FINALS 9 Flashcards
Alkenes behave as ____________ in polar reactions, donating a pair of electrons from their electron-rich C=C bond to an electrophile (Lewis acid).
nucleophiles (Lewis bases)
Carbon–carbon ___ bond is replaced by two ___ bonds.
π
σ
◦ high electron density above and below the plane of molecule
◦ more polarizable than σ bond
◦ can interact with a positively charged electrophile
alkene π bond
Alkene forms a ___________ when π electrons form a σ bond with an E+ and leave the other carbon electron-deficient.
carbocation intermediate
Electrons from a Nu- are donated to the carbocation forming a second σ bond
rate-limiting step:
carbocation formation
yields alkyl halides, RX
hydrohalogenation reactions
proposed in 1869 by Russian chemist Vladimir Markovnikov
addition of HX across a carbon–carbon multiple bond proceeds in such a way that
the proton adds to the less-substituted carbon atom
the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one the electrophile adds to the double bond to form the more stable carbocation
MARKOVNIKOV’S RULE
more alkyl groups on carbocation → more possibilities for _________ → more stable carbocation
hyperconjugation
- yields alcohols, ROH
- requires acid catalyst
- also follows Markovnikov’s rule
hydration reactions
◦ suited to large-scale industrial procedures (e.g., ethanol production from ethylene)
◦ of little value in the typical laboratory because it requires high temperatures and strongly
acidic conditions
acid-catalyzed alkene hydration
◦ electrophilic addition of Hg2+ to alkene on reaction with mercury(II) acetate (CH3CO2)2Hg/
Hg(OAc)2 in aqueous tetrahydrofuran (THF) solvent
◦ formation of mercurinium ion intermediate
◦ nucleophilic addition of water followed by loss of proton
◦ forms a stable intermediate organomercury compound
Oxymercuration
◦ treatment of intermediate organomercury compound with sodium borohydride
◦ produces alcohol
demercuration
addition of a B-H bond of borane, BH3 (a Lewis acid), to an alkene to yield an
organoborane intermediate, RBH2
oxidation of organoborane by reaction with __________, H2O2 to yield an alcohol
basic hydrogen peroxide
◦ C-H and C-B bonds form at the same time and from the same face of the alkene
◦ attachment of boron is favored at the less sterically crowded carbon atom
yields a syn non-Markovnikov product
no carbocation intermediate
yield alkyl 1,2-dihalides
halogenation reactions (addition of Br2 or Cl2)
- no carbocation intermediate
- forms cyclic halonium intermediate (bromonium or chloronium)
ADDITION OF X2 TO ALKENES: MECHANISM
◦ the bridging halogen blocks nucleophilic attack on the halogen bonded face of the original double bond
◦ rear side attack opens the cyclic halonium ion
anti stereochemistry
symmetrical bromonium ions → identical
yields two enantiomers
asymmetrical bromonium molecules → nonidentical
yields one meso compound
Disadvantage: not all alkenes take up bromine e.g., those with electron withdrawing
groups → perform ________
Baeyer’s test
◦ forms halohydrins (halo-alcohols)
Addition of HOX (HOCl or HOBr)
- increases electron density on carbon by:
forming this: C-H
or breaking one of these: C-O, C-N, C-X
reduction
- decreases electron density on carbon by:
forming one of these: C-O, C-N, C-X
or breaking these: C-H
oxidation
REDUCTION OF ALKENES:
CATALYTICHYDROGENATION
Alkenes are reduced to ________
alkanes
normally used as PtO2 (Adam’s Catalyst)
platinum
normally used as very fine powder “supported” on an inert material such as
charcoal (Pd/C)
palladium
does not occur in a homogeneous solution but instead takes place on the surface of solid catalyst particles
heterogeneous process
both H atoms add on the same more accessible face (less steric hindrance)
syn stereochemistry
much more reactive toward catalytic hydrogenation than most other unsaturated functional groups, and the reaction is therefore quite selective.
alkenes
Aldehydes, ketones, esters, and nitriles only undergo _________ under more vigorous condition
alkene hydrogenation
reduced on a large scale to produce saturated fats used in margarine and cooking products
unsaturated vegetable oils
triesters of glycerol with three long-chain carboxylic acids (fatty acids) that are generally polyunsaturated with cis stereochemistry.
vegetable oils
yields saturated fatty acids
complete hydrogenation
yields trans fatty acids (due to partial cis–trans isomerization of a remaining double bonds) → raises blood cholesterol and contributes to coronary problem
incomplete hydrogenation
OXIDATION OF ALKENES: HYDROXYLATION
oxidized to 1,2-diols
Alkenes
◦ a large molecule built up by repetitive units smaller molecules called monomers
◦ e.g., cellulose from sugar monomers, proteins from amino acid monomers, and nucleic acids from nucleotide monomers
Polymer
a synthetic alkene polymer
polyethylene
RADICAL POLYMERIZATION: MECHANISM
- Initiation
- Propagation
- Termination
Few radicals are generated on heating _________ to break the weak O-O bond.
benzoyl peroxide catalyst
The formed benzoyloxy radical loses _____ and gives a , which adds to the C=C bond of ethylene to start the _________
CO2
phenyl radical (Ph·)
polymerization process
Few radicals are generated on heating benzoyl peroxide catalyst to break the weak O-O bond.
The formed benzoyloxy radical loses CO2 and gives a phenyl radical (Ph·), which adds to the C=C bond of ethylene to start the polymerization process.
One electron from the ethylene double bond pairs up with the odd electron on the phenyl radical to form a new C-C bond, and the other electron remains on carbon
initiation
The formed carbon radical adds to another ethylene molecule to yield another radical. Repetition of the process for hundreds or thousands of times builds the polymer chain.
propagation
The chain process is ended by a reaction that consumes the radical (e.g., combination of two growing chains)
termination
(substituted ethylenes) undergo polymerization
vinyl monomers
- follows Markovnikov’s Rule
- more highly substituted, secondary radical is formed
RADICALPOLYMERIZATION
- predominant alkene synthesis reaction
- from simple precursors
◦ alcohols in biological systems
◦ either alcohols or alkyl halides in the laboratory - the opposite of addition of HBr or H2O on an alkene to form an alkyl halide or alcohol
- the loss of HBr or H2O from an alkyl halide or alcohol to form an alkene
ELIMINATION REACTIONS
- loss of HX from an alkyl halide
- occurs by reaction of an alkyl halide with strong base e.g., KOH
DEHYDROHALOGENATION
carried out by treatment of an alcohol with a strong acid
DEHYDRATION
