Organic chem

• Electrophiles:

  • electron-poor species (positively charged Ions, neutral molecules carrying partial positive charge)
  • Accepts an electron pair from electron rich site
• Nucleophiles:

  • electron rich species (negatively charged ions, neutral molecules containing lone pair of electrons)
  • donates an electron pair to an electron deficient site
• Markonikov’s rule for electrophilic addition reactions:

  • major product is the one where the hydrogen atom of an electrophile attaches itself to the carbon atom carrying a greater number of hydrogen atoms (will attach to primary>secondary>tertiary)
  • positive charge will be on carbon atom with less hydrogen atoms attached
  • tertiary carbocations most stable, primary least stable
• Electrophilic addition reactions occur for:

  • alkenes
• Reason for benzene undergoing electrophilic substitution:

  • pi electron cloud region of high electron density, resonance stabilisation makes it less reactive compared to alkenes towards electrophilic attack
  • To preserve resonance stabilised ring (substitution)
• Reagents for oxidation reactions, which compounds will be oxidised:

  • KMnO4, H2SO4, heat —> alkenes, alkyl side chain of arenes, primary and secondary alcohols, aldehydes, methanoic acid and ethanedioic acid
  • K2Cr2O7, H2SO4, heat—> primary and secondary alcohols, aldehydes, methanoic acid
  • K2Cr2O7, H2SO4, immediate distil —> primary alcohol
• Nucleophilic substitution reactions occur for:

  • alcohols
  • halogenoalkanes
  • acyl chlorides
  • esters
  • Carboxylic acids
• Nucleophilic addition reactions occur for:

  • aldehydes
  • ketones
• Effect of activating substituents on benzene:

  • electron donating groups donate electron density to benzene ring
  • increases availability of electron cloud
  • stabilises arenium ion intermediate by dispersing positive charge
  • lowers Activation energy needed, benzene ring more susceptible to electrophilic attack
• Effect of deactivating substituents on benzene ring:

  • electron withdrawing groups that withdraw electron density away from the benzene ring
  • decreases availability of electron cloud
  • destabilise arenium ion by intensifying positive charge
  • increases activation energy, benzene ring less susceptible to electrophilic attack
• halogenoalkanes:

  • carbon atom attached to halogen carries a partial positive charge (electron deficient), halogen atom is more electronegative
  • susceptible to nucleophilic attack
• alcohols:

  • partial positive carbon atom (-OH withdraws electron density), susceptible to nucleophilic attack
  • O atom has lone pair of electrons, can act as nucleophile
  • O-H bond polarised, H+ can be released (acts as Bronsted-Lowry acid)
• phenols:

  • lone pair of electrons on O can delocalise into benzene ring, gives partial double bond character
  • O-H bond polarise, H+ can be released (Bronsted-Lowy acid)
• alcohols less acidic than water and phenols
• phenols more acidic than water and alcohols:

  • 2p orbital of O carries lone pair of electrons, side-on overlap with the pi orbitals of benzene ring
  • lone pair can delocalise into benzene ring, negative charge is dispersed, phenoxide ion is stabilised
• effect of electron donating substituents on alcohols:

  • donates electron density, intensifies negative charge, destabilises conjugate base
  • weaker acid
• effect of electron withdrawing substituents on alcohol

  • withdraws electron density, disperses negative charge, stabilises conjugate base
  • stronger acid
• carbonyl compounds:

  • carbon carries partial positive charge due to highly electronegative O atom
  • carbon atom is electron deficient, susceptible to nucleophilic addition
• carboxylic acids:

  • carbonyl carbon present (carries partial positive charge, electron deficient, susceptible to nucleophilic attack)
  • O-H bond polarised, H+ can be released
• stability of conjugate base of carboxylic acids:

  • carboxylate ion forms 2 equivalent resonance structures
  • negative charge is delocalised over 2 highly electronegative oxygen atoms
• effect of electron withdrawing substituents on carboxylic acids

  • withdraws electron density, disperses negative charge, stabilises anion
  • stronger acid
• effect of electron donating substituents on carboxylic acids:

  • donates electron density, intensifies negative charge, destabilises anion
  • weaker acid
• the greater the number of electron donating substituents, the greater the extent of negative charge intensification, anion destabilised, weaker acid
• the greater the number of electron withdrawing substituents, the greater the extent of negative charge dispersal, anion stabilised, stronger acid
• the closer the ew/ed substituent, the greater its effects
• acyl chlorides and esters susceptible to nucleophilic substitution (carbonyl carbon is electron deficient)
• acyl chlorides:

  • carbonyl carbon is very electron deficient (bonded to 2 highly electronegative atoms)
  • sp2 hybridised carbon, trigonal planar, less steric hindrance
  • C-Cl bond can be broken easily (hydrolysed upon contact with water)
• Explaining cis-trans:

  • restricted rotation about the C=C bond due to presence of pi bonding
  • 2 diff groups attached to each carbon atom of the C=C
• chiral molecule: has at least 1 chiral carbon and no plane of symmetry, gives rise to a pair of isomers that are non-superimposable mirror images, chiral carbon has 4 diff groups attached
• optical activity: ability of chiral molecules with no internal plane of symmetry to rotate plane polarised light
• Alkenes cannot be reduced by LiAlH4 since source of hydrogen is from H- ions, cannot attack non-polar C=C bond, compounds with partial positive charges (like carbonyl carbons) can be reduced by LiAlH4
• why carbon atom of CN- attacks carbonyl molecule:

  • carbon atom is negatively charged, lone pair of electrons more available for nucleophilic attack
• effect of electron donating groups on amines:

  • donates electron density, increases electron density of the lone pair of electron on N
  • lone pair of electrons more available for donation to a proton, stronger base
• Effect of electron withdrawing groups on amines:

  • withdraws electron density, decreases electron density of the lone pair of electrons on N
  • Lone pair of electrons less available for donation to a proton, weaker base
• Phenylamine is the least basic:

  • lone pair of electrons on N delocalises into the benzene ring due to 2p orbital overlap with pi electron cloud
  • Lone pair of electrons less available for donation to a proton
• Amides are neutral:

  • lone pair of electrons on N delocalised into the C=O bond not available for donation