Substitution & Elimination

Sn1, Sn2, E1, E2 (basics & tips)

Benyzl > allyl

Carbocation stability:

Benzyl vs Allyl

> or < ?

HBr, HCl, HI, H2SO4, H3PO4

Weak base

Tertiary > Secondary > Primary >Methyl

Carbocation stability:

Methyl, Primary, Secondary, Tertiary

Rank from most to least

S_N2

If you have a methyl carbocation, which rxn(s) might occur?

S_N2, E2

If you have a primary carbocation, which rxn(s) might occur?

S_N1, E1, E2

If you have a tertiary carbocation, which rxn(s) might occur?

S_N1,E1

If you have a tertiary carbocation and you have a weak Nu/B then which rxn(s) might occur?

E2

If you have a tertiary carbocation and you have a strong B then which rxn(s) might occur?

polar protic

H-bonding solvent type

(H on N,O,F)

polar aprotic

No H for H-bonding

S_N2

Polar aprotic favors which rxn?

I > Br > Cl > F

Protic solvent nucleophilic strength in order starting from strongest to weakest

F > Cl > Br > I

Aprotic solvent nucleophilic strength in order from strongest to weakest

strong

When a (-) charge is present, it means _________ nucleophile.

stronger

The stronger the base, the ________ the nucleophile.

stays the same

Whatever doesn’t leave in stereochemistry does what?

Proton transfer

When H leaves, what mechanism?

S_N1

S_N1 or S_N2?

• 3° > 2° > 1°

• WEAK nucleophile

• Polar PROTIC solvent

• Racemic stereochemistry

S_N2

S_N1 or S_N2?

• 1° > 2° > 3°

• STRONG nucleophile

• Polar APROTIC solvent

• Inversion stereochemistry

double bond

In ALL elimination rxns, you ALWAYS form a what?

more

the more alkyl groups attatched to pi bonds, the ______ hyperconjugation exists.

Elimination

Which?
Elimination or Substitution?

Substitution

Which?
Elimination or Substitution?

Right; left

within the periodic table, nucleophilicity increases from _______ to ______ across the periods and down a family.

Weak bases

Examples of what?
I^–, Br^–, SCN^–, N₃^–, CH₃CO₂^– , RS^–, CN^–, Amines, etc.

Aprotic

Acetone is polar _______.

aprotic

DMF (dimethylformamide) is polar ________.

protic

H2O is polar _______.

aprotic

Acetonitrile is polar ________.

aprotic

DMSO (dimethylsulfoxide) is polar ________.

aprotic

HMPA (hexamethylphosphortriamide) is polar ________.

protic

CH3 - OH is polar _______.

protic

CH3CH2 - OH is polar _______.

protic

This is polar _______.

Non-polar solvents

These are examples of ?

strong bases

These are examples of:

Good nucleophiles

These are examples of:

S_N2

If 1° and has a good nucleophile then it is ______.

S_N1

Which mechanism?

C+ (carbocation intermediate), solvent replaces L.G.

S_N1

Which substitution rxn favors 3°?

S_N2

Which mechanism?

occurs in 1 step

No C+ (carbocation intermediate), no replaced L.G.

E1

which mechanism?

C+ (carbocation intermediate), pi bonds between alpha and beta

E2

Which mechanism?

no C+ (carbocation intermediate), pi bond between alpha and more substituted beta carbon

k[alkyl]

Rate for S_N1

k[alkyl][Nu]

Rate for S_N2

k[alkyl]

Rate for E1

k[alkyl][B]

rate for E2