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ORGANIC CHEMISTRY
FINAL REVIEW
Wednesday December 14, 2022
,Module 6: Addition Reactions of Alkenes
What is an Addition Reaction?
• Addition is when a C=C π bond is converted to 2 new σ bonds
• The π bond is an electron pair donor which can act as a base or a nucleophile
• Alkenes (double bond) are electron-rich
o Attack electron poor (δ+)
Addition vs. Elimination
• Addition is the opposite of elimination
o Addition and elimination are an equilibrium--which side is favoured depends on
thermodynamics
o The higher the temperature the more enthalpy, S, matters
• ∆G = ∆H - T∆S
• Addition reactions are favoured by enthalpy
o σ bonds are stronger (more stable) than π bonds
• 1 σ and 1 π bond in reactants become 2 σ bonds in product
▪ ∆H = bonds broken - bonds formed = -∆H = -∆G
• Addition reactions are not favoured by entropy (S): two reactants combine to form one
product = lower entropy
• At low temperatures, enthalpy dominates and addition is favoured
• At high temperatures, entropy dominates and elimination is favoured
HX Addition Reaction
• Electron-rich double bond in alkene deprotonates HX
o Forms new bond with H in location to form most stable carbocation (3˚>2˚>1˚)
• When carbocation rearrangements can occur to form a more stable
carbocation, they do
, o Formation of carbocation is RDS: anything that makes carbocation more stable
will increase rate of reaction
• X- is left as a good nucleophile
o Nucleophile X- attacks carbocation and forms new bond
• Now have alkane with new bonds to H and X
• Stereochemistry: if a new chiral center is made, both R and S configurations are
formed because carbocation is planar so nucleophile can attack from either side
• Addition of HBr and HCl work, HF and HI do not work
Acid-Catalyzed Hydration (H3O+ / H2O + H+)
• Electron-rich alkene deprotonates H3O+, producing H2O
• H bond forms in location such that most stable carbocation formed
• H2O acts as a nucleophile and attacks carbocation (like SN1), forming a new bond to
H2O+ (+ is on the O)
• Water can then acts as a weak base and deprotonate the H2O+ (forming H3O+), leaving a
bond to OH
, • H2O does not have to be used for hydration, any alcohol works (ie., MeOH) in the
presence of acid catalyst
o Alkene deprotonates acid, alcohol acts as nucleophile and deprotonates to form -
OR bond
• In presence of acid catalyst, it is possible to have the alkene and the alcohol in the same
molecule (intramolecular) such that a bond forms between carbon of alkene and oxygen
of alcohol to make a ring
o Alkene deprotonates acid, alcohol O attacks carbocation, conjugate base of acid
deprotonates OH+
• This is favourable for formation of 5-7 membered rings
Stereochemistry of Hydration
• Stereochemistry: if a new chiral center is formed, a racemic mixture of R and S are
obtained because carbocation is planar and water can attack from either side
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