The Mistake Bank
Aldehydes, Ketones & Carboxylic Acids
The King of Organic Chemistry. Watch out for the steric hindrance!
Aldehyde vs Ketone Reactivity
Nucleophilic AdditionScenario: Which is more reactive towards $HCN$? Propanal ($CH_3CH_2CHO$) or Propanone ($CH_3COCH_3$)?
Student thinks: "Ketones have two alkyl groups pushing electrons, making the oxygen more negative."
Conclusion: "Ketones are more reactive."
Aldehydes Win!
1. Steric: Ketones have two bulky alkyl groups hindering the attack.
2. Electronic: Alkyl groups ($+I$) reduce the positive charge on Carbon, making it less attractive to nucleophiles.
Answer: Propanal is more reactive.
Aldol Condensation Product
Reaction MechanismScenario: Product of Ethanal + Dilute NaOH + Heat.
Student stops at the $\beta$-hydroxy aldehyde stage.
Product: 3-Hydroxybutanal.
(Forgot the heating step!)
Heat causes Dehydration!
Heating removes a water molecule to form an $\alpha,\beta$-unsaturated aldehyde.
$$ CH_3-CH(OH)-CH_2-CHO \xrightarrow{\Delta} CH_3-CH=CH-CHO $$
Product: But-2-enal (Crotonaldehyde).
Tollens' Reagent Specificity
OxidationScenario: Reaction of $CH_2=CH-CHO$ (Acrolein) with Tollens' Reagent ($[Ag(NH_3)_2]^+$).
Student oxidizes everything.
Product: $COOH-CH(OH)-COOH$ (oxidizes double bond too).
Mild Oxidizing Agent!
Tollens' reagent is weak. It oxidizes Aldehydes to Carboxylic acids but does NOT affect Carbon-Carbon double bonds.
Product: $CH_2=CH-COOH$ (Acrylic Acid).
Grignard + Carbon Dioxide
PreparationScenario: Product of $CH_3MgBr + CO_2$ followed by hydrolysis.
Student thinks: "Grignard adds carbons."
Guesses a Ketone or Alcohol.
Carboxylic Acid Formation!
Grignard reagent attacks the electrophilic carbon in $O=C=O$.
$$ CH_3-C(=O)OMgBr \xrightarrow{H_3O^+} CH_3COOH $$
Product: Ethanoic Acid (Increases carbon chain by 1).
Acidity: Methoxy vs Nitro
Acid StrengthScenario: Compare acidity of p-Methoxybenzoic acid and Benzoic acid.
Student sees Oxygen in Methoxy ($OCH_3$) and thinks "Electronegative = -I effect = Stronger Acid".
Resonance Dominates!
The Methoxy group has a lone pair that causes a strong +R (Resonance) Effect.
This pumps electrons into the ring, destabilizing the carboxylate anion.
Answer: p-Methoxybenzoic acid is Weaker than Benzoic acid.
HVZ Reaction Requirement
Name ReactionScenario: Does 2,2-dimethylpropanoic acid undergo Hell-Volhard-Zelinsky (HVZ) reaction?
Student applies reagent ($Br_2/Red P$) blindly.
Writes product by substituting a methyl hydrogen.
Needs Alpha-Hydrogen!
HVZ reaction specifically halogenates the $\alpha$-carbon.
In $(CH_3)_3C-COOH$, the $\alpha$-carbon has no Hydrogens.
Answer: No Reaction.
Confess Your Sins!
"Did you dehydrate the aldol? Or did you accidentally oxidize a double bond?"
Did one of these catch you? Or do you have a different horror story from your last exam?
Scroll down to the comments section below and tell us:
No comments:
Post a Comment