The Carbonyl Trap

The carbonyl carbon is $sp^2$ hybridised — it's got a $\pi$ bond to oxygen and a trigonal planar geometry. But here's the key:

The carbon is electrophilic ($\delta^+$ from oxygen's electronegativity pulling electron density). Every nucleophile wants a piece of it.

Mental model: The carbonyl is a trap. Oxygen baits with its electronegativity, carbon is the spring-loaded snare. A nucleophile walks in $\rightarrow$ BOOM tetrahedral intermediate locked.

Ketones vs Aldehydes

Property Ketones Aldehydes
Carbonyl position Middle (2 R groups) End (1 H + 1 R)
Reactivity Slower Faster
Why? Steric bulk (two R groups block attack) + electronic donation (2 alkyl groups stabilise C=O, less electrophilic) Less crowded + H doesn't donate electrons

MM: Ketones are the lazy older sibling. Aldehydes are the hyper younger one who does everything first.

The inductive effect: Alkyl groups are electron-donating ($+I$ effect). Two alkyl groups in a ketone donate more electron density to the carbonyl carbon than one alkyl + H in an aldehyde. This makes the ketone carbonyl carbon less $\delta^+$, hence less electrophilic.

$$ \text{R}_2\text{C=O} \quad \text{vs} \quad \text{R(H)C=O} $$

Tetrahedral Intermediate = The "Pregnant Carbon"

When a nucleophile ($\text{Nu}^-$) attacks:

$$ \text{R}_2\text{C=O} + \text{Nu}^- \rightarrow \text{R}_2\text{C(Nu)O}^- $$

The carbonyl carbon goes from $sp^2$ (flat, trigonal planar) to $sp^3$ (tetrahedral). It bulges out.

  • If the tetrahedral intermediate collapses by kicking out a leaving group $\rightarrow$ substitution
  • If it gets protonated ($\text{H}^+$) $\rightarrow$ alcohol

MM: The carbonyl is a pop-up tent. Nucleophile is the button that collapses it into a tetrahedral lump. How it unfolds depends on what's attached.

Ketones Don't Oxidise (Easy Identification)

  • Aldehydes $\rightarrow$ oxidise to carboxylic acids ($\text{RCHO} \xrightarrow{[\text{O}]} \text{RCOOH}$)
  • Ketones $\rightarrow$ cannot oxidise (no H on carbonyl carbon to remove)

The aldehyde has a $\text{C-H}$ bond at the carbonyl that can be cleaved. A ketone has $\text{C-R}$ instead — no H to remove.

Tollens' test: Ketones give no reaction. Aldehydes give Ag mirror. $$ \text{RCHO} + 2[\text{Ag(NH}_3\text{)}_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag}\downarrow + 4\text{NH}_3 + 2\text{H}_2\text{O} $$

Fehling's test: Ketones give no reaction. Aldehydes give brick-red $\text{Cu}_2\text{O}$ precipitate.

MM: If it forms a silver mirror, it's an aldehyde. If it doesn't, it's a ketone. Simple.

Methyl Ketone Exception: Iodoform Test

A methyl ketone ($\text{CH}_3\text{COR}$) reacts with $\text{I}_2/\text{NaOH}$ to give a yellow precipitate of $\text{CHI}_3$ (iodoform).

Why: The $\alpha$-hydrogens on the methyl group are acidic enough due to the carbonyl's electron-withdrawing effect. Three $\text{I}_2$ substitutions happen sequentially, then the $\text{CI}_3$ group leaves as $\text{CHI}_3$.

$$ \text{CH}_3\text{COR} + 3\text{I}_2 + 4\text{NaOH} \rightarrow \text{CHI}_3\downarrow + \text{RCOONa} + 3\text{NaI} + 3\text{H}_2\text{O} $$

MM: If it goes yellow with $\text{I}_2/\text{NaOH}$, it's either a methyl ketone or ethanol/acetaldehyde (which oxidises to a methyl ketone first). It's the carbonyl world's party trick.

Aldol Condensation — Ketones Can Play Too

Ketones with $\alpha$-hydrogens can undergo aldol reactions:

$$ 2\text{R}_2\text{CHCOR} \xrightarrow{\text{OH}^-} \text{R}_2\text{C=C(R)COR} + \text{H}_2\text{O} $$

But:

  • Aldehydes: Aldol is fast (less steric hindrance)
  • Ketones: Aldol is slower, $K_{eq}$ lies toward reactants (need to remove product or use special conditions)

MM: Ketones can do the aldol dance, but they're reluctant partners. Aldehydes are the eager beavers. For ketones to react, you usually need heat or catalyst to push the equilibrium.

Grignard Attack — Tertiary Alcohols from Ketones

Ketones + Grignard reagent ($\text{RMgX}$) $\rightarrow$ tertiary alcohol (after hydrolysis).

$$ \text{R}_2\text{C=O} + \text{R'MgX} \xrightarrow{\text{H}_3\text{O}^+} \text{R}_2\text{C(OH)R'} $$

  • Formaldehyde + Grignard $\rightarrow$ primary alcohol ($\text{RCH}_2\text{OH}$)
  • Aldehyde + Grignard $\rightarrow$ secondary alcohol ($\text{RR'CHOH}$)
  • Ketone + Grignard $\rightarrow$ tertiary alcohol ($\text{R}_2\text{R'COH}$)

MM: More alkyl groups attached to the carbonyl = more substituted alcohol product. Ketones give the most substituted (tertiary). Each Grignard addition is a step up the substitution ladder.

Reduction of Ketones

Reagent Product Notes
$\text{NaBH}_4$ Secondary alcohol Mild, selective for carbonyls
$\text{LiAlH}_4$ Secondary alcohol Stronger, reduces everything
$\text{H}_2/\text{Pd}$ Secondary alcohol Catalytic hydrogenation
Clemmensen ($\text{Zn(Hg)/HCl}$) Methylene ($\text{CH}_2$) Complete removal of O
Wolff-Kishner ($\text{NH}_2\text{NH}_2/\text{KOH}$) Methylene ($\text{CH}_2$) Complete removal of O

MM: Want an alcohol? Use $\text{NaBH}_4/\text{LiAlH}_4$. Want to completely delete the oxygen?

$$ \text{R}_2\text{C=O} \xrightarrow{\text{Zn(Hg)/HCl}} \text{R}_2\text{CH}_2 \quad (\text{Clemmensen}) $$ $$ \text{R}_2\text{C=O} \xrightarrow{\text{NH}_2\text{NH}_2/\text{KOH}} \text{R}_2\text{CH}_2 \quad (\text{Wolff-Kishner}) $$

Clemmensen works when the molecule can survive strong acid. Wolff-Kishner works when the molecule can survive strong base/heat.

Cyclic Ketones — Ring Size Matters

  • 5-membered (cyclopentanone): Normal reactivity
  • 6-membered (cyclohexanone): Normal reactivity
  • Small rings (3-4): Unstable, ring strain adds weird reactivity
  • Large rings (7+): Behave like acyclic ketones

Strain energy in small cyclic ketones:

$$ \Delta H_{\text{strain}} (\text{cyclopropanone}) \gg \Delta H_{\text{strain}} (\text{cyclohexanone}) $$

MM: Cyclohexanone is the "Goldilocks" cyclic ketone. It's stable, well-studied, and behaves predictably. Small rings are spicy.

Big Picture Flow

                 Nucleophile attack
                       |
                 Tetrahedral intermediate
                  /              \
         Has leaving group?    No leaving group?
                 |                    |
           Substitution          Protonation
                                   |
                                Alcohol
                              /         \
                        Primary     Secondary/Tertiary
                        (from HCHO) (from RCHO / R2CO)

MM: Think of the carbonyl as a decision tree. The nucleophile hits $\rightarrow$ intermediate forms $\rightarrow$ what happens next depends entirely on what's attached and what conditions you throw at it.