Haloalkanes Flashcards
Write the overall equation for the reaction of fluorine with methane to form
trifluoromethane
CH4 + 3F2 →CHF3 + 3HF
In free radical substitution, how easy is it to control the yield of the desired haloalkane? Why?
Because it is quite difficult to control this reaction when chlorine is in excess, it is not easy to control the yield of the desired halogenoalkane. With substitution occurring in different places on the same carbon atom, or different carbon atoms, the variety of possible products places quite a limitation on this reaction as a means of chemical synthesis. Fractional distillation can be used to separate the products.
State the phases of the different chloroalkanes at room temperature
Chloromethane and chloroethane are gases at room temperature, but bigger chloroalkane molecules are liquids and useful solvents in the laboratory or in industry
What are the limitations of chloroalkanes?
They are still quite volatile and chloroalkane vapours can be harmful if breathed in
What is the main use of haloalkanes?
Halogenoalkanes are used as refrigerants
What’s a radical?
A species with an unpaired electron
What’s the name for a ‘species with an unpaired electron’?
A radical
Write free radical substitution equations to show how Cl free radicals catalyse the breakdown of O3
Cl2 → 2Cl. (in presence of UV light)
Cl. + O3 → ClO. + O2
ClO. + O3 → 2O2 + Cl.
Overall: 2O3 → 3O2
What does the b.p. of haloalkanes depend on? Also, explain how each factor affects b.p.
The boiling point of halogenoalkanes depends on:
- the halides (halogen attached)
- the length of the hydrocarbon chain
- the structure
The halides have the following boiling point order: Chloride < Bromide < Iodide. Whereas alkanes have increasing boiling points with increasing chain length. Note: chain length affects b.p. more than halide. e.g. CH3CH2Cl has higher b.p. than CH3Cl and CH3Br but not CH3I and CH3CH2Br has literally a slither lower b.p. than CH3I. This trend is constant for higher chain length haloalkanes too. Straight chains exert stronger intermolecular attractions that require higher energy to overcome, so they will have higher boiling points when compared to compact branched structures, such as secondary and tertiary halogenoalkanes
State the phase of haloalkanes at room temperature
Some of the lower chain halogenoalkanes are gases at room temperature. Methyl halides are gases at room temperature except for iodomethane, which is liquid. Chloroethane is also a gas at room temperature. All the other alkyl halides you are likely to encounter will be liquid at room temperature
Explain the pattern of b.p. observed in haloalkanes
The pattern of boiling point observed in halogenoalkanes are related to their Van der Waal’s intermolecular forces. Van der Waal’s forces depend on electron densities; the more electrons, the stronger the interaction. The larger halides and the longer alkanes all have higher electron densities, which provide strong intermolecular attractions that require more energy to overcome, hence the higher boiling points
Describe & explain the solubility of haloalkanes
Halogenoalkanes are only very slightly soluble in water as they are not polar enough and do not exhibit hydrogen bonding with water molecules. They are, however, soluble in organic solvents
What are more electronegative, halogens or carbon?
Apart from iodine, halogens are more electronegative than carbon
Which haloalkane isn’t likely to undergo nucleophilic substitution? Why?
Iodoalkane is not polar due to the similar electronegativity between carbon and iodine, so it is not likely to attract a nucleophilic substitution reaction
Describe and explain the order of halogens in a haloalkane determined by the rate of nucleophilic substitution of that haloalkane
The rate of nucleophilic substitution of the halides follows the order mapped out below:
Fluorine < Chlorine < Bromine < Iodine
This is because, despite the electronegative property that predisposes the polarity of halogenoalkane in the opposite order, it is the strength of each C-halide bond enthalpy that determines how easily broken the bond will be and therefore the rate of the reaction. The C-X bond decreases in strength in the same order above. In order to react with a halogenoalkane the carbon-halide bond must be broken. The stronger the bond, the more unreactive the halogenoalkane. In fact, carbon-fluoride is so strong it renders the molecule highly unreactive. The weaker the bond, the easier it is to break it and therefore, the faster the rate of reaction. Hence the fastest rate will involve reactions with iodoalkanes, whereas the slowest rate will involve reactions with fluoroalkanes
How is a nucleophilic substitution reaction between a haloalkane and OH- done and what is produced?
Nucleophilic substitution reaction of a halogenoalkane with an OH- group will lead to the OH- group replacing and releasing the halogen, producing an alcohol (R-OH). In this reaction, the halogenoalkane is heated under reflux with a solution of sodium or potassium hydroxide. Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture. It is important that the reaction takes place in aqueous solution. As halogenoalkanes are generally insoluble in water, a mixture solvent consisting of ethanol and water (50/50) is normally used to ensure mixing of halogenoalkane and sodium/potassium hydroxide so that the reaction can occur.
Describe the diagram showing a setup of a reflux reaction
There’s a ‘pear-shaped flask’ with the solution in it and at the bottom of it, ‘anti-bumping granules’ are labelled. Connected at the top of it is a ‘Liebig condensor’ which itself has an open top. Describing the Liebig condensor, it’s like this tube (this is attached to the pear-shaped flask) and around it is this sort of covering. The covering only has 2 exposed valves, one at the top labelled ‘cold water out’ and one at the bottom on the opposite side labelled ‘cold water in’. Underneath the pear-shaped flash there’s an upwards arrow, under which it says ‘HEAT’.
State the reagent and conditions for nucleophilic substitution between haloalkane and OH-
- Reagent: Warm, aqueous NaOH or KOH
- Conditions: Reflux in an aqueous solution of 50:50 ethanol & water
