Aldehydes, Ketones and Carboxylic Acids

Meaning

Aldehydes and ketones are organic compounds that contain a carbonyl group (a carbon-oxygen double bond, C=O). Because this carbon atom is attached to other carbon or hydrogen atoms, it forms the backbone of many biological and synthetic molecules. When that carbonyl carbon is attached to at least one hydrogen atom, we call it an aldehyde. When it is attached to two carbon-containing groups, we call it a ketone.

Definitions

1. "Aldehydes are a class of organic compounds in which a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom, and a single bond with another hydrogen atom or a carbon group." — Morrison and Boyd
2. "Ketones are organic compounds characterized by the presence of a carbonyl group bonded to two carbon atoms." — Solomons and Fryhle

Simple & Clear Explanation with Key Points

Think of the carbonyl group as a polarized magnet. Oxygen is much more electronegative than carbon. This means oxygen pulls the shared electrons of the double bond toward itself.

• As a result, oxygen develops a partial negative charge.
• Carbon develops a partial positive charge.
• Because the carbon is electron-deficient (partially positive), it acts as an easy target for anything that is electron-rich. That electron-rich species is called a nucleophile.
• This single fact explains almost all chemical reactions of aldehydes and ketones: Nucleophilic Addition.

Examples

• Formaldehyde (Methanal): Used in water solution to preserve biological specimens.
• Acetone (Propanone): Used widely as a nail polish remover and industrial solvent.

Real-Life Uses

• Fragrances and Flavors: Cinnamaldehyde gives cinnamon its distinct smell, and vanillin gives vanilla its aroma. Perfumery relies heavily on these aromatic aldehydes.
• Medicine and Steroids: Many medicines, hormones (like progesterone and testosterone), and anti-inflammatory drugs contain ketone groups which are essential for binding to biological receptors in our bodies.

Common Misconceptions and Their Corrections

• Misconception: Students often think aldehydes and ketones are the same because they both have a C=O group.
• Correction: Look at the neighbors of the carbonyl carbon! If there is at least one hydrogen attached directly to the C=O carbon, it is an aldehyde. If there are two carbon groups attached, it is a ketone. This small difference makes aldehydes much more easily oxidized than ketones.

Meaning

Nomenclature is simply the systematic naming of chemical compounds according to IUPAC rules, while structure refers to the 3D geometry of the molecule around the carbonyl carbon.

Definitions

1. "IUPAC nomenclature is a systematic method of naming chemical compounds as recommended by the International Union of Pure and Applied Chemistry." — IUPAC Gold Book
2. "The geometry around the carbonyl carbon is planar trigonal with bond angles of approximately 120 degrees due to sp2 hybridization." — Jerry March

Simple & Clear Explanation with Key Points

• Naming Aldehydes: Replace the suffix "-e" of the corresponding alkane with "-al". For example, methane becomes methanal. The carbonyl carbon is always carbon number 1.
• Naming Ketones: Replace the suffix "-e" of the corresponding alkane with "-one". For example, propane becomes propanone. Number the chain from the end closest to the carbonyl group.
• Structure: The carbon atom uses sp2 hybrid orbitals to form three sigma bonds (one to oxygen, two to other atoms). The unhybridized p-orbital overlaps with an oxygen p-orbital to form the pi bond of the carbonyl group.

Examples

• Ethanal (acetaldehyde): CH3CHO
• Butan-2-one: CH3-C(=O)-CH2-CH3

Real-Life Uses

• Industrial Synthesis: Solvents like methyl ethyl ketone (MEK) are used in surface coatings, lacquers, and printing inks because of their specific boiling points and dissolving power.

Common Misconceptions and Their Corrections

• Misconception: When numbering a chain containing an aldehyde, students sometimes start numbering from the methyl end farthest from the functional group.
• Correction: The functional group takes top priority. The carbonyl carbon of an aldehyde is always Carbon 1.

Meaning

Preparation methods are the chemical reactions used in laboratories and industries to synthesize aldehydes and ketones from simpler starting materials like alcohols, hydrocarbons, and carboxylic acid derivatives.

Definitions

1. "Oxidation of primary alcohols yields aldehydes, which can be further oxidized to carboxylic acids under harsh conditions." — Wade and Simek
2. "Rosenmund reduction is the catalytic hydrogenation of acyl chlorides over poisoned palladium to form aldehydes." — Organic Syntheses

Simple & Clear Explanation with Key Points

To make aldehydes or ketones, we usually look at oxidizing alcohols or breaking down certain unsaturated hydrocarbons.

• Oxidation: Primary alcohols give aldehydes (using mild reagents like PCC - Pyridinium chlorochromate). Secondary alcohols give ketones (using CrO3 or KMnO4).
• Ozonolysis of Alkenes: Cleaving alkene double bonds with ozone gives mixtures of aldehydes and/or ketones depending on the alkene structure.
• Hydration of Alkynes: Adding water to alkynes in the presence of mercury(II) sulfate and sulfuric acid gives ketones (except acetylene, which gives ethanal).

Examples

• Converting ethanol into ethanal using PCC.
• Converting propan-2-ol into propan-2-one using chromic anhydride.

Real-Life Uses

• Pharmaceutical Labs: Drug manufacturers use controlled oxidation methods to synthesize intermediate aldehyde and ketone compounds needed for synthesizing life-saving antibiotics.

Common Misconceptions and Their Corrections

• Misconception: Students use strong oxidizing agents like acidic KMnO4 to prepare aldehydes from primary alcohols.
• Correction: Strong oxidizing agents will not stop at the aldehyde stage; they will push the reaction all the way to a carboxylic acid. You must use mild reagents like PCC.

Meaning

Physical properties describe boiling points and solubility, while chemical properties describe how these molecules react. The central reaction mechanism here is Nucleophilic Addition.

Definitions

1. "Nucleophilic addition is a reaction where a nucleophile forms a bond with an electron-deficient carbon atom of a polar double bond." — Carey and Sundberg
2. "Boiling points of aldehydes and ketones are higher than those of non-polar compounds of comparable molecular mass due to dipole-dipole interactions." — NCERT Chemistry Textbook

Simple & Clear Explanation with Key Points

• Physical Properties: Because the C=O bond is polar, molecules of aldehydes and ketones attract each other through dipole-dipole forces. Hence, their boiling points are higher than alkanes of similar size, but lower than alcohols (because alcohols form strong intermolecular hydrogen bonds). Smaller aldehydes and ketones are soluble in water due to hydrogen bonding with water molecules.
• Mechanism of Nucleophilic Addition:
  1. Attack: A nucleophile (Nu:-) attacks the electrophilic carbon of the C=O group from a direction perpendicular to the plane of the carbonyl carbon, forming a tetrahedral alkoxide intermediate.
  2. Protonation: The alkoxide intermediate picks up a proton (H+) from the surrounding medium to form the final addition product.

Examples

• Addition of Hydrogen Cyanide (HCN) to form cyanohydrins.
• Addition of Sodium Bisulfite (NaHSO3) to form crystalline bisulfite addition compounds.

Real-Life Uses

• Resin Manufacturing: Formaldehyde reacts with phenol via nucleophilic addition followed by condensation to form Bakelite, one of the oldest synthetic plastics used in electrical switches and handles.

Common Misconceptions and Their Corrections

• Misconception: Students believe aldehydes are less reactive toward nucleophilic addition than ketones.
• Correction: Aldehydes are more reactive than ketones. Why? Because aldehydes have only one alkyl group (which pushes electrons and reduces positive charge on carbon), whereas ketones have two alkyl groups. Also, steric hindrance is less in aldehydes than in ketones.

Meaning

Reactivity refers to how easily aldehydes and ketones undergo chemical reactions, governed by electronic factors (inductive effect) and steric factors (crowding).

Definitions

1. "Steric hindrance refers to the prevention of a reaction due to the spatial crowding of groups attached to the reactive center." — Francis Carey
2. "The electron-donating inductive (+I) effect of alkyl groups decreases the positive charge on the carbonyl carbon, thereby reducing its reactivity." — Morrison and Boyd

Simple & Clear Explanation with Key Points

• Smaller size of aldehydes = Less crowding = Easy attack by nucleophiles.
• Ketones have two alkyl groups pushing electron density toward the carbonyl carbon, making it less positive and thus less reactive than aldehydes.
• Aromatic aldehydes and ketones (like benzaldehyde) are even less reactive because the carbonyl carbon is involved in resonance with the benzene ring, spreading out the positive charge.

Examples

• Formaldehyde reacts fastest with nucleophiles.
• Acetophenone reacts slower than acetaldehyde because of the phenyl ring.

Real-Life Uses

• Selective Synthesis: Chemists exploit this difference in reactivity to selectively react an aldehyde group in a molecule that also contains a ketone group.

Common Misconceptions and Their Corrections

• Misconception: Aromatic aldehydes are more reactive than aliphatic aldehydes because of the benzene ring.
• Correction: Resonance stability actually stabilizes the carbonyl carbon in aromatic compounds, making them less reactive toward nucleophiles than simple aliphatic aldehydes.

Meaning

Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH), which combines a carbonyl group and a hydroxyl group (-OH) on the same carbon.

Definitions

1. "Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group." — IUPAC
2. "The acidity of carboxylic acids is due to the resonance stabilization of the carboxylate anion formed after the loss of a proton." — Paul Bruice

Simple & Clear Explanation with Key Points

• Nomenclature: Replace the "-e" of the corresponding alkane with "-oic acid". For example, ethane becomes ethanoic acid (acetic acid).
• Acidic Nature: They are much stronger acids than alcohols or phenols. Why? Because when a carboxylic acid loses a proton, the resulting carboxylate anion (R-COO-) is highly stabilized by resonance. The negative charge is shared equally between two electronegative oxygen atoms.
• Preparation: Oxidation of primary alcohols or aldehydes using strong oxidizing agents (like KMnO4), or hydrolysis of nitriles.
• Physical Properties: High boiling points due to extensive intermolecular hydrogen bonding (they even form stable dimers in vapor phase!).

Examples

• Formic acid (Methanoic acid): HCOOH, found in ant stings.
• Acetic acid (Ethanoic acid): CH3COOH, found in vinegar.

Real-Life Uses

• Food Industry: Citric acid is used as a preservative and flavoring agent in soft drinks. Acetic acid is used as vinegar for cooking and pickling.
• Polymers: Terephthalic acid is a vital raw material used to manufacture PET (polyethylene terephthalate) plastic bottles and polyester clothing fibers.

Common Misconceptions and Their Corrections

• Misconception: Students think carboxylic acids are strong mineral acids like hydrochloric acid (HCl).
• Correction: Carboxylic acids are weak acids. They do not dissociate completely in water; they establish an equilibrium. They are only stronger than alcohols and water, not mineral acids.

• Green Chemistry Catalysts: Modern chemical industries use bio-catalysis (enzymes like lipases and oxidoreductases) to prepare chiral aldehydes and carboxylic acids without generating toxic waste.
• Flow Chemistry Reactors: Microfluidic reactors allow safe handling of dangerous oxidation reactions for synthesizing pharmaceutical intermediates on a continuous production scale.
• Gas Chromatography-Mass Spectrometry (GC-MS): Used in flavor and fragrance labs to detect trace amounts of aldehydes and ketones in essential oils.

Problem 1:

An organic compound (A) with molecular formula C2H4O gives a positive Tollens' test and reduces Fehling's solution. Identify compound (A) and write the reaction for its oxidation.

Step-by-Step Solution:
1. Analyze the molecular formula: C2H4O has a degree of unsaturation of 1. It contains 2 carbons, 4 hydrogens, and 1 oxygen.
2. Identify functional group test clues: Giving a positive Tollens' test and reducing Fehling's solution are classic characteristic tests for aldehydes.
3. Determine the structure: With 2 carbons and an aldehyde group (-CHO), the compound must be Ethanal (Acetaldehyde), CH3CHO.
4. Write the oxidation reaction: 
   CH3CHO + [O] (using KMnO4 or K2Cr2O7) -> CH3COOH (Ethanoic acid).
Answer: Compound (A) is Ethanal (Acetaldehyde), and its oxidation yields Ethanoic acid.
        

Problem 2:

Calculate the mass of silver (Ag) deposited when an excess of Tollens' reagent is treated with 0.1 moles of methanal (formaldehyde).

Step-by-Step Solution:
1. Write the balanced chemical equation:
   HCHO + 2[Ag(NH3)2]+ + 3OH- -> HCOO- + 2Ag (solid precipitate) + 4NH3 + 2H2O.
2. Determine stoichiometric coefficients: 
   From the equation, 1 mole of methanal (HCHO) produces 2 moles of silver (Ag).
3. Calculate moles of silver produced:
   Moles of Ag = 2 × Moles of HCHO = 2 × 0.1 = 0.2 moles.
4. Convert moles to mass:
   Atomic mass of Ag = 108 g/mol.
   Mass of Ag = 0.2 moles × 108 g/mol = 21.6 grams.
Answer: 21.6 grams of silver will be deposited.
        

• Aldehyde General Formula: R-CHO
• Ketone General Formula: R-CO-R'
• Carboxylic Acid General Formula: R-COOH
• Tollens' Test Equation: R-CHO + 2[Ag(NH3)2]+ + 3OH- → RCOO- + 2Ag (s) + 4NH3 + 2H2O
• Fehling's Test Equation: R-CHO + 2Cu2+ + 5OH- → RCOO- + Cu2O (red ppt) + 3H2O
• Carboxylic Acid Ionization: R-COOH + H2O ⇌ R-COO- + H3O+

Title: The Fragrance Mystery at Apex Aromas

Apex Aromas, a leading cosmetic manufacturing laboratory, ran into a major production crisis. Their flagship product—a luxury lavender and vanilla scented body lotion—began developing a sharp, vinegar-like pungent odor after sitting in warehouse storage for three weeks during summer. The quality control head, Dr. Sharma, pulled samples from the batch and decided to investigate the chemical culprits behind the ruined batches.

Dr. Sharma performed several chemical tests on the spoiled lotion. First, she tested the pH and found it had dropped significantly from 6.5 to 3.2. Next, she ran a 2,4-DNP test, which gave a deep orange precipitate, confirming the presence of carbonyl compounds. However, when she added Tollens' reagent to a fresh sample versus the spoiled sample, she noticed something fascinating. The raw ingredient stock of vanillin (an aromatic aldehyde used for fragrance) showed strong positive Tollens' tests, but the spoiled lotion showed a drastic decrease in vanillin concentration accompanied by a massive spike in vanillic acid (a carboxylic acid).

Digesting the laboratory data, Dr. Sharma realized what had happened. During the hot summer storage, atmospheric oxygen had penetrated the semi-permeable plastic containers. The mild air exposure slowly oxidized the delicate aromatic aldehyde (vanillin) into its corresponding carboxylic acid (vanillic acid). Furthermore, residual traces of primary alcohol solvents used in the formulation had also oxidized into acetic acid, explaining the sharp vinegar smell and the plummeting pH.

To fix the issue permanently, Dr. Sharma implemented three changes:

1. She replaced regular packaging with airtight aluminum-laminated tubes to completely block oxygen contact.
2. She added Vitamin E and BHT as food-grade antioxidant stabilizers to scavenge free radicals before they could oxidize the aldehydes.
3. She adjusted the storage temperature guidelines to keep warehouse temperatures below 20 degrees Celsius.

Within a month, the spoilage issues dropped to absolute zero, saving the company thousands of dollars and preserving the brand's reputation.

1. Which of the following compounds will give a positive Fehling's test?
   A) Acetone
   B) Benzaldehyde
   C) Propanal
   D) Propan-2-one
   Answer: C) Propanal
   Explanation: Fehling's test is specific for aliphatic aldehydes. Propanal is an aliphatic aldehyde, whereas acetone and propan-2-one are ketones, and benzaldehyde is an aromatic aldehyde which does not reduce Fehling's solution.


2. The hybridization of the carbonyl carbon in aldehydes and ketones is:
   A) sp3
   B) sp2
   C) sp
   D) dsp2
   Answer: B) sp2
   Explanation: The carbonyl carbon forms three sigma bonds and one pi bond, giving it a planar trigonal geometry with sp2 hybridization.


3. Which reagent is best suited to convert a primary alcohol directly into an aldehyde without oxidizing it further to a carboxylic acid?
   A) Acidified KMnO4
   B) CrO3 in water
   C) PCC (Pyridinium chlorochromate)
   D) Concentrated HNO3
   Answer: C) PCC
   Explanation: PCC is a mild, anhydrous oxidizing agent that stops oxidation at the aldehyde stage, whereas KMnO4 and HNO3 are strong oxidizing agents that push the reaction to carboxylic acids.


4. Why do carboxylic acids have higher boiling points than alcohols of comparable molecular mass?
   A) They are smaller in size
   B) They form stronger and more extensive intermolecular hydrogen bonds, often existing as stable dimers
   C) They contain two oxygen atoms instead of one
   D) They are ionic in nature
   Answer: B) They form stronger and more extensive intermolecular hydrogen bonds, often existing as stable dimers
   Explanation: Carboxylic acids form robust cyclic hydrogen-bonded dimers in both liquid and vapor states, requiring much more thermal energy to break apart.


5. The correct IUPAC name for CH3-CH(CH3)-CHO is:
   A) 2-Methylpropanal
   B) 2-Methylpropan-1-one
   C) Isobutyraldehyde
   D) Butan-2-al
   Answer: A) 2-Methylpropanal
   Explanation: The longest carbon chain has 3 carbons with an aldehyde group at carbon 1. A methyl group is attached at carbon 2, making it 2-methylpropanal.


6. Cannizzaro reaction is given by:
   A) Acetaldehyde
   B) Formaldehyde
   C) Acetone
   D) Butan-2-one
   Answer: B) Formaldehyde
   Explanation: Aldehydes that do not possess alpha-hydrogen atoms (like formaldehyde and benzaldehyde) undergo self-oxidation and reduction (Cannizzaro reaction) in the presence of concentrated alkali.


7. Nucleophilic addition to aldehydes and ketones is fastest with:
   A) Formaldehyde
   B) Acetaldehyde
   C) Acetone
   D) Diethyl ketone
   Answer: A) Formaldehyde
   Explanation: Formaldehyde has no electron-donating alkyl groups and minimum steric crowding, making its carbonyl carbon most electrophilic and reactive toward nucleophiles.


8. Which acid is the strongest among the following due to the -I effect of halogen substituents?
   A) Acetic acid
   B) Monochloroacetic acid
   C) Trichloroacetic acid
   D) Propanoic acid
   Answer: C) Trichloroacetic acid
   Explanation: The three chlorine atoms in trichloroacetic acid exert a powerful electron-withdrawing inductive (-I) effect, stabilizing the carboxylate anion significantly and increasing acidity.


9. Cyanohydrin formation when HCN reacts with a ketone is an example of:
   A) Electrophilic substitution
   B) Nucleophilic addition
   C) Free radical substitution
   D) Nucleophilic elimination
   Answer: B) Nucleophilic addition
   Explanation: The cyanide ion (CN-) acts as a nucleophile and attacks the electron-deficient carbonyl carbon in the first step.


10. Which chemical test can distinguish between Benzaldehyde and Acetaldehyde?
    A) Tollens' test
    B) 2,4-DNP test
    C) Fehling's test
    D) Sodium bisulfite test
    Answer: C) Fehling's test
    Explanation: Acetaldehyde is an aliphatic aldehyde and reduces Fehling's solution to give a red precipitate of Cu2O, whereas benzaldehyde (an aromatic aldehyde) does not reduce Fehling's solution.

1. Q1: Arrange the following compounds in increasing order of their boiling points: CH3CH2CH2OH, CH3CH2CHO, CH3CH2CH3, CH3CH2COOH.
   Answer:
   CH3CH2CH3 (Alkane: weak Van der Waals forces) < CH3CH2CHO (Aldehyde: dipole-dipole interactions) < CH3CH2CH2OH (Alcohol: hydrogen bonding) < CH3CH2COOH (Carboxylic acid: strong dimeric hydrogen bonding).


2. Q2: Explain why carboxylic acids are stronger acids than phenols even though both lose a proton to form resonance-stabilized anions.
   Answer:
   In a carboxylate anion (R-COO-), the negative charge is distributed over two highly electronegative oxygen atoms equally. In a phenoxide anion (C6H5O-), the negative charge is dispersed onto one oxygen atom and less electronegative carbon atoms of the benzene ring. Because oxygen accommodates negative charge much better than carbon, the carboxylate ion is far more stable, making carboxylic acids stronger acids.


3. Q3: Write chemical equations for the reaction of propanone with methylmagnesium bromide (CH3MgBr) followed by hydrolysis.
   Answer:
   Step 1 (Addition): (CH3)2C=O + CH3MgBr → (CH3)3C-O-MgBr
   Step 2 (Hydrolysis): (CH3)3C-O-MgBr + H2O → (CH3)3C-OH (Tert-butyl alcohol) + Mg(OH)Br.


4. Q4: What is the Aldol condensation? Give an example of a compound that undergoes Aldol condensation.
   Answer:
   Aldol condensation is a reaction where aldehydes or ketones containing at least one alpha-hydrogen atom undergo self-condensation in the presence of a dilute base (like NaOH) to form beta-hydroxy aldehydes or beta-hydroxy ketones (aldols).
   Example: Acetaldehyde (CH3CHO) undergoes aldol condensation to give 3-hydroxybutanal.


5. Q5: Why can aldehydes act as reducing agents while ketones generally do not?
   Answer:
   Aldehydes possess a hydrogen atom attached directly to the carbonyl carbon (H-C=O). This bond can be easily oxidized to an -OH group, converting the aldehyde into a carboxylic acid. Ketones have two carbon groups attached to the carbonyl carbon and lack this hydrogen atom, making them resistant to oxidation under mild conditions.


6. Q6: Account for the fact that chloroacetic acid is a stronger acid than acetic acid.
   Answer:
   The chlorine atom in chloroacetic acid is electronegative and exerts an electron-withdrawing inductive (-I) effect. This pulls electron density away from the carboxylate group, stabilizing the negative charge on the carboxylate anion and making the release of a proton easier compared to acetic acid (where the methyl group exerts a +I electron-donating effect).


7. Q7: How can you convert benzaldehyde into alpha-hydroxyphenylacetic acid?
   Answer:
   Treat benzaldehyde with Hydrogen Cyanide (HCN) to form mandelonitrile (benzaldehyde cyanohydrin). Then, hydrolyze the cyanide group (-CN) with acid to form alpha-hydroxyphenylacetic acid (mandelic acid).


8. Q8: Give a chemical test to distinguish between pentan-2-one and pentan-3-one.
   Answer:
   Iodoform Test: Pentan-2-one contains a methyl ketone group (CH3-C=O) and will react with iodine in the presence of NaOH to give a yellow precipitate of iodoform (CHI3). Pentan-3-one does not have a methyl group attached to the carbonyl carbon and will give a negative iodoform test.


9. Q9: Why are boiling points of aldehydes and ketones lower than those of corresponding alcohols?
   Answer:
   Alcohols have -OH groups capable of forming extensive intermolecular hydrogen bonding, which is a strong intermolecular force. Aldehydes and ketones are polar and exhibit dipole-dipole attractions, but these are significantly weaker than hydrogen bonds, requiring less thermal energy to break.


10. Q10: Write the structure and IUPAC name of the product formed when propene is subjected to ozonolysis followed by zinc dust and water.
    Answer:
    Ozonolysis of propene (CH3-CH=CH2) cleaves the double bond to form a mixture of ethanal (acetaldehyde) and methanal (formaldehyde).
    Structures: CH3CHO and HCHO.

• Carbonyl group (C=O) is polar due to high electronegativity of oxygen.
• Aldehydes have at least one H attached to C=O; ketones have two carbon groups.
• Nucleophilic addition is the core chemical mechanism for aldehydes and ketones.
• Aldehydes are more reactive than ketones due to less steric hindrance and fewer +I groups.
• Carboxylic acids form stable dimers via extensive intermolecular hydrogen bonding.
• Mild oxidizing agents (PCC) stop at aldehydes; strong agents push to carboxylic acids.

Memory Tricks & Mnemonics:

• "Told Fehling's Old Red Ox" (Tollens' and Fehling's tests oxidize Aldehydes to give a Red precipitate / Silver mirror).
• "CAT = Carboxylic Acid Trumps" (Carboxylic acids have priority in IUPAC naming over aldehydes and ketones).
• "AL-DE-HY-DE = Always Last End" (The aldehyde functional group -CHO is always at the end of the carbon chain, carbon number 1).

Frequently Confused Concepts:

• Aldol vs. Cannizzaro: Remember that Aldol needs Alpha hydrogens (neighbors to help out), while Cannizzaro happens when there are No alpha hydrogens.
• Oxidation Limits: Mild reagents (PCC) stop at Aldehydes. Strong reagents (KMnO4) blast straight through to Carboxylic Acids.