Chemistry

• Molecularity


  The number of molecules that collide during a step in a reaction mechanism
• Types of molecularity


  • Unimolecular
  • Bimolecular
  • Termolecular
• Unimolecular reaction


  A single reactant molecule collides
• Bimolecular reaction


  Two reactant molecules collide
• Termolecular reaction


  Three reactant molecules collide
• Rate law for unimolecular reaction

  rate = k[A]

  
• Rate law for bimolecular reaction

  rate = k[A][B]

  
• Common types of elementary reactions and their rate laws

  • A → products (Unimolecular, rate = k[A], First order)
  • A+A → products (Bimolecular, rate = k[A]2, Second order)
  • A + B → products (Bimolecular, rate = k[A][B], Second order)
  • A +A + B → products (Termolecular, rate = k[A]2[B], Third order)
  • A + B + C → products (Termolecular, rate = k[A][B][C], Third order)
• Reaction mechanism


  Explains the step-by-step processes by which organic molecules react and form products
• Nucleophile


  A species with a lone pair of electrons or a negative charge that donates electrons to form a new bond with an electrophilic atom
• Nucleophiles


  • Cl-, Br- (negatively charged)
  • H2O, ROH (neutral)

  
• Electrophile


  A chemical species (atom, ion, or molecule) that can accept an electron pair from a nucleophile to form a new bond
• Electrophiles


  • H+, Methyl cation (CH3+), Hydronium ion (H3O+) (positively charged)
  • Boron trifluoride (BF3), Aluminum chloride (ALCL3), Iron chloride (FeCL3) (neutral but have vacant orbitals)

  
• Types of reaction mechanisms in organic chemistry

  • Nucleophilic Substitution Reactions
  • Elimination, bimolecular reaction mechanism (E2)

  
• Nucleophilic Substitution Reaction


  A chemical reaction in which an electron-rich species replaces a functional group within an electron-deficient molecule

  
• Substrate


  The molecule containing the electrophile and the leaving functional group
• Leaving Group


  An atom or group that departs from the substrate as a stable molecule or ion after losing a pair of electrons
• SN2 Reaction


  Also known as bimolecular nucleophilic substitution reaction, where the nucleophile attacks the carbon atom from the opposite side of the leaving group

  
• SN2 Reactions


  • CH3Br + aq.KOH → CH3OH + KBr
  • CH3Cl + OH- → CH3OH + Cl-
  • CH3Br + CN- → CH3CN + Br-
• Reactivity of alkyl halides towards SN2 reaction

  Methyl halide ≥ 1° halide ≥ 2° halide ≥ 3° halide

  
• SN1 Reaction


  Also known as unimolecular nucleophilic substitution reaction, involving the formation of a carbocation intermediate

  
• Mechanism of SN1 Reaction

  1. Formation of Carbocation

  2. Nucleophilic Attack
• SN1 Reaction


  • (CH3)3CBr + KOH → (CH3)3COH + KBr
• Kinetics of SN1 Reaction

  Rate depends on concentration of alkyl halide, independent of nucleophile concentration

  
• Reaction of t-butyl bromide with aqueous KOH

  1. Formation of carbocation

  2. Nucleophilic attack by OH-

  3. Yields tertiary butanol

  
• SN1 reaction mechanism


  1. Formation of carbocation intermediate

  2. Nucleophilic attack by nucleophile

  
• Carbocation


  Carbon atom with a positive charge due to the loss of a bonding pair of electrons
• Elimination reaction


  1. Removal of proton

  2. Formation of C=C pi bond

  3. Breaking of bond to leaving group
• E2 elimination


  Bimolecular reaction, alkyl halide reacts with strong base like OH- to form C=C

  
• E2 elimination


  • Commonly observed in primary and certain secondary alkyl halides
  • Reaction rate depends on concentrations of alkyl halide and base

  
• E2 elimination mechanism


  1. Base attacks β-hydrogen

  2. Leaving group departs

  3. C=C bond forms

  
• E1 reaction


  Two-step process, formation of carbocation intermediate then elimination
• E1 reaction


  • Common in secondary and tertiary alkyl halides in absence of strong base
  • Reaction rate depends only on alkyl halide concentration

  
• E1 reaction mechanism


  1. Ionization to form carbocation

  2. Carbocation loses proton from adjacent carbon to form alkene

  
• SN1 vs SN2

  SN1 has unimolecular rate, forms carbocation intermediate

  SN2 has bimolecular rate, no carbocation intermediate
• E1 vs E2

  E1 is two-step with carbocation intermediate, E2 is one-step with no intermediate
• Acids have a pH less than 7.
• Electromagnetic Induction is a process in which a conductor is put in a particular position and the magnetic field keeps varying or magnetic field is stationary and a conductor is moving. This produces a Voltage or EMF (Electromotive Force) across the electrical conductor.

  
• Induced emf


  The electromotive force generated by a varying (changing) magnetic field through a coil of wire

  
• Induced current


  The current that flows in a circuit due to a changing magnetic field