(a) What is the molecular formula of 2-ethyl-3-methylbutanoic acid?
(ii) How many chiral carbon atoms are present in a molecule of 2-ethyl-3-methylbutanoic acid? If none write “none” - CIE - A-Level Chemistry - Question 5 - 2011 - Paper 1
Question 5
(a) What is the molecular formula of 2-ethyl-3-methylbutanoic acid?
(ii) How many chiral carbon atoms are present in a molecule of 2-ethyl-3-methylbutanoic acid? If... show full transcript
Worked Solution & Example Answer:(a) What is the molecular formula of 2-ethyl-3-methylbutanoic acid?
(ii) How many chiral carbon atoms are present in a molecule of 2-ethyl-3-methylbutanoic acid? If none write “none” - CIE - A-Level Chemistry - Question 5 - 2011 - Paper 1
Step 1
What is the molecular formula of 2-ethyl-3-methylbutanoic acid?
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Answer
The molecular formula of 2-ethyl-3-methylbutanoic acid is C₇H₁₄O₂.
Step 2
How many chiral carbon atoms are present in a molecule of 2-ethyl-3-methylbutanoic acid? If none write 'none'.
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Answer
There are two chiral carbon atoms in a molecule of 2-ethyl-3-methylbutanoic acid.
Step 3
State the reagent(s) that would be used for this oxidation.
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Answer
The reagent used for this oxidation is Cr₂O₇²⁻/H⁺.
Step 4
What colour change from __________________ to _________________.
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Answer
The colour change will be from orange to green.
Step 5
What could be the main organic impurity present in the sample of the acid? Explain your answer.
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Answer
The main organic impurity could be the corresponding aldehyde, as partial oxidation of the alcohol would produce aldehyde, which may remain unreacted in the sample.
Step 6
State whether a distillation apparatus or a reflux apparatus should be used. Explain your answer.
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Answer
A reflux apparatus should be used because the alcohol must be fully oxidized to form the acid; reflux allows for sustained heating and prevents loss of volatile components.
Step 7
What colour change would be seen if this were heated with the reagents you have given in (b)(i)? Explain your answer as clearly as you can.
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Answer
The colour change would not apply, as no oxidation occurs because the isomer is tertiary and cannot be oxidized.
Step 8
Draw the displayed formula of this ethyl ester.
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Answer
The displayed formula of the ethyl ester is:
H H O H
| | || |
H--C---C---C--C--OH
| | | |
H H R1 R2
where R1 and R2 are alkyl groups.
The chiral carbon atom is at the second carbon from the carbonyl and can be identified with an asterisk (*).
