Functional Groups & Nomenclature

Functional Groups for the MCAT

Part 1 of 4 — Recognition & Nomenclature

Functional groups are the vocabulary of every organic and biochemistry passage. The MCAT expects instant recognition of each group in skeletal structures, plus enough IUPAC to decode names in question stems.

The Core Inventory

Priority Order (who gets the suffix)

$\text{acid} > \text{ester} > \text{amide} > \text{nitrile} > \text{aldehyde} > \text{ketone} > \text{alcohol} > \text{amine} > \text{ether}$

The highest-priority group takes the suffix AND the lowest possible locant; everything else appears as a prefix. Example: 4-hydroxy-2-butanone (ketone outranks alcohol); 3-oxobutanoic acid (acid outranks ketone, which becomes "oxo").

Look-Alikes the MCAT Loves to Confuse

• Ester vs ether: an ester has a C=O next to the O (-CO-O-); an ether is just C-O-C
• Amide vs amine: an amide's N touches a C=O; an amine's does not — completely different basicity (Part 3)
• Aldehyde vs ketone: the -CHO hydrogen; terminal C=O carbon = aldehyde
• Hemiacetal vs acetal: OH + OR on one carbon vs OR + OR; both flag sugar chemistry
• Phenol vs alcohol: OH directly on an aromatic ring vs on sp3 carbon — a million-fold acidity gap
• Carboxylic acid vs ester: the acid's O-H; deprotonatable, H-bond donor

Amine and Alcohol Classification

Alcohols/alkyl halides classify by the carbon bearing the group (1°/2°/3° = carbons attached to that carbon). Amines classify by substituents on nitrogen: $RNH_2$ primary, $R_2NH$ secondary, $R_3N$ tertiary, $R_4N^+$ quaternary (permanently charged, no lone pair). So tert-butylamine is a PRIMARY amine on a tertiary carbon — a stock trap.

Degrees of Unsaturation

$DoU = \frac{2C + 2 + N - H - X}{2}$

Oxygen and sulfur are ignored. Each ring or pi bond counts once; a benzene ring = 4; a C=O = 1; a nitrile = 2. Compute DoU FIRST when matching a formula to candidate structures.

Group Recognition & Naming 🎯

Key Takeaways — Part 1

• Suffix priority: acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > ether; losers become prefixes (oxo-, hydroxy-, amino-)
• Look-alike pairs: ester vs ether (C=O next to O), amide vs amine (N touching C=O), hemiacetal vs acetal (OH+OR vs OR+OR), phenol vs alcohol
• Amines classify by substituents on N; alcohols/halides by the carbon — tert-butylamine is a primary amine
• Quaternary ammonium: four bonds, permanent positive charge, no lone pair
• $DoU = (2C + 2 + N - H - X)/2$; ignore O/S; benzene = 4; compute it before matching structures

Worked Examples — Recognition & Naming

Question: Aspirin is 2-acetoxybenzoic acid. From the name alone, list its functional groups and their roles.

Solution:

1. "-benzoic acid" → a carboxylic acid on a benzene ring (the suffix group, C1)
2. "2-acetoxy-" → an acetyl ester (-O-CO-CH3) on the adjacent ring carbon — named as a prefix because the acid outranks it
3. Groups: aromatic ring, carboxylic acid, ester
4. Chemistry preview: the ester hydrolyzes (aspirin acetylates COX enzymes; old aspirin smells of acetic acid), and the acid is deprotonated at physiological pH

MCAT Strategy: Decode names right-to-left: suffix = principal group, prefixes = everything else. Passage pharmacology usually turns on the most hydrolyzable or ionizable group you find.

Question: A compound is $C_4H_8O$ and shows no IR carbonyl band. What structural classes remain?

Solution:

1. $DoU = (8 + 2 - 8)/2 = 1$ → one ring OR one pi bond
2. No C=O → the degree must be a C=C or a ring
3. Candidates: unsaturated alcohols (e.g., 3-buten-1-ol), enol ethers, cyclic ethers (THF), cyclobutanol
4. IR would separate alcohols (broad 3350) from ethers (no O-H, C-O at 1000-1300)

MCAT Strategy: DoU + one spectroscopic fact eliminates most of the answer grid before any detailed analysis. Practice this two-step triage until it is automatic.

Question: Identify every functional group in the dipeptide seryl-cysteine (Ser-Cys).

Solution:

1. Backbone: one free alpha-amine (N-terminus), one amide (the peptide bond), one free carboxylic acid (C-terminus)
2. Serine side chain: primary alcohol (-CH2OH)
3. Cysteine side chain: thiol (-SH) — the sulfur analog of an alcohol, more acidic (pKa ~8) and a superior soft nucleophile
4. Note what is NOT there: the peptide N is an amide nitrogen — non-basic, non-nucleophilic

MCAT Strategy: In peptide questions, sort nitrogens into amide (backbone, inert) vs amine (N-terminus, Lys side chain, basic). That single distinction answers most "which site reacts/protonates" items.

Question: $C_3H_6O_2$ could be a carboxylic acid, an ester, or a hydroxy ketone/aldehyde. Name one example of each and one quick test to tell them apart.

Solution:

1. Acid: propanoic acid. Ester: methyl acetate ($CH_3COOCH_3$). Hydroxy carbonyl: hydroxyacetone (1-hydroxypropan-2-one) or 3-hydroxypropanal
2. IR: the acid shows the 2500-3300 O-H smear; the ester shows 1740 + strong C-O and no O-H; the hydroxy carbonyls show BOTH broad 3400 O-H and a C=O
3. Chemical test: only the acid liberates $CO_2$ from bicarbonate; only the aldehyde gives a positive Tollens

MCAT Strategy: Isomer families sharing a formula are separated by their O-H/C=O combinations — one glance at the IR's two diagnostic windows settles it.

Functional Groups for the MCAT

Part 2 of 4 — Physical Properties: Forces, Boiling Points & Solubility

Physical-property questions reduce to one skill: identify the strongest intermolecular force each functional group supports, then rank.

The Force Hierarchy

$\text{ion-ion} > \text{ion-dipole} > \text{H-bond} > \text{dipole-dipole} > \text{London dispersion}$

• Hydrogen bond donors: O-H, N-H (and to a lesser degree S-H). A donor requires H bonded directly to N, O, or F
• Acceptors only: ethers, ketones, aldehydes, esters, tertiary amines — lone pairs but no polar H. They H-bond with WATER but not with themselves
• Everything polar has dipole-dipole; everything has dispersion, which grows with surface area and molecular weight

Boiling Point Logic

Boiling separates molecules from each other, so stronger intermolecular attraction = higher bp:

• Carboxylic acids top their weight class (H-bonded dimers — effectively two H-bonds per pair)
• Alcohols and 1°/2° amides are next (self H-bonding); O-H beats N-H (oxygen is more electronegative → stronger H-bond)
• Aldehydes/ketones/esters: dipole-dipole only among themselves — mid-range
• Ethers and alkanes at the bottom; ethers barely beat comparable alkanes
• Within a family: bp rises with chain length (dispersion) and FALLS with branching (less contact surface area — neopentane boils 27 degrees below n-pentane)

Reference ladder at comparable molecular weight: acid > alcohol > amine > ketone/aldehyde > ester > ether > alkane (e.g., acetic acid 118 C vs 1-propanol 97 C vs propanal 49 C).

Solubility Logic

"Like dissolves like" quantified: each H-bonding group can drag roughly 4-5 carbons into water. So 1-butanol is somewhat soluble, 1-octanol is not; glucose (six carbons, five OH + ring O) is freely soluble. Additional levers:

• Ionization multiplies solubility: carboxylic acids dissolve in aqueous NaOH or bicarbonate (carboxylate salt); amines dissolve in dilute HCl (ammonium salt). This is the basis of acid-base extraction
• Ketones/aldehydes/ethers/esters accept H-bonds from water: modestly soluble when small, but pure samples of them cannot H-bond with each other
• Amphipathic molecules (fatty acid salts, phospholipids, detergents) bury tails and face water with heads — micelles and bilayers

Melting Points (one extra rule)

Melting also rewards crystal packing: symmetric molecules pack well and melt high (para isomers melt above ortho/meta; trans fatty acids pack and melt higher than cis-kinked ones — the membrane-fluidity story).

Properties & Forces 🎯

Key Takeaways — Part 2

• Force ladder: ion > ion-dipole > H-bond > dipole-dipole > dispersion; donors need H on N/O/F, acceptor-only groups (ethers, ketones, esters, 3° amines) H-bond with water but not themselves
• Boiling: acid dimers > alcohols > amines/amides > carbonyls > ethers > alkanes; longer chains raise bp, branching lowers it
• Solubility: ~4-5 carbons per H-bonding group; ionization (carboxylate, ammonium) makes anything soluble — the engine of acid-base extraction
• Bicarbonate pulls out acids; HCl pulls out amines; neutrals stay in the organic layer
• Melting rewards packing: para and trans isomers melt higher; cis kinks = fluid membranes

Worked Examples — Properties

Question: An ether solution contains aniline (an amine), phenol (pKa 10), benzoic acid (pKa 4.2), and anisole (an ether). Design a separation.

Solution:

1. Shake with aqueous HCl → aniline protonates to anilinium, moves to water. Set that layer aside; basify to recover aniline
2. Shake with aqueous $NaHCO_3$ → only benzoic acid (pKa 4.2) is deprotonated by this WEAK base; phenol (pKa 10) is untouched. Acidify to recover the acid
3. Shake with aqueous NaOH → now phenol deprotonates to phenoxide and transfers. Acidify to recover phenol
4. Anisole (no acidic H, no basic N) remains in ether throughout

MCAT Strategy: Bicarbonate vs hydroxide is the pKa discriminator: bicarbonate takes only true acids (pKa < ~5); NaOH also takes phenols. Sequence weak base before strong base.

Question: Rank and justify: n-pentane, neopentane, 1-butanol, butanoic acid, diethylamine.

Solution:

1. Butanoic acid: H-bonded dimer → highest (163 C)
2. 1-Butanol: O-H self H-bonding (117 C)
3. Diethylamine: N-H donor, but N-H bonds are weaker H-bonds than O-H (55 C)
4. n-Pentane: dispersion only (36 C); neopentane: same formula, spherical → least surface contact → lowest (9 C)
5. Order: acid > alcohol > amine > straight alkane > branched alkane

MCAT Strategy: When two compounds share the strongest force type, break the tie with electronegativity (O-H > N-H) or shape (linear > branched).

Question: Explain, in functional-group terms, why sodium stearate disperses grease in water while stearic acid itself does not.

Solution:

1. Stearic acid: an 18-carbon chain with one COOH — the single H-bonding group cannot carry 18 carbons → insoluble wax
2. Saponification converts COOH to $COO^- Na^+$: an ionic head worth far more hydration energy than a neutral acid
3. The amphipath assembles into micelles: tails dissolve grease in the hydrophobic core, carboxylate shells face water
4. In hard water, $Ca^{2+}$ bridges two carboxylates into an insoluble salt — soap scum

MCAT Strategy: Any "detergent/micelle/emulsion" passage is the solubility rules applied twice to one molecule: count the head's H-bonding/ionic strength against the tail's carbons.

Question: A passage compares membranes enriched in (a) saturated C18 tails, (b) cis-monounsaturated C18 tails, (c) shorter C14 saturated tails. Rank membrane fluidity at body temperature and name the governing force.

Solution:

1. Fluidity is inverse to tail packing, which is set by dispersion contact
2. (a) Straight C18 chains pack tightest → least fluid
3. (c) C14 chains have less dispersion area than C18 → more fluid than (a)
4. (b) The cis kink disrupts packing most → most fluid: order (b) > (c) > (a). Cholesterol buffers fluidity in both directions

MCAT Strategy: Chain length = amount of dispersion; cis unsaturation = packing defect. Two structural dials, one force. This exact ranking appears repeatedly in membrane passages.

Functional Groups for the MCAT

Part 3 of 4 — Acidity, Basicity & Electronic Effects

Every acidity/basicity comparison on the MCAT is decided the same way: the more stable the conjugate base, the stronger the acid. Four stabilizing effects, in rough order of power:

1. Element (which atom holds the charge): across a row, more electronegative = more stable ($CH_4 < NH_3 < H_2O < HF$); down a column, LARGER wins ($H_2O < H_2S$; HF < HCl < HBr < HI) because charge spreads over a bigger orbital
2. Resonance: delocalized charge is stabilized charge — the biggest single effect
3. Induction: nearby electronegative atoms pull charge through sigma bonds; falls off sharply with distance
4. Hybridization/orbital: more s-character holds electrons closer (sp C-H of alkynes, pKa ~25, is acidic for a hydrocarbon)

The pKa Ladder to Memorize

Why the three stars: carboxylic acid beats phenol beats alcohol — carboxylate charge is shared equally by two oxygens; phenoxide charge is delocalized into the ring but mostly onto carbons; alkoxide charge is stuck on one oxygen.

Substituent Tuning

• Electron-withdrawing groups (Cl, $NO_2$, F) increase acidity: trichloroacetic acid pKa 0.7 vs acetic 4.8; effect shrinks with distance from the acidic site
• Electron-donating groups (alkyl, $OCH_3$ by resonance) decrease acidity

Basicity: the Mirror Image

Base strength tracks lone pair availability:

• Amide nitrogen is NOT basic — the lone pair is delocalized into the C=O
• Aniline is a weak base (~million-fold weaker than alkylamines): the lone pair leaks into the ring
• Alkylamines are the strong organic bases (conjugate acid pKa 10-11)
• Pyridine: sp2 lone pair in the plane, NOT part of the aromatic pi system → basic (conj. acid pKa 5.2). Pyrrole: the lone pair IS the aromatic sextet → protonation destroys aromaticity → essentially non-basic. This pair explains histidine (imidazole has one pyridine-like N, pKa ~6 — the physiological buffer nitrogen) and the DNA bases

Henderson-Hasselbalch in One Line

$pH = pK_a + \log\frac{[A^-]}{[HA]}$

Below the pKa the protonated form dominates; above it, the deprotonated form. At physiological pH 7.4: carboxylic acids are deprotonated (anionic), amines are protonated (cationic) — amino acids are zwitterions.

Acidity & Basicity 🎯

Key Takeaways — Part 3

• Acidity = conjugate-base stability: element (row → electronegativity, column → size), resonance (strongest), induction (distance-sensitive), hybridization (s-character)
• pKa anchors: carboxylic acid 4-5 < carbonic 6.3 < thiol/ammonium/phenol 8-11 < water 15.7 < alcohol 16-18 < alpha C-H 20 < alkyne 25 < amine N-H 36 < alkane 50
• Acid > phenol > alcohol because carboxylate (2 O's) > phenoxide (ring C's) > alkoxide (1 O) in charge delocalization; EWGs raise acidity, EDGs lower it
• Basicity = lone pair availability: alkylamine > aniline >> amide (~non-basic); pyridine-type N basic, pyrrole-type N locked in the aromatic sextet
• Henderson-Hasselbalch: pH above pKa → deprotonated; at pH 7.4 acids are anions, amines are cations (zwitterionic amino acids)

Worked Examples — Acid-Base Reasoning

Question: One equivalent of NaOH is added to a molecule containing a phenol OH, an alcohol OH, and a carboxylic acid COOH. What deprotonates?

Solution:

1. Rank acidities: COOH (pKa ~4.5) > phenol (10) > alcohol (17)
2. Hydroxide (conjugate acid pKa 15.7) could deprotonate either the acid or the phenol, but with ONE equivalent, the thermodynamically most acidic proton is removed first
3. Product: the carboxylate, with phenol and alcohol intact
4. A second equivalent would take the phenol; the alcohol survives NaOH almost entirely (pKa above water's)

MCAT Strategy: "One equivalent" questions are pKa-ordering questions. Titrate protons off in strict pKa order, lowest first.

Question: Rank the acidity of butanoic acid, 2-chlorobutanoic acid, 4-chlorobutanoic acid, and 2,2-dichlorobutanoic acid.

Solution:

1. All are carboxylic acids — induction is the tiebreaker
2. More EWGs = more acidic: 2,2-dichloro > 2-chloro
3. Distance matters: chlorine at C2 (alpha) stabilizes the carboxylate far more than at C4 — 2-chloro > 4-chloro
4. Final order: 2,2-dichloro > 2-chloro > 4-chloro > parent

MCAT Strategy: Two dials for induction problems: count the withdrawing groups, then measure their distance. A closer single Cl can beat a distant pair — check both before answering.

Question: Histidine's imidazole side chain has pKa ~6.0. Explain which of its two ring nitrogens is protonated at low pH, and why imidazole (not lysine or aspartate) serves in enzyme active sites needing both acid and base catalysis at pH 7.4.

Solution:

1. Imidazole has a pyrrole-type N (lone pair in the aromatic sextet — never protonates) and a pyridine-type N (in-plane lone pair — the basic site)
2. pKa 6.0 sits within ~1.4 units of physiological pH → BOTH the protonated (acid) and neutral (base) forms are present in meaningful amounts ($\approx$ 4% protonated at pH 7.4, more in local microenvironments)
3. Lysine (pKa 10.5) is locked protonated and aspartate (3.9) locked deprotonated at pH 7.4 — each can play only one role
4. Hence catalytic triads (e.g., serine proteases) use His as the proton shuttle

MCAT Strategy: "Effective buffer/proton shuttle at pH X" = side chain with pKa within ~1 unit of X. Only histidine qualifies near neutrality.

Question: At pH 7.4 blood, a drug with a carboxylic acid group (pKa 4) and a drug with an amine (conjugate acid pKa 9) must cross lipid membranes. Which form crosses, and what fraction is available for each?

Solution:

1. Membranes admit the NEUTRAL form; ions stay in water
2. Acid drug: pH is 3.4 units above pKa → ratio deprotonated:protonated = $10^{3.4}$ ≈ 2500:1 → only ~0.04% is the neutral, absorbable HA form
3. Amine drug: pH is 1.6 units below the conjugate acid's pKa → mostly protonated (ammonium), with ~2-3% neutral free base available to cross
4. Absorption sites matter: the acidic stomach (pH ~2) leaves the acid drug neutral — weak acids absorb from the stomach, weak bases from the intestine

MCAT Strategy: Every pharmacokinetics passage is Henderson-Hasselbalch: compute pH minus pKa, convert to a power of ten, and remember only neutrals cross membranes.

Functional Groups for the MCAT

Part 4 of 4 — Functional Groups in Biomolecules: MCAT Integration

The MCAT rarely asks "name this group" directly. It asks how a group's chemistry explains a biomolecule's behavior. This part is the translation table.

The Four Biomolecule Families, as Functional Groups

Notice the pattern: every polymerization is a condensation (lose water; an OH or NH attacks a carbonyl or phosphate), and every digestion is a hydrolysis running the same linkage backward.

Group Chemistry → Biological Property

• Phosphates ionize twice (pKa ~2 and ~7): DNA's backbone is polyanionic → binds $Mg^{2+}$ and histone lysines/arginines (ammonium/guanidinium cations)
• Thiols (Cys): soft nucleophiles; pairs oxidize to disulfides (the only common covalent side-chain crosslink; reduced by DTT/2-mercaptoethanol in lab passages)
• Hydroxyls (Ser/Thr/Tyr): nucleophiles AND phosphorylation sites — kinases build phosphoesters, phosphatases hydrolyze them
• Imidazole (His): pKa ~6, the proton shuttle (Part 3)
• Guanidinium (Arg, pKa ~12.5): resonance over three nitrogens, always protonated — permanent cation for phosphate binding
• Aromatic side chains (Trp/Tyr/Phe): hydrophobic cores, 280 nm absorbance, stacking with nucleobases

Reactivity Cross-Checks (common discriminations)

• Which linkages does mild base (NaOH) cleave? Esters (fats saponify). NOT acetals (glycosides) — those need acid/enzymes. Amides resist both without heat
• Which side chains can be covalently modified? Nucleophilic ones: Cys-SH, Ser-OH, Lys-$NH_2$ (imine/amide formation), His-imidazole
• Reducing vs non-reducing sugar: free hemiacetal end vs all-acetal (sucrose is non-reducing — both anomeric carbons locked in the glycosidic bond)
• Oxidative stress passages: aldehydes (reactive electrophiles, form Schiff bases with Lys), disulfide shuffling, lipid peroxidation of cis alkenes

Passage Strategy: the Group-Scan Method

When a passage introduces an unfamiliar molecule (a drug, toxin, metabolite):

1. Scan and label every functional group in the structure
2. Ionization state at pH 7.4 (acids anionic, amines cationic)
3. Electrophiles vs nucleophiles: carbonyls, Michael acceptors, alkyl halides = electrophilic; thiols, amines, alkoxides = nucleophilic
4. Hydrolyzable linkages (ester > amide; acetal in acid) — predicts metabolism and prodrug activation
5. Then read the questions — most answers are now pre-computed

Biomolecule Integration 🎯

Key Takeaways — Part 4

• Biological linkages ARE functional groups: glycosidic = acetal, peptide = amide, fat = ester, nucleic backbone = phosphodiester; polymerize by condensation, digest by hydrolysis
• Cleavage matching: base saponifies esters only; acid/enzymes open acetals; amides resist both (proteases required)
• Side-chain toolkit: Cys thiol (soft Nu, disulfides), Ser/Thr/Tyr OH (phosphorylation), His imidazole (proton shuttle), Arg guanidinium (permanent delocalized cation), Trp/Tyr (280 nm)
• Reducing sugar = free hemiacetal; sucrose is the classic non-reducing exception
• Group-scan method: label groups → ionization at 7.4 → sort electrophiles/nucleophiles → find hydrolyzable bonds → then answer

Worked Examples — Passage Integration

Question: A passage shows aspirin: a benzene ring bearing a COOH and an O-acetyl ester. Predict its ionization at pH 7.4, its labile bond, and its acetylation chemistry.

Solution:

1. Scan: aromatic ring, carboxylic acid (pKa ~3.5), aryl ester
2. Ionization: pH 7.4 is ~4 units above the pKa → carboxylate anion dominates (ratio ~$10^4$:1)
3. Labile bond: the ester — hydrolyzed by esterases and slowly by water (aged aspirin smells of acetic acid)
4. Mechanism of action: the ester acetylates a serine OH of COX (transesterification — the serine alkoxide/OH attacks the ester carbonyl); irreversible inhibition

MCAT Strategy: The group-scan answered three exam questions before they were asked. Run it on every structure the passage draws.

Question: A meal contains starch, triacylglycerols, and protein. Match each to its linkage type, cleavage catalyst, and cleavage mechanism class.

Solution:

1. Starch: glycosidic acetals → amylases; enzymatic acetal hydrolysis (oxocarbenium-like transition state, retaining/inverting mechanisms)
2. Fat: esters → lipases; nucleophilic acyl substitution by water at the ester carbonyl
3. Protein: amides → proteases (pepsin, trypsin, chymotrypsin); acyl substitution through a tetrahedral intermediate, needing covalent (Ser) or acid-base (Asp) catalysis because amides are so stabilized
4. Common thread: all three digestions are hydrolyses of carbonyl- or acetal-family linkages

MCAT Strategy: Digestion biochemistry is Parts 1-2 of carbonyl chemistry wearing enzyme names. Identify the linkage first; the mechanism class follows automatically.

Question: A protein runs as one 100 kDa band on non-reducing SDS-PAGE but as two bands (60 + 40 kDa) after 2-mercaptoethanol treatment. Interpret in functional-group terms.

Solution:

1. 2-Mercaptoethanol is a small thiol: it reduces disulfide bonds by thiol-disulfide exchange (its SH attacks the S-S)
2. Result: the 100 kDa species was two polypeptides (60 + 40) covalently linked by at least one interchain Cys-Cys disulfide
3. SDS alone breaks noncovalent contacts only — covalent S-S survives without the reductant
4. Extracellular proteins favor disulfides (oxidizing environment); the cytosol's glutathione pool keeps thiols reduced

MCAT Strategy: "Band pattern changes with/without reducing agent" always means interchain disulfides. Count subunits from the reduced lane.

Question: You are given three unlabeled biomolecule solutions: a reducing sugar, a fatty acid salt, and a small peptide. Using only pH paper, Benedict's reagent, and dilute HCl, assign them.

Solution:

1. Benedict's (heat): only the reducing sugar's free hemiacetal opens to an aldehyde and reduces $Cu^{2+}$ → brick-red precipitate
2. pH paper: the fatty acid salt solution is basic (carboxylate is the conjugate base of a weak acid); confirm by adding HCl — the free fatty acid precipitates/clouds as an insoluble film (protonated, 4-5 carbons-per-group rule fails for a C16 chain)
3. The peptide: negative Benedict's, near-neutral zwitterionic solution, no precipitate with mild acid
4. Each assignment used one functional-group property: hemiacetal redox, carboxylate ionization/solubility, amide inertness

MCAT Strategy: Bench-test questions are property tables in disguise. For each candidate, ask which single group would react with the given reagent — the odd one out is your answer.