to build molecular orbitals, first recall that the energy of...

Molecular Orbitals for Alkyl Halide Electrophiles

Chemistry 335 Supplemental Slides: Chapter 2 1

To build molecular orbitals, first recall that the energy of the ‘starting’ atomic orbitals depends the electronegativity of the element, which you can get from the periodic table...

So therefore:

Molecular Orbitals for Alkyl Halide Electrophiles

Chemistry 335 Supplemental Slides: Chapter 2

The energy stabilization (or destabilization) that results from bonding (or antibonding) depends on energy difference and the ability of the orbitals to mix!

As we move down the periodic table, the orbitals become more diffuse and therefore have poorer overlap with first row elements like carbon.

close in energy but poor orbital overlap

far in energy but good orbital overlap

2

2.6 Predicting Substitution vs. Elimination


Remember that anything with a pair of electrons can in principle act as a base or a nucleophile. Determining whether a substitution or elimination is most likely requires evaluation of the structure of the starting material (e.g. 1o, 2o, 3o, etc.), the quality of the leaving group (e.g. I > Br > Cl) and the properties of the nucleophile/base (e.g stabilized vs. unstabilized, light vs. heavy, etc.).

poor base / good nucleophile

good base / good nucleophile

good base / poor nucleophile

Me or Bn

1o

2o

3o

A. Recall that methyl halides and benzyl halides can’t undergo elimination, since they have no a-hydrogens. Similarly, 3o alkyl halides can’t participate in SN2 reactions because they’re too hindered. So quite a few of the scenarios imagined by this table are trivial to assign.

SN2 SN2

E2 E2

B. A methyl (or benzyl) alkyl halide that is presented with a poor nucleophile will either react very slowly or else not react at all. Similarly, a 3o alkyl halide that is presented with a poor base could either react very slowly, not react at all, or undergo substitution by a different mechanism (which we’ll talk about later).

SN2 or no reaction

E2, SRN1 or no rxn

C. Electron-rich species that are good nucleophiles and poor bases will prefer to react via substitution; electron-rich species that are good bases and poor nucleophiles will prefer to react via elimination.

SN2

SN2

E2

E2

2.6 Predicting Substitution vs. Elimination


Remember that anything with a pair of electrons can in principle act as a base or a nucleophile. Determining whether a substitution or elimination is most likely requires evaluation of the structure of the starting material (e.g. 1o, 2o, 3o, etc.), the quality of the leaving group (e.g. I > Br > Cl) and the properties of the nucleophile/base (e.g stabilized vs. unstabilized, light vs. heavy, etc.).

poor base / good nucleophile

good base / good nucleophile

good base / poor nucleophile

Me or Bn

1o

2o

3o

D. Substitution is favoured for 1o alkyl halides since they are relatively unhindered, and since the competing elimination pathway would deliver a relatively unsubstituted (and therefore less stabilized) alkene.

SN2 SN2

E2 E2

SN2 or no reaction

E2, SRN1 or no rxn

SN2

SN2

E2

E2

SN2

E. 2o alkyl halides can go either way. Elimination tends to predominate for many ‘typical’ 2o alkyl halides

E2 > SN2

- Elimination is more likely for homoallylic and homopropargylic substrates (due to the possibility to create increased conjugation in the product)

but:

- exact product ratios vary with solvent, temperature, specific substrate geometry, etc.

- SN2 predominates for allylic, benzylic and propargylic 2o alkyl halides

- SN2 will be favoured by using a polar aprotic solvent or by reducing the basicity of the nucleophile

- E2 elimination is more likely for cyclic 2o alkyl halides

2.10 Roundup of Aldol-Like Reactions


not observed



Alert: Aldol and Claisen condensations are reversible. The reverse reaction in each case is often non-obvious and usually undesired.

Retro-aldol and retro-Claisen reactions are generally driven by relief of strain energy.

Consider the following synthetic route:

only observed product



Alert: Aldol and Claisen condensations are reversible. The reverse reaction in each case is often non-obvious and usually undesired.

Retro-aldol and retro-Claisen reactions are generally driven by relief of strain energy.

Consider the following synthetic route:

not observed

only observed product

2.12 Summary of Carbonyl Substitution Reactions


2.21 Carbenes & a-Eliminations


Carbenes can be singlets or triplets

triplet singlet

• Whether singlet or triplet is the ground state depends on the separation in energy between the sp2 and p orbitals...

• For simple alkyl or H substituents, triplet carbenes are lower in energy:

• But donor substituents stabilize the empty p-orbital and thus stabilize the singlet state:

to build molecular orbitals, first recall that the energy of...

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