Substitution and Elimination Rxns.

Alkyl Halides

  • Containing a halogen bonded to sp3 hybridized carbon

Primary, secondary and tertiary alkyl halides are named based on how many R groups are attached to the central carbon



Physical properties

  • Weak polar molecules, but since the rest of the molecule is non-polar, not hydrogen bonding

  • Electron deficient so lewis acid

  • Undergo substitution reactions with nucleophiles, so the halide becomes a negative leaving group

Transition state = high energy



When a bond breaks in a primary halide, the electrons go with the halide, acts a leaving group



Base attacks beta hydrogens in an alkane (beta carbons do not have halogens)



  1. Concerted mechanism → bimolecular, happens in 1 step

  2. Back attack - inversion of configuration (Walden inversion) b/c of back attack (attacking the opposite side of the halogen)



Tertiary alkyl halides will not undergo rxn

methyl/primary alkyl halides undergo the rxn faster

secondary halides undergo the rxn slower



Rate is measured in area

Nucleophilic substitution rxns are exothermic



Nucleophiles are negatively charged with a lone pair of e- to make a new bond

  • The better leaving group is a weaker base

  • Nucleophiles attacks the carbon (e- deficient) 

Bases attack protons

Atoms with less steric hindrance = stronger nucleophiles

Nucleophilicity increases down a column when in polar protic solvents

F regularly is not a good base or nucleophile



SN1

  • For tertiary alkyl halides

  • Step 1 - bond broken (slow so rate determining)

  • Nucleophile attacks positive carbon

  • 2 steps, 2 transition states

  • Kinetics is first order

  1. First the carbon is sp3, stepwise mechanism + carbocation formation

  2. Then the carbon is sp2, unimolecular transition state

  3. Racemic mixture

Since the compound is trigonal planar, the molecule can be attacked from the top or bottom - will give R and S configs



With this mechanism (SN1) = 3rd degree >> 2nd degree >> primary > methyl b/c of the stability of the 3rd degree carbocations



SN2

  • 2nd order kinetics

  • one step

  • back attack, inversion

  • Unhindered halides react faster



SN1 

  • First order

  • 2 steps

  • Trigonal planar carbocation

  • Racemic mixture produced

  • More substituted halides react fastest



Predicting the substitution

Nature of R Group

  • Methyl, primary alkyl halide = SN2

  • Tertiary = SN1

  • Secondary = either SN1/2



Nature of Nucleophile

Strong Nucleophile - SN2

Weak nucleophiles (i.e. water, ROH) - SN1



Leaving Group

  • Better leaving group will accept negative charge, Iodine is best 

  • Affect both in same way



Nature of Solvent

  • Polar protic solvents - SN1 reaction

  • Polar aprotic solvents - SN2, nucleophiles are not well solvenated 



Elimination (similar to SN2)

  • The base will grab the beta hydrogen

  • Concerted, 1 step mechanism 

  • Second order, depends on alkyl halide + base

  • Zaitsev’s rule

  • Tertiary alkyl halides undergo this reaction fastest, methyl reactions can’t because it only has 1 carbon

  • Favoured by strong bases

  • Polar aprotic solvents 



Trisubstituted = 4 R groups around the double bond, more substitutions = more stable



Trans is more stable for alkenes



E1 - unimolecular elimination

  • First order kinetics

  • Breaking carbon hydrogen bond → carboncation (tertiary, most stable)

  • Step wise 

  • Weaker bases preferred 

  • Wants a better leaving group

  • Polar protic solvent

E1 and SN1 are competing reactions so E1 mechanisms produce a mixture of results = less useful



E2

  • Dehydrohalogenation is the mechanism

  • 2nd order kinetics 

  • Concerted 

  • Exothermic 

  • Tertiary is fastest

  • Favoured by polar aprotic solvents 

Stronger base = faster E2 reaction



SN2 - methyl > primary > secondary > tertiary

SN1, E1, E2 - opposite of SN2



Tertiary alkyl halide - everything but SN2

  • Strong base - E2 mechanism 

Weak nucleophiles - SN1 + E1

Primary alkyl halides - SN2 + E2

  • Strong nucleophiles - SN2

  • Strong, sterically hindered base - E2

Secondary

  • Strong bases + nucleophiles (mixture of SN2 + E2)

  • Strong sterically hindered bases (E2)

  • Weak nucleophiles/bases - E1 + SN1