Chemistry 2213A/B Lecture Notes - Lecture 1: Nucleophile, Hydrohalogenation, Acetylide
7 May 2018
School
Department
Course
Professor
Alkene Rxn
Type
Carbocation
Formed?
Regio-
selective?
Stereo-
selective?
(Syn or
Anti)
Creates Isomers?
Additional Notes
*Hydrohalogenation
Yes - watch out for
carbocation
rearrangements
Yes - major
product comes
from most
stable carbo-
cation (shortcut:
Markovnikov’s
Rule more
stable carbo-
cation follows
this rule)
No
- RDS is electrophilic addition/
carbocation formation
- Watch out for carbocation
rearrangement (1,2-alkyl shift
or 1,2 - hydride shift) to give
more stable carbocation
- Can form racemic mixture if
neither carbocation is more
stable
*Acid-catalyzed
Hydration
Yes - watch out for
carbocation
rearrangements
Yes - major
product comes
from most
stable carbo-
cation (shortcut:
Markovnikov’s
Rule more
stable carbo-
cation follows
this rule)
No
- See additional notes for
hydrohalogenation
- In the electrophilic step, the
alkene reacts with H3O+ (H+
from acid catalyst)
- In the nucleophilic step, the
carbocation formed from the
electrophilic step reacts with
water (water acts as
nucleophile) and then H+ is
released
*Halogenation
No - a cyclic bridge
is formed
No
Yes - anti
addition
Yes - for cyclic
alkenes, a trans
isomer is formed
(due to anti-
addition)
- Stereocenters may
be formed leading
to R/S designations
(enantiomers,
diastereomers, and
meso-compounds)
- Electrophilic step: cyclic
bridge and X- are formed
- Nucleophilic step: cyclic
bridge and X- react to form final
product
- Reacts in inert solvent
- Br2 can test for presence of
C=C (one mole of Br2 reacts
with one double bond)
Hydroboration
Oxidation
No
Yes - Anti-
Markovnikov
H adds to C with
fewest Hs
attached
Yes - syn
selective
(because
C-BH2 and
C-H are
formed at
the same
time)
Yes - for cyclic
alkenes, a cis
isomer is formed
(due to syn-
addition)
- BH3 reacts with 3 equivalent
alkenes to form a trialkylborane
(B in center) {Hydroboration
step}
- B is eventually replaced by
OH- {Oxidation step}
These rxns
follow this
mechanism:
TS 1
Electrophilic
Addition (RDS)
TS 2
Nucleophilic
Addition (FS)
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Document Summary
*hydrohalogenation yes - watch out for carbocation rearrangements. Yes - major product comes from most stable carbo- cation (shortcut: Rule more stable carbo- cation follows this rule) Watch out for carbocation rearrangement (1,2-alkyl shift or 1,2 - hydride shift) to give more stable carbocation. Can form racemic mixture if neither carbocation is more stable. In the electrophilic step, the alkene reacts with h3o+ (h+ from acid catalyst) In the nucleophilic step, the carbocation formed from the electrophilic step reacts with water (water acts as nucleophile) and then h+ is released. Electrophilic step: cyclic bridge and x- are formed. Nucleophilic step: cyclic bridge and x- react to form final product. C=c (one mole of br2 reacts with one double bond) Bh3 reacts with 3 equivalent alkenes to form a trialkylborane (b in center) {hydroboration step} Yes - for cyclic alkenes, a trans isomer is formed (due to anti- addition) Stereocenters may be formed leading to r/s designations (enantiomers, diastereomers, and meso-compounds)
