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Chapter 9

CHEM 2OA3 Chapter Notes - Chapter 9: Enthalpy, Alkene, Nucleophile

Course Code
Paul Harrison

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 
Addition of two groups across a double bond
Pi bonds can function as a weak base or a weak nucleophile
 
Many cases, additions are the reverse of eliminations
oEquilibriums that are temperature dependent – eliminations are
favoured at high temperatures and additions at low
 : dominant factor is bond strength
oBonds broken – bonds formed
oAddition reactions are generally exothermic
 always negative for addition – two molecules  one
o–T(delta S) is always positive
Competition between enthalpy and
entropy terms; at low temperatures
entropy is small and enthalpy
dominates (negative G – favours
addition)), at high temperatures
entropy is large and dominates (positive G)
 !"
 "#"
oTreatment of alkenes with !$ across the pi bond
oIn non-symmetrical molecules, need to determine the placement of H
and X
H is generally placed at the vinylic position already bearing the
more Hs
Halogen is generally placed at the most substituted position
%&&rich get richer

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o%&&'!() addition in which a
hydrogen atom is installed at the move substituted vinylic position and
another group is installed at the less substituted vinylic position
oPurity of reagents a.ects whether Markovnikov or anti-Markovnikov
*" Markovnikov
"+Anti-Markovnikov – speci/cally peroxides
o: protonation can occur with either of the two regiochemical
possibilities but tertiary carbocation’s are more stable due to
 ,#!"
oInvolves the formation of a chirality center – two possible products are
Represents a pair of enantiomers produced at equal amounts =
 !"./0"
oHX additions are subject to carbocation rearrangements
oWhen rearrangements are possible, HX produce a mixture of products
1 /2!
 ! adding water across a double bond
3 possible reactions – 2 with Markovnikov and one with anti-Markovnikov
 -30
oAcid-catalyzed hydration = addition of water across a double bond, in
the presence of an acid
o^ same thing shown two ways
Brackets indicate the proton source is not consumed in the
reaction – catalyst
o is dependent on the structure of the starting alkene
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