Alkenes: Reactions with carbenes McM 7.6 cont.

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Alkenes Reactions with carbenes (McM 7.6)
cont. Alkenes Addition of radicals etc -
Polymerisation (McM 7.10, 14.7) Alkenes Formati
on by elimination reactions (McM 11.12, 11.13)
Substitution reactions Cleavage of ethers
(McM 18.5) Organometallic coupling react.
(McM 10.9, lab ex 10)
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Polymerization of dienes (McM 14.7)
Natural and synthetic rubber

• Radical polymerization
• Cationic polymerization

Acid cat polymerization of 1,3-butadiene
1,4-addition
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Hevea brasiliensis, tropical Americas. Cautchuc
resin from the Isonandra Gutta tree (South east
Asia)
Less elastic than nat. rubber Isolation
under-water cables Little use today
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Elimination reactions - Repetition

• One step
• 2. order
• Stereospecific

• Two steps
• 1. order
• 1. step rate limiting
• 1.step 1. step in SN1
• Not stereospecific

Elimination in competition with substitution
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E2 and stereochemistry
Large subst in e pos. favored
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Deuterium Isotope Effect (McM 11.14) (Kinetic
Isotope Effect)

• Important in elucidation of reaction mechanisms
• Cleavage of C-H and C-D requires different amount
  of energy

Hookes Law, Stretching frequencies, IR
Relationship between Streching frequency (n) and
zero-point energy (E0) E0 1/2 hn E0 C-H 18
kJ/mol E0 C-D 13 kJ/mol Higher activation energy
for cleavage of C-D Cleavage of C-H will be faster
TS
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Substitution reactions - Repetition

• One step
• 2. order
• Stereospecific - Inversion
• Sterical hindrance

• Two steps
• 1. order
• 1. step rate limiting
• 1.step 1. step in E1
• Not stereospecific - Racimisation
• Stabilisation of carbocation

X Leaving group, Godd leaving group - weak base
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C-C Bond Formation - Organometallic Coupling
Reactions (McM 10.9, Lab ex 10)
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Grignard reagents and Organolithium reagents
Reacts with carbonyls Strong bases
RMgX or RLi does not react well with alkyl
halides in substitutions
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Organocuprates (McM 10.9)
Gilman reagent Dialkylcuprate
(R alkyl, alkenyl, alkynyl, aryl)
Participate in substitution with alkylhalides
(Cl, Br, I)
Also reaction with alkenyl halides and
arylhalides (sp2)
Not SN1 or SN2
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Palladium catalyzed coupling reactions (Lab ex.
10)
Alkenyl or aryl halides Pd complex from aklyl
halides unstable
Met -ZnX, -B(OH)2, -SnR3 (-MgX)
Alkynyl, alkenyl, aryl, alkyl reacts (depending
on the metal used)