SEMI FINAL STAGE 2024
Ghana National College: 49 points
St. Augustine's College: 52 points
Wesley Girls' High School: 37 points
QUESTION
Give the systematic name of the most stable saturated isomer of the alcohol with molecular formula C$_5$H$_{10}$O which cannot be oxidized by potassium dichromate.
ANSWER: 1-methylcyclobutanol
SOLUTION:
1. Analyze the molecular formula and constraints:
- Formula: $C_5H_{10}O$ has a degree of unsaturation (Index of Hydrogen Deficiency) equal to 1.
- Constrained as a "saturated alcohol": This means there are no $C=C$ double bonds, so the unsaturation must come from a ring structure (a cyclic alcohol).
- Constrained as "cannot be oxidized by potassium dichromate": This requires the alcohol to be tertiary ($3^\circ$), meaning the $-OH$ group must be attached to a carbon that has no hydrogen atoms attached directly to it.
2. Determine ring stability:
- A 5-membered ring (cyclopentanol) would have the $-OH$ group on a secondary ($2^\circ$) carbon, which can be oxidized.
- A 4-membered ring with a methyl branch on the same carbon holding the $-OH$ group creates a tertiary ($3^\circ$) alcohol.
This structure is 1-methylcyclobutanol.
- A 3-membered ring with an alcohol group and multiple methyl substituents would suffer from much higher ring strain than a 4-membered ring, making the 4-membered ring the most stable saturated option.
3. Final identification:
- Structure: A cyclobutane ring with both a $-CH_3$ group and an $-OH$ group on carbon-1.
- This matches the formula $C_5H_{10}O$, represents a tertiary cyclic alcohol resistant to oxidation, and is the most stable ring system capable of meeting all parameters.
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PRACTICE QUESTIONS
1. QUESTION: Give the systematic name of the tertiary alcohol with molecular formula $C_4H_{10}O$.
ANSWER: 2-methylpropan-2-ol
SOLUTION:
It is the only $C_4H_{10}O$ alcohol whose –OH carbon has three alkyl groups.
2. QUESTION: Give the systematic name of the tertiary alcohol with molecular formula $C_5H_{12}O$.
ANSWER: 2-methylbutan-2-ol
SOLUTION:
The –OH is on C2, which carries a methyl, an ethyl and a methyl group.
3. QUESTION: Give the systematic name of the secondary alcohol with molecular formula $C_3H_8O$.
ANSWER: Propan-2-ol
SOLUTION:
The –OH is on the middle carbon.
4. QUESTION: Give the systematic name of the only alcohol with molecular formula $C_2H_6O$.
ANSWER: Ethanol
SOLUTION:
Its isomer, methoxymethane, is an ether.
5. QUESTION: Give the systematic name of the secondary alcohol with molecular formula $C_4H_{10}O$.
ANSWER: Butan-2-ol
SOLUTION:
The –OH is on the second carbon of a four-carbon chain.
6. QUESTION: Give the systematic name of the tertiary alcohol $(CH_3)_2C(OH)CH_2CH_2CH_3$.
ANSWER: 2-methylpentan-2-ol
SOLUTION:
The longest chain has five carbons, with the –OH and a methyl group on C2.