JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 20

Basic Strength: Definitions

A Bronsted-Lowry base accepts a proton and a Bronsted-Lowry acid donates one. When an acid loses its proton what remains is its conjugate base, and when a base gains a proton the result is its conjugate acid. The stronger the acid, the weaker (and more stable) its conjugate base. A Lewis base donates an electron pair (NH\(_3\)) and a Lewis acid accepts one (AlCl\(_3\), H\(^+\)).
Basicity is measured by K\(_b\) for \(\ce{B + H2O <=> BH+ + OH-}\); pK\(_b\) = -log K\(_b\), so a smaller pK\(_b\) means a stronger base. In the same way acidity is measured by K\(_a\) and pK\(_a\) = -log K\(_a\); a smaller pK\(_a\) means a stronger acid.
A base is strong when the lone pair is easily available, or when the conjugate acid it forms is well stabilised. Basicity falls across a period as the donor atom becomes more electronegative (for example NH\(_3\) > H\(_2\)O).

Basic Strength of Aliphatic Amines

1. In the gas phase or in non-polar solvents the +I effect decides: 3\(^\circ\) > 2\(^\circ\) > 1\(^\circ\) > NH\(_3\).
2. In water the ammonium ion is also stabilised by hydrogen bonding with solvent. More alkyl groups mean fewer N-H bonds and less solvation, so the +I gain is partly cancelled. The observed orders are R\(_2\)NH > RNH\(_2\) > R\(_3\)N > NH\(_3\) for R = CH\(_3\), and R\(_2\)NH > R\(_3\)N > RNH\(_2\) > NH\(_3\) for R = C\(_2\)H\(_5\).
3. Hybridisation of nitrogen: a lone pair in an sp\(^3\) orbital (amine) is held less tightly than in sp\(^2\) (imine) and sp (nitrile), so basicity falls in the order amine > imine > nitrile. An electronegative or unsaturated group near the nitrogen lowers basicity: CH\(_3\)CH\(_2\)CH\(_2\)NH\(_2\) > CH\(_2\)=CHCH\(_2\)NH\(_2\) > HC\(\equiv\)CCH\(_2\)NH\(_2\).
4. A cyclic amine whose alkyl groups are tied back is more basic than the open-chain amine of similar type, because the lone pair is better exposed.
5. Amidines, RC(=NH)NH\(_2\), are strongly basic. The imine nitrogen takes the proton and the cation is stabilised by resonance between the two nitrogens, so the doubly bonded nitrogen is the more basic one.

Aromatic Amines and Heterocycles

The lone pair of aniline sits in a 2p orbital in conjugation with the ring, so it is partly delocalised and aniline is a much weaker base than ammonia or cyclohexylamine. In pyridine the lone pair lies in an sp\(^2\) orbital in the plane of the ring, outside the 6-pi-electron system, so it is available and pyridine is more basic than aniline. In pyrrole the lone pair is part of the aromatic sextet; protonation would destroy aromaticity, so pyrrole is an extremely weak base.

Order: pyridine > aniline > pyrrole

Substituted anilines: electron-releasing groups (+M, +I, hyperconjugation) raise basicity; electron-withdrawing groups (-M, -I) lower it. Almost every ortho substituent weakens the base (ortho effect): it hinders solvation of the -NH\(_3^+\) group in the conjugate acid and crowds the nitrogen. Very small groups such as -OH and -NH\(_2\) escape this.

Typical pK\(_b\) values in water: methylamine 3.38, dimethylamine 3.27, trimethylamine 4.22, ethylamine 3.29, diethylamine 3.00, triethylamine 3.25, benzylamine 4.70, aniline 9.38, N-methylaniline 9.30, N,N-dimethylaniline 8.92.

Basic Strength of Anions

The conjugate base of a weaker acid is the stronger base. A rough order for carbon, nitrogen, oxygen and halogen anions is: R\(_3\)C\(^-\) > R\(_2\)CH\(^-\) > RCH\(_2^-\) > CH\(_3^-\) > Ph\(^-\) > PhCH\(_2^-\) > NH\(_2^-\) > Ph\(_2\)CH\(^-\) > Ph\(_3\)C\(^-\) > H\(^-\) > RC\(\equiv\)C\(^-\) > RC(=O)CH\(_2^-\) > RO\(^-\) > HO\(^-\) > PhO\(^-\) > RCOO\(^-\) > H\(_2\)O > Cl\(^-\).

Reactions of Amines as Bases

Amines react with water to give alkylammonium hydroxides, so their solutions are alkaline; the solution of an alkylamine precipitates brown Fe(OH)\(_3\) from ferric chloride. With acids amines form water-soluble ionic salts: \(\ce{RNH2 + HCl -> RNH3+Cl-}\), \(\ce{2RNH2 + H2SO4 -> (RNH3)2SO4}\), and aniline gives anilinium chloride (aniline hydrochloride) with HCl.

Acidic Strength

A compound is a strong acid when its conjugate base is stable. Factors that stabilise the anion are resonance delocalisation, electron-withdrawing groups nearby, greater s-character of the carbon bearing the charge and a more electronegative or larger atom carrying it. Acid strength is therefore best judged by drawing the conjugate base.

Hydrocarbons: acidity rises with s-character of the C-H carbon, so HC\(\equiv\)CH > H\(_2\)C=CH\(_2\) > H\(_3\)C-CH\(_3\).

Phenols are about a million times more acidic than ethanol (pK\(_a\) 10 against 15.9) because the phenoxide ion is stabilised by resonance. Electron-withdrawing groups at ortho or para positions (-NO\(_2\), -CN, -CHO) strengthen the acid; electron-donating groups (-CH\(_3\), -OCH\(_3\)) weaken it. Phenols with a nitro group at the meta position are helped only by the -I effect, so the order is para > ortho > meta > phenol for nitrophenols (the ortho isomer is weakened by intramolecular hydrogen bonding). Cresols are weaker than phenol, and the methyl group is least harmful at the meta position.

Acidity of Carboxylic Acids

The carboxylate ion is stabilised by two equivalent resonance structures with the charge spread equally over both oxygens, so carboxylic acids are far more acidic than phenols and alcohols. Electron-withdrawing groups raise acidity and electron-releasing groups lower it; the effect weakens with distance from the COOH group.
Decreasing acidity: CF\(_3\)COOH > CCl\(_3\)COOH > CHCl\(_2\)COOH > NO\(_2\)CH\(_2\)COOH > NCCH\(_2\)COOH > FCH\(_2\)COOH > ClCH\(_2\)COOH > BrCH\(_2\)COOH > HCOOH > ClCH\(_2\)CH\(_2\)COOH > C\(_6\)H\(_5\)COOH > C\(_6\)H\(_5\)CH\(_2\)COOH > CH\(_3\)COOH > CH\(_3\)CH\(_2\)COOH.
Effect of the group on acidity (increasing): Ph < I < Br < Cl < F < CN < NO\(_2\) < CF\(_3\). A dicarboxylic acid with the two COOH groups closer together is more acidic in its first dissociation.

Maleic and Fumaric Acids

The first dissociation constant of maleic acid (cis) is greater than that of fumaric acid (trans), because the monoanion of maleic acid is stabilised by an intramolecular hydrogen bond. The second dissociation constant of maleic acid is smaller than that of fumaric acid, because in the maleate dianion the two negative carboxylate groups sit close together and repel each other. So K\(_1\)(maleic) > K\(_1\)(fumaric), but K\(_2\)(fumaric) > K\(_2\)(maleic).

Substituted Benzoic Acids and the Ortho Effect

Formic acid is stronger than benzoic acid, and phenylacetic acid is stronger than acetic acid. A para electron-donating group always makes a benzoic acid weaker than benzoic acid itself, and weaker than its meta isomer. A meta withdrawing group makes it stronger. Typical order: p-nitro > p-chloro > benzoic acid > p-methyl > p-methoxy.
Almost every ortho-substituted benzoic acid is stronger than benzoic acid and than its meta and para isomers. This is the ortho effect, a mix of steric crowding (the COOH is twisted out of the ring plane so it cannot be conjugated, which favours loss of H\(^+\)) and electronic effects. For example o-toluic acid > benzoic acid > m-toluic acid > p-toluic acid, and o-hydroxybenzoic acid (salicylic acid) is much stronger than its isomers because the anion is stabilised by an intramolecular hydrogen bond.
Steric inhibition of resonance (SIR) can also reduce acidity: in 2,6-dimethyl-4-nitrophenol the two methyl groups twist the NO\(_2\) group out of the plane of the ring, so it cannot withdraw by -M and the compound is a weaker acid than 3,5-dimethyl-4-nitrophenol. Picric acid (2,4,6-trinitrophenol) is as acidic as a strong carboxylic acid.

Feasible Acid-Base Reactions

A reaction of the type NaX + HY gives NaY + HX is feasible only when HY is the stronger acid than HX; the weaker acid is displaced as a gas, liquid or precipitate. Examples: 2NaCl + H\(_2\)SO\(_4\) gives Na\(_2\)SO\(_4\) + 2HCl (feasible), but Na\(_2\)SO\(_4\) + 2HCl does not react; CH\(_3\)COONa + CH\(_3\)SO\(_3\)H gives acetic acid (feasible), but CH\(_3\)COONa + PhOH does not react. Carboxylic acids (stronger than H\(_2\)CO\(_3\)) dissolve in NaHCO\(_3\) with evolution of CO\(_2\); phenols (weaker than H\(_2\)CO\(_3\)) do not, but they dissolve in NaOH.

Tautomerism

Tautomerism is the existence of one compound as two or more readily interconvertible isomers that differ in the position of an atom, usually hydrogen. The two forms are in dynamic equilibrium and each may be isolated and gives its own tests. The commonest case is keto-enol tautomerism, which needs an sp\(^3\) carbon carrying at least one hydrogen attached to a carbonyl (also imine, nitroso or nitro) group. The alpha hydrogen moves to the more electronegative atom and the double bond shifts: \(\ce{H-C-C=O <=> C=C-OH}\).

Keto-Enol Tautomerism: Catalysis and Enol Content

Base catalysis: the base removes an alpha hydrogen to give an enolate ion (the common conjugate base of the keto and enol forms), which is protonated on oxygen to give the enol or on carbon to return to the carbonyl compound. Acid catalysis: the carbonyl oxygen is protonated, and loss of an alpha proton from this cation gives the enol.
A simple aldehyde or ketone contains very little enol. The enol content increases with factors that stabilise the enol: conjugation (resonance), hyperconjugation and intramolecular hydrogen bonding. Thus a compound with an active methylene group between two carbonyl groups has a high enol content: acetylacetone is about 75 % enol, while ethyl acetoacetate is only about 7 to 8 % because the ester group withdraws electrons less strongly than a ketone. The more acidic the active methylene group, the greater the enol content. The enol content is higher in non-polar solvents, and the keto form is favoured in polar solvents.

Racemisation and Deuterium Exchange

Because of enolisation, the alpha hydrogens of a carbonyl compound are exchanged by deuterium in D\(_2\)O (acid or base catalysed), and an optically active compound whose chiral centre is an alpha carbon is racemised, since the planar enol (or enolate) has lost the stereocentre and can be reprotonated from either face.

Other Tautomeric Systems

Imine and enamine: an imine R\(_2\)CH-CR=NR is in equilibrium with the enamine R\(_2\)C=CR-NHR; the enamine is stable only when the nitrogen has no hydrogen. Nitroso and oxime: a primary or secondary nitroso compound rearranges to the more stable oxime. Nitro and aci-nitro: a nitroalkane with an alpha hydrogen (RCH\(_2\)NO\(_2\)) is in equilibrium with the aci form RCH=N(O)OH, with the nitro form being far more stable. Lactam and lactim: an amide N-H with an adjacent C=O is in equilibrium with the iminol form N=C-OH.

Worked Examples

Example 1. Arrange p-nitrophenol, phenol, p-cresol and cyclohexanol in decreasing order of acidity.
Cyclohexanol has no resonance stabilisation of its anion, so it is the weakest acid. Among the phenols the -M, -I nitro group stabilises the phenoxide most; the methyl group of p-cresol destabilises it. Order: p-nitrophenol > phenol > p-cresol > cyclohexanol.

Example 2. Which is the stronger base, aniline or N-methylaniline in non-aqueous medium?
In a non-polar medium only the +I effect matters, and the methyl group raises the electron density on nitrogen, so N-methylaniline is the stronger base (pK\(_b\) 9.30 against 9.38 in water).

Example 3. Order the acids FCH\(_2\)COOH, Cl\(_2\)CHCOOH, CH\(_3\)COOH and ClCH\(_2\)CH\(_2\)COOH.
Two chlorine atoms on the alpha carbon give the largest -I effect, then one fluorine on the alpha carbon, then one chlorine placed one carbon farther away, and finally the methyl group (+I): Cl\(_2\)CHCOOH > FCH\(_2\)COOH > ClCH\(_2\)CH\(_2\)COOH > CH\(_3\)COOH.

Example 4. Which of the following has the highest enol content: CH\(_3\)COCH\(_3\), CH\(_3\)COCH\(_2\)COCH\(_3\) or CH\(_3\)COCH\(_2\)COOC\(_2\)H\(_5\)?
The diketone gives an enol stabilised by conjugation and an intramolecular hydrogen bond, and its CH\(_2\) is the most acidic. So acetylacetone has the highest enol content, then the keto ester, then acetone.