Halogenoalkane Reactions
Nucleophilic substitution
Mechanisms
Rate-Determining Step
• A reaction is only as fast as the slowest step
• This is called the Rate-Determining Step
• We are concerned with the number of molecules
involved in the rate determining step
Questions:
• If a molecule IS NOT involved in the RDS, will its
concentration affect the rate?
• If a molecule IS involved in the RDS, will its
concentration affect the rate?
Transition States
• Reactions can have steps between the reactant
and the product
• This can be an intermediate or a transition state
• An intermediate is an in between product that
usually doesn’t last very long
• A transition state is not a compound but when
the reactants are colliding while bonds break and
form
• These are at the top of the energy diagram
Mechanism
• Steps of a reaction
• Rate determining step is the slowest step in a rxn
• Can have SN1 or SN2 mechanism
• Depends on molecularity of rate determining step
• SN1 has one molecule SN2 has molecules in the
slow step
Things that affect HOW a reaction
proceeds:
• Steric hindrance: how easy is it to get at what
you are attacking
• Are there lots of things blocking it?
• Is there a clear shot at it?
• How stable is the intermediate?
Nucleophile
• Loves positive or negative?
• Reagent that attacks at the center of a positive
charge
• Does so by donating an electron pair
• Review: What else donates an electron pair?
Nucleophiles
• Who has an electron pair to donate?
• Who is going to have a positive (or partial
positive)charge?
• Halogenoalkanes have C-X bond
• Polar, so partial charges
• Are halogenoalkanes more/less reactive than
regular alkanes?
Halogenoalkanes
• Undergo nucleophilic substitution reactions
• Nucleophile attacks C and substitutes for the
halogen
• Ex: R-X (l) + OH- (aq)  R-OH (l) + X – (aq)
Hydrolysis reactions form alcohols
• From biology: what is hydrolysis?
• In alkali solutions (often NaOH) with heat
Example: hydrolysis of bromobutate
C4H9 – Br (l) + OH-  …
C4H9 – Br (l) + OH-  C4H9 – OH (aq) + Br – (aq)
SN1
First order nucleophilic substitution
The slow step is unimolecular (only involves one species)
The slow step is when C-X bond undergoes heterolytic fission.
(hetero= each ends up with a different than original # electrons )
Draw the structural formula for 2-chloro, 2- methyl propane
The reaction intermediates are X- and C+ ( a carbocation)
Draw the structural formula of the carbocation.
More SN1
Next step: OH – attacks C+ to yield product
• This step is very quick
• What is the OH- called?
SN1 mechanism are common with 30 carbocations
• More stable as charge distributed by carbon
neighbors
• Also original form is sterically hindered for attack
of a nucleophile
Kinetics
• The Rate Determining Step involves only the
halogenoalkane
• So [Nu-] has no effect on rate
• Ex C4X  C4+ + X: - slow
•
C4+ + Nu: -  C4Nu fast
• Faster than SN2 reactions
Hint: Writing one exam is faster than 2
• Cleaning one room is faster than 2
guweb2.gonzaga.edu
SN2
• Second order nucleophilic substitution
• The slow step is bimolecular: involves 2 species
(second order)
• The slow step is the attack of the nucleophile on
the C-X bond
• No carbocation intermediate exists
• A transition state exists while bonds break and
form
• The substitution occurs in one step
SN2 Example
• Nu:- + CRH2– X  [Nu:- - CRH2 -X]  Nu-CRH2 + X:-
• Rate depends on [Nu] AND [R-X]
• Common for 10 halogenoalkanes
• Carbocation would be unstable
• No steric hindrance to attack of C-X
chemgapedia.de
Review at your own pace:
• http://www.chemguide.co.uk/physical/basicra
tes/energyprofiles.html Kinetics
• http://www.chemguide.co.uk/mechanisms/nu
csub/whatis.html#top Nucleophilic
Substitution
• http://www.chemguide.co.uk/mechanisms/nu
csub/hydroxide.html#top Nu- rxns with OH-