Substitution and Elimination

What makes a good leaving group?

A positive charge or a conjugate base of an acid with pka<0

Typical leaving groups

Br^-, Cl^-, I^-, RSO₃^-

Iodomethane

Common name methyl iodine

2-Choloropropane

Common name isopropyl chloride 

1-Bromopropane

Common name n-propyl bromide 

Alkyl halide can be converted to 

SN2 mechanism

Rate of SN2

Rate decreases as more R groups are added to the alpha carbon

methyl>1*>2*>3*(basically unreactive)

SN2 energy diagram

SN2 nucleophile strength

Needs a fast nucleophile, reaction will not proceed with a slow one

Strong nucleophiles

I^- , Br^-, Cl^-, HS^-, RS^-, HO^-, RO^-, N=C^-

Weak nucleophiles

H₂O, ROH

SN1 Mechanism

Protic solvents

Aprotic solvents

Preferred SN2 solvent

Polar aprotic because they do not bond with the nucleophile and instead raise the energy of the nucleophile, increasing the reaction rate due to smaller Ea

Preferred SN1 solvent

Polar protic because they stabilize the intermediate

Reactions with primary substrate

Reactions with tertiary substrate

E2 mechanism

Can E2 proceed with a tertiary substrate?

Yes because it is abstracting a H from a beta carbon, which is not affected as much by steric hindrance

What makes an alkene stable?

Trans will be more stable than cis. Cis has more steric strain, and is less favorable

More substituted means more stable since alkyl groups can also stabilize the neighboring sp2-hybridized carbon atoms of a π bond because of electron cloud delocalization through hyperconjugation 

Trans bond in cycloalkenes

Cycloalkenes comprised of fewer than seven carbon atoms cannot accommodate a trans π bond. These rings can only accommodate a π bond in a cis configuration

Bridged bicyclic compounds

A bridgehead carbon cannot have a double bond if the trans bond is being incorporated into a small ring, it is only possible if the bottom ring has at least eight carbons 

Regioselectivity of E2

Pi bond will form connecting to all beta positions, but there will be major and minor products, generally the more substituted alkene will be the major product

Zaitsev 

More substituted alkene in an E reaction. Major product when a strong, non-bulky base is used.

Hofmann

Less substituted alkene in an E reaction. Major product when a strong, bulky base is used 

Common sterically hindered bases