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Alkene Addition Reactions

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Chapter 6
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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


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