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Reactions of alkenes arise from __.
Because sigma bonds are more stable than pi bonds, the most common rxns of double bonds do what?
- the reactivity of the C=C
- transform the pi bond into a sigma bond
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__ is an example of an addition reaction, one of the three major reaction types. The three are __, __, __.
- hydrogenation
- addition
- elimination
- substitution
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What happens in a n additon?
What happens when an alkene undergoes addition?
two molecules combine to form one product molecule
two groups add to the carbon atoms of the double bond and the carbons become saturated
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In many ways, addition is the reverse of __, in which one molecule splits into two fragment molecules. in a __, one fragment replaces another fragment in a molecule.
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__ is the most common reaction of alkenes. A wide vvariety of functional groups can be formed by __ to the double bonds of alkenes.
- addition
- adding suitable reagents
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In principle, many different reagents can add to a double to form more stable products. What does that mean?
Not all of these reactions have convenient __, however. How do we overcome this?
the rxns are energetically favorable
rates
adding a catalyst, such as platinum, palladium, or nickel, which causes the rxn to take place at a rapid rate
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Some reagents react with C=C without the aid of a __.
Structure of the pi bond:
- although the electrons in the sigma bond are tightly held, the pi bond is __.
- the pi bonding electrons are spread __, and they are __.
- catalyst
- delocalized above and below the sigma bond
- loosely held
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A strong __ has an affinity for these loosely held electrons. It can do what to do what, leaving one of the carbon atoms with what? In effect, the double bond has reacted as a __, doing what?
- electrophile
- pull them away to form a new bond
- only three bonds and a positive charge: a carbocation
- nucleophile
- donating a pair of electrons to the electrophile
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Most addition reactions involve a second step in which what?
In the product, both the __ and the __ are bodned to teh carbo atoms that were connected by the double bond. This type of rxn requires a __ to attract the electrons of the pi bond and generate a __ in the __. Most alkene reactions fall into this large class of __ to alkenes.
a nucleophile attacks the carbocation, forming a stable addition product
- electrophile
- nucleophile
- strong electrophile
- carbocation
- rate-limiting step
- electrophilic additions
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A wide variety of electrophilic additions involve similar mechanisms.
First, a strong __ attracts the loosely held electrons from the pi bond of the alkene. The electrophile forms a __ to one of the carbons of the double bond, while the other carbon becomes a __. The __ (a strong electrophile) reacts with a nucleophile (often a weak nucleophile) to form another sigma bond.
- electrophile
- sigma bond
- carbocation x2
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Steps of electrophilic addition to alkenes
- 1) attack of the pi bond on the electrophile forms a carbocation
- 2) attack by a nucleophile gives the addition product
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Regiochemistry
- synonym
- meaning
- result
- - orientation of addition
- - which part of the reagent adds to which end of the =
- - one of the two possible orientations of addition results preferentially over the other
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Protonation of one carbon atom of a double bond gives a carbocation on the carbon atom that was __. Therefore, the proton adds to the end of the double that is __.
- not protonated
- less substituted to give the
- more substituted carbocation (more stable)
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Ionic addition of HX to an Alkene Steps
- 1) protonation of the pi bond forms a carbocation
- 2) attack by the halide ion gives the addition product
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Markovnikov rule
addition of a proton acid to the double bond of an alkene results in a product with the acid proton bonded to the carbon atom that already holds the greater number of hydrogen atoms
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Reactions that follow the Markovnikov rule are said to follow __ and give the __.
- Markovnikov orientation
- Markovnikov product
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Extended version of markovnikov's rule
in an electrophilic addition to an alkene, the electrophile adds in such a way as to generate the most stable intermediate
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Reactions that give products that are opposite to those expected from Markovnikov's rule are said to be __.
When are they most likely to occur?
- anti-Markovnikov reactions
- when the reagents or solvents came from old supplies that had accumulated peroxides from exposure to the air
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__ give rise to free radicals that initiate the addition, causing it to occur by a __. The O-O bond in __ is rather __, so it can break to give two __.
- peroxides
- radical mechanism
- peroxides
- weak
- alkoxy radicals
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__ initiate the anti-Markovnikov addition of HBr
alkoxy radicals R-O⋅
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Free radical addition of HBr to Alkenes
- Initiation: formation of reactions
- Propagation: a radical reacts to generate another radical
- 1) a bromie radical adds tot he double bond to generate an alkyl radical on the more subbed carbon atom
- 2) the alkyl radical abstracts a hydrogen atom from HBr to generate the product and a bromine radical
- --The bromine radical generated in Step 2 goes on to react with another molecule of alkene in Step 1, continuing the chain
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The bromine radical in free radical addition of HBr to alkenes lacks __ in its valence shell, makijng it __ and __. It adds to a __, forming a new free radical with the __ on a carbon atom.
This free radical reacts with an HBr molecule to form a __ and generate another __.
- an octet of electrons
- electron-deficient and electrophilic
- double bond
- odd electron
- C-H bond
- bromine radical
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The regenerated bromine radical reacts with another molecule of hte alkene, continuing the __. Both of the propagation steps are moderately __, allowing them to proceed faster than the __.
- chain reaction
- exothermic
- termination steps
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As we saw in the protonation of an alkene, the electrophile adds to the __ and the unpaired electron appears on the more __ to give the more stable free rad. This intermediate reacts with HBr to give the __, in which H has added to the more substituted end of the double bond: the end that started with fewer hydrogens
- less subbed end of the double bond
- subbed C
- anti-Markovnikov
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Both mechanisms for the addition of HBr to an alkene (with and without peroxides) follow our extended statement of Markovnikov's rule, which is __.
What are the electrophiles?
electrophile must add to teh elss subbed end of hte double bond to give the mroe stable intermediate, either a carbcation or a free rad.
- Ionic reaction: H+
- peroxide-catalyzed free rad: Br
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True or False:
If just a tiny bit of peroxide is present, all products demonstrate anti-Markovnikov mechanisms.
- False:
- a mixture of Mark and anti-Mark occurs
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If an appreicable amt of peroxide is present, the rad chain is __ that the uncatalyzed iionic reaction that only the __ is observed.
- way faster
- anti-Mark product
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The reversal of orientation in the presence of peroxides is called the __. It occurs only with the addition of HBr to alkenes. The peroxide effect is not seen with HCl because the reaction of an alkyl radical with HCl is strongly __.
- peroxide effect
- endothermic
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Similarly, the peroxide effect is not observed with HI because the reaction of an iodine atom with an alkene is strongly __
endothermic
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