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OrgChem – Carboxylic Acid
Carboxylic Acid
Structure & Formation:
Carboxylic acid contains Carboxyl group:
Carboxyl group = Carbonyl group + Hydroxyl group
= + –OH
Behavior of the carbonyl group and hydroxyl group modified by each other
-Hydroxycarboxylic acid can be prepared by hydrolysis of -hydroxynitriles
Hydrolysis of nitriles by alkali gives carboxylate ion
Carboxylic acid can be recovered by addition of acid to the solution
If 2° or 3° haloalkanes are used instead of 1° haloalkane, alkene is formed
CN– is a strong base and leads to elimination rather than substitution
Oxidation of 1° alcohols by strong oxidizing agents can give carboxylic acid
Aldehydes are formed as an intermediate
The oxidation of aldehyde can undergo in even milder condition to carboxylic acid
Oxidation of alkylbenzenes by strong oxidizing agents
Oxidation takes place at phenylmethyl carbon
Oxidation requires the abstraction of a phenylmethyl hydrogen
2-Methyl-2-phenylpropane have no phenylmethyl H and resistant to side chain oxidation
Any alkylbenzene will become Benzoic acid
If the side chain contains aldehydic or ketone group, strength of the oxidizing agent
determines the product
The KMnO4 / OH– is the strongest oxidizing agent
Oxidation of methyl ketone & alcohol
Methyl ketone w/ or alcohol w/ are liable to undergo triiodomethane
reaction
Resulting carboxylate is one carbon less than the original carbon skeleton
Acidity:
Carboxylic acids are weak acids w/carboxyl group responsible for the acidity
Carboxylic acid molecule in water will dissociate into ions:
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The carboxylate ion is energetically stabilized by the spreading of the negative charge
over a C and two O
Acidity: [Mineral acids] H2CO3 R–OH
R–COO– ion is stabilized by spreading of negative charge to two highly electronegative O
ion is stabilized by the delocalization of charge of the benzene ring
Stability of conjugate anion: [Mineral acids] CO32–
R–O–
Acidity of carboxylic acid is affected by the group attached to –COOH
Electron attracting / withdrawing groups (e.g. Cl) increase the acidity
O–H bond of RCOOH is weakened
Stabilized conjugate anion by spreading out negative charge on RCOO– ion
Electron releasing groups (e.g. CH3–) reduce the acid strength
Pushing electrons to electron-deficient carbonyl C will strengthening the O–H bond
The electron-donating substituent will push e– toward the electron-rich COO– group to
intensify to negative charge
Destabilizes the resulting anion
Reactions
Salt formation:
RCOOH is able to react w/metal carbonates, metal hydrogencarbonates, and some reactive metal
Acids liberate CO2 from carbonates and hydrogencarbonates as effervescence
Phenols have no reaction w/CO32– or HCO3–
Acids react w/reactive metals and give off H2
Carboxylic acid reacts with ammonia to give ammonium salts
Strong heating dehydrates the ammonium salt and gives amide
The carboxylic acid salt can be displaced by stronger acids:
RCOO–Na+ + HCl RCOOH + NaCl
Reduction:
Carboxylic acid resistant to ordinary reducing agents
Carboxylic acid can be reduced to alcohol by lithium tetrahydridoaluminate
Primary alcohols is directly formed
The C=C, CC or parts are unaffected
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Acid Derivatives
Acyl Chloride:
Acyl chloride (Alkanoyl chloride):
Reacting RCOOH w/reagents PCl5, PCl3 or SOCl2:
If the Acyl Chloride has a high boiling point, PCl5 is used
B.P. of PCl5 = 107°C
Using fractional distillation, PCl5 given out first, then acyl chloride
If the Acyl Chloride has a low boiling point, PCl3 is used
B.P. of PCl5 = 200°C
Acyl chloride can be obtained by distillation
If the Acyl Chloride has a intermediate boiling point, SOCl2 is used
Gaseous SO2 & HCl pass off first
The Cl is readily substituted by other nucleophiles like OH, OR, NH2, etc.
The carbonyl carbon has charge that can be attacked by the nucleophiles
Acyl chloride can be hydrolyzed rapidly by cold water:
Aromatic alkanoyl chlorides is hydrolyzed much more slowly
The carbonyl carbon is less susceptible to nucleophilic attack
The acyl chloride is less soluble
Hydrolysis is faster w/alkali as OH is a stronger nucleophile
Acyl chlorides react w/alcohols and phenols to form esters
A base catalyst is required for phenol to react w/aromatic alkanoyl chlorides
Basic medium can provide a more powerful nucleophile
Acyl chlorides react w/ammonia to give amides
Ammonolysis:
Alkanamide formed
Low temperature and large excess of ammonia solution is required
To moderate the reaction
To prevent the formation of side product
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Reaction of acyl chlorides w/amines gives amides
1° amine gives N-substituted amides
2° amine gives N,N-disubstituted amides
Acyl chlorides react w/anhydrous sodium salts of acids to give acid anhydrides
ALKANOLYATION is the process of replacing a H in –OH or –NH2 group by an alkanoyl group
BENZOYLATION or BENZENECARBONYLATION is the process of replacing a H in –OH or –NH2 group by
an alkanoyl group
ACETYLATION: Alkanolyation or benzoylation
Acid anhydrides:
Acid anhydrides:
Reacting RCOOH w/alkanoyl chlorides in presence of pyridine
Pyridine is used to remove the HCl formed
Shifts the equilibrium toward the product side
It can also be formed by refluxing alkanoyl chloride w/sodium salt of carboxylic acid
Reaction similar to that of acyl chlorides but w/lower reactivity
Reaction of acid anhydrides often gives carboxylic acids as products
It can be hydrolyzed by water slowly to give carboxylic acids:
(RCO)2O + H2O 2 RCOOH
It reacts w/alcohols & phenols to give esters
(RCO)2O + R'OH RCOOR' + RCOOH
Heating is usually required
A more powerful phenoxide ion is provided if reacting w/phenols in the alkaline medium
It reacts w/ammonia & amines to give amides
(RCO)2O + 2 NH3 RCONH3 + RCOONH4
Amides:
Acid anhydrides:
Reacting RCOOH w/aqueous ammonia
Ammonium carboxylate is form as an intermediate
Heating gives primary amides
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The water formed will hydrolyze the RCOO–NH4+ back to RCOOH
excess RCOOH is added to shift the equilibrium to the product side
It is readily hydrolyzed on refluxing w/dil acid or alkali
Can be served as a test between amine and amide
Only amide reacts w/caustic soda and liberates ammonia gas
Heating amides w/dehydrating agent leads to the formation of nitrile
Dehydrating agent: Phosphorus(V) oxide, P2O5
Amides react w/KOH(aq) and Br2 to give 1° amines
Hofmann degradation
The amine product is one carbon less
Amides can be reduced to 1° amines
By lithium tetrahydridoaluminate (LiAlH4) or sodium in ethanol (RO–Na+)
Esters:
Ester:
Reacting RCOOH w/alcohols in the presence of acid catalyst
The reaction is called esterification
Ester often has a lower BP than alcohol or acid
Ester can be distilled out of the reaction mixture
High activation energy required for esterification
Reaction is too slow without the acid catalysis
Ester can be hydrolyzed in acidic medium
Reverse of esterification, excess water is required to shift the equilibrium
If in alkaline medium, the hydrolysis also occur
The carboxylic acid formed will be reacted w/OH– immediately
The equilibrium will shift to product side w/higher extend
Alkaline hydrolysis1 is much faster
Ester can be reduced by LiAlH4
Primary alcohols formed:
1
Alkaline hydrolysis = Saponification
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