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Chapter 27.5-27.6

CHEM 112 Chapter Notes - Chapter 27.5-27.6: Addition Reaction, Nucleophile, Halogenation

Course Code
CHEM 112
Gang Wu

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Chemistry Week 9
Addition reactions are typically exothermic
High electron density regions: potentially can react with electrophiles
There are different types of addition reactions
1. Addition of hydrogen (hydrogenation)
Occurs when hydrogen is added to an alkene reactant
The double bond is made into a single bond or the triple bond is made into a double bond and two hydrogen molecules are
With a lidlar’s catalyst the olecule is cis Z
With a sodium and liquid ammonia the bond is trans E
2. Addition of Hydrogen Halides (HX)
In the first step, partially positive H atom adds to one of the carbon atoms
of the double bond preferably the least substituted carbon
This produces a carbocation
When h is added to the least substituted carbon, the positive charge on
the carbocation ends up on the carbon atom that is most able to stabilize
the positive charge typically the most substituted carbon
In the second step, the Br ion acts as a nucleophile and attacks the
positively charged carbon atom of the carbocation
3. Addition of Water (hydration)
Creates an alcohol
Carried out in acidic solutions
Involves the formation of a carbocation followed by a
nucleophilic attack by the OH group
In acids there is excess of H3O+
Hydrogen from H3O+ attacks carbon to form
carbocation and water
The water molecule acts as a nucleophile and attacks
the carbocation
The proton is then removed yielding the alcohol
4. Addition of halogens (halogenation)
Vicinal dihalide when two halogens are bonded to adjacent carbons
Germinal dihalide when two halogens bind to the same carbon
Nonaqueous solvent
Formation of a bridged halonium ion rather than a carbocation
In the halonioum ion, the halogen atom is bonded to two carbon atoms and the halogen atom in the halonium ion have
complete octets
The electron pair in the pi bond is directed towards a chlorine atom
The electron pair from the chlorine atom is directed to one of the
carbon atoms and the Cl-Cl bond breaks
Then the Cl ion attacks a carbon atom in the bridge from the back
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