8. Aldehydes, Ketones and Carboxylic Acids

What you will learn

• Name carbonyl compounds and draw their local geometry.
• Choose preparation routes at the intended oxidation level.
• Organise carbonyl reactions into addition, reduction, oxidation and alpha-carbon chemistry.
• Connect carboxylate resonance with acidity.
• Track carbons through acid preparation and decarboxylation.
• Use paired 2D/3D models while keeping reaction conditions and identification-test limits explicit.

Big idea: carbonyl carbon is a useful meeting point between structure, polarity and reaction planning.

A planar C=O group offers a predictable site for bond formation. Changing its neighbours changes whether the compound behaves as an aldehyde, ketone or carboxylic acid. Changing its oxidation level opens another set of routes without necessarily changing the skeleton.

Keep a carbon map beside complicated reactions. Mark the original carbonyl carbon, any new carbon introduced, and any carbon removed. This small habit prevents many plausible-looking but incorrect product drawings.

The carbonyl carbon is a structural landmark: it is planar, bonded to oxygen, and commonly electron-poor.

An aldehyde has at least one hydrogen attached to its carbonyl carbon. A ketone has two carbon groups attached there. A ring elsewhere in the molecule does not change this definition. The group attached to the carbonyl carbon, not the total molecular formula alone, decides the class.

A simple aldehyde parent uses -al. Its carbonyl carbon belongs to the parent chain and takes the terminal position. A ketone uses -one and a locant where required. When an aldehyde group is attached to a ring and its carbon cannot be included in that ring parent, a carbaldehyde name may be appropriate. Recognise common names, but practise returning to a clear structural formula.

The carbonyl carbon is approximately trigonal planar. The sigma framework and pi bond make rotation around C=O unlike rotation around an ordinary single bond. Oxygen draws electron density towards itself, so a nucleophile is attracted to the carbon end and the oxygen can accept a proton or bind an electrophile.

Identify the class of $\mathrm{CH_3CH_2COCH_2CH_3}$.

Locate C=O. The carbonyl carbon is attached to a carbon group on each side, not to hydrogen.

Result: A ketone, pentan-3-one. Writing the carbonyl near the middle of a chain is a clue, but the attached groups provide the definition.

1. What local geometry is typical around carbonyl carbon?
2. Which carbonyl end is attacked by a nucleophile in the ordinary addition picture?

Answers

1. Approximately trigonal planar.
2. The electron-poor carbon end.

A preparation route must choose both the carbon skeleton and the correct oxidation level.

Controlled oxidation or dehydrogenation of a primary alcohol can give an aldehyde. A secondary alcohol gives a ketone through the corresponding simple oxidation. Strong conditions may overoxidise an aldehyde further, so naming an oxidant without its stopping conditions can be insufficient.

$$\mathrm{CH_{3}CH_{2}CH_{2}OH} \xrightarrow{\mathrm{Cu},\Delta} \mathrm{CH_{3}CH_{2}CHO} + \mathrm{H_{2}}$$

$$\mathrm{CH_{3}CH(OH)CH_{2}CH_{3}} \xrightarrow{\mathrm{Cu},\Delta} \mathrm{CH_{3}COCH_{2}CH_{3}} + \mathrm{H_{2}}$$

Oxidative cleavage of an alkene can create carbonyl fragments. Under reductive ozonolysis work-up, assign oxygen to each former double-bond carbon and preserve the carbon groups already attached there. A carbon that carried hydrogen can give an aldehyde; one carrying two carbon groups can give a ketone. Different work-up conditions can change the final oxidation level.

Rosenmund reduction converts an acyl chloride to an aldehyde using a moderated palladium catalyst, limiting further reduction. Selected nitrile reduction/hydrolysis routes also give aldehydes. Controlled hydride reagents can reduce suitable esters or nitriles to that level under their specific conditions; do not assume a reagent that normally gives an alcohol will stop at an aldehyde.

$$\mathrm{CH_{3}CH_{2}COCl} + \mathrm{H_{2}} \xrightarrow{\mathrm{Pd/BaSO_4}} \mathrm{CH_{3}CH_{2}CHO} + \mathrm{HCl}$$

Selected side-chain oxidation methods convert an aromatic methyl group to a formyl group under controlled conditions. Strong ordinary benzylic oxidation may instead continue to the carboxylic acid. The reagent and work-up define the target level.

An acyl chloride can react with a suitable organocadmium reagent to give a ketone. A nitrile can react with a Grignard reagent and then hydrolyse to a ketone. These routes introduce a carbon group at a carbonyl-derived centre. Strongly nucleophilic reagents can continue reacting with an already formed carbonyl, so reagent selectivity matters.

Friedel–Crafts acylation introduces an acyl group onto a suitable aromatic ring using a Lewis-acid system followed by work-up. It gives an aryl ketone. Ring deactivation and other functional groups can limit its scope.

$$\mathrm{C_{6}H_{6}} + \mathrm{CH_{3}CH_{2}COCl} \xrightarrow{\mathrm{AlCl_3,\ then\ workup}} \mathrm{C_{6}H_{5}COCH_{2}CH_{3}} + \mathrm{HCl}$$

Predict the reductive-ozonolysis fragments of pent-2-ene.

Split the carbon–carbon double bond. One carbon retains a methyl group and hydrogen; the other retains an ethyl group and hydrogen. Each becomes a carbonyl carbon.

Result: Ethanal and propanal. Count the carbons in both fragments to check that the original skeleton has been accounted for.

1. Which alcohol class gives a ketone by ordinary simple carbonyl oxidation?
2. Why does an ozonolysis question need its work-up stated?

Answers

1. Secondary alcohols.
2. The work-up controls whether initially formed carbonyl fragments remain or undergo further oxidation.

Carbonyl compounds are polar, but they do not hydrogen-bond to identical molecules in the same donor–acceptor pattern as alcohols.

The carbonyl dipole strengthens attractions relative to suitable nonpolar molecules. A carbonyl oxygen can accept hydrogen bonds from water. An ordinary aldehyde or ketone without another donor group has no O–H hydrogen to donate to an identical molecule. Its boiling point therefore often lies above a comparable hydrocarbon but below a comparable hydrogen-bonding alcohol.

Small aldehydes and ketones can mix well with water because of favourable oxygen–water interactions. Increasing the nonpolar carbon portion commonly reduces water solubility. A carbonyl group is not a guarantee that a large molecule is water-soluble.

Compare compounds fairly. Molecular mass, branching, shape and other functional groups can change the result. Boiling point depends on escaping from a liquid; melting point also depends strongly on crystal packing. Neither property is decided by the carbonyl class name alone.

Compare the main intermolecular possibilities for an alkane, a ketone and an alcohol of broadly comparable size.

The alkane mainly contributes dispersion. The ketone adds a strong carbonyl dipole and can accept hydrogen bonds from suitable partners. The alcohol can also donate through O–H.

Result: The alcohol often has the strongest self-associated hydrogen-bonding network; the ketone commonly lies between the other two in boiling point, with structure-dependent exceptions.

In an exam explanation, make the link explicit: identify the interaction, explain how structure enables it, then connect that interaction to the property. Naming “hydrogen bonding” without identifying a donor and acceptor is not a complete explanation.

1. Can propanone accept a hydrogen bond from water?
2. Why can a longer-chain ketone be poorly water-soluble despite its polar carbonyl?

Answers

1. Yes, at its carbonyl oxygen.
2. Its growing nonpolar portion can dominate the balance of mixing interactions.

Most carbonyl reactions begin by asking whether the C=O group is being added to, reduced, oxidised or used to activate a neighbouring carbon.

A nucleophile attacks carbonyl carbon, shifting the pi electron pair towards oxygen. Proton transfer then gives the addition product appropriate to the conditions. Steric crowding and electron donation from attached carbon groups often make ketones less reactive than comparable aldehydes. Conjugation with an aromatic ring can also alter electrophilicity.

$$\mathrm{CH_{3}CHO} + \mathrm{HCN} \longrightarrow \mathrm{CH_{3}CH(OH)CN}$$

Bisulfite can form addition compounds with suitable aldehydes and some ketones. Alcohol addition under acid can give a hemiacetal and then an acetal with another alcohol molecule. Nitrogen nucleophiles can give condensation derivatives after addition and loss of water.

Semicarbazide supplies another condensation derivative, a semicarbazone with the characteristic $\mathrm{C=NNHCONH_2}$ connection. Check which nitrogen forms the carbonyl-derived double bond.

$$\mathrm{CH_{3}CHO} + 2 \mathrm{CH_{3}OH} \xrightarrow{\mathrm{H^+}} \mathrm{CH_{3}CH(OCH_{3})_{2}} + \mathrm{H_{2}O}$$

$$\mathrm{CH_{3}CHO} + \mathrm{CH_{3}NH_{2}} \longrightarrow \mathrm{CH_{3}CH=NCH_{3}} + \mathrm{H_{2}O}$$

Reduction of C=O to an alcohol retains the oxygen. A different reduction that replaces the carbonyl oxygen by two hydrogens gives a methylene group. Clemmensen conditions use an acidic metal system; Wolff–Kishner conditions pass through a hydrazone in a basic heated system. Choose conditions compatible with the other groups present.

$$\mathrm{CH_{3}CH_{2}CHO} + \mathrm{H_{2}} \xrightarrow{\mathrm{catalyst}} \mathrm{CH_{3}CH_{2}CH_{2}OH}$$

Aldehydes commonly oxidise to carboxylic acids without breaking the carbon skeleton. Ordinary ketones resist the mild aldehyde oxidants; stronger conditions may cleave carbon–carbon bonds. A Tollens-type reagent can give silver with suitable reducing compounds, so the observation is useful but not unique proof of one exact aldehyde structure. Fehling-type tests are especially useful for many aliphatic aldehydes; aromatic aldehydes and special ketones require care in interpreting simple rules.

$$2 \mathrm{CH_{3}CHO} + \mathrm{O_{2}} \longrightarrow 2 \mathrm{CH_{3}COOH}$$

The haloform reaction identifies the methyl-ketone pattern, or a precursor that oxidises to it under the test conditions. The methyl group adjacent to the carbonyl becomes the haloform carbon. Iodoform gives a characteristic yellow solid; not every ketone has the required pattern.

Tollens reagent is an ammoniacal silver(I) system and gives metallic silver with suitable reducing substrates. Fehling-type reagent contains complexed copper(II) in alkaline solution and can give copper(I) oxide with a positive ordinary aldehyde test. Ethanol and suitable methyl-carbinol alcohols can also respond to iodoform conditions after oxidation, so the test is not restricted to molecules that began as ketones.

$$\mathrm{CH_{3}COCH_{2}CH_{3}} + 3 \mathrm{I_{2}} + 4 \mathrm{NaOH} \longrightarrow \mathrm{CH_{3}CH_{2}COONa} + \mathrm{CHI_{3}} + 3 \mathrm{NaI} + 3 \mathrm{H_{2}O}$$

A hydrogen on the carbon next to C=O can be removed in an appropriate acid/base pathway because the resulting intermediate is stabilised by the carbonyl system. Aldol chemistry forms a new carbon–carbon bond between an enol/enolate-derived carbon and another carbonyl carbon.

$$2 \mathrm{CH_{3}CHO} \xrightarrow{\mathrm{dilute\ base}} \mathrm{CH_{3}CH(OH)CH_{2}CHO}$$

$$\mathrm{CH_{3}CH(OH)CH_{2}CHO} \xrightarrow{\Delta} \mathrm{CH_{3}CH=CHCHO} + \mathrm{H_{2}O}$$

Two different enolisable carbonyl reactants can give several crossed-aldol combinations. Selectivity improves when the reactants and conditions restrict which partner acts as the nucleophile and which as the electrophile. Follow the new C–C bond and retain the alpha-hydrogen requirement rather than memorising a mixture count without its assumptions.

Aldehydes without an alpha hydrogen can undergo Cannizzaro disproportionation in a suitable concentrated-base setting. One molecule is oxidised while another is reduced. The carbonyl group on an aromatic ring also affects electrophilic substitution: aldehyde and acyl substituents commonly deactivate the ring and direct meta.

$$2 \mathrm{C_{6}H_{5}CHO} + \mathrm{NaOH} \longrightarrow \mathrm{C_{6}H_{5}COONa} + \mathrm{C_{6}H_{5}CH_{2}OH}$$

A compound is a ketone but fails the iodoform test. Does that contradict its classification?

The test needs a specific methyl-ketone arrangement or a suitable precursor. A ketone can have other carbon groups on both sides of C=O.

Result: No. Functional-group class is broader than the structural pattern tested by iodoform.

1. What structural feature permits the ordinary aldol pathway?
2. Why can benzaldehyde take part in Cannizzaro chemistry?
3. Does Clemmensen reduction retain carbonyl oxygen in the organic product?

Answers

1. At least one participating carbonyl reactant must supply a suitable alpha hydrogen/enol or enolate pathway.
2. Its carbonyl has no alpha-hydrogen-bearing carbon attached.
3. No. It converts the carbonyl group to a methylene group under the applicable conditions.

Carbonyl compounds are useful because their polarity and reactivity can be selected separately for different tasks.

Small ketones are common industrial solvents. Aldehydes and ketones also serve as intermediates for making larger molecules, polymer precursors and other functional groups. The carbonyl group provides a predictable place to form bonds or change oxidation level.

Use examples to explain a property, not to imply universal suitability. A solvent’s ability to dissolve a material depends on the complete molecule and the mixture. Its reactivity, volatility and exposure hazards can limit a use even when its solvency is good.

Why might a carbonyl compound be useful as a synthesis intermediate even when it is not the final desired functional group?

Nucleophilic addition can make a new bond at the carbonyl carbon. Reduction or oxidation then changes the functional group while retaining a planned carbon skeleton.

Result: The group is a controllable reaction handle, not merely an endpoint.

The next sections turn to carboxylic acids. They still contain a carbonyl, but the neighbouring hydroxyl group changes both the dominant reactions and acid–base behaviour.

1. Give one reaction type that makes a carbonyl useful as a synthesis handle.
2. Why is the whole molecular structure relevant to solvent choice?

Answers

1. Nucleophilic addition, controlled oxidation/reduction, or an alpha-carbon bond-forming route.
2. It determines interactions, volatility, reactivity and other practical properties.

A carboxyl group combines carbonyl and hydroxyl connections into one delocalised functional group.

A simple chain carboxylic acid uses an -oic acid ending. The carboxyl carbon belongs to the parent chain. Two carboxyl groups can require a -dioic acid description. When the carbonyl carbon lies outside a selected ring parent, a carboxylic acid name may be used. Established common names remain important, but always connect them with a structure.

The carboxyl carbon is approximately trigonal planar. The carboxylate conjugate base distributes negative charge over its two oxygens in a resonance description. A single drawing with one C=O and one C–O⁻ is a contributor, not evidence that a free carboxylate permanently selects one oxygen for the entire charge.

Count the parent-chain carbons in CH₃CH₂COOH.

Include the carbon inside the carboxyl group, not only the hydrocarbon fragment to its left.

Result: 3 carbons; the acid is propanoic acid.

1. Are the two C–O bonds in a free carboxylate described fully by one localised single/double drawing?
2. What geometry is typical around its carboxyl carbon?

Answers

1. No. Equivalent resonance contributions distribute bonding and charge.
2. Approximately trigonal planar.

Acid preparation often changes oxidation level without changing carbon count, but nitrile and carbon-dioxide routes can build a longer skeleton.

Oxidation of a primary alcohol or aldehyde can give a carboxylic acid. Follow the original functional carbon into the carboxyl group. Suitable alkylbenzene side chains with a benzylic hydrogen can oxidise to a ring carboxyl group under strong conditions; the requirement for an accessible benzylic hydrogen matters.

Nitrile hydrolysis turns the nitrile carbon into the carboxyl carbon. If a nitrile was first made by replacing a haloalkane group with cyanide, the route has introduced an additional carbon. Amide hydrolysis also gives a carboxylic acid or its salt, depending on medium, with the appropriate nitrogen-containing product.

$$\mathrm{CH_{3}CH_{2}CN} + 2 \mathrm{H_{2}O} + \mathrm{H}^{+} \longrightarrow \mathrm{CH_{3}CH_{2}COOH} + \mathrm{NH_{4}}^{+}$$

A Grignard reagent can react with carbon dioxide. Acidic work-up gives a carboxylic acid containing the added carbon from carbon dioxide. The following balanced overall representation summarises hydrolytic conversion of the magnesium intermediate; the detailed work-up can use an acidic aqueous medium.

$$\mathrm{CH_{3}CH_{2}MgBr} + \mathrm{CO_{2}} + \mathrm{H_{2}O} \longrightarrow \mathrm{CH_{3}CH_{2}COOH} + \mathrm{MgOHBr}$$

Hydrolysis of acyl halides or acid anhydrides provides acids. Ester hydrolysis also supplies an acid in an acidic work-up; alkaline hydrolysis first gives a carboxylate salt. The medium determines which protonation state is present before isolation.

$$\mathrm{CH_{3}CH_{2}COCl} + \mathrm{H_{2}O} \longrightarrow \mathrm{CH_{3}CH_{2}COOH} + \mathrm{HCl}$$

$$\mathrm{CH_{3}COOCH_{2}CH_{3}} + \mathrm{H_{2}O} \xrightarrow{\mathrm{H^+}} \mathrm{CH_{3}COOH} + \mathrm{CH_{3}CH_{2}OH}$$

Plan a route from a two-carbon alkyl halide to a three-carbon carboxylic acid.

Use cyanide substitution to add the nitrile carbon, then hydrolyse the nitrile. Check both carbon count and substitution suitability of the starting halide.

Result: A suitable ethyl halide can give propanenitrile, then propanoic acid. The extra carbon comes from cyanide, not from water.

1. Where does the new carbon come from in Grignard carboxylation?
2. Does primary-alcohol-to-acid oxidation normally add a new carbon?

Answers

1. Carbon dioxide.
2. No. The functional carbon is retained at a higher oxidation level.

Carboxylic acids combine a polar group with both hydrogen-bond donation and acceptance, but their carbon skeleton still matters.

Carboxylic acids can form strongly associated hydrogen-bonded structures, including dimers in suitable environments. This association helps explain their relatively high boiling points compared with many similarly sized compounds. The dimer picture is context-dependent; water and ionisation change the surrounding interaction network.

Small acids can mix readily with water. A longer nonpolar carbon chain commonly reduces water solubility. Their salts can behave very differently because charged carboxylate groups interact strongly with water, although a sufficiently large nonpolar portion can still matter.

Why can a neutral acid and its sodium salt show very different water solubility?

The neutral molecule relies on molecular interactions and may have a large nonpolar portion. The salt introduces an ionic carboxylate group and its associated hydration.

Result: Changing protonation state changes the relevant interactions; the same carbon skeleton does not imply the same solubility.

Use the words association and dissociation carefully. Hydrogen-bonded dimerisation reduces the number of independent neutral particles in an appropriate solvent. Acid ionisation produces ionic species in an appropriate medium. Either can alter apparent molar mass in a colligative measurement, but in opposite idealised directions.

1. Why do carboxylic acids often have high boiling points for their size?
2. How can solvent change an acid’s colligative behaviour?

Answers

1. Strong intermolecular association, including hydrogen bonding, raises the energy needed to separate molecules.
2. It can favour neutral association or ionisation, changing the effective particle count.

Carboxylic acid chemistry separates into proton transfer, substitution at the acyl centre, and reactions that remove or modify the carbon skeleton.

Acid strength compares the equilibrium tendency to donate a proton in a stated solvent. Carboxylate stabilisation helps make carboxylic acids stronger than typical alcohols and phenols. Electron-withdrawing groups commonly increase acidity by stabilising the conjugate base; their effect usually weakens with distance along a saturated chain.

$$K_a=\frac{a_{\rm H^+}a_{\rm A^-}}{a_{\rm HA}},\qquad pK_a=-\log_{10}K_a$$

Compare model acids with $K_a=2.0\times10^{-4}$ and $2.0\times10^{-5}$.

The larger constant favours ionisation more strongly under the same convention. The logarithmic scale runs in the opposite numerical direction.

Result: Their $pK_a$ values are 3.699 and 4.699. The first has a 10-fold larger $K_a$.

Carboxylic acids react with bases to give salts. Carbonate or bicarbonate reactions also release carbon dioxide, providing a useful distinction from much weaker phenols under the usual test conditions.

$$\mathrm{CH_{3}CH_{2}COOH} + \mathrm{NaHCO_{3}} \longrightarrow \mathrm{CH_{3}CH_{2}COONa} + \mathrm{CO_{2}} + \mathrm{H_{2}O}$$

Acyl substitution replaces the hydroxyl-derived group at the carboxyl carbon. Appropriate dehydrating conditions can form an anhydride; alcohols give esters under suitable catalysis. Reaction with ammonia first gives an ammonium carboxylate; heating under suitable conditions can then give an amide.

$$2 \mathrm{CH_{3}COOH} \xrightarrow{\mathrm{dehydrating\ conditions}} \mathrm{(CH_{3}CO)_{2}O} + \mathrm{H_{2}O}$$

$$\mathrm{CH_{3}COONH_{4}} \xrightarrow{\Delta} \mathrm{CH_{3}CONH_{2}} + \mathrm{H_{2}O}$$

Phosphorus chlorides and thionyl chloride can convert an acid into an acyl chloride. Read each reagent formula accurately: phosphorus pentachloride and phosphorus trichloride have different stoichiometries. An equation must conserve both atoms and charge.

$$\mathrm{CH_{3}COOH} + \mathrm{PCl_{5}} \longrightarrow \mathrm{CH_{3}COCl} + \mathrm{POCl_{3}} + \mathrm{HCl}$$

$$3 \mathrm{CH_{3}COOH} + \mathrm{PCl_{3}} \longrightarrow 3 \mathrm{CH_{3}COCl} + \mathrm{H_{3}PO_{3}}$$

$$\mathrm{CH_{3}COOH} + \mathrm{SOCl_{2}} \longrightarrow \mathrm{CH_{3}COCl} + \mathrm{SO_{2}} + \mathrm{HCl}$$

A sufficiently powerful reducing system such as lithium aluminium hydride can convert a carboxylic acid to a primary alcohol after work-up. This is different from sodium borohydride’s usual mild carbonyl scope. Decarboxylation removes a carboxyl-derived carbon as carbon dioxide or a carbonate product, depending on the process.

$$\mathrm{CH_{3}CH_{2}COONa} + \mathrm{NaOH} \xrightarrow{\mathrm{soda\ lime},\Delta} \mathrm{CH_{3}CH_{3}} + \mathrm{Na_{2}CO_{3}}$$

Kolbe electrolysis oxidises carboxylate ions and can couple the organic fragments after loss of carbon dioxide. The carbon skeleton of the organic product therefore depends on the fragment remaining after the carboxyl carbon is removed.

$$2 \mathrm{CH_{3}COO}^{-} \longrightarrow \mathrm{CH_{3}CH_{3}} + 2 \mathrm{CO_{2}} + 2 e^{-}$$

The Hell–Volhard–Zelinsky transformation halogenates an appropriate alpha position of a carboxylic acid using its specific halogen/phosphorus reagent system. An alpha hydrogen is required for the usual route. An aromatic carboxyl group generally deactivates the ring and directs electrophilic substitution towards meta positions.

$$\mathrm{CH_{3}CH_{2}COOH} + \mathrm{Br_{2}} \xrightarrow{\mathrm{P,\ then\ workup}} \mathrm{CH_{3}CHBrCOOH} + \mathrm{HBr}$$

Why does sodium propanoate give a two-carbon hydrocarbon in the shown soda-lime route?

Identify the carboxyl carbon as the carbon transferred into the carbonate product. The remaining ethyl fragment gains hydrogen.

Result: The organic skeleton loses the carboxyl carbon. Ethane, not propane, is the hydrocarbon product.

1. Which has stronger acidity in the same convention: a larger or smaller pKa?
2. What acid structural feature is needed for the usual alpha-halogenation route?
3. Does making an ester necessarily mean the carboxyl carbon disappears?

Answers

1. A smaller pKa.
2. An appropriate alpha hydrogen.
3. No. Esterification retains the acyl carbon skeleton.

Carboxylic acids are useful both as final materials and as starting points for an entire family of acyl derivatives.

Carboxylic acids and their derivatives contribute to polymers, coatings, chemical processing and many synthesis routes. A compound with two suitable functional groups can link into a larger structure, while a monofunctional acid may act as an end group, solvent component or precursor.

Converting an acid into an acyl chloride, ester or amide changes reactivity without necessarily changing its carbon skeleton. That makes acid derivatives a planning network. Choose the desired reactivity and compatibility rather than treating every derivative as an equivalent bottle of the same functional group.

A synthesis needs to attach an acyl group to a weak nucleophile. Why might an acyl chloride be considered instead of the unactivated acid?

A more reactive acyl derivative can undergo substitution more readily under appropriate conditions. The actual reagent choice must still be compatible with the other groups and the work-up.

Result: Activate the acyl centre deliberately; the acid is not automatically the most effective reagent for every acylation.

A complete uses answer links structure to function. For example, a difunctional monomer supports chain building, while an acyl derivative supports a selected substitution. Avoid treating industrial importance as evidence that every use or exposure is benign.

1. Why are difunctional acid-derived monomers useful in polymer chemistry?
2. What should guide selection of an acid derivative in synthesis?

Answers

1. They can connect at more than one site, allowing chain formation with suitable reaction partners.
2. The required reactivity, selectivity and compatibility with other functional groups.

Try these original questions without looking at the answers. Show equations, conditions and units when relevant.

8.1 Name $\mathrm{CH_3CH_2COCH_2CH_3}$ and classify it.

8.2 Draw a labelled planar carbonyl centre and explain the polarity.

8.3 Predict reductive-ozonolysis fragments of hex-3-ene.

8.4 What transformation defines Rosenmund reduction?

8.5 Predict the alcohol class after reducing an ordinary acyclic ketone.

8.6 Why is a ketone often less reactive in nucleophilic addition than a comparable aldehyde?

8.7 What structural change follows cyanohydrin formation?

8.8 Distinguish an oxime from a hydrazone by their nitrogen-derived groups.

8.9 When might acidic versus basic deoxygenation conditions matter?

8.10 Does every ketone give an iodoform test? Explain.

8.11 Predict the aldol addition product of two ethanal molecules.

8.12 Why can two different enolisable aldehydes give a complicated crossed-aldol mixture?

8.13 Why does benzaldehyde undergo Cannizzaro rather than the ordinary self-aldol pathway?

8.14 Plan a one-carbon extension from an ethyl Grignard reagent to an acid.

8.15 Explain why carboxylate is stabilised relative to an ordinary alkoxide.

8.16 Calculate pKa for a model acid with $K_a=5.0\times10^{-5}$.

8.17 Explain the bicarbonate distinction between an ordinary carboxylic acid and phenol.

8.18 Give the carbon-count outcome of soda-lime decarboxylation of sodium butanoate.

8.19 State the structural requirement for the usual Hell–Volhard–Zelinsky alpha-halogenation.

8.20 Plan how to distinguish an aldehyde, an ordinary ketone and a carboxylic acid without treating one test as unique proof.

Answers

8.1 Pentan-3-one; ketone.

8.2 Use a trigonal-planar carbon with C=O and two further groups. Oxygen attracts electron density; carbon is the ordinary electrophilic end.

8.3 Two propanal molecules, because each double-bond carbon retains an ethyl group and hydrogen.

8.4 Controlled reduction of an acyl chloride to an aldehyde with an appropriately moderated catalyst.

8.5 Secondary alcohol.

8.6 It commonly has greater steric crowding and greater electron donation from its two carbon substituents.

8.7 The carbonyl carbon becomes tetrahedral and gains both hydroxyl and a cyanide-derived carbon connection.

8.8 An oxime has $\mathrm{C=NOH}$; a hydrazone has $\mathrm{C=NNH_2}$ in the simplest derivative.

8.9 Other functional groups can be unstable in one medium; Clemmensen and Wolff–Kishner conditions differ even when they target the same carbonyl-to-methylene change.

8.10 No. The required methyl-ketone pattern or an oxidisable precursor is needed.

8.11 $\mathrm{CH_3CH(OH)CH_2CHO}$.

8.12 Either can form the nucleophilic partner and either can act as the carbonyl electrophile.

8.13 It lacks a suitable alpha hydrogen and can undergo disproportionation in the appropriate concentrated-base conditions.

8.14 React with carbon dioxide, then use appropriate acidic work-up to obtain propanoic acid.

8.15 Its negative charge and bonding are delocalised over two oxygens in the resonance description.

8.16 $4.301$.

8.17 The carboxylic acid is strong enough under the usual conditions to release carbon dioxide from bicarbonate; phenol normally is not.

8.18 A three-carbon hydrocarbon, propane; the carboxyl carbon is removed from the organic skeleton.

8.19 An appropriate alpha hydrogen on the acid substrate.

8.20 Combine acidity/bicarbonate behaviour, a carbonyl-derivative test, a suitable aldehyde oxidation test and structural information. Interpret known exceptions and use the complete evidence rather than one colour change.

Quick-check answers appear directly after each topic. The selected explanations below show how to check the reasoning, rather than memorise a final line.

Calculate pKa for a model acid with $K_a=5.0\times10^{-5}$.

$4.301$.

Explain the bicarbonate distinction between an ordinary carboxylic acid and phenol.

The carboxylic acid is strong enough under the usual conditions to release carbon dioxide from bicarbonate; phenol normally is not.

Give the carbon-count outcome of soda-lime decarboxylation of sodium butanoate.

A three-carbon hydrocarbon, propane; the carboxyl carbon is removed from the organic skeleton.

State the structural requirement for the usual Hell–Volhard–Zelinsky alpha-halogenation.

An appropriate alpha hydrogen on the acid substrate.

Plan how to distinguish an aldehyde, an ordinary ketone and a carboxylic acid without treating one test as unique proof.

Combine acidity/bicarbonate behaviour, a carbonyl-derivative test, a suitable aldehyde oxidation test and structural information. Interpret known exceptions and use the complete evidence rather than one colour change.