1. The structural feature present in aldehydes, ketones, and carboxylic acids is:
ⓐ. An amino group
ⓑ. A carbonyl group
ⓒ. An ether linkage
ⓓ. A carbon-carbon triple bond
Correct Answer: A carbonyl group
Explanation: A carbonyl group contains a carbon atom doubly bonded to oxygen, represented by \(\mathrm{C=O}\). Aldehydes and ketones contain this group as their principal functional group. A carboxylic acid also contains a carbonyl group, but it is joined to a hydroxyl group on the same carbon. Ether and amino groups do not contain a \(\mathrm{C=O}\) bond. The presence of oxygen alone is not enough to classify a compound as a carbonyl compound.
2. In an aldehyde, the characteristic functional group is represented by:
ⓐ. \(\mathrm{-OH}\)
ⓑ. \(\mathrm{-COOH}\)
ⓒ. \(\mathrm{-CO-}\)
ⓓ. \(\mathrm{-CHO}\)
Correct Answer: \(\mathrm{-CHO}\)
Explanation: The aldehyde group is \(\mathrm{-CHO}\), in which the carbonyl carbon is directly bonded to a hydrogen atom. This group normally occurs at the end of a carbon chain. The notation \(\mathrm{-COOH}\) represents a carboxyl group, while \(\mathrm{-OH}\) represents a hydroxyl group. A carbonyl carbon bonded to two carbon groups belongs to a ketone rather than an aldehyde. The hydrogen attached directly to the carbonyl carbon is called the aldehydic hydrogen.
3. The general ketone notation is \(\mathrm{R-CO-\_\_\_}\). The missing symbol is:
ⓐ. \(\mathrm{R'}\)
ⓑ. \(\mathrm{H}\)
ⓒ. \(\mathrm{OH}\)
ⓓ. \(\mathrm{O}\)
Correct Answer: \(\mathrm{R'}\)
Explanation: A ketone is commonly represented as \(\mathrm{R-CO-R'}\). The symbols \(\mathrm{R}\) and \(\mathrm{R'}\) represent carbon-containing groups, and they may be identical or different. Thus, the carbonyl carbon in a ketone is bonded to two other carbon atoms. Replacing \(\mathrm{R'}\) with hydrogen would produce an aldehyde group. Replacing it with hydroxyl would produce a carboxyl group instead.
4. Assertion: The \(\mathrm{C=O}\) bond of a carbonyl group is polar.
Reason: Oxygen attracts the shared electron pair more strongly than carbon because oxygen is more electronegative.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Oxygen is more electronegative than carbon, so it draws the bonding electron density toward itself. The oxygen atom consequently develops a partial negative charge, while the carbonyl carbon develops a partial positive charge. The bond may be represented as \(\mathrm{C^{\delta+}=O^{\delta-}}\). This unequal charge distribution makes the carbonyl bond polar. The polarity later explains why electron-rich species are attracted toward the carbonyl carbon rather than toward the carbonyl oxygen.
5. Consider the following statements.
Statement I: Aldehydes and ketones contain a carbonyl group.
Statement II: A carboxyl group contains carbonyl and hydroxyl parts attached to the same carbon.
Statement III: Every oxygen-containing organic compound is a carbonyl compound.
ⓐ. Statements I and III only
ⓑ. Statements II and III only
ⓒ. Statements I and II only
ⓓ. Statements I, II, and III
Correct Answer: Statements I and II only
Explanation: Aldehydes and ketones are carbonyl compounds because both contain a \(\mathrm{C=O}\) group. A carboxyl group, \(\mathrm{-COOH}\), contains carbonyl and hydroxyl parts on one carbon atom. Statement III is false because alcohols and ethers contain oxygen without containing a carbonyl bond. For example, ethanol has the formula \(\mathrm{CH_3CH_2OH}\) but has no \(\mathrm{C=O}\) group. Classification depends on the bonding arrangement of oxygen, not merely on its presence in the molecular formula.
6. Successive oxidation of ethanol first under controlled conditions and then more strongly follows the sequence:
ⓐ. \(\mathrm{CH_3CH_2OH \rightarrow CH_3COCH_3 \rightarrow CH_3COOH}\)
ⓑ. \(\mathrm{CH_3CH_2OH \rightarrow CH_3CHO \rightarrow CH_3COOH}\)
ⓒ. \(\mathrm{CH_3CH_2OH \rightarrow HCHO \rightarrow HCOOH}\)
ⓓ. \(\mathrm{CH_3CH_2OH \rightarrow CH_3COOH \rightarrow CH_3CHO}\)
Correct Answer: \(\mathrm{CH_3CH_2OH \rightarrow CH_3CHO \rightarrow CH_3COOH}\)
Explanation: Ethanol is a primary alcohol, so controlled oxidation converts it into ethanal. Further oxidation raises the oxidation level of the same carbon and produces ethanoic acid. The carbon skeleton remains composed of two carbon atoms throughout the sequence. Propanone cannot form because it contains three carbon atoms and is obtained from a secondary alcohol precursor. Careful control is needed when ethanal is the desired product because aldehydes are readily oxidised further.
7. Reduction of propanone produces:
ⓐ. Propan-1-ol
ⓑ. Propanal
ⓒ. Propanoic acid
ⓓ. Propan-2-ol
Correct Answer: Propan-2-ol
Explanation: Reduction of a ketone converts its carbonyl group into a secondary alcohol group. In propanone, the carbonyl carbon is bonded to two methyl groups, so after reduction it becomes the carbon carrying the \(\mathrm{-OH}\) group in propan-2-ol. The three-carbon skeleton is retained during this conversion. Propan-1-ol would be formed by reducing propanal rather than propanone. Reduction lowers the oxidation level of the carbonyl carbon instead of producing a carboxylic acid.
8. Match each compound in Column I with its description in Column II.
| Column I | Column II |
|---|
| P. Methanal | 1. One-carbon aldehyde |
| Q. Propanone | 2. Three-carbon ketone |
| R. Methanoic acid | 3. One-carbon carboxylic acid |
| S. Ethanal | 4. Two-carbon aldehyde |
ⓐ. P-1, Q-2, R-3, S-4
ⓑ. P-3, Q-2, R-1, S-4
ⓒ. P-1, Q-4, R-3, S-2
ⓓ. P-4, Q-2, R-1, S-3
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: Methanal, \(\mathrm{HCHO}\), is the aldehyde containing one carbon atom. Propanone, \(\mathrm{CH_3COCH_3}\), is a three-carbon ketone. Methanoic acid, \(\mathrm{HCOOH}\), is the one-carbon carboxylic acid. Ethanal, \(\mathrm{CH_3CHO}\), contains two carbon atoms and an aldehyde group. Similar carbon counts do not imply the same functional class because the bonding pattern around the oxygen-containing group must also be examined.
9. In general organic formulas such as \(\mathrm{R-CHO}\) and \(\mathrm{R-COOH}\), the symbol \(\mathrm{R}\) usually represents:
ⓐ. An aryl group with no alkyl part
ⓑ. An alkyl group or carbon residue
ⓒ. A hydroxyl group bonded to carbon
ⓓ. The oxygen atom of a carbonyl group
Correct Answer: An alkyl group or carbon residue
Explanation: The symbol \(\mathrm{R}\) is used as a convenient representation of an alkyl group or the remaining carbon-containing part of a molecule. It allows a general reaction or structure to be written without specifying every carbon and hydrogen atom. An aryl group is often represented separately by \(\mathrm{Ar}\). The symbol \(\mathrm{R}\) does not represent oxygen or a hydroxyl group. Its exact identity may change from one compound to another while the functional group under discussion remains the same.
10. A compound is required that gives an aldehyde on controlled oxidation without changing its carbon skeleton. The suitable starting class is:
ⓐ. A primary alcohol
ⓑ. A secondary alcohol
ⓒ. A tertiary alcohol
ⓓ. An ether
Correct Answer: A primary alcohol
Explanation: In a primary alcohol, the carbon bearing the hydroxyl group is attached to only one other carbon atom, except in methanol. Controlled oxidation removes hydrogen and forms an aldehyde group while retaining the carbon framework. A secondary alcohol instead produces a ketone because its functional carbon is attached to two carbon groups. Tertiary alcohols do not possess the required hydrogen on the hydroxyl-bearing carbon for direct carbonyl formation by simple oxidation. The oxidising conditions must be controlled because the aldehyde can undergo further oxidation to a carboxylic acid.
11. Oxidation of a secondary alcohol most directly produces:
ⓐ. An aldehyde
ⓑ. A carboxylic acid
ⓒ. An ether
ⓓ. A ketone
Correct Answer: A ketone
Explanation: A secondary alcohol has the general arrangement \(\mathrm{R-CH(OH)-R'}\). Oxidation removes hydrogen from the hydroxyl group and from the same carbon, producing \(\mathrm{R-CO-R'}\). Since the carbonyl carbon remains bonded to two carbon groups, the product is a ketone. An aldehyde is normally obtained from a primary alcohol, not a secondary alcohol. The carbon skeleton is usually retained during this conversion.
12. In the general notation \(\mathrm{Ar-CHO}\), the symbol \(\mathrm{Ar}\) represents ______.
ⓐ. An alkyl group
ⓑ. A hydroxyl group
ⓒ. An aryl group
ⓓ. An aldehydic hydrogen
Correct Answer: An aryl group
Explanation: The symbol \(\mathrm{Ar}\) represents an aryl group derived from an aromatic ring. For example, \(\mathrm{C_6H_5-}\) is a phenyl group and may be represented by \(\mathrm{Ar}\). The notation \(\mathrm{Ar-CHO}\) therefore describes an aldehyde whose formyl group is directly attached to an aromatic group. An alkyl group is more commonly represented by \(\mathrm{R}\). The distinction between \(\mathrm{R}\) and \(\mathrm{Ar}\) becomes useful when comparing aliphatic and aromatic carbonyl compounds.
13. Conversion of ethanol into ethanal is classified as oxidation mainly because:
ⓐ. Hydrogen is lost as the carbon-oxygen bond order rises
ⓑ. Hydrogen is gained as the carbon-oxygen bond order falls
ⓒ. One carbon atom is added to the carbon skeleton
ⓓ. The functional carbon undergoes a decrease in oxidation state
Correct Answer: Hydrogen is lost as the carbon-oxygen bond order rises
Explanation: Ethanol contains a carbon-oxygen single bond at the functional carbon, whereas ethanal contains a carbon-oxygen double bond. During the conversion, hydrogen is removed from the alcohol molecule. Increasing bonding to oxygen and decreasing bonding to hydrogen are both signs of oxidation in organic chemistry. The number of carbon atoms remains unchanged, so carbon-chain growth is not involved. The oxidation state of the functional carbon rises rather than falls.
14. The oxidation numbers of the functional carbon in ethanol, ethanal, and ethanoic acid respectively are:
ⓐ. \(-1,-1,+1\)
ⓑ. \(+1,-1,+3\)
ⓒ. \(-3,+1,+3\)
ⓓ. \(-1,+1,+3\)
Correct Answer: \(-1,+1,+3\)
Explanation: \( \textbf{Bond-accounting rule:} \) For the carbon being examined, each bond to hydrogen contributes \(-1\), each single bond to oxygen contributes \(+1\), each carbon-oxygen double bond contributes \(+2\), and a carbon-carbon bond contributes \(0\).
\( \textbf{Ethanol carbon:} \) In \(\mathrm{CH_3CH_2OH}\), the carbon attached to \(\mathrm{OH}\) has two carbon-hydrogen bonds and one carbon-oxygen single bond.
\[
x=-2+1=-1
\]
\( \textbf{Ethanal carbon:} \) In \(\mathrm{CH_3CHO}\), the aldehyde carbon has one carbon-hydrogen bond and one carbon-oxygen double bond.
\[
x=-1+2=+1
\]
\( \textbf{Ethanoic-acid carbon:} \) In \(\mathrm{CH_3COOH}\), the carboxyl carbon has one carbon-oxygen double bond and one carbon-oxygen single bond.
\[
x=+2+1=+3
\]
\( \textbf{Final answer:} \) The sequence is \(-1,+1,+3\). Each oxidation step raises the oxidation number of the functional carbon by \(2\).
15. For the structure \(\mathrm{CH_3CH(CH_3)CH_2CHO}\), the longest parent chain that contains the aldehyde carbon has:
ⓐ. Three carbon atoms
ⓑ. Four carbon atoms
ⓒ. Five carbon atoms
ⓓ. Six carbon atoms
Correct Answer: Four carbon atoms
Explanation: The parent chain must include the carbon atom of the \(\mathrm{-CHO}\) group. Beginning with that carbon and following the longest continuous path gives a chain of four carbon atoms. The remaining \(\mathrm{CH_3}\) group is treated as a branch rather than as part of a five-carbon continuous chain. Counting all carbon atoms in the molecule is not the same as identifying the longest continuous parent chain. The aldehyde carbon cannot be omitted merely to obtain a different chain arrangement.
16. In \(\mathrm{CH_3CH_2CH_2CH_2CHO}\), the alpha and beta carbons are respectively:
ⓐ. Carbon \(3\) and carbon \(4\)
ⓑ. Carbon \(1\) and carbon \(2\)
ⓒ. Carbon \(2\) and carbon \(3\)
ⓓ. Carbon \(4\) and carbon \(5\)
Correct Answer: Carbon \(2\) and carbon \(3\)
Explanation: The aldehyde carbon is assigned carbon number \(1\). The carbon directly beside it is carbon \(2\), and this is the alpha carbon. The next carbon is carbon \(3\), which is the beta carbon. Greek-letter positions are measured outward from the functional-group carbon even though that carbon itself is not called the alpha carbon. This relationship later helps identify alpha hydrogens involved in reactions next to a carbonyl group.
17. The roles of \(\mathrm{CN^-}\) and the carbonyl carbon during nucleophilic addition are:
ⓐ. \(\mathrm{CN^-}\) is an electrophile, and the carbonyl carbon is a nucleophile
ⓑ. \(\mathrm{CN^-}\) is an acid, and the carbonyl carbon is a base
ⓒ. \(\mathrm{CN^-}\) is a nucleophile, and the carbonyl carbon is an electrophile
ⓓ. Both species act as nucleophiles
Correct Answer: \(\mathrm{CN^-}\) is a nucleophile, and the carbonyl carbon is an electrophile
Explanation: Cyanide ion possesses an available electron pair and can donate that pair to form a new covalent bond. It therefore behaves as a nucleophile. The carbonyl carbon is electron-deficient because the carbon-oxygen bond is polarised toward oxygen. It accepts electron density and acts as an electrophile. The interaction joins an electron-rich species to an electron-poor centre. Acidity and basicity alone do not adequately describe the bond-forming roles in this step.
18. In a mechanism sketch, a curved arrow starts from a lone pair on \(\mathrm{CN^-}\) and ends at a carbonyl carbon. The arrow represents:
ⓐ. Transfer of an electron pair from carbonyl carbon to cyanide
ⓑ. Transfer of an electron pair from cyanide to carbonyl carbon
ⓒ. Transfer of a proton from carbonyl carbon to cyanide
ⓓ. Transfer of one electron from oxygen to carbon
Correct Answer: Transfer of an electron pair from cyanide to carbonyl carbon
Explanation: A full curved arrow in an organic mechanism represents the movement of an electron pair. Its tail begins at the electron source, which is the lone pair on \(\mathrm{CN^-}\). Its head points toward the atom where a new bond is being formed, namely the carbonyl carbon. The nuclei are not shown moving along the arrow. A single-electron movement would require a fish-hook arrow rather than the ordinary full curved arrow.
19. Benzaldehyde and phenylacetaldehyde differ structurally because:
ⓐ. Benzaldehyde: \(\mathrm{C_6H_5CHO}\); phenylacetaldehyde: \(\mathrm{C_6H_5COCH_3}\)
ⓑ. Benzaldehyde: \(\mathrm{C_6H_5CH_2CHO}\); phenylacetaldehyde: \(\mathrm{C_6H_5CHO}\)
ⓒ. Benzaldehyde: \(\mathrm{C_6H_5COCH_3}\); phenylacetaldehyde: \(\mathrm{C_6H_5CHO}\)
ⓓ. Benzaldehyde: \(\mathrm{C_6H_5CHO}\); phenylacetaldehyde: \(\mathrm{C_6H_5CH_2CHO}\)
Correct Answer: Benzaldehyde: \(\mathrm{C_6H_5CHO}\); phenylacetaldehyde: \(\mathrm{C_6H_5CH_2CHO}\)
Explanation: Benzaldehyde has the structure \(\mathrm{C_6H_5CHO}\), so the aldehyde carbon is bonded directly to the aromatic ring. Phenylacetaldehyde has the structure \(\mathrm{C_6H_5CH_2CHO}\), placing a methylene group between the ring and the aldehyde group. In the strict structural classification, direct attachment of \(\mathrm{-CHO}\) to an aromatic ring characterises an aromatic aldehyde. Both compounds still contain an aldehydic hydrogen. Merely having a phenyl group somewhere in a molecule does not make the formyl group directly ring-bound.
20. Methanal occupies a special boundary position among aldehydes because it:
ⓐ. Has both alkyl and aryl groups attached to its carbonyl carbon
ⓑ. Contains two carbon atoms and two aldehyde groups
ⓒ. Contains no alkyl or aryl group but is still an aldehyde
ⓓ. Contains one carbon atom and is the simplest ketone
Correct Answer: Contains no alkyl or aryl group but is still an aldehyde
Explanation: Methanal has the molecular formula \(\mathrm{HCHO}\) and contains only one carbon atom. Its carbonyl carbon is bonded to hydrogen rather than to an alkyl or aryl group. It is nevertheless an aldehyde because the carbonyl carbon carries hydrogen. A ketone requires two carbon-containing groups attached to the carbonyl carbon, so a one-carbon ketone cannot exist. The common name of methanal is formaldehyde.
