Additions to Alkenes
Anti-Markovnikoff
HCl
HBr
H2SO4
H2O
Hg(OAc)2
H2O
Br2
Br2
1)
2) NaBH4
Cl2
H2O
OsO4
BH3
1)
2) NaOH,
H2O2
R
R R
H
R
R R
H
R
R R
H
R
R R
H
R
R R
H
R
R R
H
R
R R
H
R
RR
H
HOH
Sometimes you might see BH3•THF or B2H6 used here: it's the same reagent in a slightly different form.
The base (can be NaOH, KOH, identity unimportant) helps make H2O2 more reactive. The reaction is anti -
Markovnikoff because the H–B bond is polarized toward H (electronegativity of H = 2.2, B = 2.0) - the H
adds to the carbon best able to stabilize positive charge (i.e. the most substituted one).
R
HO H
R
RH
R
Cl H
R
RH
R
Br H
R
RH
R
RR
H
Br
Br
R
RR
H
HO
Br
R
RR
H
Cl
Cl
R
RR
H
HO OH
R
R R
H
O
OOH
R
R
RR
H
O
H2
Pd/C
R
R R
H
The catalyst can vary - you might see Pt or Ni as well. All provide the same product with the same
stereochemistry.
R
RR
H
HH
This reaction goes through 3-membered "mercurinium" ion. The NaBH4 step removes the
mercury. While the addition is anti, the overall reaction is stereorandom because this step involves a carbon
based free radical (usually not discussed). Alternatively, an alcohol used in place of water will produce an ether.
HBr
peroxides
(RO-OR)
H
R H
HR
HBr
H
HH
RCO3H is a peroxyacid. A common peroxy acid for this reaction is m-CPBA
syn addition
Markovnikoff syn + anti
R
HO H
R
RH
Markovnikoff syn + anti
Markovnikoff syn + anti
Markovnikoff syn + anti
N/A anti addition
Markovnikoff anti addition
N/A anti addition
N/A syn addition
N/A syn addition
N/A syn addition
R
R R
H
O3
CH3
SCH3
O
R
ROR
H
O3O
R
ROR
OH
Anti-Markovnikoff syn + anti
Strong acid protonates the alkene, generating free carbocation. Watch out for
possibility of rearrangements when a tertiary carbocation could be generated through a 1,2 shift.
HSO4– anion is not strongly nucleophilic, hence it does not add. Gives a mixture of syn and anti products
due to the free carbocation.
HCl and HBr (as well as HI, not pictured) protonate the alkene to give a free
carbocation which can then be trapped by the halide anion. Gives a mixture of syn and anti
The key detail in these reactions is solvent: water and alcohol solvents will form the
halohydrin products (the ones containing the OH and Br). All other solvents (you might
see CCl4, CHCl3, hexane, etc. ) provide the dibromide.
As with bromination, above. Although not depicted, use of water or alcohol as solvent
will also lead to formation of the halohydrin product (also anti).
Osmium is a transition metal. The tools won't be given in this course to fully understand how this reaction
works. Occasionally a second reagent like NaHSO3, H2S, or Na2S2O3 is also given as a reactant in this
reaction - minor detail, it's used to remove the osmium from the hydroxyl groups.
O
OOHCl
(m-chloroperoxybenzoic acid). If H3O+, heat is written afterwards, this
is opening of the epoxide to give the diol (anti-selective)
Peroxides generate the Br• radical, which adds to the double bond in the way that will generate the
most stable radical (i.e. the radical will go on to the most substituted carbon). This explains the
selectivity for the anti-Markovnikoff product. It gives a mixture of syn and anti because it goes through
a free radical process.
m-CPBA
Reductive workup: Zinc (Zn), or dimethyl sulfide (DMS, Me2S) is a reducing agent. It reduces excess ozone, allowing for isolation
of the aldehyde.
H2O2
"Master Organic Chemistry"
masterorganicchemistry.com
Oxidative workup: Hydrogen peroxide is used to obtain the carboxylic acid instead of the aldehyde.
+
+
Br
R
R R
H
Br or H2O/ROH depending on solvent
Bromonium ion mechanism
CH2I2
Zn/Cu
R
R R
HR
RR
H
C
Hydroboration
Oxymercuration
Acid-catalyzed addition of H2O
(hydration)
Addition of HX
Addition of HX
Bromination
Halohydrin Formation
Chlorination
Dihydroxylation
Epoxidation
Hydrogenation
Radical addition of HBr
Ozonolysis (Reductive workup)
Ozonolysis (Oxidative Workup)
Cyclopropanation N/A syn addition
This reaction goes through addition of a carbene (actually, "carbenoid") to
the double bond. The reaction is stereospecific.
Another set of conditions to provide a cyclopropane is CHCl3 with strong
base (NaOH), which makes the dichlorocyclopropane.
Omissions, Mistakes, Suggestions?
james@masterorganicchemistry.com
This sheet copyright 2019, James A. Ashenhurst
masterorganicchemistry.com
Note - this sheet is not meant to be comprehensive. Your course
may provide additional material, or may not cover some of the
reactions shown here. Your course instructor is the final authority.
KMnO4
cold, dilute
R
R R
HR
RR
H
HO OH
N/A syn addition Keywords are "cold, dilute". NOTE: If "heat" or "acid" is mentioned in the conditions, the diol will
be cleaved to provide carbonyl compounds (same reaction as ozonolysis with oxidative workup,
below).
Dihydroxylation
Reaction "Regiochemistry" "Stereochemistry"
Can also use KMnO4 and acid
H H
R
R R
H
(or Zn/H+)
NOTE: "anti" hydroxylation can be achieved through epoxidation followed by treatment with NaOH (basic)
or aqueous acid ( H3O+ )