Aldehydes & Ketones – Electrophilicity, Oxidation, Physical Properties

Overview of the Carbonyl Functional Group

• Carbonyl: C=O double bond between carbon & oxygen.
  • Shared motif in many functional groups: aldehydes, ketones, carboxylic acids, esters, amides, anhydrides, etc.
• Dual reactivity makes carbonyls central for MCAT chemistry:
  • Can act as nucleophile (e.g.
    condensation, enolate chemistry—covered next chapter).
  • Can act as electrophile (e.g.
    nucleophilic addition—focus of this chapter).

Aldehydes vs. Ketones – Structural Definition

• Aldehyde (R–CHO)
  • Carbonyl carbon bonded to 1 alkyl (or aryl) group + 1 H.
  • Always terminal on a chain or ring.
• Ketone (R–CO–R')
  • Carbonyl carbon bonded to 2 alkyl/aryl groups.
  • Therefore never terminal.
• Consequences of structure:
  • Aldehydes experience less steric hindrance and fewer electron-donating effects ⇒ generally more electrophilic & more reactive than ketones.

Real-World Presence & Odor Examples

• Volatile carbonyls contribute to characteristic aromas/flavors:
  • Cinnamon: cinnamaldehyde
  • Vanilla: vanillin
  • Cumin: cuminaldehyde
  • Dill: carvone (a ketone)
  • Ginger: zingerone (contains both carbonyl & other groups)

IUPAC & Common Nomenclature

Aldehydes

• Replace terminal –e of parent alkane with –al.
• 1–5-carbon common names:
  • $CH_2O$ = formaldehyde
  • $CH_3CHO$ = acetaldehyde
  • $CH<em>3CH</em>2CHO$ = propionaldehyde
  • $CH<em>3(CH</em>2)_2CHO$ = butyraldehyde
  • $CH<em>3(CH</em>2)_3CHO$ = valeraldehyde
• As substituent prefix ➝ oxo- (e.g. 3-oxohexanoic acid).
• Attached to ring ➝ add -carbaldehyde suffix (e.g. benzaldehyde is technically benzenecarbaldehyde).

Ketones

• Replace –e with –one; indicate carbonyl position by # (lowest possible).
  • Example: $CH<em>3COCH</em>3$ = 2-propanone (a.k.a. acetone / dimethyl ketone)
  • $CH<em>3COCH</em>2CH_3$ = 2-butanone (ethyl methyl ketone)
• Common naming: list two alkyl groups alphabetically + “ketone” (e.g. ethyl methyl ketone).
• As substituent prefixes: oxo- or keto-.

Physical Properties Governed by Carbonyl Dipole

• Carbonyl oxygen more electron-withdrawing than alcohol’s single-bonded O ⇒ larger dipole moment.
• Intermolecular dipole–dipole interactions raise boiling points compared with non-polar alkanes of similar MW.
• However, no H-bond donor (no O–H) ⇒ BP elevation < alcohols.
  • Trend: BP{alkanes} < BP{carbonyls} < BP_{alcohols}.

Electrophilicity & Reactivity

• C=O oxygen pulls electron density ⇒ partial positive charge on carbonyl C.
• Good electrophilic target for nucleophiles (cyanide, hydride reagents, organometallics, etc.).
• Aldehyde vs. Ketone Reactivity:
  • Aldehydes: less steric bulk + only one e-donating alkyl group.
  • Ketones: two alkyl groups donate e-density (+ steric crowding) ⇒ lower electrophilicity.

Formation by Oxidation (Laboratory/MCAT Focus)

Aldehydes

• Partial oxidation of primary alcohols using PCC (pyridinium chlorochromate) only:
  $RCH<em>2OH \xrightarrow{C</em>5H<em>5NHCrO</em>3Cl\,(PCC)} RCHO$
• Stronger or aqueous oxidants (e.g. $KMnO<em>4$, $CrO</em>3/H<em>2SO</em>4$) will further oxidize to carboxylic acid $RCOOH$.

Ketones

• Oxidation of secondary alcohols; reaction generally stops at ketone because a C–C bond must be broken to oxidize further.
• Common reagents (all acceptable on MCAT):
  • $Na<em>2Cr</em>2O<em>7$ / $H</em>2SO_4$ (sodium dichromate)
  • $K<em>2Cr</em>2O<em>7$ / $H</em>2SO_4$ (potassium dichromate)
  • $CrO_3$ (chromium trioxide) in aqueous acid (Jones oxidation)
  • PCC (anhydrous, milder)
    $R<em>2CHOH \xrightarrow{[O]} R</em>2C=O$

Connections to Upcoming Material

• Next chapter ➝ enolate chemistry: carbonyl α-hydrogen acidity allows carbonyls to serve as nucleophiles after deprotonation.
• Understanding electrophilicity here sets stage for nucleophilic additions (cyanohydrin formation, hydration, acetal/ketal formation, etc.).

Practical & Conceptual Takeaways

• Recognize oxidation levels:
  • Primary alcohol → Aldehyde → Carboxylic Acid
  • Secondary alcohol → Ketone (stop)
• Choose reagent wisely (PCC for aldehyde isolation).
• Carbonyl electrophilicity underpins many carbon–carbon bond-forming reactions used in synthesis.
• Real-world smell/taste ties help recall structures (e.g. cinnamon = aldehyde).

Quick Reference – Numerical/Formula Highlights

• PCC formula: $C<em>5H</em>5NHCrO_3Cl$
• Dichromate salts: $Na<em>2Cr</em>2O<em>7$ & $K</em>2Cr<em>2O</em>7$.
• Trend: BP{alkane} < BP{aldehyde/ketone} < BP_{alcohol} due to dipole vs. hydrogen bonding.
• Electrophilicity order (least hindered most reactive): Aldehyde > Ketone.