shellybellyjelly
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Is there any rule about which hydrogen in alkane will be replaced by a halogen atom during substitution reaction as per below? Thank you in advanced!!!!!
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Organic Chem: Substitution Reaction (1 Viewer)
There is a rule and a reasoning behind that but for the content taught specifically in the HSC syllabus it doesn't explain it.
In most scenarios for HSC based questions if they require you to react something like that where multiple products could be formed, it will normally occur in a reaction pathway question. In these the choice of compound that could form due to that reaction is restricted by whatever compound they have following after that, so in that sense they won't usually ask you to draw all the 3 things that could be made for your example. So for example in the diagram below:
That first reaction they will then restrict it to assuming it only makes the 3-chloropentane as that would be what makes the pentan-3-ol. Although, in some cases it might not be true to what is made in majority in real life, that's as far as what most of the syllabus goes to.
For a simplified actual reasoning (some of the stuff is left out but it explains it in a way you can understand it with HSC knowledge):
- Not all the sites where we can substitute the Cl are the same, therefore in the end there will be a different percentage of each possible compound in the final mixture.
- Secondary and tertiary substituted carbons (similar to how you learn about primary, secondary, tertiary alcohols), tend to be more preferred for substitution over the primary carbons due to the greater stability of an intermediate that is formed.
- So for the pentane the most likely products will be 2-chloropentane and 3-chloropentane and some 1-chloropentane will be made.
You can learn more about it in detail here: Halogenation of Alkanes - Chemistry LibreTexts , but for HSC they won't require you to make the decision behind which one is more likely to form as there is no syllabus dot-point that asks for predict the major product of organic reactions, so they will make these things clear when making exam questions.
In most scenarios for HSC based questions if they require you to react something like that where multiple products could be formed, it will normally occur in a reaction pathway question. In these the choice of compound that could form due to that reaction is restricted by whatever compound they have following after that, so in that sense they won't usually ask you to draw all the 3 things that could be made for your example. So for example in the diagram below:
That first reaction they will then restrict it to assuming it only makes the 3-chloropentane as that would be what makes the pentan-3-ol. Although, in some cases it might not be true to what is made in majority in real life, that's as far as what most of the syllabus goes to.
For a simplified actual reasoning (some of the stuff is left out but it explains it in a way you can understand it with HSC knowledge):
- Not all the sites where we can substitute the Cl are the same, therefore in the end there will be a different percentage of each possible compound in the final mixture.
- Secondary and tertiary substituted carbons (similar to how you learn about primary, secondary, tertiary alcohols), tend to be more preferred for substitution over the primary carbons due to the greater stability of an intermediate that is formed.
- So for the pentane the most likely products will be 2-chloropentane and 3-chloropentane and some 1-chloropentane will be made.
You can learn more about it in detail here: Halogenation of Alkanes - Chemistry LibreTexts , but for HSC they won't require you to make the decision behind which one is more likely to form as there is no syllabus dot-point that asks for predict the major product of organic reactions, so they will make these things clear when making exam questions.
The choice of halogen will also have an effect.
For example, chlorination of propane will produce two chloropropane isomers, 1-chloropropane (45%) and 2-chloropropane (55%). This shows a substantial preference for substitution at the internal, rather than the terminal, carbon as the product distribution if the site of substitution was random would be 75% 1-chloropropane and 25% 2-chloropropane.
Bromination is even more selective, the monobromopropane isomers being produced from free radical bromination of propane are 97% 2-bromopropane against 3% 1-bromopropane.
We do not consider fluorination as it tends to produce an explosion, and also ignore iodination as it is very slow.
More substituted sites are even more favoured. Chlorination of methylpropane yields 2-chloro-2-methylpropane in 65% yield over 1-chloro-2-methylpropane despite the substitution sites being present in 9:1 ratio for the primary over the tertiary site.
Alkanes with more sites, like pentane, will yield a mixture of 3-halo, 2-halo, and 1-halo isomers. The questions will then likely be constrained either by a subsequent reaction (as in Jazz's example) or by separating the products, as in:
Question
2-methylbutane, C5H12, reacts with chlorine in the presence of light to yield 3 products, A, B, and C, all with formula C5H11Cl. A, B and C each react with aqueous potassium hydroxide to yield X, Y, and Z. When these are tested with zinc chloride and concentrated hydrochloric acid, the mixture containing Y becomes cloudy almost instantly, the mixture containing Z develops cloudiness over several minutes, while the mixture containing X remains clear.
(a) Provide the structures and IUPAC names for A, B, and C.
(b) Which of X, Y, and Z would not cause a colour change when added to acidified potassium dichromate solution? Justify your answer.
(c) Rank X, Y, and Z in terms of the their boiling points and explain your answer.
(d) Draw the structure of an alkene that could be converted to X by a one-step hydration reaction. Which of Y and Z would not be produced in this reaction? What would be the major product of the reaction. Support you answer with suitable chemical equations and reasoning.
For example, chlorination of propane will produce two chloropropane isomers, 1-chloropropane (45%) and 2-chloropropane (55%). This shows a substantial preference for substitution at the internal, rather than the terminal, carbon as the product distribution if the site of substitution was random would be 75% 1-chloropropane and 25% 2-chloropropane.
Bromination is even more selective, the monobromopropane isomers being produced from free radical bromination of propane are 97% 2-bromopropane against 3% 1-bromopropane.
We do not consider fluorination as it tends to produce an explosion, and also ignore iodination as it is very slow.
More substituted sites are even more favoured. Chlorination of methylpropane yields 2-chloro-2-methylpropane in 65% yield over 1-chloro-2-methylpropane despite the substitution sites being present in 9:1 ratio for the primary over the tertiary site.
Alkanes with more sites, like pentane, will yield a mixture of 3-halo, 2-halo, and 1-halo isomers. The questions will then likely be constrained either by a subsequent reaction (as in Jazz's example) or by separating the products, as in:
Question
2-methylbutane, C5H12, reacts with chlorine in the presence of light to yield 3 products, A, B, and C, all with formula C5H11Cl. A, B and C each react with aqueous potassium hydroxide to yield X, Y, and Z. When these are tested with zinc chloride and concentrated hydrochloric acid, the mixture containing Y becomes cloudy almost instantly, the mixture containing Z develops cloudiness over several minutes, while the mixture containing X remains clear.
(a) Provide the structures and IUPAC names for A, B, and C.
(b) Which of X, Y, and Z would not cause a colour change when added to acidified potassium dichromate solution? Justify your answer.
(c) Rank X, Y, and Z in terms of the their boiling points and explain your answer.
(d) Draw the structure of an alkene that could be converted to X by a one-step hydration reaction. Which of Y and Z would not be produced in this reaction? What would be the major product of the reaction. Support you answer with suitable chemical equations and reasoning.