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Organic Chemistry
5th Edition
Paula Yurkanis Bruice
Chapter 4
Reactions of Alkenes:
Electrophilic
Addition Reactions
and
Addition of Hydrogen
Amy Parente
Penn State Altoona
Altoona, PA
Reactions of Alkenes
Electrophilic Addition Reactions (3.6)
Reaction of alkenes with HBr:
Step 1 - Slow addition of electrophilic proton to nucleophilic alkene
(shown via “attack” by Nu:  electrons) forms a carbocation intermediate
Step 2 - rapid reaction of positively charged carbocation intermediate
(E+) with negatively charged bromide ion (Nu:) generates the alkyl halide
Reactions of Alkenes
General themes for alkene reactions:
Loosely held  electrons of carbon-carbon double bond are attracted to an E+
Reactions start with addition of E+ to one of the sp2 carbons of the alkene
(often forming a carbocation intermediate) and concludes with the addition of
a Nu: to the other sp2 carbon
E+ or point of E+
addition - YELLOW
Nu: or point of Nu:
addition - GREEN
End result -  bond breaks and two new  bonds are formed - one with E+ and
one with Nu:
Importance - alkenes can be a synthetic “hub” for a variety of other
compounds, using a variety of reagents to supply different E+ and Nu:
4.1 - Addition of a Hydrogen Halide
Addition of a hydrogen halide (E+) to an alkene generates an alkyl halide:
“RULE” - For alkenes with the same substituents on both sp2 carbons (as is shown
here), there is no specificity (i.e., no RULE) for which sp2 carbon forms a new  bond
with the E+ and which forms a new  bond with the Nu:
4.1 - Addition of a Hydrogen Halide
What happens for alkenes that do not have the same substituents on sp2 carbons?
Two possible alkyl halide products
Experimental findings - the only product of the reaction is tert-butyl chloride…
4.1 - Addition of a Hydrogen Halide
…The only product of the reaction is tert-butyl chloride… why?
We can use the mechanism of the reaction (3.6) to understand why…
Step 1 - formation of carbocation intermediate (two possible intermediates)
The sp2 carbon that does not become attached to the proton becomes the positively
charged carbon in the carbocation - tert-butyl cation (upper) or isobutyl cation (lower)
Is there any difference between these two carbocations?
RULE - The intermediate that forms faster during the rate-determining step (here the
first step) will be the predominant product - the tert-butyl cation is formed faster
4.2 - Assessing Carbocation Stability
Can we identify any structural-based trends to explain the apparent stabilities?
Primary carbocation - one alkyl substituent
Secondary carbocation - two alkyl substituents
Tertiary carbocation - three alkyl substituents
RULE - the greater the number of alkyl substituents bonded to the positively charged
carbon, the more stable the carbocation is (potential maps show how positive charge
is “shared" among more atoms)
4.2 - Assessing Carbocation Stability
How do alkyl groups decrease the concentration of positive charge on the carbon?
Recall - Positive charge on an sp2 carbon atom of a carbocation occupies an
empty p orbital
For any attached alkyl group, the orbital of the adjacent C-H  bond can overlap
with the empty p orbital (hyperconjugation - delocalization of electrons by
overlap of a  bond orbital with an empty orbital on an adjacent carbon atom)
A methyl cation has no
such hyperconjugation
options, and the carbon
atom alone must bear all
of the positive charge
(unstable)
4.2 - Assessing Carbocation Stability
How do alkyl groups decrease the concentration of positive charge on the carbon?
Difference from hyperconjugation to explain
stability in staggered conformers:
Conformer stability - interaction of a
C-H  bond orbital with an empty 
(antibonding) orbital
Carbocation stability - interaction of a
C-H  bond orbital with an empty p orbital
Nonetheless, either case can be explained
using molecular orbital diagrams:
4.2 - Assessing Carbocation Stability
Since we are considering molecular orbital theory and orbital overlap, we must also
consider the geometry of these interacting orbitals
RULE - orbital interactions (such as hyperconjugation) will only occur if the  bond
orbital and the empty p orbital have the proper orientations
Side-to-side overlap requires parallel orbitals
Elimination of the  bond allows for free C-C 
bond rotation
RULE - The degree that hyperconjugation is able to stabilize carbocations is directly
proportional to the number of C-H (red dots) and C-C  bonds that can overlap with
the empty p orbital
4.3 - Transition State Structure
RULE - the structure of the transition state lies somewhere between the structure of
the reactants and the structure of the products (3.7)
… where is “somewhere”?
Hammond Postulate - the transition state is more similar in structure to the species
to which it is more similar in energy
Exergonic reaction - TS is most like
(similar in energy to) reactants
Endergonic reaction - TS is most
like (similar in energy to) products
If reactants and products have similar
energies (∆G° ~0), transition state
structure is truly halfway between
4.3 - Transition State Structure
Can we use this to then explain why the tert-butyl cation is formed faster than the
isobutyl cation?
Rate reflects likelihood
of reaction occurring
(height of barrier)
Carbocation formation
is endergonic, so TS
structure will resemble
products (carbocation
intermediate)
The TS will have a significant amount of positive charge on a carbon atom
The same factors contributing to stabilization of the carbocation intermediate will
affect stability of the transition state - because the tert-butyl cation is more stable
than the isobutyl cation, the TS leading to its formation is also more stable
RULE - for electrophilic addition reactions, stable carbocations are formed faster
Note - the difference in stability of the transition states is not as great as the
difference in stabilities of the carbocation intermediates…. why?
The amount of positive charge on the carbon atom is not as great in the TS (the
breaking of the  bond and formation of a new  bond is not complete)
4.3 - Transition State Structure
For electrophilic addition reactions, carbocation formation is rate-limiting:
If the difference in rates for carbocation formation is small, both products will be
formed, but the major product will be that from the more stable carbocation
If the difference in rates for carbocation formation is large, the only product
formed will be that from the more stable carbocation
4.4- Regioselective Reactions
RULE - For electrophilic addition reactions of alkenes with different substituents on
sp2 carbons, the major product of the reaction is the one that results from formation of
the more stable carbocation (formed more rapidly)
The rate for formation of primary carbocations never allows for their generation RULE - products from primary carbocations are never found
4.4- Regioselective Reactions
RULE - the distribution of products of electrophilic addition reactions correlate with
carbocation stability
Configurational isomer…?
Products from different (2° vs 3°) carbocations are constitutional isomers molecules with the same molecular formula, but differing in how atoms are connected
Regioselective reaction - a reaction in which two or more constitutional isomers
could be obtained as products, but one predominates.
4.4- Regioselective Reactions
Degrees of regioselectivity:
Moderate to highly - reaction of 2° versus 3° carbocations
Completely - reaction of 1° versus 3° carbocations
None - reaction of two 2° carbocations
4.4- Regioselective Reactions
RULE - For electrophilic addition reactions, the major product of the reaction is the
one that results from formation of the more stable carbocation (formed more rapidly)
H+ adds preferentially to C-1 because that results in a 2° carbocation that is more
stable than a 1° carbocation
RULE (stated another way) - for all alkene electrophilic addition reactions, the E+
adds to the sp2 carbon that is bonded to the greater number of hydrogens Markovnikov’s rule
H+ adds preferentially to C-1 because C-1 is bonded to 2 hydrogens
4.4- Regioselective Reactions
An introduction to “retrosynthesis”…
4.4- Regioselective Reactions
4.4- Regioselective Reactions
4.4- Regioselective Reactions
4.5 - Acid-Catalyzed Addition Reactions
Addition of hydrogen halides to alkenes:
H+ = E+ (Y+, step 1)
X- (halide) = Nu: (Z-, step 2)
Can we have other E+/Nu: combinations?
Other “YZ” combinations may effectively provide a Nu:, but may not have a
suitable E+:
Water - (Y = H+, Z = H2O) - lone-pair electrons on oxygen make a great Nu:,
but unlike acidic hydrogen halides, water is not acidic enough (pKa = 14) to
provide the electrophilic H+
Alcohols - (Y = H+, Z =ROH) - lone-pair electrons on oxygen also make for a
a great Nu:, but alcohols are also not acidic enough (pKa ~15) to provide the
electrophilic H+
4.5 - Acid-Catalyzed Addition Reactions
Solution? Inclusion of an acid (most often H2SO4, pKa = -5) provides the
electrophilic H+ to generate a carbocation that reacts with the Nu:
RULE - water and alcohols add to alkenes only if an acid is present
Addition of water to an alkene generates an alcohol
- hydration reaction
RULE - the strongest acid that can exist in water is H3O+:
RULE - for any reaction acid-catalyzed mechanism that you draw, even if the proton
comes from another molecule (H2SO4), use H3O+ as the proton donor
4.5 - Acid-Catalyzed Addition Reactions
Reaction mechanisms for the acid-catalyzed addition of water to an alkene:
Three steps (first two are the same as addition of hydrogen halide to an alkene):
Note that H3O+ (not H2SO4) is
used to supply the electrophilic
proton
Step 1 - in the RDS of the reaction the electrophile (H+) adds to the sp2 carbon
that is bonded to the greater number of hydrogen atoms (Markovnikov’s Rule)
Step 2 - the nucleophile (oxygen of H2O) adds to the carbocation forming a
protonated alcohol (with hydrogen halides, the Nu: was missing its proton)
Step 3 - the protonated alcohol loses a proton because pH of the solution (even
with H2SO4) is greater than the pKa of the protonated alcohol (-2.5, Section 1.18)
4.5 - Acid-Catalyzed Addition Reactions
Concern over “side-reactions” - are there any other potential Nu: in solution?
Acid conjugate base (HSO4- counterion) - the concentration of this species is
much less than that of H2O (remember 55 M!), so a carbocation is much more
likely to encounter water in this fast step 2 than the counterion.
Why not consider using -OH as our Nu:? We could simplify things by eliminating our
third step of the reaction….
If we are performing an acid-catalyzed electrophilic addition reaction, how
appreciable do you think the [-OH] is? Even in pure water, it is only 10-7 M!
RULE - when you are doing reactions, always think about what is present in your
solution - either for “successful” reactions, or for competing side-reactions.
4.5 - Acid-Catalyzed Addition Reactions
RULE - when writing reaction mechanisms, always be sure to balance atoms and
charges:
The hydration of an alkene is an acid-catalyzed
reaction… H+ is a catalyst in the reaction
H+ addition - step 1, an electrophilic proton is needed to start the reaction
H+ removal - step 3, a proton is removed from the protonated alcohol
H+ balance? Considering the overall reaction, a proton is not consumed
Catalyst - a species that increases the rate of the reaction, but is not consumed
Rate increases happen because the free energy of activation is decreased (3.7)
Catalysts cannot affect the equilibrium of a reaction (∆G°)
RULE - catalysts increase the rate at which a product is formed (kinetics) but do not
affect the amount of product formed at equilibrium (thermodynamics)
4.5 - Acid-Catalyzed Addition Reactions
RULE - water and alcohols add to alkenes only if an acid is present
Addition of an alcohol to an alkene generates an ether
Looking for trends - comparing addition reactions of water and alcohols:
Acid-catalyzed electrophilic addition reaction - E+ is same (H+) - same step 1
Nu: is ROH instead of HOH - Nu: is different - different steps 2 and 3
RULE - for reactions that have similar chemistry, look to make simple modifications to
generate the “new” mechanism
4.5 - Acid-Catalyzed Addition Reactions
Reaction mechanisms for the acid-catalyzed addition of an alcohol to an alkene:
Three steps (same as that seen for water, but Nu: has changed):
Note now that CH3OH2+ (not H3O+) is
used to supply the electrophilic proton
- this is possible because protonated
methanol is produced in the reaction
Step 1 - in the RDS of the reaction the electrophile (H+) adds to the sp2 carbon
that is bonded to the greater number of hydrogen atoms (Markovnikov’s Rule)
Step 2 - the nucleophile (oxygen of ROH) adds to the carbocation forming a
protonated ether (with water, we generated a protonated alcohol)
Step 3 - the protonated ether loses a proton because pH of the solution (even
with H2SO4) is greater than the pKa of the protonated alcohol (-3.6)
4.6 - Carbocation Rearrangements
RULE - the electrophile (H+) adds to the sp2 carbon that is bonded to the greater
number of hydrogen atoms (Markovnikov’s Rule)
Exceptions/Modifications to the RULES….??
Addition of hydrogen bromide to 3-methyl-1-butene:
Expected outcomes:
NO 1-bromo-3-methylbutane product (Nu: addition to 1° carbocation)
YES 2-bromo-3-methylbutane product (Nu: addition to 2° carbocation)
Unexpected outcome
2-bromo-2-methylbutane as a major product (Nu: addition to a 3° carbocation…)
4.6 - Carbocation Rearrangements
Exceptions/Modifications to the RULES….??
Addition of hydrogen chloride to 3,3-dimethyl-1-butene:
Expected outcomes:
NO 4-chloro-2,2-dimethylbutane product (Nu: addition to 1° carbocation)
YES 3-chloro-2,2-dimethylbutane product (Nu: addition to 2° carbocation)
Unexpected outcome
2-chloro-2,3-dimethylbutane as a major product (Nu: addition to a 3°
carbocation… AND a different skeletal structure - did the methyl group move!!!)
4.6 - Carbocation Rearrangements
RULE - carbocations can rearrange if they become more stable as a result of the
rearrangement
Addition of hydrogen bromide to 3-methyl-1-butene:
Expected outcomes:
YES 2-bromo-3-methylbutane product (Nu: addition to 2° carbocation)
Unexpected outcome - a 1,2-hydride shift (movement of H with electrons)
2-bromo-2-methylbutane as major product from Nu: addition to a 3° carbocation
RULE - carbocations rearrangements can occur by H:- (hydride) movement to an
adjacent carbon atom (C1 to C2)
4.6 - Carbocation Rearrangements
RULE - carbocations can rearrange if they become more stable as a result of the
rearrangement
Addition of hydrogen chloride to 3,3-dimethyl-1-butene:
Expected outcomes:
YES 3-chloro-2,2-dimethylbutane product (Nu: addition to 2° carbocation)
Unexpected outcome - a 1,2-methyl shift (movement of CH3 with electrons)
2-chloro-2,3-dimethylbutane major product from Nu: addition to a 3° carbocation
RULE - carbocations rearrangements can occur by CH3:- movement to an adjacent
carbon atom (from C1 to C2 - should have been called methide shift, but it wasn’t!)
4.6 - Carbocation Rearrangements
Is there some way to know or predict whether or not these shifts occur?
RULE - shifts can only occur between adjacent carbon atoms (1,3 shifts not seen)
RULE - shifts will only occur to generate a more stable carbocation (shifts between 2°
and 2° carbocations are not seen)
4.6 - Carbocation Rearrangements
One more kind of rearrangement (variation on the 1,2-methyl shift) - ring expansion
Factors to consider for shifts in ring expansion:
Rearrangements will occur to produce more stable carbocations (2° to 3°, etc)
Rearrangements will also occur to produce less strained rings (4-membered to 5membered, etc).
We will be studying many other reactions that generate carbocations….
RULE - whenever a reaction leads to the formation of a carbocation, you must check
its structure for the possibility of rearrangement.
4.7 - Halogen Addition to Alkenes
Addition of water/alcohols to alkenes:
+H+ = E+ (Y+, acid in step 1)
X- (halide) = Nu: (Z-, step 2)
-H+ = deprotonation (step 3)
Can we have other E+/Nu: combinations?
Other “YZ” combinations may effectively provide a Nu:, but it may not be obvious
that they have a suitable E+:
Halogens - (Y = X+, Z = -X) - the bond joining halogens is relatively weak
(Table 3.2 - about half that of C-H bond) so it is easy to generate a Nu: and
E+ from a diatomic halogen gas.
4.7 - Halogen Addition to Alkenes
Reaction mechanisms for the addition of bromine (a halogen) to an alkene:
Two steps (similar to what we’ve seen for hydrogen halides - but building in a twist):
Step 1 - formation of a cyclic bromonium ion
The  electrons of the alkene (Nu:) approach the the electrophile (one
bromine atom of Br2)… as this E+ accepts those electrons (forming  bond),
the  bond between the two bromine atoms must break
Although the electrons could be left as a lone pair on bromine (generating a
carbocation), the proximity of a Nu: and E+ facilitate their interaction and
attachment of bromine to the other sp2 carbon atom.
Notice that while this compound looks “strange,” it doesn’t violate any RULES (not
more than 8 electrons about bromine, but three bonds mean a formal charge of +1)
4.7 - Halogen Addition to Alkenes
Reaction mechanisms for the addition of bromine (a halogen) to an alkene:
Two steps (similar to what we’ve seen for hydrogen halides - but building in a twist):
Step 1 - formation of a cyclic bromonium ion
Step 2 - the cyclic bromonium ion intermediate,
though isolatable, is unstable (and like most
carbocations has significant positive charge on
carbon atoms - see potential maps), and quickly
reacts with the remaining Nu: in solution
(bromide ion) producing a vicinal dibromide vicinal means substituents are on adjacent
carbon atoms (vicinus - Latin for “near”)
4.7 - Halogen Addition to Alkenes
Why form this unique cationic intermediate?
Comparison of stabilities
Carbocation - least stable - not all atoms
have complete octets (1.3)
Bromonium ion - more stable - proximity of
bromine’s electron cloud to sp2 carbon
atoms allows for bond formation that
completes octets
Note - even though the formal charge resides on
bromine, potential maps indicate that the bulk of
the positive charge still remains with the carbon
atoms (further reactivity reflects carbocation
character of carbon atom)
4.7 - Halogen Addition to Alkenes
Building on the theme… addition of chlorine (a halogen) to an alkene:
Reaction mechanism
Step 1 - formation of a cyclic chloronium
ion intermediate
Step 2 - reaction with the remaining Nu: in
solution (chloride ion) produces a vicinal
dichloride
Solvents (like CH2Cl2) are often
used - can dissolve reactants but
will not participate in the reaction
RULE - halogenation of alkenes proceeds without carbocation formation - there are
no carbocation rearrangements to consider with these reactions
4.7 - Halogen Addition to Alkenes
What about the other halogens - F2 and I2?
Fluorine reacts explosively with alkenes - not a useful synthetic route
Iodine addition is not thermodynamically favorable (endergonic - equilibrium lies
to left) - vicinal diiodides are unstable at room temperature, decomposing back to
alkenes with release of iodine.
4.7 - Halogen Addition to Alkenes
Information provided by writing organic reactions (≠mechanisms):
Reactants to left of arrow
Products to right of arrow
Reaction conditions (solvent, temperature, catalysts) are written above/below the
arrow
Sometimes only organic (carbon-containing) compounds are written in the
“reactant” spots, with other reagents written above/below the arrow
RULE - look at all materials present in an organic reaction to identify what is reactant
and what is either solvent or catalyst (if the products are given, this can be helpful)
4.7 - Halogen Addition to Alkenes
Inert solvents are often
used because they do
not participate in the
reaction
Concern over “side-reactions” - are there any other potential Nu: in solution? (4.5)
What if water was used as a solvent? Water can provide a good Nu: (H2O) after the addition of the halogen E+, the solvent can now act as the Nu:, reacting
with the chloronium ion intermediate - vicinal halohydrin formation
The solvent is
present at higher
concentrations and
generates the major
product
4.7 - Halogen Addition to Alkenes
Reaction mechanism for halohydrin formation:
Three steps (similar to what was seen for halogen addition, but Nu: has changed):
Step 1 - formation of a cyclic bromonium (or chloronium) ion intermediate - this
step does not change, because without acid, Br+ or Cl+ are only E+ in solution
Step 2 - reaction of unstable cyclic bromonium (or chloronium) ion with any Nu: it
encounters - solvent is highest concentration so Nu: is H2O producing a
protonated halohydrin
Step 3 - as we’ve seen for protonated ethers and alcohols, the protonated
halohydrin loses a proton because pH of the solution is greater than the pKa of
the protonated halohydrin
4.7 - Halogen Addition to Alkenes
Is there any regioselectivity to this process?
We will always get a vicinal halohydrin - when the Nu: breaks the cyclic cationic
intermediate the  bond that is formed to the Nu: forces that carbon to break its 
 bond to the E+
RULE - you will never see products with the halide and OH on same carbon
Why did water attack the carbon atom that it did to end up with the bromine (E+)
on the carbon with the greater number of hydrogens?
4.7 - Halogen Addition to Alkenes
Consider stability of transition states of step 2, where the C-Br bond is breaking, and
a C-O bond is forming:
For either transition state (red arrows above, or Nu: attack on other carbon
atom), the C-Br bond is broken to a greater extent than the C-O bond is formed
This means that the carbon atom of
the breaking C-Br bond will have to
take on a significant amount of the
positive charge - the carbon atom
better able to do this is that which is
more substituted (bonded to fewer
hydrogen atoms)
Note that this is consistent with Markovnikov’s rule - the E+ ends up on the sp2
carbon that is bonded to the greater number of hydrogens (meaning the Nu: adds to
the carbon with the fewer number of hydrogens)
4.7 - Halogen Addition to Alkenes
Concern over “side-reactions” - any other potential Nu: in solution?
Counterions - remember ions like Na+ and K+ cannot form covalent bonds, so
they do not react with organic compounds - counterions are often omitted when
writing chemical equations
4.8 - Oxymercuration-Reduction
There are more ways to obtain the products of the addition of water or alcohols to
alkenes (i.e., generate alcohols or ethers, respectively)
Important to have multiple “synthetic routes” because when you start having
multiple functional groups, but only want one to react, you need to choose a
reaction that will be selective
Oxymercuration-Reduction - another way to add water to an alkene to generate an
alcohol
Numbers indicate these reactions are run sequentially
Reaction 1 - alkene is treated with mercuric acetate in aqueous tetrahydrofuran (THF)
Reaction 2 - sodium borohydride is added to the reaction mixture
Advantages:
Does not require acidic conditions that may be harmful to organic molecules
Does not involve carbocations, so rearrangements (and multiple products) do not
occur
4.8 - Oxymercuration-Reduction
Reaction mechanism for oxymercuration - Reaction 1:
Three steps (similar to what was seen for halohydrin formation, but E+ has changed):
Step 1 - formation of a cyclic mercurinium ion
intermediate - concluded from the fact that no
carbocation rearrangements occur
Step 2 - reaction of unstable cyclic mercurinium ion with any Nu: it encounters THF is an inert solvent, so Nu: is H2O producing a protonated alcohol - we see
the regioselectivity as shown for the same reasons as in halohydrin formation
Step 3 - as we’ve seen for protonated halohydrins, the protonated alcohol loses
a proton because pH of the solution is greater than the pKa of the protonated
alcohol
4.8 - Oxymercuration-Reduction
Reaction mechanism for oxymercuration - Reaction 2:
At this point, while we have an alcohol functional group added to our molecule (and
elimination of alkene), we still have a covalent C-Hg bond where we need a C-H
Unclear mechanism of action…
Reduction reaction - any reaction that increases the number of C-H bonds or
decreases the number of C-O, C-N or C-X bonds (X = halogen) in a compound
Sodium borohydride (NaBH4) - converts the C-Hg bond into a C-H bond (a
reduction reaction).
RULE - oxymercuration-reduction reaction of an alkene yields the same product
as would the acid catalyzed addition of water:
Hg/Hydrogen (E+) is added to the sp2 carbon bonded to the greater number
of hydrogens, and -OH adds to the other sp2 carbon
4.8 - Alkoxymercuration-Reduction
Reaction mechanism for alkoxymercuration - Reaction 1:
Two reactions (and four steps) similar to what was seen for oxymercurationreduction, but now Nu: has changed:
Just like using an alcohol instead of water with acid-catalyzed addition reactions
will generate ethers instead of alcohols, the same is true here:
Mercuric trifluoroacetate
works even better!
Reaction 1 (3 steps) - reaction of an unstable cyclic mercurinium ion with an
alkoxy Nu: (alcohol) produces a protonated ether (with expected
regioselectivity) that rapidly deprotonates
Reaction 2 (2 steps) - reduction with sodium borohydride yields the
corresponding ether.
4.9 - Epoxides Using Peroxyacids
Epoxide - ether in which the oxygen atom is incorporated into a three-membered ring
Peroxyacid - carboxylic acid with an extra oxygen atom
Section 1.4 - when drawing Lewis structures, an O-O bond is weak, are rarely found:
Overall reaction (alkene to expoxide) transfers oxygen from the peroxyacid to alkene
Oxidation reaction - increases the number of C-O, C-N, or C-X (X= halogen)
bonds or decreases the number of C-H bonds.
4.9 - Epoxides Using Peroxyacids
Reaction mechanism for epoxidation of an alkene:
Concerted reaction - all
the bond-making and bondbreaking processes occur in
the same step
All addition reactions have
Nu: reacting with E+ (part
1), generating a cationic
species (new E+) that reacts
with a second Nu: (part 2)
Electrophile - oxygen atom of OH group of peroxyacid is electron-deficient
Nucleophile (two) - the  bond electrons of an alkene (part 1), as well as the
electrons left from the O-H bond (part 2)
Mechanism is similar to that for
halogenation (bromonium ion) and
oxymercuration (mercurinium ion):
Lone pair from E+ becomes Nu:
that adds to other sp2 carbon
Difference - the epoxide is
stable (no further reaction)
4.9 - Epoxides Using Peroxyacids
Epoxide nomenclature:
Generation of common names… (red)
Addition of the word “oxide” to the common name of the alkene (3.2) assuming
the oxygen atom is where the  bond would be
Generation of systematic (IUPAC) names … (blue)
Method 1 - 3-membered oxygen-containing ring = “oxirane” with oxygen in 1position, and naming of alkyl substituents on remaining 2- and 3- carbons
Method 2 - Named as an alkane, with an “epoxy” prefix identifying the carbons to
which oxygens are attached - parent chain must contain both epoxide carbons
4.10 - Hydroboration-Oxidation
Electrophile – electron-deficient atoms or molecules (“electron-loving” - phile is the
Greek suffix for “loving”) - electrophiles look for a pair of electrons (3.6)
Although they commonly are, electrophiles do not have to be positively charged, or
even partially positively charged
Borane (BH3)
Neutral, non-polar electrophilic molecule because the boron atom only has six
electrons in its valence shell rather than a complete octet
Like a carbocation, boron is sp2 hybridized with an empty p orbital available to
accept an electron pair
4.10 - Hydroboration-Oxidation
Addition of peroxyacids to alkenes:
Peroxyacid = E+ (Y+)
Peroxyacid = Nu: (Z-)
Y = Z, cyclic, concerted mechanism
Can we have other E+/Nu: combinations?
What about E+ and Nu: combinations that are from different molecules (like we
saw for halohydrin formation)
E+ - boron (reaction 1)
Nu: - aqueous solution of sodium hydroxide and hydrogen peroxide
Hydroboration-oxidation - addition of borane to an alkene followed by hydroxide
and hydrogen peroxide treatment generates an alcohol
4.10 - Hydroboration-Oxidation
So we have yet another reaction to generate alcohols…??
All reactions we’ve covered thus far - the electrophile adds to the sp2 carbon that
is bonded to the greater number of hydrogens
So how do we get different products here? Differences in what is the E+:
Acid-catalyzed addition of water - H+ is E+, and H2O is Nu:
Hydroboration-oxidation - BH3 is E+ (with OH taking its place in later steps),
and H- is Nu:
RULE - Hydroboration-oxidation is one way to make primary alcohols (placing the
-OH on the less substituted carbon atom) - note E+ still on less substituted
4.10 - Hydroboration-Oxidation
Experimental considerations:
Borane is made from diborane (B2H6) - flammable, toxic, explosive gas
Generally used as a dissolved
solution in THF (BH3/THF) - THF
satisfies boron’s requirement for
an extra pair of electrons, and
provides a source of BH3 in situ
4.10 - Hydroboration-Oxidation
Reaction mechanism for hydroboration-oxidation - Reaction 1
Like we saw for addition of peroxyacids to alkenes, this is a concerted reaction:
Electrophile - boron atom of borane is electron-deficient
Nucleophile (two) - the  bond electrons of an alkene (part 1), as well as the
hydride left from borane (part 2)
Addition of Nu: and E+ to the alkene in one step (concerted reaction) means that
no intermediate is formed
Boron (E+) adds to the carbon bonded to the greater number of hydrogens
(consistent with other addition reactions we’ve seen)
4.10 - Hydroboration-Oxidation
Boron (E+) adds to the carbon bonded to the greater number of hydrogens - but there
are not any carbocations formed - so why this apparent regioselectivity?
Like we did with oxymercuration, let’s consider stability of possible transition states,
where we are forming two bonds, a C-B bond and a C-H bond
For either transition state, the C-B bond is formed to a greater extent than the CH bond
Meaning - the sp2 carbon that does not become attached to boron has a partial
positive charge ( bond is broken, but little  bond has formed - carbocation-like)
Inference - follow carbocation stability rules - the addition of an E+ (either B for
hydroboration, or H+ for hydrogen halides) take place at the same (lesser
substituted) sp2 carbon to form the more stable transition state
RULE - charged character of transition states can greatly influence regioselectivity
4.10 - Hydroboration-Oxidation
Reaction mechanism for hydroboration-oxidation - Reaction 1 (continued):
At this point, we have generated an alkylborane - a boron atom with one alkyl group,
but with two more reactive hydride equivalents:
Alkylborane molecules react
just as borane does with
alkenes, and generate a
dialkylborane
Dialkylborane molecules
react just as borane (or alkyl
borane molecules) does with
alkenes, and generate a
trialkylborane
However… Alkylborane (RBH2) is bulkier than borane (R is bulkier than H), and
dialkylborane (R2BH) is even bulkier - this suggests a second reason for the addition
to a lesser substituted carbon atom:
First - achieve the most stable carbocation-like transition state (previous slide)
Second - there is more room at a less substituted sp2 carbon atom for an E+ to
attach - steric effects (steric hindrance - steric effect caused by bulky groups
at the site of the reaction making it difficult for reactants to approach each other)
4.10 - Hydroboration-Oxidation
Reaction mechanism for hydroboration-oxidation - Reaction 2:
After the hydroboration reaction (trialkylborane) - aqueous sodium hydroxide and
hydrogen peroxide (HOOH) are added to replace the boron with an OH group:
This will put the Nu: (-OH) where the E+ was - on the less substituted sp2
carbon atom
Oxidation reaction - increases the number of C-O, C-N, or C-X (X= halogen)
bonds or decreases the number of C-H bonds.
RULE (not RULE breaking!) - Use of hydroboration-oxidation to yield primary
alcohols still obeys all previous rules for E+ addition, placing the E+ on the least
substituted carbon atom (similarity), but since Nu: replacement of E+ occurs via a
different mechanism, the Nu: also ends up on this least substituted carbon atom
(difference)
4.10 - Hydroboration-Oxidation
Reaction mechanism for hydroboration-oxidation - Reaction 2:
Step 1 - Nu: (HOOH) adds to E+ (R3B)
Step 2 - 1,2-alkyl shift displaces a -OH ion
Step 3 - Repeat steps 1-2 two times to generate (RO)3B
Steps 4-5 - Nu: (-OH) adds to E+ (RO)3B, and alkoxide ion is eliminated
Step 6 - Protonation of alkoxide ion (shown from substituted borane, but could easily
be from H2O (solvent))
Step 7 - Repeat steps 4-6 two times to generate 3 ROH and -B(OH)4
4.10 - Hydroboration-Oxidation
Overall reaction:
RULE - 1 mol of BH3 reacts with 3 mol of alkene to form 3 mol of alcohol
RULE - The OH ends up on the sp2 carbon that was bonded to the greater number of
hydrogens because it replaces boron which was the original E+ in the reaction (obeys
our E+-addition reaction rules - E+ add to the less substituted sp2 carbon of alkenes)
RULE - no carbocation intermediates, mean no rearrangements in products - allows
for the generation of primary alcohols:
4.11 - Addition of Hydrogen to Alkenes
In the presence of a metal catalyst (platinum, palladium, or nickel), H2 adds to the
double bond of an alkene to form an alkane
Reduction reaction - decreases the number of C-O, C-N, or C-X (X= halogen)
bonds or increases the number of C-H bonds.
H-H bond is very strong (104 kcal/mol) - the energy barrier for this reaction is
enormous - catalysts accelerate the reaction by weakening the H-H bond
Pt and Pd are used in a finely divided state adsorbed on charcoal (Pt/C,
Pd/C), or often as PtO2 (Adams catalyst)
4.11 - Addition of Hydrogen to Alkenes
(Catalytic) Hydrogenation - addition of hydrogen - generally requiring a catalyst
Heterogeneous catalysts - the finely powdered metals above do not dissolve in the
reaction - since they are not consumed in the reaction, they can easily be recovered
History - Nickel is much less expensive than Pt/Pd - accidental discovery by Paul
Sabatier made catalytic hydrogenation on an industrial scale feasible
Conversion of plant oils into solids (margarine) - hydrogenated fats
Fats - solids at RT, contain fewer/no carbon-carbon double bonds (saturated fats)
Oils - liquids at room temperature, contain more carbon-carbon double bonds
(unsaturated oils - polyunsaturated oils have many double bonds)
Natural geometry of double bonds = cis
Catalytic hydrogenation of plant oils (soybean, safflower) eliminates some or all
of the double bonds producing shortening and margarine
Creamy and solid consistency; very stable to oxidation - long shelf life
Isomerization of double bonds is possible under catalytic hydrogenation
conditions - trans double bonds (trans fats)
Trans fats are bad - unsaturated features (double bonds) allows cells to use them in
cell membranes, but saturated “shape” (linear) affects their ability to work properly
4.11 - Addition of Hydrogen to Alkenes
Reaction mechanism for hydrogenation - details not well understood:
Metal surface interactions - adsorption of H2, complexation of alkene by overlap
of p orbitals with vacant metal orbitals
Bond making and breaking events occur on metal surface
Formation of alkane reduces affinity for metal surface and alkane diffuses away
Visualize as a radical reaction:
4.11 - Addition of Hydrogen to Alkenes
Heat of hydrogenation - heat released in a hydrogenation reaction (exothermic)
Reaction coordinate diagrams cannot be drawn without understanding the
mechanism - relative energies of product and reactant can still be compared:
Since all three alkenes form the same product (same energy), the differences in
heat of hydrogenation must be a reflection of differences in reactant energies
The alkene losing the most heat must be the least stable (highest energy)
The alkene losing the least heat must be the most stable (lowest energy)
RULE - the greater the stability of a compound, the lower its energy, and smaller its
heat of hydrogenation
4.11 - Addition of Hydrogen to Alkenes
Can we identify any structural-based trends to explain the apparent stabilities?
The most stable alkene has a total of three alkyl substituents on its sp2 carbons
The least stable alkene has only one alkyl substituent on its sp2 carbons
RULE - the more alkyl substituents bonded to the sp2 carbons of an alkene, the
greater its stability (said another way, the fewer hydrogens bonded to the sp2 carbons
of an alkene, the greater its stability)
4.11 - Addition of Hydrogen to Alkenes
Can we identify any structural-based trends to explain the apparent stabilities?
What about the observation that trans-2-butene has a smaller heat of hydrogenation
than cis-2-butene - can we infer that the trans isomer is more stable?
4.11 - Addition of Hydrogen to Alkenes
What about the observation that trans-2-butene has a smaller heat of hydrogenation
than cis-2-butene - can we infer that the trans isomer is more stable?
YES - large substituents on the same side of the molecule (cis isomer) have
electron clouds that can interfere with each other - steric strain, destabilizing
In fact, placing two alkyl substituents on the same sp2 carbon atom has the same
effect:
4.12 - Reactions and Synthesis
Alkenes in summary - pages 194 - 195 - “SUMMARY OF REACTIONS” section:
Why they react (constant) -  electrons are good nucleophiles
Reagents they react with (varied) - E+ (hydrogen halides, water, alcohols,
halides, peroxyacids, mercuric acid with sodium borohydride, borane with
hydroxide and hydrogen peroxide, hydrogen)
Mechanism of alkene reactions (constant) - addition of an E+ to the sp2 carbon
atom that is bonded to the greater number of hydrogens (least substituted)
AB
Remember, as we are studying reactions (why, with what, and how A reacts),
we are simultaneously studying synthesis (how can we make B)
Reactions of alkenes - addition reactions - the addition of atoms (or groups of
atoms) to the two sp2 carbons of the double bond - Chapter 4
Synthesis of alkenes - elimination reactions - the elimination of atoms (or
groups of atoms) from two adjacent sp3 carbons to generate a double bond
SUMMARY OF REACTIONS
Electrophilic addition reactions - the first step is the addition of an electrophile to the
sp2 carbon that is bonded to the greater number of hydrogens (least substituted)
Addition of hydrogen halides (H+ is electrophile; Section 4.1)
Acid-catalyzed addition of water and alcohols (H+ is electrophile; Section 4.5)
SUMMARY OF REACTIONS
Electrophilic addition reactions - the first step is the addition of an electrophile to the
sp2 carbon that is bonded to the greater number of hydrogens (least substituted)
Addition of halogen (Br+ or Cl+ is electrophile; Section 4.7)
SUMMARY OF REACTIONS
Electrophilic addition reactions - the first step is the addition of an electrophile to the
sp2 carbon that is bonded to the greater number of hydrogens (least substituted)
Oxymercuration-reduction and alkoxymercuration-reduction (Hg is electrophile
and is subsequently replaced by H; Section 4.8)
SUMMARY OF REACTIONS
Electrophilic addition reactions - the first step is the addition of an electrophile to the
sp2 carbon that is bonded to the greater number of hydrogens (least substituted)
Addition of a peroxyacid (O is electrophile; Section 4.9)
Hydroboration-oxidation (B is electrophile and is subsequently replaced by OH;
Section 4.10)
Addition of hydrogen (Section 4.11)
How to Study….
Why they react (constant) - understanding is key - little memorization
Mechanism of alkene reactions (constant) - understanding is key - little
memorization
Reagents they react with (varied) - these are the “verbs” of organic chemistry,
and you will need to “memorize” these compounds - “wheel diagrams”