Carboxylic Acids & Derivatives — Intuition Note

1. Definition — The Carboxyl Group

The carboxyl group is a carbonyl ($C=O$) and a hydroxyl ($-OH$) on the same carbon: $-COOH$.

$$R-COOH$$

The carbonyl carbon is sp2 hybridised. The real story is what happens when it loses a proton:

$$\ce{R-COOH <=> R-COO^- + H+}$$

The carboxylate ion is resonance-stabilised — the negative charge is shared equally between both oxygen atoms. This is why carboxylic acids ($pK_a \sim 4-5$) are much stronger acids than alcohols ($pK_a \sim 16-18$) or even phenols ($pK_a \sim 10$).

MM: The carboxylate ion is the calm one at a party who doesn't care which hand holds the drink — the negative charge freely bounces between both oxygens. An alkoxide is rigid — the charge sits on one oxygen and stays there. Resonance = stability.

Acidity Comparison

$$pK_a:\ Carboxylic\ (\sim4.8) < Phenol\ (10.0) < Alcohol\ (16-18)$$

Why: The carboxylate anion's negative charge is delocalised over two oxygen atoms. Phenoxide's charge is delocalised over oxygen + three ring carbons (less effective). Alkoxide's charge is stuck on one oxygen.

IUPAC Naming

Replace -e of parent alkane with -oic acid.

Common name	IUPAC name	pKa
Formic acid	Methanoic acid	3.75
Acetic acid	Ethanoic acid	4.76
Benzoic acid	Benzenecarboxylic acid	4.20

2. Physical Properties — H-Bonding on Steroids

Dimer Formation

Carboxylic acids form dimers in non-polar solvents — two molecules held together by two H-bonds:

R-C=O --- HO-C-R
    \       /
     O-H --- O

This effectively doubles the molecular weight in non-polar media, which is why acetic acid (MW 60) boils at 118 °C while propan-1-ol (MW 60) boils at 97 °C.

MM: Carboxylic acids don't like to be alone. In non-polar solvent, they pair up like buddies holding both hands — twice as hard to pull apart.

Boiling Point Hierarchy

$$Carboxylic\ acid > Alcohol > Aldehyde/Ketone > Ether > Alkane$$

Solubility

Lower members (up to 4-5 C) are water-soluble
The -COOH group forms strong H-bonds with water
Solubility drops as the hydrophobic R chain lengthens

3. Acidity Factors — What Makes a Carboxylic Acid Stronger

Inductive Effect of Substituents

Acid	pKa	What's happening
$CF_3COOH$	~0.2	Three F atoms pull electron density hard
$ClCH_2COOH$	2.87	Cl pulls electron density → stabilises anion
$HCOOH$	3.75	Reference (no alkyl group)
$CH_3COOH$	4.76	CH3 is EDG → destabilises anion
$CH_3CH_2COOH$	4.87	More EDG than CH3

EWG (electron-withdrawing): Halogens, -NO2, -CN — pull electron density away from the carboxylate → stabilise negative charge → more acidic (lower pKa).

EDG (electron-donating): Alkyl groups — push electron density toward the carboxylate → destabilise negative charge → less acidic (higher pKa).

Distance Matters

$$ClCH_2COOH\ (pK_a\ 2.87) > ClCH_2CH_2COOH\ (pK_a\ 4.08) > ClCH_2CH_2CH_2COOH\ (pK_a\ 4.52)$$

The inductive effect fades as the EWG gets further from the carboxyl group. Like a magnet — stronger close up, weaker at distance.

4. Identification — Limited Tests

Carboxylic acids don't have flashy colour-change tests like carbonyls. Identification relies on:

Test	What happens	Why it works
NaHCO3/Na2CO3	CO2 bubbles (effervescence)	Acid + carbonate → CO2 + salt + water
Blue litmus	Turns red	Acidic
Odour	Sharp, vinegar-like (short chain)	Volatile acids

Key distinction: Phenol does NOT give CO2 with NaHCO3. Carboxylic acid does. This is THE test that separates them.

Carboxylic acid + NaHCO3 → CO2↑ (fizz) 
Phenol + NaHCO3 → no reaction

5. Preparation — Making Carboxylic Acids

Method	Starting material	Reagent	Product
1° alcohol oxidation	$RCH_2OH$	$K_2Cr_2O_7/H^+$ or $KMnO_4/H^+$	$RCOOH$
Aldehyde oxidation	$RCHO$	Any oxidant	$RCOOH$
Nitrile hydrolysis	$RCN$	$H_3O^+$, heat	$RCOOH$
Grignard carboxylation	$RMgX$	$CO_2$, then $H_3O^+$	$RCOOH$
Kolbe-Schmitt	Phenol + $CO_2$	$NaOH$, then $H^+$	Salicylic acid

Grignard Carboxylation — Carbon Extension

$$RMgX + CO_2 \rightarrow RCOOMgX \xrightarrow{H_3O^+} RCOOH$$

This is how you extend a carbon chain by one. Example: bromoethane ($C_2H_5Br$) → propanoic acid ($C_2H_5COOH$).

Nitrile Hydrolysis

$$RCN + 2H_2O \xrightarrow{H^+} RCOOH + NH_3$$

Nitriles are made from haloalkanes ($R-X + CN^- \rightarrow RCN$), so this is a two-step route: haloalkane → nitrile → carboxylic acid.

6. Reactions — The Nucleophilic Acyl Substitution Family

The Core Mechanism

All carboxylic acid derivatives react via nucleophilic acyl substitution. The key difference from aldehyde/ketone chemistry:

Aldehydes/ketones undergo nucleophilic addition (tetrahedral intermediate gets protonated). Carboxylic acid derivatives undergo nucleophilic acyl substitution (tetrahedral intermediate kicks out a leaving group).

       R-C=O           Nu⁻          R-C=O
          |          ------>           |
          L                         Nu
     (leaving group)           (tetrahedral intermediate)
                                      |
                                      ↓
                                 R-C=O  +  L⁻
                                    |
                                   Nu

Reactivity Order

$$Acyl\ chloride > Acid\ anhydride > Ester \sim Carboxylic\ acid > Amide$$

Derivative	Leaving group	How good is it leaving?
Acyl chloride ($RCOCl$)	$Cl^-$	Excellent (weak base)
Acid anhydride ($(RCO)_2O$)	$RCOO^-$	Good (resonance-stabilised)
Ester ($RCOOR'$)	$R'O^-$	Poor (strong base)
Carboxylic acid ($RCOOH$)	$HO^-$	Very poor
Amide ($RCONH_2$)	$NH_2^-$	Terrible (very strong base)

The trend follows leaving group ability. Better leaving group = more reactive derivative.

MM: Think of the derivatives as a slide. Acyl chlorides are at the top — super reactive, desperate to dump Cl⁻. Amides are at the bottom — they hold onto NH₂⁻ like a toddler with a favourite toy.

6a. Acyl Chlorides ($RCOCl$) — The Most Reactive

Preparation: $$RCOOH + SOCl_2 \rightarrow RCOCl + SO_2 + HCl$$ $$RCOOH + PCl_5 \rightarrow RCOCl + POCl_3 + HCl$$

Reactions:

With	Product	Notes
$H_2O$	$RCOOH$	Vigorous hydrolysis
$ROH$	$RCOOR'$ (ester)	Exothermic, no catalyst needed
$RNH_2$	$RCONHR'$ (amide)	Fast, high yield
Gilman reagent ($R'_2CuLi$)	$RCOR'$ (ketone)

Acyl chlorides are so reactive they react with water in the air. Use anhydrous conditions when you want them to react with something else.

6b. Acid Anhydrides ($(RCO)_2O$) — The Reactive but Milder Cousin

Preparation: $$2RCOOH \xrightarrow{P_2O_5} (RCO)_2O + H_2O$$

Reactions: Same pattern as acyl chlorides but milder. Reacts with alcohols, amines, water — but slower, more controlled.

Cyclic anhydrides (succinic, maleic, phthalic) come from dicarboxylic acids:

O=C1CCC(=O)O1       succinic anhydride
O=C1C=CC(=O)O1      maleic anhydride

6c. Esters ($RCOOR'$) — The Fischer Esterification

Preparation (Fischer Esterification): $$RCOOH + R'OH \xrightleftharpoons[H^+]{\Delta} RCOOR' + H_2O$$

Equilibrium reaction — $K_c \approx 4$ for simple cases
Push forward with excess alcohol or remove water
Acid catalyst required (H2SO4 or HCl)

Reactions of Esters:

Reaction	Reagent	Product
Acidic hydrolysis	$H_3O^+$, heat	$RCOOH + R'OH$
Saponification	$OH^-$, heat	$RCOO^- + R'OH$
Reduction	LiAlH4	Two alcohols ($RCH_2OH + R'OH$)
Transesterification	$R''OH$, catalyst	$RCOOR'' + R'OH$
Grignard (excess)	$2R''MgX$	Tertiary alcohol ($R_2R''COH$)

Saponification is the basis of soap-making — fats (triesters of glycerol) are hydrolysed with NaOH to give soap (fatty acid salts) and glycerol. Irreversible — the carboxylate salt cannot re-form the ester.

MM: Fischer esterification is a tug-of-war between acid + alcohol and ester + water. Neither side wins easily — that's why it's an equilibrium. But saponification (base) is a knockout punch — the carboxylate forms and cannot go back.

6d. Amides ($RCONH_2$, $RCONHR'$, $RCONR'_2$) — The Most Stable

Preparation: $$RCOCl + 2NH_3 \rightarrow RCONH_2 + NH_4Cl$$ $$RCOOR' + NH_3 \rightarrow RCONH_2 + R'OH$$

Reactions:

Reaction	Reagent	Product
Acidic hydrolysis	$H_3O^+$, heat	$RCOOH + NH_3$
Basic hydrolysis	$OH^-$, heat	$RCOO^- + NH_3$
Dehydration	$P_2O_5$	$RCN$ (nitrile)
Hofmann rearrangement	$Br_2/NaOH$	$RNH_2$ (amine, loses CO)

The Hofmann rearrangement is the classic named reaction: $$RCONH_2 + Br_2 + 4NaOH \rightarrow RNH_2 + 2NaBr + Na_2CO_3 + 2H_2O$$

An amide loses its carbonyl carbon as CO2 and becomes an amine with one fewer carbon. This is how you go from amide → primary amine.

6e. Other Carboxylic Acid Reactions

Salt formation: $$RCOOH + NaOH \rightarrow RCOONa + H_2O$$

Reduction (LiAlH4): $$RCOOH \xrightarrow{LiAlH_4} RCH_2OH$$

LiAlH4 reduces carboxylic acids all the way to primary alcohols. NaBH4 does not reduce carboxylic acids (it's milder).

Decarboxylation: $$RCOCOOH \xrightarrow{\Delta} RCHO + CO_2$$

$\beta$-Keto acids and malonic acid derivatives decarboxylate readily on heating. The $\beta$-carbonyl stabilises the transition state.

7. Derivative Interconversion Map

                         Carboxylic Acid
                         /     |      \
                        ↓      ↓       ↓
                     Acyl Cl  Ester   Amide
                       |        |       |
                       ↓        ↓       ↓
                   Anhydride  R-OH    Nitrile
                       |                ↓
                       ↓              Amino acid?
                    Ester/Amide

The directional nature:

Upward → harder (need special reagents/conditions)
Downward → easier (more reactive derivatives go to less reactive ones)

For example: Amide → carboxylic acid is easy (hydrolysis). Carboxylic acid → amide requires activation (make acyl chloride first, then react with amine).

8. Named Reactions Summary

Reaction	What it does	Exam likelihood
Fischer esterification	$RCOOH + ROH \rightleftharpoons RCOOR' + H_2O$	Very high
Saponification	$RCOOR' + OH^- \rightarrow RCOO^- + R'OH$	Very high
Hofmann rearrangement	$RCONH_2 \xrightarrow{Br_2/NaOH} RNH_2$	High
Gabriel synthesis	Phthalimide → primary amine	Medium
HVZ (Hell-Volhard-Zelinsky)	$\alpha$-halogenation of carboxylic acids	Medium
Kolbe-Schmitt	Phenol + CO2 → salicylic acid	Low

9. Exam Watch — Carboxylic Acid + Stereochemistry

From exam leaks: Section B5 combines Carboxylic Acids with Stereochemistry. Past year reference: PYP 22/23 Q15 (Carbonyl + Carboxylic + Stereochem).

Tutorial 9, Question 2 style — "puzzle-type" questions where you're given limited info and need to work out the structure.

What to watch for:

Chiral centres in carboxylic acids (e.g. 2-chloropropanoic acid has a stereocenter at C2)
R/S configuration of chiral carboxylic acids
Fischer projections of $\alpha$-hydroxy or $\alpha$-amino acids
Reaction mechanisms — the exam leak says "remember reaction mechanisms for every reaction"
Derivative interconversion sequences (e.g. acid → acyl chloride → ester → amide → Hofmann → amine)

Carbonyl Compounds — Precursors; same core electrophilic carbonyl chemistry
Alcohol & Phenol — Esterification partners; reduction products
Amines & Amino Acids — Form amides; Hofmann rearrangement connects them
Polymer Chemistry — Polyesters and polyamides (nylon, PET)
Stereochemistry — Chiral centres in substituted acids