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
State the reagent, conditions, product (including why this product is useful) and general equation for nucleophilic substitution between haloalkane and CN-
- Reagent: Warm ethanolic sodium or potassium cyanide
- Conditions: Reflux in ethanol (no water)
- Product: Halogen and Nitrile (R-CN) which extends the carbon chain by one carbon atom
- General Equation: R-X + OH- -> ROH + X-
Describe the importance of nucleophilic substitutions between haloalkanes and cyanide
The importance of this reaction is in its ability to extend the length of the carbon chain by one carbon atom. There are not many simple ways of making new carbon-carbon bonds. The cyanide group can later react with water or acid and be easily converted into carboxylic acids or amines, respectively
State the reagent, conditions (including why it’s like this), product and general equation for nucleophilic substitution between haloalkane and NH3
- Reagent: Warm, concentrated ammonia in ethanol solution in excess
- Conditions: In a sealed tube because if it is done under reflux, the ammonia gas will only escape through the condenser
- Product: Amine (R-NH2 or its salt if reacting with the acid it produced)
- General Equation: R-X + 2NH3 -> RNH2 + NH4^+ + X-
Describe the mechanism for nucleophilic substitution between haloalkane and ammonia
- N of NH3 has a lone pair. An arrow from lone pair to a C next to the halogen of the haloalkane which is marked slightly positive. The halogen is marked slightly negative. Arrow from bond between halogen and C atom to halogen.
- The NH3 has now replaced the halogen which is an independent X-. Arrow from lone pair of N of another NH3 that’s pointing to N in the main molecule. This N has a + sign on it. Arrow from bond between N+ and one of its H atoms towards said H atom.
- Final molecules shown: amine, NH4^+, X-
Explain the stages of the nucleophilic substitution between ammonia and haloalkane
The reaction happens in two stages. In the first stage, a salt of a primary amine is formed - alkylammonium halide. The second ammonia removes a hydrogen ion from the alkylammonium ion to leave an amine. The reaction is reversible and the more ammonia there is in the mixture, the more the forward reaction is favoured
If a primary amine is produced in nucleophilic substitution between haloalkane and ammonia, what will likely occur?
The primary amine produced in the reaction is also a nucleophile that contains a lone pair of electrons that can produce a series of amines . The primary amine can attack another halogenoalkane molecule and produce a secondary amine which will attack another to produce a tertiary amine and eventually an ionic quaternary ammonium salt
Give the general equations for the production of secondary & tertiary amines and quaternary ammonium salt from a primary amine
- Secondary Amine: RNH2 + RX -> R2NH + HX
- Tertiary Amine: R2NH + RX -> R3N + HX
- Quaternary Ammonium Salt: R3N + RX -> R4N+ + X-
In nucleophilic substitution between ammonia and haloalkane where ammonia is in excess…?
- The forward reaction is favoured
- Haloalkanes will more likely connect with another ammonia molecule rather than a primary amine and therefore, prevent the creation of the amine series
In nucleophilic substitution between ammonia and haloalkane where less ammonia is in mixture…?
- The forward reaction is less favoured
- Haloalkanes will more likely connect with another primary amine molecule produced rather than ammonia and therefore, the creation of the amine series is more likely
What mechanism between which reactants produce alcohol?
Nucleophilic substitution between haloalkane and water
What mechanism between which reactants produce nitrile?
Nucleophilic substitution between haloalkane and cyanide
What mechanism between which reactants produce amine?
Nucleophilic substitution between haloalkane and ammonia
What is nucleophilic substitution?
A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ− atom leaves molecule and is replaced by a nucleophile
What’s the name for ‘A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ− atom leaves molecule and is replaced by a nucleophile’?
Nucleophilic Substitution
Explain when elimination occurs instead of nucleophilic substitution
Eliminations of a halide can occur in halogenalkanes, instead of substitutions, in the presence of hydroxide (OH-) under different conditions, such as when the solvent is alcohol instead of water. In this case, the hydroxide will act like a normal base instead of a nucleophile, reacting, removing an H+ from the haloalkane, and producing an alkene instead of an alcohol
State the reagent, conditions, product and general equation for elimination between haloalkane and OH-
- Reagent: Warm ethanolic sodium or potassium hydroxide
- Conditions: Reflux in alcoholic solution
- Product: Alkene and halide
- General Equation: CnH2n+1X (l) + OH- (alc) -> CnH2n (g) + H2O (l) + X- (aq)
In a reaction with conditions of reflux in alcoholic solution, what happens if one of the reactants is a haloalkane with more than 1 halogen in it?
If a haloalkane contains more than one halide in an unsymmetrical structure, another proton on the molecule will also be open to attack, producing mixtures of isomers of alkenes
What factors affect whether elimination or nucleophilic substitution predominantly occurs in a reaction vessel between OH- and a haloalkane? (3)
- Solvent
- Temperature
- OH- concentration
- Type of Haloalkane
In a haloalkane-OH- reaction, which mechanism will take place if the solvent used is water?
Nucleophilic Substitution
In a haloalkane-OH- reaction, which mechanism will take place if the solvent used is ethanol?
Elimination
In a haloalkane-OH- reaction, which mechanism will take place if temperature is on the lower side?
Nucleophilic Substitution
In a haloalkane-OH- reaction, which mechanism will take place if temperature is on the higher side?
Elimination
In a haloalkane-OH- reaction, which mechanism will take place if OH- concentration is on the lower side?
Nucleophilic Substitution
In a haloalkane-OH- reaction, which mechanism will take place if OH- concentration is on the higher side?
Elimination
In a haloalkane-OH- reaction, which mechanism will take place if a primary haloalkane is being reacted?
Mainly nucleophilic substitution
In a haloalkane-OH- reaction, which mechanism will take place if a secondary haloalkane is being reacted?
Both nucleophilic substitution and elimination
In a haloalkane-OH- reaction, which mechanism will take place if a tertiary haloalkane is being reacted?
Mainly elimination
