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123 terms

Kaplan MCAT OChem Ch. 5: Alkenes, Alkynes, and Elimination Reactions

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unsat. fats
fatty carbon chains w/ one or more DBs
double bonds
considered func. groups
alkenes
more reactive than alkanes
simplest alkene
ethene (ethylene)
alkynes
at least one triple bond
alkynes
unstable and very reactive
alkenes and alkynes
contain pi bonds => many similar properties
pi bonds
commonly formed through elimination rxns
alkenes
sometimes called olefins
alkenes
described cis, trans, E, and Z
ethylene
https://www.softchalkcloud.com/lesson/files/E4hyMb8JpR79i3/Ethylene-2D.png
isobutylene
http://www.wavesignal.com/o_chem/images/Alkene10.gif
propylene
http://upload.wikimedia.org/wikipedia/commons/d/d1/Propylene.PNG
alkenes
physical properties are similar to those of alkanes
trans-alkenes
have higher MP that cis since more symm => better packing in solid state
trans-alkenes
have lower BP than cis alkenes since less polar
polarity
asymm dist'n of electrons in a molecule => molecule has one partially neg. region and one partially pos. region
alkenes
unequal electron dist'n => creates dipole moments pointing from electropositive alkyl groups toward electronegative alkene (sp3 donates to sp2, 3 has less s-character than sp2, s can be found at pos. nucleus => more stable)
trans-2-butene
two dipole moments are oriented in opposite directions => cancel each other => no net dipole moment and nonpolar
cis-2-butene
addition of two smaller dipoles => has net dipole moment
alkene
polarity => add. intermolecular forces => raise BP
two mechs of elimination (unimolecular and bimolecular elimination )
e1 and e2
elimination rxns
way to synthesize alkenes, of either alcohols or alkyl halides
elimination rxns
carbon backbone kicks off (or eliminates) a H and halide (dehydrohalogenation) or a molecule of water (dehydration) => DB
dehydrohalogenation
C backbone kicks off H and halide
dehydration
C backbone kicks off water
E1 (unimolecular elimination)
two step process

rate of reactions depends on conc. of only one species, the substrate
E1 (unimolecular elimination) steps
1. LG leaves and form of carbocation

2. proton on adjacent carbon (beta- carbon) is moved by WB => DB formed
E2 (bimolecular elimination)
if strong base present, this is more likely
E1 (unimolecular elimination) factors
polar protic solvents

ability to form stable carbocation

highly branched carbon chains

good LGs

absence of a good Nu
E1 (unimolecular elimination)
higher temps favor this pathway
E2 (bimolecular elimination)
...
E1 (unimolecular elimination)
typically favor that more stable alkenes as major product (more highly sub)
E2 (bimolecular elimination)
one step

rate depends on two species, the sub and the base (Nu)
E2 (bimolecular elimination)
SB (ex: ethoxide ion) removes proton => halide ion anti to proton leaves => DB
E2 (bimolecular elimination)
has two possible prod.

DB can form on either side of departing halide, but more sub. DB is large % of products; if can form either geo isomers, trans predominates since more stable
E2 vs. Sn2 (e2)
steric hindrance is imortant

highly sub carbon chains (form stable alkenes), undergo this and other rarely
E2 vs. Sn2 (e2)
bulk of base (Nu) has hard time getting to backside of alpha carbon (w/ LG attached)

much easier to pluck off H from neighboring chain
E2 vs. Sn2 (e2)
strong base favor this over other

pull off a beta-H before can reach alpha-carbon => this rxn
E2 vs. Sn2 (sn2)
weak Lewis base (strong Nu) favored
E1 vs. Sn1 (e1)
done by controlling conditions, like factors like polarity of solvent, or most important, the temp.
e1
least likely mech of the four
Reactions of alkenes
reduction

electrophilic addition

addition of HX

addition of x(2)

add of h20

free radical add.

hydroboration

oxidation
types of oxidation in alkenes
potassium permanganate

ozonolysis

percarboxylic acids

polymerization
oxidation
loses H => form DB

often gains O
reduction
getting more H
type of reduction rxn
catalytic hydrogenation
catalytic hydrogenation
reducing alkene by adding molecular H to DB w/ aid of metal catalyst
typical catalysts of catalytic hydrogenation
Pt, Pd, Ni, sometimes rhodium, iridium, or ruthenium
catalytic hydrogenation
takes place on surface of metal

one face of pi bond becomes coordinated to the metal surface where molecular H is bond => rxn takes place where two touch (syn addition)
catalytic hydrogenation
this type of addition known as syn addition
stereospecific rxns
reaction where only one stereoisomer is formed
type of electrophilic addition rxns (alkenes)
addition of HX

addition of x(2)

add of h20
electrophilic addition rxns (alkenes)
pi bond broken w/o breaking the sigma bond (add rxns)
electrophilic addition rxns (alkenes)
since electrons of pi bond are particularly reactive => easily attacked by molecules seeking to accept electron pair (Lewis acids, electrophiles)
electrophiles
lovers of electrons
Lewis Acids
Accept Electrons
Lewis Bases
donate electrons
addition of HX
DB acts like Lewis base and reacts w/ partially positive hydrogen of HX
addition of HX
first step yields carbocation intermediate
addition of HX
in cases where alkene is asymm => initial protonation produces most stable carbocation

alkyl sub. stabilize carbocations

forms slow step
addition of HX
Mark's rule
Mark's rule
produce most stable carbocation
addition of HX
2nd step, halide ion combines w/ carbocation => alkyl halide
addition of X2
addition of this to DB is a rapid process
addition of X2
used a diagnostic tool to test presence of DB
addition of X2
1st step: DB as N => attacks 1/2 of X2 => X- as LG and cyclic halonium ion that dissipates pos charge w/ trinuclear intermediate
addition of X2
2nd step: X- attacks ion on on opposite face => dihalo compound
addition of X2
if rxn in Nu solvent, cyclic halonium ion attacked by this first before halogen ion => halo alcohol
Addition of H2O
water added to DB under acidic conditions (sulfuric acid)
Addition of H2O
DB protonated by Mark's rule => reacts w/ water => protoanted alcohol => lose proton => alcohol
Addition of H2O
performed in low temp, since at high, reversed rxn favored (dehydration)
Addition of H2O
can also be achieved under mild conditions w/ oxymercuration-reduction
free radical additions
add HX to DB using free radical intermediates
free radical additions
disobey Mark's rule because X radical add first to DB => most stable free radical, making halogen end up on least sub. carbon
free radical additions
useful for HBR, not for HI and HCl since energetically unfavorable
always determines favored products
most stable intermediate and least energetic transition state
hydroboration
first step: B2H6 (diborane) adds to DB => B atom (LA) to less sterically hindered, at same time H transferred to adjacent carbon
free radical additions
second step: oxidation-hydrolysis w/ peroxide (H2O2) and aq. base (-OH) => transfers water to bond w/ born => alcohol
oxidation
if reagent has bunch of oxygen => changes that it's an oxidizing agent
potassium permanganate (oxidation) KMnO4
alkenes ca n be oxidized w/ this, depending on rxn conditions => end w/ diff products
potassium permanganate (oxidation) KMnO4
if condition mild, using cold, dilute, basic KMnO4 => 1,2 diols (vicinal diols)
vicinal diols or glycols
1,2 diols

syn addition
potassium permanganate (oxidation) KMnO4
using hot, basic, sol of this, followed by an acid wash

1. nonterminal cleaved => to form two molar equivalents of carb acid
2. terminal cleaved => carb acid and CO2
4. if nonterminal is disub => ketone formed
ozonolysis
more selective than hote, acidic, KMnO4
ozonolysis
cleaves DB, but only oxidizes the carbon to aldehyde (or ketone if starting molecule is disub)
ozonolysis
under reducing conditions (Zn/H+ or (CH3)2S
ozonolysis
under oxidizing conditions (H peroxide) yields same products as hot, acidic KMnO4
ozonolysis
can obtain alcohols if reduce aldehyde or ketone products w/ mild reducing agent, such as NaBH4 or LiAH4
percarb acids
alkenes oxidized by this, which are strong oxidizign agents
percarb acids
peroxyacetic acid (CH3CO3H) and MCPBA commonly used
percarb acids
products are epoxides (oxiranes)
percarb acids
syn additon
epoxides
products of percarb acids
polymerization
creation of long, high MW chains (polymers) composed of monomers
polymerization
usually occur through a radical mech, although anionic and even cationic are observed too
monomers
repeating units that polymers are composed of
polymerization
some require high temp and pressure
heat present
consider possible radical mech
alkynes
one or more C-C triple bonds
alkynes
180 degrees straight lines from sp hybridization
common name from ethyne
acetylene
physical properties of alkynes
to make triple bonds, go through two rounds of elimination of geminal (twins) or vicinal dihalides (requires high temp and SB though)
physical properties of alkynes
more useful method is adding already existing triple bon into a new carbon skeleton

terminal TB coverted into Nu by reomving acidic H w/ SB (NaNH2 or n-BuLi) => acetylide ion => ion performs Nu displacement on primary alkyl halides at room temp.
terminal alkynes
are fairly acidic
rxns of alkynes
reductions

addtions

hydroboration

oxidation
type of addition rxns of alkynes
electrophilic

free radical
reduction (alkynes)
alkynes can be hydrogenated (reduced) w/ catalyst to give alkanes

if just want alkenes => stop this after one equivalent of H (partial hydrogenation)
reduction (alkynes)
use Lindlar's catalyst, w/ palladium on BaSO4 w/ quinoline, a heterocyclic aromatic poison
reduction (alkynes)
w/ Lindlar's catalyst, since rxn occurs on metal surface, product is cis-isomer
reduction (alkynes)
second method (just to alkene), use Na in liquid ammonia at temp. below -33 C (BP of ammonia) => trans isomer via free radical mech
addition, electrophilic (alkynes)
follows Mark's rule
addition, electrophilic (alkynes)
can be stopped at intermediate (alkene)
addition, electrophilic (alkynes)
can go all the way to alkane, just need two equivalents
free radical, electrophilic (alkynes)
anti-Mark
free radical, electrophilic (alkynes)
reaction product is usually trans-isomers, since intermediate vinyl radical can isomerize to its more stable form
hydroboration
addition of boron on TB => syn
hydroboration
boron bound to 3 diff sub
hydroboration
boron addition followed by acetic acid wash, boron atom removed => each sub w/ have proton from acetic acid => cis-alkene
hydroboration
w/ terminal alkynes, disub borane used to prevent further boration of vinylic intermediate to an alkane => vinylic borane can be oxidatively cleaved w/ H2O2, creating an intermediate vinyl alcohol (an enol) => tautomerizes to more stable carbonyl compound (via keto-enol taut)
oxidation (alkynes)
can be oxidatively cleaved w/ either hot, basic, KMnO4 (followed by acidification) or w/ ozone
oxidation (alkenes)
w/ ozone reducing cond (Zn/CH3COOH) => aldehyde or ketones

oxidizing condition (H2O2) => carb acids
oxidation (alkynes)
w/ ozone - carb acid or CO2

TB adds two oxygne to each carbon

terminam alkyne - CO2