Substitution Reactions

Question 1

Substrate

Answer 1

molecule that undergoes substitution

Question 2

Nucleophile

Answer 2

“attacking” Lewis base (can be neutral or anionic)

Question 3

Electrophile

Answer 3

carbocation or positive partial end of polar C-X bond

Question 4

Leaving Group

Answer 4

displaced halide (negative partial end of polar C-X bond)

Question 5

Describe a substitution reaction

Answer 5

nucleophile “attacks” electrophile with the displacement of a leaving group

Question 6

SN1

Answer 6

two separate steps: bond breaking to leaving group, then bond making to nucleophile

Question 7

SN2

Answer 7

one step: bond making to Nucleophile and bond breaking of leaving group simultaneously

Question 8

Key feature of SN2

Answer 8

1. “Back side attacks” - Nu must approach 180deg from C-LG bond
2. “Inversion of configuration” - sp3 carbon is turned inside out (potentially changes stereochemistry)

Question 9

Key Feature of SN1

Answer 9

1. “Either side attack” - Nu can approach C+ from left or right
2. “Scrambling of Configuration” - sp3 carbon in product is racemic (potentially changes stereochemistry)

Question 10

Describe carbocation rearrangement

Answer 10

• Unstable carbocations can rearrange to form more stable carbocations
• 1,2 -alkyl or halide shift (migrate with its pair of electrons)
• original site of shifting group attachment becomes new carbocation
• Only SN1

Question 11

When does carbocation rearrangement happen?

Answer 11

ASK LAURA!

Question 12

SN1 substrate selectivity

Answer 12

resonance > tertiary C > secondary C > primary C&raquo_space; methyl

Question 13

SN2 substrate selectivity

Answer 13

• all about that steric hindrance around reacting site (the less the better)

methyl > primary C > secondary C (tricky)&raquo_space;> neopentyl (1* C but really hindered) > tertiary carbon

Question 14

SN1 nucleophile selectivity

Answer 14

No effect on rate, weak/neutral nucleophiles are OK (nucleophile not involved in rate determining step)

Question 15

Solvolysis

Answer 15

substitution by solvent molecule (weak/neutral nucleophile like hydrolysis or methanolysis)

Question 16

SN2 nucleophile selectivity

Answer 16

• larger effect, the stronger the Nu the better
  
  • Anion better Nu
  • Less electronegative better Nu
  • NO resonance better Nu
  • Less hindered better Nu

Question 17

Polar protic

Answer 17

• solvent possessing a H-bond donor

tends to dissolve all ions very strongly

Question 18

Polar aprotic

Answer 18

• solvent that does not possess an H-bond donor

tends to dissolve only cations

Question 19

SN1 solvent selectivity

Answer 19

polar protic > polar aprotic&raquo_space;> non-polar

• solvate carbocations with exposed negative dipoles

Question 20

SN2 solvent selectivity

Answer 20

polar aprotic&raquo_space;> polar protic > non-polar

-aprotic enhances, protic reduces reactivity of anionic Nu

Question 21

Polar Solvents

Answer 21

• good R+ cation solvation (protic solvents best)

- carbocations are stabalized when solvated more effectively (smaller deltaG)

Question 22

Aprotic Solvent

Answer 22

• enhances reactivity of Nu
• nucleophilicity follows basic trends
• in a weak solvent cage:
  
  • large and delocalized anions less reactive
  • smaller ions more reactive as nucleophiles

Question 23

Protic Solvent

Answer 23

• hinders reactivity of nucleophile
• strongerst H-bonds to small electronegative Nu
• in a strong solvent cage:
  
  • anions less reactive
  • larger ions more reactive as nucleophiles

Question 24

What kinds of hydrocarbons are not reactive for both SN1 and SN2?

Answer 24

Vinyl, Ethynyl, and Aryl Halides