chem231 - ch.10 - radicals & alkyl halides

radical

a reactive intermediate w/ a single unpaired e^-

heterolysis

2 e^- movement

homolysis

1 e^- movement

• half arrow

radical stability: hybridization

R = C

• want 3°

• weakest/lowest BDE → most stable/fastest forming radical

radical stability: resonance

more resonance stabilized → more stable

• e^- coming together to form double bond

radical stability example

alkane halogenation

alkanes react w/ halogen (Cl₂ or Br₂), + heat/light (hv), H atom of alkane is replaced by halogen atom to form alkyl halide product

• initiation → form halide radical

• propagation → that halide will abstract & form alkyl radical

• termination → reform halide radical into product

  • always have same # of radicals

halogenation: initiation

• weakest bond breaks

• homolytic cleavage to form Br radical

halogenation: propagation

forms new radicals

• hydrogen abstraction

  • Br radical makes bond w/ 1 e^- from H bond

  • other H e^- goes to C

• halogen abstraction

  • 1 e^- from Br—Br form bond w/ newly made carbon radical

  • other e^- from Br—Br goes to Br → new Br radical

    • Br radical can be used as reactant in hydrogen abstraction

• happens over & over again (chain reaction)

• the product of the previous step is used as the reactant in the next step

halogenation: termination

when you run out of Br, the 2 radicals come together

• 3 options

  • 2 Br radical e^- come together & form Br—Br bond

  • H₃C radical + Br radical come together → H₃CBr

    • these 2 preferred

  • 2 H₃C radicals come together → H₃C—CH₃

    • dimerization - not wanted, but possible

radical indicators

• peroxides - oxygen—oxygen bond is weak

  • radical forms w/ heat

• hv (light) - light E can break X—X bond → radicals

radical thermodynamics

• only Cl₂ & Br₂

  • Br₂ is slower, but more selective → more useful

  • both exothermic

• F₂ too explosive

• I₂ too slow, NR

halogenation selectivity

• exothermic reactions → TS resemble reactants

• endothermic reactions → TS resemble products

  • don’t get good selectivity for better radical b/c exothermic

racemic

solid lines in skeletal

• represent both dash & wedge (50:50)

• implied w/ lines

  • must write if dash/wedge present

allylic bromination

best C—H bond to be abstracted

• ↑ stable radical → ↓ bond + ↑ resonance stabilized

  • don’t put e^- on double bond (DB)

N-bromosuccinimide (NBS)

if molecule has double bond you do not want to react w/ Br₂, use NBS + hv to do radical halogenation

• generates small amt of Br₂

  • small [Br₂]

organometallic compounds

• grignard - RMgX

• organolithium - RLi

  • R = C

• solvent has to be inert → b/c SB + SNu

  • are strong bases

• reagents becomes an alkane

elemental form

represented by “ ° “ next to element

• not charged

grignard

RMgX

• Mg is “inserted” between R—X

• X = halide

  • “ R^- Mg⁺X “

organolithium

RLi

• “ R^- Li⁺ “

  • acts like this, but isn’t

• Li exchanges w/ halide

organometallic compound examples