CH3CH2CH=CHCH3 +H2O—H+ mechanism of formation of alcohols from alkene?
Answers
Answer:
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
Oxidative Cleavage
C=C --> C=O + O=C
acidic KMnO4 causes cleavage
ozone (O3) causes cleavage
sometimes useful degradation method to identify unknown compounds
Polymers
long chains of repeating units (monomers)
n CH2=CH2 --(init)--> (init)-(CH2-CH2)n-
n=100-10,000 polyethylene
has properties like a very long alkane
many polyalkenes are commercially important materials and plastics
e.g., PVC, Teflon, Orlon
Chain Reactions
polymerization occurs by a free radical chain mechanism
initiation - generation of the first free radical from an initiator
R-O-O-R --(heat)--> 2 R-O·
(initiators have one weak bond)
Chain Reactions
propagation - radical adds to a p bond
RO· + CH2=CH2 ---> RO-CH2-CH2·
note that the product is also a radical
RO-CH2-CH2· + CH2=CH2 ---> RO-CH2-CH2-CH2-CH2· ---> etc.
typically this occurs hundreds or thousands of times
(until radicals recombine - termination)
Substituted Monomers
radical additions follow the Markovnikov Rule:
add radicals to form the more stable radical intermediate
radical stability is like cation stability: 3° > 2° > 1°
this leads to polymers with alternating substituents
Vinyl Polymers
polyvinyl chloride
polypropylene
polystyrene