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Chapter 6
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The planarity of thesp2 hybridizedcarbons and theirsubstituents exposes the π-bond to attack
from above or belowthe plane
The higherelectronegativity of
the sp2 hybridizedcarbons makes themhave a slight negativecharge
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Hydrogens on the double bond have the largestpositive charge
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Syn hydrogenation
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Breaking a H-H σ bond and the C-C π bond Making 2 C-H σ bonds
Bonds in the product are stronger
Reaction is exothermic ΔH° is negative
Heat of hydrogenation is the amount of heatevolved so it has a positive sign
Without a catalyst the activation barrier ishigh so that uncatalyzed hydrogenation isvery slow
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Pt, Pd, Ni, Rh◦ Reaction is heterogeneous and occurs exclusively at
the interface between the solid and liquid phase
◦ Catalyst is finely divided to increase surface area
Rapid at room temperature
Usually in high yield and only one product
Solvent chosen based on reactant solubility
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Hydrogen is absorbed on the surface of the catalysts Strong H σ -bond is broken and two weak H-metal σ -
bonds are formed Alkene is absorbed on the surface π bond is broken and two weak C-metal σ -bonds are
formed An H atom diffuses on the surface until it encounters
the alkene and the weak metal atom H and C bondsare replaced by a strong C-H σ -bond
A second H diffuses until it encounters the free
radical and the weak metal atom H and C bonds arereplaced by a strong CH σ -bond The alkene desorbs and the catalysis site is ready for
adsorption of another alkene
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1 3
2 4
5
S t e r e o c
h e m i c a l I m p l i c a t i o
n s
H y d r o g e n s
a d d t o s a m e s
i d e o f d o u b l e b
o n d
Syn-coplanareclipsed
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Unsubstituted TetrasubstitutedMonosubstituted Trisubstituted
cis-disubstituted trans-disubstituted
Highest Lowest
Heat of Hydrogenation
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Unsubstituted TetrasubstitutedMonosubstituted Trisubstituted
cis-disubstituted trans-disubstituted
Least stable Most stable
Stability of Double Bond
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Alkane product is formed in the synconfiguration with alkene substitutients ineclipsed positions
In 1,2 substituted cycloalkenes, the additionexclusively forms the cis-isomer
In alkenes with bulky substitutents (egbridged-cyclalkanes) steric effects force the
less hindered face of the double bond againstthe metal surface, so that is the face thehydrogens will add to.
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Bridged ringstructure oncyclohexene makesan extremely rigid
structure
Methyl groups oncyclobutane ringoverhang the doublebond restrictingaccess
More hindered faceaway from catalystsurface
Less hindered facetowards catalystsurface
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Steric effects can influence reactivity ◦ Blocking the formation of the reactive
intermediate◦ What would happen if we added two
methyls to the other cyclobutane apex? Previously we saw steric effectsinfluencing the structure and stability
of molecules or intermediates◦ Zaitsev’s Rule◦ E vs Z isomers
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The world accordingto Markovnikov
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Occurs in variety of non-polar and polarsolvents at low temperatures (-30°C)◦ Pentane, benzene, dichloromethane, chloroform,
acetic acid
The weaker the hydrogen halide bond (themore acidic the hydrogen) the faster thereaction rate◦ HF << HCl < HBr < HI
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Hydrogen halide (electrophile) acts as an acidand protonates the π-bond of the alkeneforming a carbocation in the rate limitingstep.
Carbocation(conjugate acid)
Alkene(base)
Hydrogen halide(acid)
Anion(conjugate
base)
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Electron rich π-orbitals, slightly negatively charged
Positively charged end of HBr dipole
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Alkyl groups are electron releasing The more alkyl groups on the double bond,
the more electron rich the π-bond becomes
The more electron rich, the more attractive toelectrophilic attack, increasing reactivity
The positive charge develops on the carbonthat bears the most electron releasing alkyl
groups
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Halide ion acts as nucleophile and attacks theelectrophilic carbocation forming alkyl halidein a fast step
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In an unsymmetrically substituted alkene, the hydrogen adds tothe carbon with the greatest number of hydrogens, and thehalogen adds to the carbon with the fewest number of hydrogens
Protonation of the double bond leads to a carbocation, soregioselectivity comes from the stability of the carbocation withmore stable one favored
Final product has slightly higher energy since there are stericeffects of the alkyl with the halide substitutent
Proton attachs here
More stable 2°carbocationforms here
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Exothermic reactions Primary transition state has much higher
activation energy than secondary transitionstate.
So transition state with secondarycarbocation characterisitics is more likely toform and is immediately captured by the
nucleophilic attack by the anion Secondary and primary halide possible
products differ little in energy
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Carbocation intermediates can undergohydride shifts to lead to the more stabletertiary carbocation
Presence of products due to rearrangmentsupports argument that carbocations arereaction intermediates
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Sulfuric acid (strong acid conditions) can also addto alkenes by a similar mechanism of protonationfollowed by addition of the hydrogen sulfateanion to the most stable carbocationintermediate
Markovnikov’s rule is followed: H adds to thecarbon with the most H’s
Cleavage of the O-S bond at high temperature inthe presence of water produces the alcohol andregenerates the sulfuric acid.
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More stable secondarycarbocation is formed andHydrogen sulfate anion adds
slow
heat
Nucleophilic attack
Electrophilic attack
hydration
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1. Hydronium ion acts as the electrophileprotonating the double bond leading tomore stable carbocation Rate-determining step
•
Markovnikov Rule is followed• The more stable the carbocation, the faster the
reaction rate
2. Water acts as nucleophile adding to formalkyloxonium ion
3. Water acts as Bronsted base to deprotonatethe alkyloxonium ion and forming thealcohol regenerating the hydronium ion
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50% H2SO4/H20
Hydroniumion acts aselectrophile
Water acts asnucleophile
Water acts asBrønsted base
Hydroniumion isregenerated
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Acid catalyzed hydration is the reverse of acidcatalyzed dehydration of alcohols Principle of Microscopic Reversibility applies Le Chatelier’s Principle applies
◦ A system at equilibrium adjusts so as to minimize a
stress applied to it
Adding water favors formation of alcohol
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Or Markovnikov does not Rule
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Decomposition of peroxides ROOR’ ◦ Intentional addition of peroxides◦ Unintentional formation of alkylperoxides when
oxygen is not excluded◦ O-O bond is weak yielding two alkoxy radicals◦ Alkoxy radical attacks the HBr, pulling off H to form
the alcohol, leaving neutral Br atom
Photochemical decomposition of HBr
Bonds split so each moiety takes one electron◦ Use single barb arrows to illustrate the process
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Peroxide initiator decomposes to form alkoxyradical (happens only once)
Alkoxy radical pulls H off HBr leaving Br atom
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Br atom electrophilically attacks the doublebond
Alkyl radical attacks HBr and removes Hleading to product and Br atom
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Free radicals combine to form neutralmolecules
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More stable alkyl radical sets regioselectivity
Opposite to Markovnikov’s rule since Br addsfirst in the less stable position
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Secondary alkyl radicals more stable thanprimary alkyl radicals so H adds to the C withthe most substitution
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HBr is only hydrogen halide that reacts byeither electrophilic or free radical addition
HI and HCl always add by electrophilic mechanism
The Big Idea
Use HBr to under varying conditions tochoose whether the product followsMarkovnikov Rule or goes opposite toMarkovnikov’s Rule
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Lowering the Activation Barrier
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•
Anti-Markovnikov hydration in two steps
•Borane dimerizes readily and then stabilized by the ether, often THF
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Four center transition state has carbocation character, sopositive charge is favored on most substituted carbon,leading to regioselectivity
NO chance for rearrangements in transition state
Steric effects favor boron on less substitute C
Repeats for each H on the boron
Addition is synEmpty porbital
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Nucleophilic attack of hyperperoxide anionon boron
Alkyl group migratesfrom boron to oxygenduring hydroxide
elimination withpreservation of stereochemistry
Water in electrophilicattack forms another 4center transition stateyielding the alcohol
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Works with Cl2 or Br2 at room temperature in varietyof organic solvents No rearrangements so either carbocations are not
intermediates OR nucleophilic capture is faster thanrearrangement
Anti addition is observed Formation of 3-center bromonium ion intermediate
Br is polarizable
Bromonium ionintermediate
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The more substituted the alkene, the fasterthe reaction rate◦ Consistent with rate determining step being
electron flow from alkene to the halogen
◦ Alkyl electron releasing groups stabilize thetransition state for bromonium ion formation
Bridging of the bromine in the bromoniumion forces the bromine anion into attack on
the opposite face of the double bond leadingto trans addition in cycloalkenes
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Halogen addition in water adds X and OH 1st step is formation of halonium bridged ion
2nd step is nucleophilic attack by water
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Electrophilic attack
Nucleophilic attack on more substituted C
Deprotonation
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Follows Markovnikov Rule in that nucleophile adds to moresubstituted carbon
Addition is in cycloalkenes will be anti because bridgedhalonium ion prevents syn approach
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Overall transformation : C=C to epoxide Reagent : a peracid or peroxyacid, RCO3H Regioselectivity : not relevant since both new
bonds are the same, C-O Stereoselectivity : syn since the two new C-O σ
bonds form at the same time from the peracid. The reaction is an example of a concerted
process (all bonding changes occur in one step) Since the reaction is concerted the
stereochemistry of the alkene is preserved in the
product.◦ For example if the alkyl groups of the alkene are cis -
then they are also cis - in the epoxide.
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Start at the C=C as thenucleophile, make abond to the slightlyelectrophilic O
break the weak O-O
make a new C=O break the original C=O
to make a new O-H bond,
break the original O-H
to form the new C-O bond ! (phew !) bondto give the epoxide.
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Most common reagent is peroxyacetic acid Solvents are acetic acid , dichloromethane or
chloroform
Substitutions increase rate of reaction sincealkyl groups release electrons to the doublebond◦ Implies peroxyacid acts as an electrophilic reagent
towards the double bond
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Overall reaction Common method for formation of aldehydes
and ketones
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Heated in presence of oxygen or peroxides aschain initiators
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Alkyl groups stabilize the carbocation intermediate