CH3 – CH-CH2 - CHg
Ň H₂
Answers
Explanation:
Alkene Addition Reactions
pi bonds undergo addition reactions
CH2=CH2 + HCl --> CH3CH2Cl
in general,
C=C + HX --> H-C-C-X
alkenes react with hydrogen halides to form alkyl halides
Addition of HX to Alkenes
cyclohexene + HBr --> bromocyclohexane
1-methylcyclohexene + HBr --> 1-bromo-1-methylcyclohexane (not 1-bromo-2-methylcyclohexane)
Reaction Notation
reactants -------> products
focus on the organic reactants and products
show reagents over the arrow
show solvent and conditions under the arrow
(or show full balanced reaction)
Orientation of Addition
regiochemistry:
specific orientation of addition
(which C gets H, which gets X?)
alkene additions are regioselective:
one direction of addition is usually preferred
Markovnikov's Rule
the original:
add H to the C with more H's
(or to the C with fewer alkyl groups)
the reason:
add H+ to form the more stable cation
CH3CH=CH2 + HCl --->
CH3CH+CH3 (not CH3CH2CH2+)
---> CH3CHClCH3 (not CH3CH2CH2Cl)
Tues, Feb. 13
Carbocations
structure: trigonal (sp2)
stability: 3° > 2° > 1°
more alkyl groups stabilize a cation by electron donation to the electron-deficient (6-electron) carbocation
Markovnikov Addition
Hydration of Alkenes
alkene + water --> alcohol
CH2=CH2 + H2O --(H+)--> CH3CH2OH
mechanism:
step 1:
addition of H+ electrophile to pi bond
step 2:
addition of H2O nucleophile to cation
Hydration Mechanism
Halogenation of Alkenes
CH2=CH2 + Cl2 ---> Cl-CH2-CH2-Cl
mechanism:
Cl2 is an electrophile (adds Cl+)
then Cl- is a nucleophile
Anti Addition
anti stereochemistry: two new groups are added to opposite sides of the original pi bond
cyclopentene + Br2 ---> trans-1,2-dibromocyclopentane (no cis)
anti - describes the process
trans - describes the product
Bromonium Ion
carbocations can be stabilized by bonding to a neighboring Br
(also works with Cl, but less favorable)
Reduction of Alkenes
reduction - addition of H2
(or removal of O)
CH2=CH2 + H2 ---> CH3-CH3
R-O-H + H2 ---> R-H + H2O
Catalytic Hydrogenation
CH2=CH2 + H2 ---> CH3-CH3
requires an active catalyst, typically Pt, Pd, Ni, PtO2
reaction occurs on the surface
both Hs are delivered to the same side of the pi bond
Syn Addition
syn stereochemistry: two new groups are added to the same side of the original pi bond
1,2-dimethylcyclohexene + H2 --(cat)-->cis-1,2-dimethylcyclohexane(no trans)
syn - describes the process
cis - describes the product
Oxidation of Alkenes
oxidation - addition of O
(or removal of H2)
RCH2OH ---> RCH=O ---> RCOOH
there are a wide variety of oxidizing agents:
O2, O3, KMnO4, CrO3, Na2Cr2O7
metals in high positive oxidation states
Hydroxylation
alkene + KMnO4 --(base)--> 1,2-diol
addition of two OH groups is syn
cyclopentene --> cis-1,2-cyclopentanediol
alkenes are typically prepared by elimination reactions
loss of HX from alkyl halides
(promoted by strong base)
loss of H2O from alcohols
(promoted by strong acid)
eliminations are the reverse of additions
Dehydrohalogenation
Dehydration
Zaitsev Rule
predicts regiochemistry
the major product in an elimination reaction is the more substituted alkene
(generally more stable)
Conjugated Dienes
two double bonds separated by one single bond
overlap their p orbitals into an extended (conjugated) molecular orbital
more stable than separate pi bonds
e.g., 1,3-butadiene
1,2- and 1,4-Additions
electrophilic additions (HX, X2, etc.) often add at opposite ends (1,4) of a conjugated diene
Allylic Carbocations
initial electrophilic addition occurs at end (not middle) of a conjugated diene
the resultant cation retains three overlapping p orbitals (stabilized)
allyl - position next to a C=C bond
(vinyl - position on a C=C bond)
Allylic Resonance
allylic cation has two Lewis structures (resonance forms)
the actual structure is a hybrid
the allyl cation is more stable than normal alkyl cations (due to resonance)
Resonance Forms
each resonance form is a correct Lewis structure
no atoms change (only electrons)
equivalent resonance forms are most important for stabilization
nonequivalent resonance forms may also contribute
actual structure is a resonance hybrid
Alkynes - Structure
carbon-carbon triple bond
sp hybridization (linear)
no cis-trans possibilities
the two pi bonds are perpendicular
high electron density
(usually more reactive than alkenes)
Alkynes - Nomenclature
-yne suffix (with number)
rules similar to alkenes
with both -enes and -ynes, suffix is -enyne and numbering is from the end closer to a multiple bond
(E)-4-hexen-1-yne
Alkyne Additions
similar to alkenes but more reactive
Markovnikov Rule is followed
excess reagent gives double addition
single addition is usually possible
single addition gives alkene product, which may be cis (syn addition) or trans (anti addition) or nonspecific
Reduction of Alkynes
excess H2 + catalyst gives alkanes
Lindlar catalyst gives cis-alkenes
Halogenation of Alkynes
first addition of X2 is anti
product is trans-dibromoalkene
Hydration of Alkynes
initial product is an enol, which is typically unstable
an enol isomerizes to a ketone
(tautomers - a special kind of isomer, where the only difference is the placement of one hydrogen)
Alkyne Acidity