4. A, B and C all have the formula C₄H₈ - CIE - A-Level Chemistry - Question 4 - 2017 - Paper 1
Question 4
4. A, B and C all have the formula C₄H₈. They all decolourise bromine and are structural isomers of each other.
(a) State the name of the process by which A, B and ... show full transcript
Worked Solution & Example Answer:4. A, B and C all have the formula C₄H₈ - CIE - A-Level Chemistry - Question 4 - 2017 - Paper 1
Step 1
State the name of the process by which A, B and C could be obtained from C₄H₈.
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Answer
The process is known as cracking.
Step 2
Draw the structures of these three structural isomers.
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Answer
The structural isomers of C₄H₈ include:
But-1-ene:
But-2-ene:
Isobutylene (2-methylpropene):
Step 3
Explain the meaning of the term geometrical isomerism.
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Answer
Geometrical isomerism refers to the existence of compounds with the same molecular formula and connectivity of atoms but differing spatial arrangements around a double bond or rings, leading to different physical and chemical properties.
Step 4
Draw the displayed formula of A and use it to show the mechanism of the reaction of A with HBr.
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Answer
The displayed formula of A (But-2-ene):
CH₃-CH=CH-CH₃
Mechanism:
Draw the double bond (
Show the attack of HBr on the double bond, with dipole indicating positive end of H interacting with the double bond.
Show the formation of a carbocation and the subsequent addition of Br to the carbocation.
Step 5
State the meaning of the term chiral centre.
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Answer
A chiral centre, or chiral carbon, is a carbon atom that is bonded to four different atoms or groups, leading to non-superimposable mirror images (enantiomers).
Step 6
Name B.
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Answer
B is named but-2-ene.
Step 7
Draw these isomers using the conventional three-dimensional representation.
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Answer
The optical isomers of X:
(R)-2-bromobutane:
(S)-2-bromobutane:
Step 8
Explain why X is produced in higher yield than Y.
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Answer
X is produced in higher yield than Y due to the stability of the transition state formed during the reaction with HBr. The formation of a more stable carbocation intermediate from B, which has a chiral center, leads to favored production of X.
Step 9
Name C.
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Answer
C is named but-1-ene.
Step 10
Draw the displayed formula of each of the structural isomers produced by the reaction of C with HBr.
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Answer
The displayed formula of the structural isomers from the reaction of C with HBr include:
