L1 - Nucleophillic Substitution

Understandings

• SN1 represents a nucleophilic unimolecular substitution reaction and SN2 represents a nucleophilic bimolecular substitution reaction.
• SN1 involves a carbocation intermediate.
• SN2 involves a concerted reaction with a transition state.
• For tertiary halogenoalkanes the predominant mechanism is SN1 and for primary halogenoalkanes it is SN2. Both mechanisms occur for secondary halogenoalkanes.
• The rate determining step (slow step) in an SN1 reaction depends only on the concentration of the halogenoalkane, rate = k[halogenoalkane].
• For SN2, rate = k[halogenoalkane][nucleophile]. SN2 is stereospecific with an inversion of configuration at the carbon.
• SN2 reactions are best conducted using aprotic, non-polar solvents and SN1 reactions are best conducted using protic, polar solvents.

Applications and Skills

• Explanation of why hydroxide is a better nucleophile than water.
• Deduction of the mechanism of the nucleophilic substitution reactions of halogenoalkanes with aqueous sodium hydroxide in terms of SN1 and SN2 mechanisms.
• Explanation of how the rate depends on the identity of the halogen (ie the leaving group), whether the halogenoalkane is primary, secondary or tertiary and the choice of solvent.
• Outline of the difference between protic and aprotic solvents.

Heterolytic Fission

• These are reactions in which the bond between the carbon atom and the halogen atom breaks by heterolytic fission.
• This means that when the bond breaks, one atom gets a lone pair of electrons, the other gets none. The result is that ions are formed

Nucleophilic Substitution

• The most common reaction of the halogenoalkanes is nucleophilic substitution (SN1 and SN2 mechanisms)
• The type of mechanism depends on the nature of the halogenoalkane - primary halogenoalkanes react via SN2 and tertiary halogenoalkanes via SN1.
• The reason for the different mechanisms lies with the stability of the intermediate tertiary carbocation which forms in the case of the SN1 mechanism, whereas a primary carbocation would not be stable encouraging SN2.
• Another factor encouraging the SN1 mechanism in tertiary halogenoalkanes is the steric hindrance on approaching nucleophile experiences, preventing easy access to the partially positive carbon atom

SN1

• First, due to the electron withdrawing effect of the halogen, the carbon-halogen bond breaks heterolytically.
• This forms a carbonium ion intermediate, which is stabilised by the presence of three +I (positively inductive) methyl groups.

SN2

• Rather than completely breaking the bond, the polar bond between the halogen and carbon produces a partial +ve charge on the carbon.
• This is enough to attract a nuleophile to form a high energy transition state, which effectively has 5 bonds, one to the nucleophile, one with the halogen and 3 others.
• This is the rate determining step, hence the second order reaction.
• The halide ion then breaks off heterolytically forming CH3Nu + Cl-.

• When nucleophile enters the molecule, a Walden inversion in which the shape of the molecule inverts. Similar to analogy of inverting umbrella

Rate of Reaction

An important reaction step is the breaking of the carbon-halogen (C-X) bond

The rate of reaction depends on the strength of the C-X bond

C-I 238 kJmol^-1 weakest - easiest to break

C-Br 276 kJmol^-1

C-Cl 338 kJmol^-1

C-F 484 kJmol^-1 strongest - hardest to break

Proving this Experimentally

• Water is a poor nucleophile but it can slowly displace halide ions

C₂H₅Br(l) + H₂O(l) ——> C₂H₅OH(l) + H⁺ (aq) + Br¯(aq)

• If aqueous silver nitrate is shaken with a haloalkane (they are immiscible) the displaced halide combines with a silver ion to form a precipitate of a silver halide.
• The weaker the C-X bond the quicker the precipitate appears.

Ag+ (aq) + X¯(aq) ——> AgX(s)

AgCl white ppt AgBr cream ppt AgI yellow ppt

Summary

This depends on:

• the identity of the halogen (F, Cl, Br or I)
• the nature of the halogenoalkane (1º, 2º or 3º)

The type of halogen determines the bond strength between the carbon and the halogen. F-C is the strongest and consequently fluoroalkanes are the least reactive (slowest rate). The strength of the C-X bonds is a function of the bond length. Longer bonds = weaker bonds.

Primary (1º) halogenoalkanes tend to react via the SN2 mechanism which is slower. Consequently the order of reactivity is 3º > 2º > 1º.

For SN2 -

Rate Determining Step: depends on concentration of the halogenoalkane and nucleophile

• Rate = k[halogenoalkane][nucleophile]

For SN1 -

Rate Determining Step: depends only on concentration of the halogenoalkane

• Rate = k[halogenoalkane]

Solvents

• Polar solvents have large dipole moments (aka “partial charges”); they contain bonds between atoms with very different electronegativities, such as oxygen and hydrogen.
• Non polar solvents contain bonds between atoms with similar electronegativities, such as carbon and hydrogen (think hydrocarbons, such as gasoline). Bonds between atoms with similar electronegativities will lack partial charges; it’s this absence of charge which makes these molecules “non-polar”.

• Protic solvents have O-H or N-H bonds. Why is this important? Because protic solvents can participate in hydrogen bonding, which is a powerful intermolecular force. Additionally, these O-H or N-H bonds can serve as a source of protons (H+).
• Aprotic solvents may have hydrogens on them somewhere, but they lack O-H or N-H bonds, and therefore cannot hydrogen bond with themselves.

Solvents and Nucleophilic Substitution

SN1

• S_N1 best performed in protic, polar solvents. (Hydrogen bonding, polar). Example is water.

SN2

• S_N2 best performed in aprotic, non-polar solvents. (No OH or NH groups thus no hydrogen bonding, no polar). Example is propanone.