem associated with CFC’s?
They are responsible for the thinning of the ozone layer (in the stratosphere)
Ozone absorbs significant proportions of the Suns ultraviolet radiation and as a result protects us from skin cancer.
What is the chemical symbol for ozone?
O3
What does the term radical mean?
A species with unpaired electrons
What are the two overall equations for the depletion of the ozone layer by the halogen radical?
2O3 ➡️(UV) 3O2
O3 + O• ➡️ 2O2
