Alkenes Properties Nomenclature Addition Reactions
Ethylene
Electron Rich -Bond
Cis and Trans no rotation about -bond
Elements or Degrees of Unsaturation: -Bond or Ring
Halogens replace Hydrogens
Saturated compounds with Oxygen and Nitrogen
Determine the # elements of Unsaturation
Alkene Nomenclature
Cyclic alkenes
Alkyl Groups with -Bonds
Alkylidene Groups Double Bonds Fused to Rings
Polyenes
Name these Alkenes
cis-trans Isomers
E/Z System
What’s My Name?
(Z)-4-ethyl-5-isopropyl-4-nonene
Establish Priority of Substituents on Each sp 2 Carbon
Priorities with Multiple Bonds
E or Z?
Name These
Preparation of Alkenes E2 - Elimination reactions of alkyl halides and tosylates (or mesylates) E1 - Acid catalyzed dehydration of alcohols
Hydrogenation Data Helps to Determine Stability H hydrogenation of Alkenes
Enthalpy Change Shows Relative Energy of Alkene
Both cis and trans 2-Butene are Hydrogenated to Butane
“E” is More Stable than “Z” by 2.3 KJ/mol
Relative Stabilities of Alkenes
Hyperconjugation bond associates with adjacent C-H bond
The Addition Reaction
HBr Addition
Markovnikov’s Rule The addition of H-X across a double bond results in the more highly substituted alkyl halide as the major product.
Addition of HBr or HCl Markovnikov Addition
Regiochemistry Determined by Stability of Intermediate
3 o Carbocation forms Preferentially
Carbocation Stability more highly substituted, lower energy
Asymmetric Center is Generated Racemic Mixture Formed
Definitions Regioisomers – two constitutional isomers that could result from an addition reaction. Regiospecific – only one regiosisomer forms at the expense of the other. Regioselective – both regioisomers are formed, but one is formed in preference.
Determine the major product:
Rearrangements
HBr Addition with RO-OR Anti-Markovnikov
Free-Radical Mechanism
Addition of Br 2
More Definitions Stereospecific – only one stereoisomer is formed at the expense of the other (e.g. trans vs. cis) Stereoselective – one stereoisomer is formed preferentially over the other.
Anti-Addition Stereospecific Reaction Anti-addition
Bromonium Ion Intermediate
Trans is formed exclusively No Meso is formed (cis)
Bromonium Ion is Opened Equally from Both Sides
Brominations Often Generate Asymmetric Centers
trans alkene + anti addition = MESO
Provide a Mechanism
Halohydrin Formation Addition of Br – OH Stereospecific & Regiospecific
Unsymmetrical Bromonium ion H 2 O opens ring at more hindered site
Br and OH are trans in anti addition
Catalytic Hydrogenation syn addition
Mechanism
Syn Addition of H 2
Reactions that Generate Chirality Centers Hydrogenation, syn
Hydrogenation all alkene bonds are reduced
Asymmetric Induction
Preparation of (L)-Dopa for Treatment of Parkinson’s
10 pt. problem 7.(10) Consider the compound 3(S),5,5- trimethylcyclohexene. Upon reduction with H 2 on a 1% Pt/C catalyst, the resulting product has an absolute configuration of (R). Draw the equation for this reaction, clearly drawing the starting alkene and the product alkane and explain why the absolute configuration is completely inverted in this reaction.
Hydration Addition of H 2 O
Oxymercuration Hydration Markovnikov addition Regiospecific Reaction
Oxymercuration Mechanism
Hydroboration Hydration Anti-Markovnikov Syn addition
Hydroboration
Draw the Major Products
2 Complementary Hydration Reactions
Cyclopropanation
Simmons-Smith Reaction
In situ Preparation of Carbene
Two MCAD Inhibitors
MCPA and SPA
Ozonolysis Alkene Cleavage
Mechanism of Ozonolysis
Addition polymers
Air Pollution Reacts with Tires
O 3 reacts with all alkene Bonds
Problem An unknown compound A, C 8 H 16, reacts with H 2 on a 1% Pt/C catalyst to form B (C 8 H 18 ). B has two chiral centers but is optically inactive due to the presence of an internal plane of symmetry. Treatment of A with O 3 followed by Zn/HOAc affords butanone only. Identify A and B and draw B in a Fischer projection.
What is the Structure of Limonene? Solomons 8.39
C=C bonds become carbonyl groups
Acid Catalyzed Dimerization Solomons 8.58
Hoffmann Product via E1