JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 09

Reactants and Bond Cleavage

In a reaction mechanism the molecule that supplies carbon to the new bond is the substrate and the other reactant is the reagent. A mechanism describes, step by step, how electrons move, how energy changes and how fast each step runs. Four ideas are needed to follow any mechanism: how the bond breaks, what attacks, which intermediate forms, and which electronic effects operate.

In heterolysis of C-X, the atom with the larger electronegativity takes the pair. Chlorine takes it from carbon (giving a carbocation), while a metal such as Cu or Mg gives up the pair to carbon (giving a carbanion).

Attacking Reagents

Ambident nucleophiles such as cyanide or nitrite can attack through two different atoms. Neutral nucleophiles are a good test of understanding: water and ammonia are nucleophiles because of their lone pairs, while SO3 is an electrophile.

Reaction Intermediates

Carbocations may rearrange by a hydride or alkyl shift to give a more stable ion. Carbenes (divalent carbon with six electrons) and nitrenes are other reactive intermediates.

Inductive Effect

1. The permanent displacement of sigma electrons along a carbon chain caused by an atom or group of different electronegativity. It acts through sigma bonds and fades rapidly with distance, becoming negligible after about three carbons.
2. Groups that attract sigma electrons more than hydrogen show a -I effect: \(\ce{-NO2}\) > \(\ce{-CN}\) > \(\ce{-COOH}\) > \(\ce{-F}\) > \(\ce{-Cl}\) > \(\ce{-Br}\) > \(\ce{-I}\) > \(\ce{-OH}\) > \(\ce{-C6H5}\).
3. Groups that push sigma electrons show a +I effect: alkyl groups, with the order tertiary > secondary > primary > methyl, and negatively charged groups such as \(\ce{-O-}\) and \(\ce{-COO-}\).
4. Acid strength: a -I group stabilises the conjugate base, so chloroacetic acid is stronger than acetic acid and acidity falls as the group moves away from the COOH. A +I group weakens an acid.
5. Base strength of amines: a +I group raises the electron density on nitrogen and so basicity; in water solvation also matters, giving the order secondary > primary > tertiary for methylamines and secondary > tertiary > primary for ethylamines. In the gas phase tertiary > secondary > primary > ammonia.
6. Stability of cations is raised by +I and lowered by -I groups; the reverse holds for anions.

Resonance (Mesomeric) Effect

1. When a molecule cannot be written correctly with a single Lewis structure, it is a hybrid of several resonance (canonical) structures. Only electrons move, never the atoms. The hybrid is more stable than any one structure, and the difference in energy is the resonance energy.
2. Rules for writing resonance structures: the atoms stay in the same positions; the number of paired and unpaired electrons is unchanged; every atom of the second period keeps to the octet; the structures should have the same or nearly the same energy to contribute well.
3. More stable contributors have more covalent bonds, complete octets, the smaller charge separation and any negative charge on the more electronegative atom.
4. +M (+R) groups donate a lone pair or pi electrons into the system: \(\ce{-NH2}\), \(\ce{-OH}\), \(\ce{-OR}\), \(\ce{-X}\) (weakly), \(\ce{-NHCOR}\).
5. -M (-R) groups withdraw pi electrons: \(\ce{-NO2}\), \(\ce{-CN}\), \(\ce{-CHO}\), \(\ce{-COR}\), \(\ce{-COOH}\), \(\ce{-SO3H}\).
6. A donor group makes the ortho and para positions electron rich; a withdrawing group makes them electron poor. This explains directing effects.
7. Halogens are -I and +M: the -I effect dominates, so they deactivate the ring, but the +M effect directs the attack to ortho and para.
8. Steric inhibition of resonance: a bulky group twists the donor out of the plane of the ring and cuts off the effect (for example in N,N-dimethyl-2,6-dimethylaniline).
9. Bond lengths in a resonance hybrid are between single and double (for example C-O in carboxylate, N-O in a nitro group are equal).

Applications of Resonance

1. Stability of carbocations: a positive carbon next to a lone pair (as in CH3-CH(+)-OCH3, or the cation next to nitrogen) or next to a pi bond (allyl, benzyl) is stabilised by delocalisation.
2. Stability of carbanions: delocalisation of the negative charge, especially onto oxygen (enolate), makes the carbanion more stable.
3. Acidity of phenol: the phenoxide ion is stabilised by delocalising the charge into the ring, so phenol is more acidic than alcohols. -M groups at ortho and para (nitro) raise the acidity much more than at meta. Carboxylate ions are stabilised by two equivalent structures.
4. Basicity of amines: in aniline the lone pair is delocalised into the ring, so aniline is much less basic than cyclohexylamine. A p-nitro group reduces basicity further; a p-methoxy group raises it slightly.
5. Reactivity of aryl halides: the lone pair of chlorine in chlorobenzene gives the C-Cl bond some double-bond character, making it shorter and harder to break.
6. Stability of alkenes and dienes: conjugated dienes are more stable than isolated dienes.

Aromaticity

1. A species is aromatic if it is cyclic, planar, fully conjugated (every ring atom has a p orbital) and has \(4n+2\) pi electrons (Huckel rule, n = 0, 1, 2 ...).
2. Antiaromatic species are cyclic, planar and conjugated with \(4n\) pi electrons and are unusually unstable (cyclobutadiene, cyclopentadienyl cation, oxirene).
3. Non-aromatic species have an sp3 atom that breaks the conjugation or are not planar. Cyclooctatetraene has 8 pi electrons but puckers into a tub shape to avoid being antiaromatic.
4. Examples of aromatic species: benzene, cyclopentadienyl anion, tropylium cation (cycloheptatrienyl cation), pyridine, pyrrole, furan, thiophene, naphthalene, the cyclopropenyl cation and the cyclobutadiene dication.

Hyperconjugation

1. Delocalisation of the sigma electrons of a C-H bond (alpha hydrogen) into the adjacent empty p orbital, pi bond or half-filled p orbital. It is also called no-bond resonance or the Baker-Nathan effect.
2. Conditions: an sp2 carbon (or cation, or radical centre) attached to an sp3 carbon bearing at least one hydrogen.
3. The number of hyperconjugative structures equals the number of alpha hydrogens: methyl 3, ethyl 2, isopropyl 1, tert-butyl 0. Order of hyperconjugation: \(\ce{CH3}\) > \(\ce{CH3CH2}\) > \((\ce{CH3})_2\ce{CH}\) > \((\ce{CH3})_3\ce{C}\), although the +I order is the reverse.
4. Uses: stability of carbocations (tertiary > secondary > primary), stability of alkenes (more substituted is more stable), stability of radicals, heat of hydrogenation, dipole moment of propene and toluene, bond length (the C-C bond next to a double bond is shortened, the C=C is slightly lengthened) and the ortho-para directing effect of alkyl groups.
5. Deuterium has weaker hyperconjugation than hydrogen (stronger C-D bond), which is the reason for the Baker-Nathan order.

Electromeric Effect

1. The complete and temporary transfer of a shared pi pair to one of the atoms of a multiple bond when an attacking reagent approaches. It is shown only in the presence of a reagent and disappears when the reagent is removed.
2. +E effect: the pi electrons move toward the atom that bonds to the attacking species (electrophile attacks), for example in addition of HX, hydroboration and hydration of an alkene.
3. -E effect: the pi electrons move away from the atom that bonds to the attacking species, for example when a nucleophile such as cyanide attacks a carbonyl carbon and the pair moves onto oxygen.
4. Only compounds with a double or triple bond show this effect. If the inductive or mesomeric effect has already polarised the bond, the electromeric effect usually acts in the same direction.

Tautomerism

1. Tautomers are structural isomers in rapid dynamic equilibrium that differ in the position of a hydrogen atom (and of a double bond). They are different compounds, whereas resonance structures are only different pictures of one molecule.
2. Keto-enol tautomerism needs a carbonyl compound with an alpha hydrogen. Ketones and aldehydes without an alpha hydrogen (benzaldehyde, benzophenone, di-tert-butyl ketone, methanal) do not enolise.
3. The enol content is high where the enol is stabilised: in 1,3-dicarbonyl compounds such as pentane-2,4-dione by intramolecular hydrogen bonding and conjugation, and in phenol (almost entirely enol form).
4. Other tautomeric pairs: nitroalkane and aci-nitro form, imine and enamine, amide and imidic acid, and the cyanide and isocyanide pair, which are better described as functional isomers.
5. Tautomerism is catalysed by acids and bases and explains the reactions of enolates.

Comparison of Acid and Base Strengths