Some Basic Principles of Organic Chemistry MCQs with Answers – Part 3 (NEET Chemistry)

Explanation: \(\textbf{Interpret the common name:}\) Styrene contains a benzene ring attached directly to a two-carbon group possessing one C=C double bond. The substituent has the structure \(\mathrm{-CH=CH_2}\). This is an ethenyl group, commonly called vinyl. Therefore, the corresponding systematic structural name is ethenylbenzene. \(\textbf{Determine the molecular formula:}\) The monosubstituted benzene ring contributes six carbon atoms and five hydrogen atoms. The ethenyl group contributes two carbon atoms and three hydrogen atoms. Therefore, \[ N_C=6+2=8 \] \[ N_H=5+3=8 \] The molecular formula is consequently \(\mathrm{C_8H_8}\). \(\textbf{Count pi bonds in the aromatic ring:}\) The benzene ring contains a delocalised aromatic pi-electron system. In its conventional Kekulé representation, it is depicted with three alternating double bonds. This corresponds to three pi-bond contributions in introductory bond counting. \(\textbf{Count the side-chain pi bond:}\) The ethenyl substituent contains one additional C=C double bond. This contributes one pi bond. Therefore, \[ N_{\pi}=3+1 \] \[ N_{\pi}=4 \] \(\textbf{Check the other possible structures:}\) Ethylbenzene contains a saturated ethyl substituent, \(\mathrm{-CH_2CH_3}\). Its molecular formula is \(\mathrm{C_8H_{10}}\), so it is not styrene. Ethynylbenzene contains a C≡C triple bond in its substituent and has a different hydrogen count. Therefore, neither represents the given structure. The correct interpretation requires recognising the common-name structure, distinguishing an ethenyl group from ethyl and ethynyl groups, and combining aromatic and side-chain unsaturation. \(\textbf{Final answer:}\) \[ \boxed{\mathrm{C_8H_8},\quad N_{\pi}=4} \]

Explanation: \(\textbf{Identify the characteristic linkages:}\) The compound contains two oxygen atoms, each connected through single bonds to two carbon atoms. They therefore form ether linkages rather than alcoholic hydroxyl groups. One oxygen connects the parent carbon framework to a methyl group. The corresponding substituent is methoxy. The other oxygen connects the parent framework to an ethyl group. The corresponding substituent is ethoxy. \(\textbf{Select the parent framework:}\) The longest continuous carbon chain contains five carbon atoms. The parent is consequently pentane. The methoxy and ethoxy groups are treated as substituents on this parent. \(\textbf{Compare numbering directions:}\) Numbering from the left places methoxy at carbon 2 and ethoxy at carbon 4. Numbering from the right places ethoxy at carbon 2 and methoxy at carbon 4. Both directions give the locant set \(\{2,4\}\). The ordinary lowest-set comparison therefore remains tied. \(\textbf{Resolve the alphabetical tie:}\) Ethoxy precedes methoxy alphabetically. When the locant sets remain equivalent, the alphabetically earlier substituent receives the lower available locant. Therefore, ethoxy receives position 2 and methoxy receives position 4. The preferred name is 2-ethoxy-4-methoxypentane. Neither oxygen-containing substituent should be represented as an ordinary ethyl or methyl group because its attachment to the parent occurs through oxygen. The result requires recognising the ether groups, selecting the parent and applying the alphabetical numbering tie-breaker.

Explanation: \(\textbf{Identify the principal characteristic groups:}\) The molecule contains two alcoholic hydroxyl groups and one carbon–carbon double bond. Both hydroxyl groups are attached to saturated carbon atoms. The principal naming treatment therefore uses the diol suffix. The double bond is represented by the en component of the parent name. \(\textbf{Determine the parent chain:}\) The continuous carbon chain contains five carbon atoms. All five atoms belong to the parent framework containing the hydroxyl groups and the double bond. The parent is consequently a pentene-derived diol. \(\textbf{Compare numbering from the alkene end:}\) Numbering from the terminal alkene carbon places the double bond at position 1. The two hydroxyl groups then occupy positions 3 and 4. The hydroxyl locant set is therefore \(\{3,4\}\). \(\textbf{Compare numbering from the methyl end:}\) Numbering from the opposite end places the hydroxyl groups at positions 2 and 3. The double bond then begins at carbon 4. The hydroxyl locant set becomes \(\{2,3\}\). \(\textbf{Apply the correct priority:}\) The hydroxyl groups determine the principal suffix and receive priority in the relevant numbering comparison. The set \(\{2,3\}\) is preferred over \(\{3,4\}\). Therefore, numbering begins at the methyl end despite assigning the double bond locant 4. The correct systematic name is pent-4-en-2,3-diol. The molecule still contains the double bond; the larger alkene locant does not imply that unsaturation has been overlooked. This example demonstrates why the lowest double-bond locant cannot always override the locants of a higher-priority characteristic group.

Explanation: \(\textbf{Reconstruct the carbon framework:}\) The structure contains a four-carbon sequence with a methyl branch and an ethyl branch. However, the four-carbon sequence as written is not necessarily the longest eligible parent chain. A continuous path can be traced from the left terminal methyl group through the two substituted carbons and into the ethyl branch. This path contains five carbon atoms. Therefore, the appropriate parent is pentane rather than butane. \(\textbf{Identify the remaining substituents:}\) The additional methyl group attached to the second carbon remains outside the selected parent chain. The methyl group appearing at the right end of the original written formula also becomes a substituent on the selected five-carbon chain. Consequently, the preferred parent contains two methyl substituents. \(\textbf{Determine their positions:}\) Numbering from the end giving lower locants places the methyl groups at positions 2 and 3. Numbering from the opposite end would give positions 3 and 4. The lower set is \(\{2,3\}\). Therefore, the preferred name is 2,3-dimethylpentane. \(\textbf{Check the molecular formula:}\) The pentane parent contributes five carbon atoms. Two methyl substituents contribute two additional carbon atoms. Thus, \[ N_C=5+2=7 \] The structure is saturated and acyclic, so its molecular formula is \(\mathrm{C_7H_{16}}\). Every carbon atom satisfies ordinary tetravalency. The student's proposed name identifies the connectivity through a shorter parent chain but does not follow the preferred longest-chain selection. The error concerns parent selection rather than chemical impossibility. \(\textbf{Final answer:}\) \[ \mathrm{C_7H_{16}} \]

Explanation: \(\textbf{Identify the nitrogen environment:}\) The nitrogen atom is directly attached to three carbon-containing groups. These are an ethyl group, a methyl group and a butan-2-yl group. No hydrogen atom is directly bonded to nitrogen. Therefore, the compound is a tertiary amine. Its classification depends on the number of organic groups attached to nitrogen rather than on the substitution degree of any individual carbon atom. \(\textbf{Select the principal carbon framework:}\) For the ordinary substitutive amine name, the longest carbon chain directly associated with nitrogen contains four carbon atoms. The corresponding parent is butane. Nitrogen is attached to carbon 2 of this chain. Therefore, the parent amine name is butan-2-amine. \(\textbf{Identify the nitrogen substituents:}\) The remaining carbon-containing groups directly attached to nitrogen are ethyl and methyl. Their attachment positions are indicated using the locant N. Therefore, the substituent prefixes are N-ethyl and N-methyl. Ethyl precedes methyl alphabetically. \(\textbf{Construct the complete name:}\) The preferred systematic name is N-ethyl-N-methylbutan-2-amine. The compound is not a secondary amine because its nitrogen bears three organic substituents rather than two. A butan-1-amine parent would incorrectly place the amino attachment at a terminal carbon. The distinction between an N-substituent locant and an ordinary carbon-chain locant is essential. An alkyl group attached directly to nitrogen must not be treated as if it were attached to a carbon atom of the parent chain. The correct name follows from the nitrogen substitution class, parent-chain selection and placement of the N-substituent prefixes.

Explanation: \(\textbf{Identify the aromatic parent:}\) The compound contains one benzene ring with three substituents. No higher-priority characteristic group is specified to determine a separate retained parent. Therefore, the ring is named as a trisubstituted benzene. The substituents are bromo, chloro and methyl. \(\textbf{Compare possible ring numbering patterns:}\) The provisionally numbered arrangement gives substituent positions 1, 2 and 4. The corresponding locant set is \(\{1,2,4\}\). Renumbering the ring from other substituted positions may give sets such as \(\{1,2,5\}\) or \(\{1,3,4\}\). These alternatives do not improve on \(\{1,2,4\}\). Therefore, the lowest available substituent-locant set is retained. \(\textbf{Assign the substituents to the selected positions:}\) With the preferred numbering, bromine occupies carbon 1. Methyl occupies carbon 2. Chlorine occupies carbon 4. The position of one substituent cannot be changed independently without changing the actual ring connectivity. \(\textbf{Apply alphabetical presentation:}\) The prefixes are written in the order bromo, chloro and methyl. Therefore, the name must begin with 1-bromo, followed by 4-chloro and 2-methyl. The preferred systematic name is 1-bromo-4-chloro-2-methylbenzene. The name placing chlorine at position 2 would describe a different substitution pattern. The alternatives based on less favourable numbering do not follow the lowest-locant rule. This example illustrates that aromatic naming requires both symmetry-aware numbering and separate alphabetical ordering of substituent prefixes.

Explanation: \(\textbf{Identify the complete unsaturated parent:}\) The carbon framework contains seven carbon atoms in one continuous open chain. It possesses two C=C double bonds and one C≡C triple bond. All three multiple bonds belong to the seven-carbon parent. The systematic name must therefore contain both dien and yne components. \(\textbf{Number from the left:}\) The first double bond begins at carbon 1. The triple bond begins at carbon 4. The second double bond begins at carbon 6. The combined multiple-bond locants are \[ \{1,4,6\} \] \(\textbf{Number from the right:}\) The second double bond now begins at carbon 1. The triple bond begins at carbon 3. The first double bond begins at carbon 6. The combined locant set becomes \[ \{1,3,6\} \] \(\textbf{Select the numbering direction:}\) Both sets begin with locant 1. At the next point of difference, locant 3 is lower than locant 4. Therefore, the direction giving \(\{1,3,6\}\) is preferred. The resulting name is hepta-1,6-dien-3-yne. \(\textbf{Count the pi bonds:}\) Each C=C double bond contributes one pi bond. The C≡C triple bond contributes two pi bonds. Therefore, \[ N_{\pi}=1+1+2 \] \[ N_{\pi}=4 \] The triple bond must not be counted as one pi bond merely because it is one multiple-bond position. Likewise, the two double bonds remain separately represented in the dien component. The answer requires comparing competing multiple-bond locant sequences before calculating the complete pi-bond count. \(\textbf{Final answer:}\) \[ N_{\pi}=4 \]

Explanation: \(\textbf{Identify the characteristic groups:}\) The ring contains a ketone carbonyl group. An additional hydroxyl group is attached to a neighbouring saturated carbon. Therefore, the compound contains both ketone and alcohol functionality. The hydroxyl group does not convert the ketone into a carboxylic acid because it is not bonded directly to the carbonyl carbon. \(\textbf{Select the principal suffix:}\) The ketone has higher nomenclature priority than the alcohol. Consequently, the ketone determines the principal -one suffix. The hydroxyl group is represented by the hydroxy prefix. \(\textbf{Number the ring:}\) The carbonyl carbon receives position 1. The neighbouring hydroxyl-bearing carbon receives position 2. Therefore, the systematic name is 2-hydroxycyclohexan-1-one. \(\textbf{Examine the hydroxyl-bearing carbon:}\) Carbon 2 is tetrahedral and forms four sigma bonds. It is attached directly to hydroxyl and hydrogen. Its remaining bonds lead around the ring in two different directions. One direction immediately reaches the ketone carbonyl carbon. The other direction initially reaches a saturated methylene carbon. These two ring paths are constitutionally different. Therefore, carbon 2 has four different substituent environments and is stereogenic. \(\textbf{Check the carbonyl carbon:}\) The ketone carbonyl carbon is \(sp^2\)-hybridised and approximately trigonal planar. It is not a tetrahedral stereogenic centre. The molecule can consequently possess stereochemical distinctions arising from carbon 2 rather than from the carbonyl carbon. The correct conclusion requires combining principal-group naming with an independent test of the ring substituent paths.

Explanation: \(\textbf{Identify the aldehyde attachment:}\) The molecule contains a terminal \(\mathrm{-CHO}\) group. However, the aldehyde carbonyl carbon lies outside the five-membered ring. It is attached directly to one ring carbon rather than forming part of the cyclic framework. Therefore, the cyclic aldehyde is named using the carbaldehyde suffix. \(\textbf{Select the cyclic parent:}\) The ring contains five carbon atoms and one C=C double bond. Its cyclic parent is consequently derived from cyclopentene. The ring carbon carrying the external aldehyde group receives position 1. \(\textbf{Determine the double-bond locant:}\) The C=C double bond occurs between ring carbons 2 and 3. Therefore, it receives locant 2. Numbering in the opposite direction would give a higher locant for the same double bond. The preferred unsaturated cyclic framework is cyclopent-2-ene. \(\textbf{Locate the methyl group:}\) The methyl substituent is attached to carbon 3. It is therefore represented by the prefix 3-methyl. The complete name is 3-methylcyclopent-2-enecarbaldehyde. \(\textbf{Check the alternatives:}\) A cyclopentenal name that treats the external aldehyde carbon as though it occupied ring position 1 misrepresents the described framework. Changing the double-bond locant to 3 also gives an unfavourable numbering description. Placing methyl at position 2 would change the substituent attachment. The correct nomenclature requires distinguishing an aldehyde carbon outside a ring from a carbonyl carbon that forms part of a cyclic parent. The multiple-bond and substituent locants are then assigned within the numbered ring.

Explanation: \(\textbf{Select the principal group:}\) Both structures contain a carboxylic acid group. The carboxyl carbon must be included in the parent chain and receives position 1. Each molecule contains five carbon atoms and one C=C double bond. The parent names are therefore derived from pentenoic acid. \(\textbf{Name P:}\) Starting from the carboxyl carbon, the neighbouring methylene carbon is position 2. The double bond begins at position 3. Therefore, P is pent-3-enoic acid. \(\textbf{Name Q:}\) The double bond of Q lies between positions 4 and 5 when numbering begins at the carboxyl carbon. Therefore, Q is pent-4-enoic acid. \(\textbf{Examine stereoisomerism in P:}\) Each doubly bonded carbon of P possesses hydrogen and a different carbon-containing substituent. Therefore, both alkene carbons have two different groups. P can consequently exhibit geometrical isomerism about its C=C bond. \(\textbf{Examine stereoisomerism in Q:}\) Q contains a terminal \(\mathrm{CH_2=}\) group. The terminal doubly bonded carbon bears two identical hydrogen atoms. It therefore fails the different-substituent condition. Q cannot exhibit geometrical isomerism about that double bond. \(\textbf{Integrate the differences:}\) P and Q are position isomers because their molecular formulas and principal acid groups are identical while their double-bond positions differ. Moving the double bond changes the geometrical-isomerism possibilities without changing the characteristic carboxylic acid group. The correct answer requires applying acid-based numbering and then independently testing substituent identities at both doubly bonded carbons.

Explanation: \(\textbf{Select the unsaturated parent:}\) The molecule contains a six-carbon continuous chain with two C=C double bonds. Both double bonds belong to the selected parent framework. The parent is therefore a hexadiene. The additional methyl group is treated as a substituent. \(\textbf{Compare numbering directions:}\) Numbering from the terminal alkene end places the double bonds at positions 1 and 3. The methyl substituent then occupies position 5. Numbering from the opposite end would place the double bonds at positions 3 and 5. The methyl substituent would then occupy position 2. The multiple bonds must receive the lower applicable locants. Therefore, the direction producing 1,3-diene is preferred. The systematic name is 5-methylhexa-1,3-diene. \(\textbf{Examine the double-bond arrangement:}\) The relevant portion of the structure is \[ \mathrm{CH_2=CH-CH=CH-} \] The two double bonds are separated by exactly one carbon–carbon single bond. This alternating multiple-bond arrangement is characteristic of conjugated double bonds. The double bonds are not cumulated because they do not share one common carbon atom. They are not isolated because more than one single bond does not separate them. \(\textbf{Interpret the structural consequence:}\) The consecutive \(sp^2\)-hybridised carbon atoms provide adjacent unhybridised \(p\) orbitals. These orbitals can participate in conjugated pi-electron delocalisation. The methyl branch at carbon 5 does not interrupt the conjugated four-carbon portion. Thus, correct parent numbering and recognition of the alternating bonding pattern lead to the selected answer.

Explanation: \(\textbf{Identify the characteristic groups:}\) The molecule contains one \(\mathrm{-COOH}\) group and one \(\mathrm{-CHO}\) group. The first is a carboxylic acid. The second is an aldehyde. Both groups are attached externally to the benzene ring through their carbon atoms. \(\textbf{Select the principal parent:}\) The carboxylic acid has higher nomenclature priority than the aldehyde. Therefore, the principal parent is benzoic acid. The benzene-ring carbon bonded directly to the carboxyl group receives position 1. \(\textbf{Identify the aldehyde substituent treatment:}\) The additional \(\mathrm{-CHO}\) group is attached to the aromatic ring through its carbonyl carbon. Because it is not selected as the principal characteristic group, it is represented by the prefix formyl. The prefix oxo would not appropriately describe this external one-carbon aldehyde substituent in the selected benzoic acid framework. \(\textbf{Assign the ring position:}\) The formyl group occupies ring position 4 relative to the carboxylic acid attachment. The substituents are consequently para to one another. The systematic name is 4-formylbenzoic acid. \(\textbf{Check the alternative principal-group treatments:}\) Naming the compound as a carboxy-substituted benzaldehyde would select the lower-priority aldehyde as the principal group. Calling it a formylphenol would incorrectly introduce phenolic functionality. The molecule has no hydroxyl group attached directly to a benzene-ring carbon. Its hydroxyl oxygen belongs to the carboxylic acid group. The correct name requires distinguishing the external aldehyde carbonyl from a ketone-like oxo substituent and applying the appropriate principal-group priority.

Explanation: \(\textbf{Reconstruct the carbon framework:}\) The pentan-2-ol parent contains five carbon atoms. Two additional ethyl substituents are attached to carbon 3. Each ethyl substituent contributes two carbon atoms. Therefore, \[ N_C=5+2(2) \] \[ N_C=9 \] \(\textbf{Determine the molecular formula:}\) The structure is saturated and acyclic. It contains one alcoholic hydroxyl group and no additional heteroatom. The corresponding general formula is \(\mathrm{C_nH_{2n+2}O}\). For nine carbon atoms, \[ N_H=2(9)+2 \] \[ N_H=20 \] Therefore, the molecular formula is \(\mathrm{C_9H_{20}O}\). \(\textbf{Identify the hydroxyl-bearing carbon:}\) Carbon 2 is attached to hydroxyl and hydrogen. It is also attached to the terminal methyl group at carbon 1. Its fourth attachment is to carbon 3, which bears three ethyl-containing directions in the complete structure. Thus, the four groups attached to carbon 2 are hydroxyl, hydrogen, methyl and the substituted carbon framework. These are different substituent environments. Carbon 2 is consequently a stereogenic tetrahedral centre. \(\textbf{Examine the highly substituted carbon:}\) Carbon 3 is attached directly to carbon 2 and to three ethyl groups. It is a quaternary carbon. However, the three ethyl groups attached to carbon 3 are identical. Carbon 3 therefore does not possess four different substituents and is not stereogenic. \(\textbf{Integrate the distinction:}\) The presence of identical groups at carbon 3 does not make the complete carbon-3-containing substituent identical to the methyl group attached to carbon 2. The stereogenic character of carbon 2 must be tested using the four groups directly attached to that carbon. Thus, the compound contains nine carbon atoms and has a stereogenic hydroxyl-bearing carbon. \(\textbf{Final answer:}\) \[ \mathrm{C_9H_{20}O} \]

Explanation: \(\textbf{Identify the principal carbon chain:}\) The structure contains a continuous seven-carbon chain. This chain extends from the left terminal methyl group to the right terminal methyl group. Alternative paths through the branched substituent do not provide a longer parent chain. Therefore, the principal parent is heptane. \(\textbf{Locate the simple substituent:}\) A methyl group is attached to the third carbon when the parent chain is numbered from the left. The same group would occupy position 5 if numbered from the right. \(\textbf{Identify the branched substituent:}\) The substituent attached to the fourth carbon is \(\mathrm{-CH(CH_3)_2}\). Its attachment to the parent occurs through the middle carbon of a three-carbon framework. The systematic substituent name is therefore propan-2-yl. It is not an unbranched propyl group attached through a terminal carbon. \(\textbf{Choose the numbering direction:}\) Numbering from the left gives substituent locants 3 and 4. Numbering from the right gives locants 4 and 5. The set \(\{3,4\}\) is preferred. Therefore, methyl is at carbon 3 and propan-2-yl is at carbon 4. \(\textbf{Construct the systematic name:}\) The methyl prefix precedes the propan-2-yl prefix alphabetically. The preferred name is 3-methyl-4-(propan-2-yl)heptane. The familiar common substituent descriptor isopropyl may help identify the branch, but it should not displace the requested systematic substituent form. Option D would attach an unbranched propyl group and therefore represents different connectivity. The alternatives using locant 5 for methyl employ the less favourable parent numbering direction. The correct name requires interpreting the attachment atom within a branched substituent, not merely counting the number of carbons in that substituent.

Explanation: \(\textbf{Identify the principal characteristic group:}\) The molecule contains the arrangement \(\mathrm{-C(=O)-NH-}\). Nitrogen is directly attached to a carbonyl carbon. Therefore, the compound is an amide rather than an ordinary amine. The molecule also contains an alcoholic hydroxyl group attached to saturated carbon. The amide determines the principal naming treatment. \(\textbf{Select the acid-derived parent:}\) The carbonyl carbon is attached to an ethyl group. Including the carbonyl carbon, the acid-derived framework contains three carbon atoms. Therefore, the parent amide is propanamide. \(\textbf{Identify the nitrogen substituent:}\) The amide nitrogen is attached to one hydrogen atom and a \(\mathrm{-CH_2CH_2OH}\) group. This group contains two carbon atoms and one hydroxyl group. Numbering the substituent from its attachment to nitrogen places the hydroxyl group at carbon 2. The complete substituent name is consequently 2-hydroxyethyl. Its attachment to the amide nitrogen is indicated by the locant N. \(\textbf{Construct the name:}\) The correct name is N-(2-hydroxyethyl)propanamide. The hydroxyl group is not located at carbon 3 of the propanamide parent. It belongs to the separate substituent attached through nitrogen. \(\textbf{Determine the amide class:}\) The nitrogen is attached to the carbonyl carbon, one organic substituent and one hydrogen. Therefore, the structure is a secondary amide. A tertiary amide would have two organic substituents on nitrogen in addition to the acyl attachment and would contain no N–H bond. The correct answer requires distinguishing the amide parent from its nitrogen-attached substituent and numbering the hydroxyl group within that substituent.

Explanation: \(\textbf{Verify record P:}\) The structure contains a terminal aldehyde group. The carbonyl carbon must receive position 1. The parent chain contains four carbon atoms, and the methyl branch occupies carbon 3. Therefore, 3-methylbutanal is correct. \(\textbf{Verify record Q:}\) The compound contains an ordinary ether linkage. The longer carbon group forms a three-carbon propane parent. The oxygen-attached ethyl group is represented by the ethoxy prefix. The ethoxy group is attached at carbon 2. Therefore, 2-ethoxypropane correctly represents the structure. \(\textbf{Verify record R:}\) The structure contains five carbon atoms in one continuous chain. The C≡C triple bond receives locant 2 when numbering begins from the right. Numbering from the other end would assign locant 3. Therefore, pent-2-yne is the correct systematic name. \(\textbf{Verify record S:}\) The molecule contains a carboxylic acid group. The carboxyl carbon must receive position 1. The adjacent methylene carbon is position 2. The C=C double bond begins at position 3. Therefore, the correct name is pent-3-enoic acid, not pent-2-enoic acid. \(\textbf{Interpret the naming error:}\) The incorrect name could arise from numbering the carbon chain from the hydrocarbon end or overlooking the carboxyl carbon in the parent count. However, carboxylic acid numbering begins at the principal acid carbon. The position of the multiple bond must then be determined using that numbering. Records P, Q and R satisfy their respective naming rules. Only record S assigns an incorrect double-bond locant. The comparison illustrates that aldehyde, ether, alkyne and carboxylic acid names require different structural decisions, even though each ultimately uses a systematic parent and appropriate locants.

Explanation: \(\textbf{Establish the equivalence criterion:}\) Two names may appear different merely because the same carbon framework has been numbered from opposite directions. A genuine constitutional difference requires different atom-to-atom connectivity. The comparison must therefore reconstruct each structure independently of its written locants. \(\textbf{Examine P:}\) In 2,2,3-trimethylbutane, two methyl branches occur at carbon 2 and one at carbon 3. Reversing the four-carbon parent chain converts the substituent positions into 2,3,3. Therefore, both names represent the same molecule. The preferred numbering is 2,2,3 because it gives the lower locant sequence. \(\textbf{Examine Q:}\) The ethyl branch occupies the middle carbon of the five-carbon parent. Reversing this parent preserves ethyl at position 3 but interchanges methyl positions 2 and 4. Therefore, the two descriptions again represent identical connectivity. \(\textbf{Examine R:}\) In 2,3-dimethylpentane, the methyl groups occupy adjacent parent-chain carbons. Reversing the chain gives locants 3 and 4. It does not produce the 2,4-disubstitution pattern. In 2,4-dimethylpentane, the methyl-bearing carbons are separated by an intervening carbon. Therefore, the two molecules possess genuinely different carbon skeletons. \(\textbf{Examine S:}\) In a cyclopentane ring, positions 1 and 5 are adjacent because the ring is closed. Reversing the numbering direction converts the 1,5-description into 1,2. Thus, S also describes identical constitutional connectivity. Only R compares two distinct compounds. The decisive method is to test the equivalence of complete connectivities under chain reversal or ring renumbering, rather than interpreting different locant sequences as automatic evidence of isomerism.

Explanation: \(\textbf{Select the principal functional group:}\) Both P and Q contain an aldehyde group directly attached to the benzene ring. The aldehyde remains the principal characteristic group in the stated naming comparison. The appropriate parent name is therefore benzaldehyde. The ring carbon connected to the aldehyde group is assigned position 1. \(\textbf{Identify the substituent in P:}\) The substituent at ring position 3 is \(\mathrm{-CH_2OH}\). Its hydroxyl oxygen is bonded directly to saturated methylene carbon. This represents an alcoholic hydroxyl group rather than a phenolic hydroxyl group. As a substituent on the benzaldehyde parent, the group is described as hydroxymethyl. Therefore, P is 3-(hydroxymethyl)benzaldehyde. \(\textbf{Interpret the structural change:}\) Replacing the hydroxyl hydrogen with a methyl group creates the arrangement \(\mathrm{-CH_2-O-CH_3}\). The oxygen is now bonded through single bonds to two carbon atoms. Consequently, the side chain contains an ether linkage rather than a free alcoholic hydroxyl group. The complete substituent is methoxymethyl. \(\textbf{Name Q:}\) The benzaldehyde parent remains unchanged. The substituent still occupies ring position 3. Thus, Q is 3-(methoxymethyl)benzaldehyde. \(\textbf{Check the retained functional group:}\) The aldehyde carbonyl and its direct attachment to the ring are unaffected by the stated replacement. Neither compound contains a phenolic hydroxyl group. Q is not simply 3-methoxybenzaldehyde because that structure would have oxygen directly bonded to the aromatic ring. Here, a methylene carbon remains between the ring and the ether oxygen. The naming decision therefore depends on preserving the principal group while recognising the changed connectivity within the side chain.

Explanation: \(\textbf{Identify the principal functional group:}\) The compound contains the characteristic \(\mathrm{-C(=O)-O-C-}\) arrangement. It is therefore an ester. The ester linkage determines the principal naming treatment. The nitrogen-containing group is an additional substituent on the acid-derived carbon framework. \(\textbf{Determine the alcohol-derived portion:}\) The singly bonded ester oxygen is attached to \(\mathrm{-CH_2CH_3}\). This is an ethyl group. Consequently, the ester name begins with ethyl. \(\textbf{Determine the acid-derived portion:}\) The carbonyl carbon is attached to a two-carbon chain. Including the ester carbonyl carbon, the acid-derived parent contains three carbon atoms. The corresponding carboxylate name is propanoate. The carbonyl carbon receives position 1. The neighbouring carbon carrying the nitrogen substituent is position 2. \(\textbf{Identify the complete nitrogen substituent:}\) The nitrogen attached to carbon 2 is also bonded to a methyl group and hydrogen. The complete substituent is therefore methylamino rather than simply amino. The systematic ester name is ethyl 2-(methylamino)propanoate. \(\textbf{Determine the nitrogen substitution degree:}\) Nitrogen is directly attached to two carbon-containing groups. One connection is to the propanoate framework. The other is to the methyl group. Nitrogen also retains one N–H bond. It consequently has the bonding environment of a secondary amine. The nitrogen is not part of an amide because it is not directly attached to the carbonyl carbon. The alternatives interchange the ester and nitrogen-attached alkyl groups or omit the methyl substituent from nitrogen. The correct answer requires dividing the ester into its two naming portions before separately classifying the amino nitrogen.

Explanation: \(\textbf{Select the unsaturated parent of X:}\) X contains two C=C double bonds connected through a central saturated carbon. A five-carbon chain passes through both double bonds. A longer seven-carbon path can be traced through the four-carbon side chain, but that path contains only one of the double bonds. Under the applicable parent selection for this unsaturated hydrocarbon, the chain incorporating the maximum number of multiple bonds is preferred. Therefore, the five-carbon chain containing both double bonds is selected. \(\textbf{Name the starting structure:}\) The two double bonds occupy positions 1 and 4. The four-carbon side chain is attached to carbon 3. It is an unbranched butyl substituent. Thus, X is 3-butylpenta-1,4-diene. \(\textbf{Identify the hydrogenation change:}\) Complete hydrogenation removes the two C=C pi bonds. Each double bond becomes a carbon–carbon single bond. No carbon–carbon sigma connection is broken or rearranged. The central carbon consequently remains connected to three carbon-containing branches. \(\textbf{Reconstruct the saturated product:}\) After hydrogenation, the two original ethenyl portions become ethyl portions. The central carbon is now bonded to two ethyl groups, one butyl group and hydrogen. The longest continuous chain passes through one ethyl branch, the central carbon and the four-carbon butyl branch. It contains seven carbon atoms. The remaining ethyl branch is attached to carbon 3 when numbered from the nearer end. Therefore, Y is 3-ethylheptane. \(\textbf{Check conservation of carbon atoms:}\) The starting molecule contains five parent-chain carbons and four side-chain carbons. The product therefore retains nine carbon atoms. Its saturated acyclic formula is \(\mathrm{C_9H_{20}}\), consistent with complete hydrogenation of the two double bonds in \(\mathrm{C_9H_{16}}\). The important inference is that changing the degree of unsaturation changes the applicable parent-chain selection even though the underlying carbon–carbon sigma connectivity remains unchanged. The preferred parent of X contains five carbons, while that of Y contains seven. This is a nomenclature change resulting from the altered bonding conditions, not a carbon-skeleton rearrangement.