1. An organic compound is classified as an alcohol when its hydroxyl group is bonded directly to:
ⓐ. an aromatic \(\mathrm{sp^2}\)-hybridised carbon
ⓑ. an \(\mathrm{sp^3}\)-hybridised carbon
ⓒ. an oxygen atom joined to two carbon groups
ⓓ. the carbon atom of a carbonyl group
Correct Answer: an \(\mathrm{sp^3}\)-hybridised carbon
Explanation: In an alcohol, the hydroxyl group \(\mathrm{-OH}\) is directly bonded to an \(\mathrm{sp^3}\)-hybridised carbon atom. Such a carbon normally forms four single bonds and belongs to a saturated part of the molecule. If \(\mathrm{-OH}\) is attached directly to an aromatic-ring carbon, the compound belongs to the phenol family instead. Attachment to a carbonyl carbon produces a different functional group, such as a carboxylic acid. The hybridisation of the carbon carrying \(\mathrm{-OH}\) is therefore central to identifying an alcohol.
2. The structural feature that identifies a phenol is:
ⓐ. oxygen bonded to two alkyl groups
ⓑ. \(\mathrm{-OH}\) bonded to a saturated side-chain carbon
ⓒ. \(\mathrm{-OH}\) bonded to the carbon of a carbonyl group
ⓓ. \(\mathrm{-OH}\) bonded to an aromatic-ring carbon
Correct Answer: \(\mathrm{-OH}\) bonded to an aromatic-ring carbon
Explanation: A phenol contains a hydroxyl group directly attached to a carbon atom of an aromatic ring. In phenol itself, the structure is represented as \(\mathrm{C_6H_5OH}\). Merely having both a benzene ring and an \(\mathrm{-OH}\) group somewhere in the molecule does not necessarily make the compound a phenol. If the hydroxyl group is on a saturated side-chain carbon, the compound is an alcohol. Direct attachment to the aromatic ring gives phenols their characteristic structure and reactivity.
3. The general structural representation of an ether is:
ⓐ. \(\mathrm{R-O-R'}\)
ⓑ. \(\mathrm{R-COOH}\)
ⓒ. \(\mathrm{R-CHO}\)
ⓓ. \(\mathrm{R-CO-R'}\)
Correct Answer: \(\mathrm{R-O-R'}\)
Explanation: An ether contains one oxygen atom bonded to two carbon-containing groups. These groups are represented by \(\mathrm{R}\) and \(\mathrm{R'}\), and they may be identical or different. The oxygen atom lies between the two groups rather than appearing as a terminal hydroxyl group. The structures \(\mathrm{R-COOH}\), \(\mathrm{R-CHO}\), and \(\mathrm{R-CO-R'}\) represent carboxylic acids, aldehydes, and ketones respectively. The absence of an \(\mathrm{O-H}\) bond separates ethers structurally from alcohols and phenols.
4. Anisole, \(\mathrm{C_6H_5-O-CH_3}\), is best classified as:
ⓐ. an aliphatic alcohol
ⓑ. a phenol
ⓒ. a diaryl ether
ⓓ. an aryl alkyl ether
Correct Answer: an aryl alkyl ether
Explanation: In anisole, oxygen is bonded to a phenyl group \(\mathrm{C_6H_5-}\) on one side and a methyl group \(\mathrm{CH_3-}\) on the other. It therefore has the ether linkage \(\mathrm{R-O-R'}\). The oxygen does not carry a hydrogen atom, so anisole is neither an alcohol nor a phenol. One side is aryl and the other is alkyl, making it an aryl alkyl ether. Anisole is also commonly represented by its systematic name, methoxybenzene.
5. In the general representations \(\mathrm{R-OH}\) and \(\mathrm{Ar-OH}\), the symbols \(\mathrm{R}\) and \(\mathrm{Ar}\) denote, respectively:
ⓐ. an aryl group and an alkyl group
ⓑ. a hydroxyl group and a carbonyl group
ⓒ. an alkyl group and an aryl group
ⓓ. a hydrogen atom and an oxygen atom
Correct Answer: an alkyl group and an aryl group
Explanation: The symbol \(\mathrm{R}\) commonly represents an alkyl group or a saturated carbon-containing residue in this context. The symbol \(\mathrm{Ar}\) represents an aryl group derived from an aromatic system. Thus, \(\mathrm{R-OH}\) is the general representation of an alcohol, while \(\mathrm{Ar-OH}\) represents a phenol. Reversing these symbols would also reverse the structural distinction between saturated and aromatic attachment. The notation focuses attention on the type of carbon group directly bonded to oxygen.
6. In neutral alcohols, phenols, and ethers, oxygen normally shows a valency of ______.
ⓐ. \(2\)
ⓑ. \(1\)
ⓒ. \(3\)
ⓓ. \(4\)
Correct Answer: \(2\)
Explanation: Oxygen has six valence electrons and usually forms two covalent bonds to complete its octet in these compounds. In an alcohol or phenol, oxygen is bonded to one carbon atom and one hydrogen atom. In an ether, it is bonded to two carbon atoms. The remaining four valence electrons occur as two lone pairs on oxygen. Valency counts the number of bonds formed, so the two lone pairs do not raise oxygen’s valency above \(2\).
7. Three structures are listed below:
P: \(\mathrm{CH_3CH_2OH}\)
Q: \(\mathrm{C_6H_5OH}\)
R: \(\mathrm{CH_3OCH_3}\)
The functional-group families of P, Q, and R are, respectively:
ⓐ. phenol, alcohol, ether
ⓑ. alcohol, ether, phenol
ⓒ. ether, phenol, alcohol
ⓓ. alcohol, phenol, ether
Correct Answer: alcohol, phenol, ether
Explanation: In P, the hydroxyl group is attached to an \(\mathrm{sp^3}\)-hybridised carbon, so P is an alcohol. In Q, the hydroxyl group is directly attached to an aromatic ring, making Q a phenol. In R, oxygen is bonded between two methyl groups, so R is an ether. All three contain oxygen, but the atoms directly bonded to oxygen are different. Examining the immediate bonding environment of oxygen is more reliable than classifying only by molecular formula.
8. A neutral oxygen atom in dimethyl ether, \(\mathrm{CH_3-O-CH_3}\), has:
ⓐ. one sigma bond and three lone pairs
ⓑ. two sigma bonds and two lone pairs
ⓒ. three sigma bonds and one lone pair
ⓓ. two sigma bonds and no lone pairs
Correct Answer: two sigma bonds and two lone pairs
Explanation: The oxygen atom in dimethyl ether forms one sigma bond with each methyl carbon. It therefore forms two sigma bonds in total. Oxygen contributes six valence electrons, and after bond formation it retains four non-bonding electrons as two lone pairs. This arrangement gives oxygen a complete octet and no formal charge in the ordinary Lewis structure. Ignoring the lone pairs would give an incomplete description of both its geometry and chemical behaviour.
9. The polarity of \(\mathrm{C-O}\) and \(\mathrm{O-H}\) bonds is represented most appropriately by:
ⓐ. \(\mathrm{C}^{\delta-}-\mathrm{O}^{\delta+}\) and \(\mathrm{O}^{\delta+}-\mathrm{H}^{\delta-}\)
ⓑ. \(\mathrm{C-O}\) and \(\mathrm{O-H}\) with no partial charges
ⓒ. \(\mathrm{C}^{\delta+}-\mathrm{O}^{\delta-}\) and \(\mathrm{O}^{\delta-}-\mathrm{H}^{\delta+}\)
ⓓ. \(\mathrm{C}^{\delta-}-\mathrm{O}^{\delta-}\) and \(\mathrm{O}^{\delta+}-\mathrm{H}^{\delta+}\)
Correct Answer: \(\mathrm{C}^{\delta+}-\mathrm{O}^{\delta-}\) and \(\mathrm{O}^{\delta-}-\mathrm{H}^{\delta+}\)
Explanation: Oxygen is more electronegative than both carbon and hydrogen. It attracts the shared electron pairs more strongly and consequently develops a partial negative charge, \(\delta^-\). The bonded carbon in a \(\mathrm{C-O}\) bond and hydrogen in an \(\mathrm{O-H}\) bond acquire partial positive charges, \(\delta^+\). These are partial charges caused by unequal electron sharing, not full ionic charges. The direction of bond polarity must therefore point toward oxygen in both bonds.
10. Around the oxygen atom in an alcohol or ether, the electron-pair arrangement and the observable atomic arrangement are best described as:
ⓐ. trigonal planar electron-pair arrangement and linear atomic arrangement
ⓑ. linear electron-pair arrangement and tetrahedral atomic arrangement
ⓒ. tetrahedral electron-pair arrangement and trigonal planar atomic arrangement
ⓓ. tetrahedral electron-pair arrangement and bent atomic arrangement
Correct Answer: tetrahedral electron-pair arrangement and bent atomic arrangement
Explanation: Oxygen in these compounds has four regions of electron density: two bond pairs and two lone pairs. Four electron pairs arrange themselves approximately tetrahedrally to reduce repulsion. Molecular geometry, however, is named using the positions of bonded atoms and does not display lone pairs as atoms. Since only two bonded atoms surround oxygen, the visible arrangement at oxygen is bent. Confusing electron-pair geometry with molecular geometry would incorrectly describe the structure as tetrahedral.
11. A Lewis structure shows a central oxygen atom bonded by single bonds to two carbon groups, with two lone pairs remaining on oxygen. This arrangement represents:
ⓐ. a phenol group, \(\mathrm{Ar-OH}\)
ⓑ. an ether group, \(\mathrm{R-O-R'}\)
ⓒ. an alcohol group, \(\mathrm{R-OH}\)
ⓓ. a carbonyl group, \(\mathrm{C=O}\)
Correct Answer: an ether group, \(\mathrm{R-O-R'}\)
Explanation: The described oxygen atom is bonded to two carbon-containing groups and has no bond to hydrogen. This is the defining Lewis-structure pattern of an ether. In an alcohol or phenol, one of oxygen’s two bonds would be an \(\mathrm{O-H}\) bond. A carbonyl group would contain a double bond between carbon and oxygen rather than two separate carbon-oxygen single bonds. Tracking the atoms directly attached to oxygen distinguishes these functional groups without relying on their names.
12. The terms monohydric, dihydric, and trihydric classify alcohols according to:
ⓐ. the number of carbon atoms in the longest chain
ⓑ. the degree of the carbon carrying the first hydroxyl group
ⓒ. the number of carbon-carbon bonds in the molecule
ⓓ. the number of alcoholic hydroxyl groups in one molecule
Correct Answer: the number of alcoholic hydroxyl groups in one molecule
Explanation: Hydricity refers to the number of alcoholic \(\mathrm{-OH}\) groups present in a molecule. An alcohol with one hydroxyl group is monohydric, one with two is dihydric, and one with three is trihydric. Carbon-chain length does not determine this classification. The degree of the hydroxyl-bearing carbon is used for primary, secondary, and tertiary classification instead. A long-chain molecule can still be monohydric when it contains only one alcoholic hydroxyl group.
13. Methanol, ethane-1,2-diol, and propane-1,2,3-triol are classified, respectively, as:
ⓐ. dihydric, monohydric, and trihydric alcohols
ⓑ. monohydric, trihydric, and dihydric alcohols
ⓒ. monohydric, dihydric, and trihydric alcohols
ⓓ. trihydric, dihydric, and monohydric alcohols
Correct Answer: monohydric, dihydric, and trihydric alcohols
Explanation: Methanol, \(\mathrm{CH_3OH}\), contains one hydroxyl group and is therefore monohydric. Ethane-1,2-diol contains two hydroxyl groups, one on each carbon, so it is dihydric. Propane-1,2,3-triol contains three hydroxyl groups and is trihydric. The prefixes mono-, di-, and tri- refer directly to hydroxyl-group count. The numbers of carbon atoms in these examples happen to increase, but that is not the basis of classification.
14. Consider the following statements.
Statement I: A four-carbon alcohol containing one \(\mathrm{-OH}\) group is monohydric.
Statement II: A two-carbon alcohol containing two \(\mathrm{-OH}\) groups is dihydric.
Statement III: The number of carbon atoms determines whether an alcohol is monohydric or polyhydric.
ⓐ. Statements II and III only
ⓑ. Statements I and III only
ⓒ. Statements I, II, and III
ⓓ. Statements I and II only
Correct Answer: Statements I and II only
Explanation: Statement I is valid because a molecule with one alcoholic hydroxyl group is monohydric, regardless of its four-carbon chain. Statement II is also valid because two alcoholic hydroxyl groups make the compound dihydric. Statement III reverses the actual basis of classification. Carbon count and hydroxyl-group count are independent structural features. Hydricity must be assigned by counting the \(\mathrm{-OH}\) groups attached to suitable saturated carbon atoms.
15. Match each structure in Column I with its classification in Column II.
| Column I | Column II |
|---|
| P. \(\mathrm{CH_3OH}\) | 1. Dihydric alcohol |
| Q. \(\mathrm{HOCH_2CH_2OH}\) | 2. Alcohol containing six hydroxyl groups |
| R. \(\mathrm{HOCH_2CHOHCH_2OH}\) | 3. Monohydric alcohol |
| S. \(\mathrm{HOCH_2(CHOH)_4CH_2OH}\) | 4. Trihydric alcohol |
ⓐ. P-3, Q-1, R-4, S-2
ⓑ. P-1, Q-3, R-2, S-4
ⓒ. P-3, Q-4, R-1, S-2
ⓓ. P-2, Q-1, R-4, S-3
Correct Answer: P-3, Q-1, R-4, S-2
Explanation: Structure P has one hydroxyl group, so it matches the monohydric classification in 3. Structure Q contains two hydroxyl groups and therefore matches the dihydric classification in 1. Structure R has three hydroxyl groups, making it trihydric and matching 4. In S, the two terminal \(\mathrm{CH_2OH}\) groups and four internal \(\mathrm{CHOH}\) groups give six hydroxyl groups in total. Counting every explicitly shown \(\mathrm{-OH}\) group gives the mapping without relying on carbon-chain length.
16. The condensed structure \(\mathrm{CH_3CH(OH)CH_2CH_2OH}\) represents a:
ⓐ. monohydric alcohol because only one \(\mathrm{-OH}\) group is terminal
ⓑ. trihydric alcohol because the chain contains four carbon atoms
ⓒ. dihydric alcohol because the molecule contains two \(\mathrm{-OH}\) groups
ⓓ. phenol because one hydroxyl group is bonded to an internal carbon
Correct Answer: dihydric alcohol because the molecule contains two \(\mathrm{-OH}\) groups
Explanation: One hydroxyl group appears in the \(\mathrm{CH(OH)}\) unit and another appears in the terminal \(\mathrm{CH_2OH}\) unit. The molecule therefore contains two alcoholic hydroxyl groups. A hydroxyl group does not need to be terminal to be counted in hydricity. The four-carbon chain does not make the compound tetrahydric or trihydric. Both hydroxyl-bearing carbons are saturated, so the compound is a dihydric alcohol rather than a phenol.
17. A molecule has the structure \(\mathrm{HOCH_2-C(OH)(CH_3)-CH_2OH}\). Its branching and hydroxyl-group count show that it is:
ⓐ. monohydric because only the central \(\mathrm{-OH}\) group is counted
ⓑ. dihydric because the equivalent terminal \(\mathrm{-OH}\) groups count as one
ⓒ. trihydric because the molecule contains three \(\mathrm{-OH}\) groups
ⓓ. tetrahydric because four carbon atoms are present
Correct Answer: trihydric because the molecule contains three \(\mathrm{-OH}\) groups
Explanation: The left terminal \(\mathrm{HOCH_2-}\) group contributes one hydroxyl group. The central \(\mathrm{C(OH)}\) unit contributes a second, and the right terminal \(\mathrm{-CH_2OH}\) group contributes a third. Equivalent groups must still be counted separately when they occur at different positions in the molecule. Branching changes the carbon skeleton but not the rule used to determine hydricity. Three alcoholic hydroxyl groups make the compound trihydric.
18. A mixture contains \(0.20\,mol\) of ethane-1,2-diol and \(0.10\,mol\) of propane-1,2,3-triol. The total amount of alcoholic hydroxyl groups present is:
ⓐ. \(0.30\,mol\)
ⓑ. \(0.70\,mol\)
ⓒ. \(0.40\,mol\)
ⓓ. \(0.50\,mol\)
Correct Answer: \(0.70\,mol\)
Explanation: \( \textbf{Identify the hydroxyl-group counts:} \)
Ethane-1,2-diol contains \(2\) hydroxyl groups per molecule.
Propane-1,2,3-triol contains \(3\) hydroxyl groups per molecule.
\( \textbf{Hydroxyl groups from the diol:} \)
\[
0.20\,mol\times2=0.40\,mol
\]
\( \textbf{Hydroxyl groups from the triol:} \)
\[
0.10\,mol\times3=0.30\,mol
\]
\( \textbf{Combine the two contributions:} \)
\[
0.40\,mol+0.30\,mol=0.70\,mol
\]
\( \textbf{Final answer:} \) The mixture contains \(0.70\,mol\) of alcoholic hydroxyl groups. Adding only the amounts of the two alcohols would give \(0.30\,mol\), which counts molecules rather than hydroxyl groups.
19. Substance X contains four \(\mathrm{-OH}\) groups, each attached to an \(\mathrm{sp^3}\)-hybridised carbon. Substance Y contains eight carbon atoms but only one such \(\mathrm{-OH}\) group. The appropriate classifications are:
ⓐ. X is polyhydric and Y is monohydric
ⓑ. X is monohydric and Y is polyhydric
ⓒ. X and Y are both monohydric because each is one compound
ⓓ. X is tetra-carbon and Y is octahydric
Correct Answer: X is polyhydric and Y is monohydric
Explanation: Substance X has several alcoholic hydroxyl groups, so it belongs to the broad class of polyhydric alcohols. More specifically, its four hydroxyl groups could also be described using a tetraol name where appropriate. Substance Y contains only one alcoholic hydroxyl group and is therefore monohydric, even though its carbon chain is much longer. Hydricity does not measure molecular size or the number of carbon atoms. A large carbon skeleton can carry fewer hydroxyl groups than a much smaller polyhydric molecule.
20. In classifying a monohydric alcohol as primary, secondary, or tertiary, the deciding structural feature is:
ⓐ. the total number of carbon atoms in the molecule
ⓑ. the carbon-group count at the carbinol carbon
ⓒ. the position of the longest branch in the carbon chain
ⓓ. the total number of hydrogen atoms attached to oxygen
Correct Answer: the carbon-group count at the carbinol carbon
Explanation: The carbon atom directly bonded to the \(\mathrm{-OH}\) group is called the carbinol carbon. If this carbon is bonded to one other carbon group, the alcohol is primary. Attachment to two carbon groups makes it secondary, while attachment to three makes it tertiary. Branching elsewhere in the molecule does not determine the class. The classification is based on the immediate carbon environment around the carbinol carbon.
