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IQ3. What are the products of reactions of hydrocarbons, and how do they react?
IQ3. What are the products of reactions of hydrocarbons, and how do they react?
3.1 Reactions of unsaturated hydrocarbons:
- combustion [combination with O2 to produce CO2 and H2O]
- addition
3.2 Reactions of saturated hydrocarbons
- combustion [combination with O2 to produce CO2 and H2O ]
- substitution
3.1 investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to hydrogen (H2), halogens (X2), hydrogen halides (HX) and water (H2O)
Addition Reactions
Addition Reactions
Alkenes are more chemically reactive than alkanes. The double bond is a site of high electron density and will readily combine with species with high electronegativities (eg N, O, F, Cl, Br, I).
Ethene (ethylene) is a small molecule with a highly reactive double bond. This means it can be readily transformed into many useful products.
1. Hydrogenation – Production of ethane
Hydrogenation is the process of adding hydrogen atoms across a double bond in an alkene (or across a triple bond in an alkyne). Ethene can be hydrogenated to produce ethane. A metal catalyst, eg Ni or Pt, is often used.
CH2=CH2 + H2 (Pt catalyst) → CH3-CH3
2. Halogenation - Production of dichloroethane
Halogenation is the process of adding halogen atoms (F, Cl, Br, I) across a double bond in an alkene (or across a triple bond in an alkyne). Ethene can be halogenated with chlorine to produce 1,2-dichloroethane. No catalyst is needed.
CH2=CH2 + Cl2 → CH2Cl-CH2Cl
3. Hydrohalogenation - Production of chloroethane
Hydrohalogenation is the process of adding one hydrogen and one halogen atom (H-F, H-Cl, H-Br, H-I) across a double bond in an alkene (or across a triple bond in an alkyne).
Example: ethene can be hydrohalogenated with hydrogen chloride to produce chloroethane.
CH2=CH2 + HCl → CH3-CH2Cl
NB To work out which carbon will take the hydrogen and which will take the halogen, we use Markovnikov’s Rule. This rule states that the hydrogen atom from the hydrogen halide will bond with the carbon which has the greater number of hydrogens prior to the addition reaction.
4. Hydration - Production of Industrial Alcohol (ethanol)
Hydration is the process of adding water molecules, or the equivalent of water molecules, to a substance. Ethylene can be hydrated to produce ethanol when heated with a dilute sulfuric acid (H2SO4) catalyst.
eg CH2=CH2 + H-OH (H+ catalyst) → CH3-CH2OH
ATAR Notes Key Point: Unsaturated hydrocarbons undergo addition reactions by ‘opening’ double bonds (like a bridge opening) and incorporating the additional molecule. (Silove, 2018, p. 61)
The second (pi) bond of the double bond breaks, allowing each C atom to bond individually with another atom or group.
Extra for experts
Alkenes.pptView videos:
- OC#14 https://www.youtube.com/watch?v=MtRdeqv7jRw&list=UUkdi7YOXBGAapx_dR40UoFQ&index=18 [11.50]
- High reactivity https://youtu.be/G3QMD0t-YRw 11.15
Practical 3.1 Properties of Hydrocarbons
Introduction
There is a distinct difference between the chemistry of alkenes and that of alkanes.
- Alkenes are unsaturated hydrocarbons that react readily by the process of addition.
- Alkanes are saturated hydrocarbons that react slowly by the process of substitution.
This difference in reactivity is used as the chemical basis for this experiment.
Compare the reactivities of alkenes with alkanes in bromine water. Write chemical equations for all reactions observed.
Materials
1x bromine water (dropper bottle)
1x safety glasses
test tubes + rack + labels
hydrocarbon test samples (e.g. cyclohexene, cyclohexane/hexane*)
waste container for the safe disposal of test samples
Construct a table to show the name of each compound tested, whether it is an alkane or alkene and what you observed after the addition of bromine water.
Write a conclusion based on your observations.
Background
- Bromine water has a distinctive brown colour.
- Saturated: Containing only single carbon-to-carbon bonds.
- Unsaturated: Containing at least one double carbon-to-carbon bond.
- Alkenes react spontaneously with bromine water due to their unsaturated nature.
- The reaction between an alkene and bromine involves the breaking open of the alkene’s double bond and the inclusion of bromine in its structure (an addition reaction).
- This results in the production of an alkane with two bromine functional groups.
- Therefore, when alkenes come into contact with bromine water, they cause it to decolourise.
- Alkanes do not react spontaneously with bromine water due to their saturated nature.
- If placed in ultraviolet light, a substitution reaction may occur between an alkane and bromine water.
- This results in the production of hydrogen bromide and an alkane with a single bromine functional group.
- Therefore, when alkanes come into contact with bromine water and are exposed to ultraviolet light, they decolourise bromine water.
Experiment
1. Place 5 mL of cyclohexane and cyclohexene in separate, labelled 10 mL test tubes.
2. Add 2 mL of bromine water to each test tube using a dropper.
3. Stopper and gently agitate each test tube to the same extent.
4. Allow the test tubes to rest for 30 seconds.
5. Observe and record any colour changes.
The hydrocarbon forms an immiscible layer at the top of the bromine water.
TASK 3.1.1
In the margarine industry, alkenes are often hydrogenated to convert unsaturated oils into solid fats that have a greater proportion of saturated molecules.
a) Using ethene as an example, write an equation for this reaction and state the type of reaction this represents. (2 marks)
b) Describe a test that could be used to confirm that all the ethene has been converted. (2 marks)
2. (2011, Q11)
Which compound can form when bromine water reacts with propene?
(A) 1-bromopropane
(B) 2-bromopropane
(C) 1,1-dibromopropane
(D) 1,2-dibromopropane
(2009, Q6)
Bromine, Br2, dissolves in unsaturated hydrocarbons and reacts immediately. Which of the following is the best description of this process?
(A) Bromine is polar and reacts by adding bromine atoms across the double bond.
(B) Bromine is polar and reacts by substituting hydrogen atoms with bromine atoms.
(C) Bromine is non-polar and reacts by substituting hydrogen atoms with bromine atoms.
(D) Bromine is non-polar and reacts by adding bromine atoms across the double bond.
4. (2011, Q1)
Which of the following industrial processes is used to produce ethanol from ethylene?
(A) Hydration
(B) Dehydration
(C) Addition polymerisation
(D) Condensation polymerisation
5. Use an example to demonstrate your understanding of Markovnikov’s rule.
6. PAST HSC QS (IF NO MARKS ARE INDICATED, THE QS ARE MULTIPLE CHOICE. THERE ARE CURRENTLY 20 MC QS PER PAPER, BUT IN PAST YEARS THERE WERE 15).
2016 Q15
2015 Q25A 3
2013 Q18
2011 Q11
2010 Q24A 2
2006 Q20
2017 Q7
2016 Q15
2011 Q11
2010 Q24B 2
2009 Q6
2008 Q16B 3
2005 Q16A 1; 16B 2; 16C 2
7. VIEW and complete Bromine Water HSC Qs answers (see below) https://drive.google.com/drive/u/1/folders/1CqBwsw9AA__fbPcDBxAtRqI7n4WXfwd7
3.2 write equations and construct models to represent the reactions of saturated hydrocarbons when substituted with halogens.
Substitution Reactions
Substitution Reactions
Spelling error means he needs to be there himself, I think!
Substitution Reactions
Substitution Reactions
The low reactivity of alkanes means they do not readily react with halogens unless in the presence of ultraviolet light. Even so, the reaction is not an addition reaction, it is a substitution reaction. One of the halogen atoms substitutes for (or replaces) a hydrogen atom and a haloalkane is formed as a result. A second product will also form from the combination of the substituted hydrogen with the remaining halogen atom.
Ethane + chlorine → chloroethane
CH3—CH3 + Cl2 → CH3—CH2Cl + HCl
The chlorination of methane does not necessarily stop after one substitution (chlorination). It may actually be very hard to get a mono-substituted chloromethane. Instead di-, tri- and even tetra-chloromethanes are formed.
One way to avoid this problem is to use a much higher concentration of methane in comparison to chloride. This reduces the chance of a chlorine radical running into a chloromethane and starting the mechanism over again to form a dichloromethane. Through this method of controlling product ratios one is able to have a relative amount of control over the product.
dichloromethane CH3—CH2Cl + Cl2 → CH2Cl—CH2Cl + HCl
Extra for experts
Halogenation mechanism. In the methane molecule, the carbon‐hydrogen bonds are essentially non-polar covalent bonds. The halogen molecule has a totally non-polar covalent bond. UV light contains sufficient energy to break the weaker non-polar chlorine‐chlorine bond (∼58 kcal/mole), but it has insufficient energy to break the stronger carbon‐hydrogen bond (104 kcal/mole).
The breaking of the chlorine molecule leads to the formation of two highly reactive chlorine free radicals (chlorine atoms). A free radical is an atom or group that has a single unshared (unpaired) electron. It is represented by the element symbol with a dot to the centre right (the unpaired electron).
Cl·
In the bond that is ruptured, each of the originally bonded atoms receives one electron. The chlorine free radicals that form are in a high‐energy state and react quickly to complete their octets and liberate energy. (Once the high‐energy chlorine free radicals are formed, the energy source (UV light or heat) can be removed. The energy liberated in the reaction of the free radicals with other atoms is sufficient to keep the reaction running.)
When a chlorine free radical approaches a methane molecule, a fission of a carbon‐hydrogen bond occurs. The chlorine free radical combines with the liberated hydrogen free radical to form hydrogen chloride and a methyl free radical. The methyl free radical then combines with another chlorine free radical and forms chloromethane.
View videos:
TASK 3.2.1
1. Excess fluorine is added to methane in the presence of UV light. What would be the expected products? Justify your response. (3 marks)
2. Intense conditions are required to undergo halogenation of an alkane. Which of the following do not contribute to these conditions?
A) High temperatures (400oC)
B) UV light
C) High halogen concentration
D) Concentrated acid catalyst
3. Explain why the chlorination of methane, if not carefully controlled, can produce tetrachloromethane. In your answer, include relevant reactions. (3 marks)
ANS
The chlorination of methane does not necessarily stop after one substitution (chlorination). It may actually be very hard to get a mono-substituted chloromethane. Instead di-, tri- and even tetra-chloromethanes are formed.
One way to avoid this problem is to use a much higher concentration of methane in comparison to chloride. This reduces the chance of a chlorine radical running into a chloromethane and starting the mechanism over again to form a dichloromethane. Through this method of controlling product ratios one is able to have a relative amount of control over the product.
dichloromethane CH3—CH2Cl + Cl2 → CH2Cl—CH2Cl + HCl
12C7 Pearson Ch11 Answers.pdfGoogle Sites
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