Ch 6 reactions

Question 1

What type of reaction is this?

What is the order of reactivity for substrates and why?

What happens to stereochemistry?

Answer 1

S_N2

CH₃X > 1º > 2º (3º alkyl halides cannot react via S_N2). This is due to steric hindrance on the substrate making back-side attack increasingly difficult with greater substitution on the substrate.

S_N2 results in stereochemical inversion of configuration (back-side attack turns the sp³ tetrahedron inside out).

Question 2

What factors affect nucleophilicity?

Answer 2

Charge: species with negative charge better Nucs than similar neutral species (a base is always a stronger Nuc than its conj acid).

Electronegativity: Nuc trends opposite electronegativity trends. Nucleophilicity goes DOWN as you go right across a period because elements hold their nonbonding electrons more tightly.

Size: Nucleophilicity increases DOWN a column. It follows an INCREASE in size/polarizability and a DECREASE in electronegativity.

Steric Effects: Bulky nucleophiles are too fat to attack an electrophile. Ex: tert-butoxide is a great base but a crappy nucleophile.

Solvent: polar aprotic (no -OH/NH) good for S_N2 because no H-bonding blocking the attack. polar protic (-OH/-NH somewhere) good for S_N1 because they can H-bond with ions.

Question 3

What type of reaction is this?

What are the product(s)?

What is the order of reactivity for substrates and why?

What happens to stereochemistry?

Answer 3

S_N1

Two products due to hydride shift. Rearrangement possible (methyl/hydride shifts).

3º > 2º > (1º > CH₃X are unlikely). Alkyl groups enhance the S_N1 reaction by stabilizing the carbocation intermediate.

S_N1 results in racemization (Nuc can attack either side of E⁺) - formation of products showing both retention and inversion of configuration.

Question 4

Which will undergo S_N2 reaction faster?

Answer 4

Less branching on the neighboring carbon.

Question 5

Which will undergo SN2 reaction faster?

Answer 5

Iodide is a better LG.

Question 6

Which will undergo S_N1 solvolysis (in aqueous EtOH) more rapidly?

Answer 6

Secondary carbocation more stable than primary carbocation.

Question 7

Which will undergo SN1 solvolysis (in aqueous EtOH) more rapidly?

Answer 7

Secondary benzylic carbocation more stable than secondary carbocation.

RESONANCE-STABILIZATION

Question 8

Which will undergo SN1 solvolysis (in aqueous EtOH) more rapidly?

Answer 8

Secondary allylic carbocation more stable than secondary carbocation.

RESONANCE-STABILIZATION