exam #3 Flashcards
most common bases used in elimination reactions are
alkoxides
common bases used in dehydration
sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide, potassium tert-butoxide
alkenes are classified by
number of carbon atoms bonded to the carbons of the double bond
stability of an alkene increases as the number of
R groups bonded to the double bond carbons increases
higher s character =
how much easier an atom accepts electron density
trans isomer
when R groups are on opp sides
cis isomer
when R groups are on same side
stereoisomers on C=C bond is possible when
two groups on each C must be different
what is more stable? trans or cis alkenes?
TRANS b/c groups bonded to double bond carbons are further apart (reducing steric interactions)
E2
bimolecular elimination
E1
unimolecular eliminiation
E2 and E1 differ in
timing of bond cleavage and formation (analogous to SN2 and SN1)
most common mechanism for dehydrohalogenation?
E2
rate = k[(CH3)3CBr][-OH]
all bonds are broken and formed in a single step
E2 generally has
strong, neg charged bases (-OH, -OR)
uses DBN and DBU
what are DBN and DBU?
sterically hindered nitrogen basese
base appears in (x) so rate of E2 rxn (y) as strength of (z)
rate equation
increases
base increases
when the base strength increases so does the
E2 reaction rate
the better the leaving group, the
faster the E2 rxn
polar aprotic solvents
increase E2 rxn rates
SN2 and E2 mechanisms differ in
how the R group affects the rxn rate
as the number of R groups on the carbon with the leaving group increases,
rate of E2 rxn increases
when alkyl halides have two or more different Beta-carbons,
more than one alkene product is formed
one of the product usually predominates when this happens
major product is more
STABLE (has more substituted double bond)
zaitsev rule
formation of major and minor product when alkyl halides have 2+ different beta-carbons
zaitsev rule:
the major product in beta-elimination has the more substituted double bond
regioselective:
when it yields predominantly or exclusively one constitutional isomer when more than one is
when a mixture of stereoisomers is possible from a dehydrohalogenation…
the major product is the more stable stereoisomer
stereoselective:
when rxn forms predominantly / exclusively one stereoisomer when 2+ are possible
e2 rxn is stereoselective bc
one stereoisomer is formed preferentially
rate of E1 rxn increases as
number of R groups on carbon w/ leaving group increases
strength of the base determines
whether a rxn is E1 or E2
strong bases favor
E2
weaker bases favor
E1
E1 rxns are
regioselective
- favor formation of the more substituted, more stable alkene
zaitsev’s rule applies
SN1 and E1 have the same
first step (formation of a carbocation)
SN1 and E1 differ in
what happens to the carbocation
E1 competes w/
SN1
E1 rxns of alkyl halides are much less
useful than E2 rxns
transition state of an e2 rxn consists of
four atoms from an alkyl halide
one H atom
two C atoms
leaving group (x)
all aligned in a plane
two ways for C–H and C–X to be
coplanar
syn periplanar
H and X are on same side
anti periplanar
H and X are on opp sides
E2 elimination require the
anti periplanar geometry
anti periplanar arrangement has a
staggered conformation
two electron-rich groups are far apart
PREFERRED GEOMETRY
syn periplanar arrangement has an
eclipsed formation
two electron-rich groups are close
strength of base is the
MOST important factor in determining mechanism for elimination
a single elimination rxn produces a
pi bond of an alkene
two consecutive elimination rxns produce two pi bonds of an alkene
alkynes are prepared by
two successive dehydrohalogenation rxns
two elimination rxns are needed to
remove two moles of HX from a dihalide substrate
vicinal dihalide or a geminal dehalide –>
two different starting materials can be used in elimination rxns that remove two moles of HX from a dihalide substrate
to synthesize alkenes,
stronger bases are needed
typical bases used -NH2 (amide)
good nucleophiles that are weak bases favor
substitution over eliminiation
why do good nucleophiles favors sub over elimination?
certain anions generally give products of substitution b/c they are good nucleophiles/weak bases
e.g. I-, Br-, HS-, -CN, CH3COO-
bulky non-nucleophilic bases favor
elimination over substitution
KOC(CH3)3, DBU, DBN are
too sterically hindered to attack tetravalent carbon but are able to remove a small proton
alcohols contain
hydroxy group (OH) bonded to an sp3 hybridized carbon
epoxides are
ethers having O atom in a three-membered ring
epoxides are also called
oxiranes
an epoxide is a special type of
ether
when an OH group is bonded to a ring,
the ring is numbered beginning with the OH group
b/c the functional group is at C1,
the 1 is usually omitted from the name
the ring is then numbered in a clockwise/counterclockwise to
give the next substituent the lowest number
how to name alcohols (common names)
1) name all carbon atoms of the molecule as a single alkyl group
2) add word ‘alcohol’, separating words with a space
simple ethers (naming them)
name both alkyl groups bonded to the O, arrange names alphabetically, add word ‘ether’
for symmetrical ethers, name alkyl group and add “di-“ prefix
more complex ethers are named using IUPAC system using following rules
name simple alkyl group (–yl to —oxy)
name remaining alkyl group as an alkane
cyclic ethers have an
O in the ring
common example: THF
alcohols/ethers/epoxides exhibit
dipole-dipole interactions b/c they have a bent structure w/ two polar bonds
alcohols are capable of
intermolecular H bonding
alcohols are more polar than
ethers and epoxides
what affects H bonding?
steric hindrance
preparation of ethers by
williamson ether synthesis
an alkoxide salt is
needed to make an ether
alkoxides can be prepared from alcohols by a
bronsted-lowry acid-base rxn
sodium ethoxide is prepared by
treating ethanol w/ NaH
NaH is a very good base for forming alkoxide b/c
the by-product of the rxn, H2, is a gas that just bubbles out of the rxn mixture
halohydrins:
organic compounds that contain both a hydroxy group and a halogen atom on adjacent carbons
in halohydrins, an intramolecular version of the
williamson ether synthesis can occur to form epoxides
OH group in alcohols is a
POOR leaving group
halogen atom serves as a
GOOD leaving group in alkyl halides
for an alcohol to undergo nucleophilic substitution, OH must be
converted into a better leaving group
by using acid, -OH can become H20 (good leaving group)
dehydration:
B-elimination rxn in which OH and H are removed from alpha and beta carbon atoms respectively
dehydration is typically carried out using
H2SO4 + other strong acids
POCl3 (in presence of amine base)
typical acids used for alcohol dehydration
H2SO4
TsOH
more substituted alcohols (x) more easily
DEHYDRATE
increasing reactivity
when an alcohol has 2-3 beta-carbons, dehydration is
regioselective and FOLLOWS zaitsev’s rule
the more substituted alkene is the major product when
a mixture of constitutional isomers is possible
secondary and tertiary alcohols react by
e1 mechanism
primary alcohols react by
e2 mechanism
primary carbocations are highly
unstable (need carbocation intermediate to complete dehydration rxn)
primary alcohols undergo
HYDRATION and then e2 mechanism
rearrangement
when carbocation intermediates will be converted into a more stable carbocation by a shift of H or an alkyl group
1,2 shift can convert
a less stable carbocation into a more stable carbocation
rearrangements are not unique to
dehydration rxns
can occur whenever a carbocation is formed as a reactive intermediate
dehydration can also be done via
POCl3 and pyridine (an amine base) in place of H2So4 / TsOH
POCl3 serves the same role as
a strong acid does in acid-catalyzed dehydration
converts poor LG (-OH) into a good LG
then dehydration –> e2
substitution reactions do NOT occur with alcohols unless
-OH is converted into a good leaving group
reaction of alcohols w/ HX (X = Cl, Br, I) is a general method to prepare
primary, secondary, tertiary alkyl halides
more substituted alcohols usually react more rapidly with
HX
order of reactivity can be rationalized by considering the
rxn mechanisms involved
depends on R group structure
hydrogen halides reactivity increases with
INCREASING acidity
b/c Cl- is a poorer nucleophile than
Br- or I-, the rxn of primary alcohols with HCl occurs only when an additional lewis acid catalyst (usually ZnCl2) is added
complexation of ZnCl2 with the O atom of the alcohol makes a
very good leaving group that facilitates the SN2 rxn
when a primary or secondary alcohol is treated with SOCl2 and pyridine,
an alkyl chloride is formed with HCl and So2 as by products
treatment of a primary or secondary alcohol with PBr3 forms
an alkyl halide
alcohols can be converted into
alkyl tosylates
alkyl tosylate is composed of
alkyl group R (derived from an alcohol)
tosylate is a good LG
tosyl group (CH3C6H4SO2-) is known as
Ts
through TsCl, alcohols are converted to
tosylates in the presence of pyridine
converts poor LG (-OH) into a good one (-OTs)
tosylate is a good LG b/c of its
conjugate acid, p-toluenesulfonic acid is a STRONG ACID
alkyl tosylates have good
LGs b/c they undergo nuc substitution and beta-elimination
alkyl tosylates are treated with
STRONG nucs (SN2) and bases (E2)
for ethers to undergo sub/elimination, their poor LG must be
converted into a good LG by rxn with strong acids (HBr and HI)
HBr & HI can provide
nucleophiles, Br- and I-
H2SO4 does not have a
nucleophile
add H20 for hydrolysisw
when ehters react with HBr or HI,
both C–O bonds are cleaved and two alkyl halides are formed as products
mechanisms of ether cleavage
SN1 or SN2 (depends on R’s identity)
when secondary or tertiary alkyl groups are bonded to the ether oxygen,
C–O bond is cleaved by an Sn1 mechanism (involves carbocation)
with methyl or primary R groups,
C–O bonds are cleaved by SN2
negatively charged nucleophiles attack
SN2-like (less hindered) 1>2>3
epoxides in acids use
SN1-like mechanism
reactions of epoxides
ring opening of an epoxides (either with a strong nuc or acid) is regioselective
one constitutional isomer is the major or exclusive product
site selectivity of these 2 rxns is exactly opposite
nucleophile attacks a
carbon atom –> substitution product
bronsted-lowry base removes a proton to
form a pi bond –> elimination product
nucleophiles that are weak bases
-SH
Br-
-CN
I-
CH3CO2-
sub is favored over elimination
strong bulky bases
-OC(CH3)3
DBU
DBN
e2 elimination favored
strong nucs and bases
-OH
-OR
SN2 and E2 favored
weak nucs and bases
H20
ROH
SN1 and E1 favored
how to classify alkenes
count the number of R groups bonded to the C=C
with 2 groups on the C=C, alkene is cis or trans
alkene stability increases
when # of R groups bonded to double bond carbons increases
trans or cis alkenes are more stable?
trans
how to find products/major product of an elimination rxn
1) identify alpha and beta carbons
2) remove halogen and substitute carbons
3) more substituted product is favored
more substituted alkenes are favored in
E2/E1 reactions (Zaitsev rule)
PRIMARY ALKYL HALIDE reacts with
strong nuc (SN2)
strong bulky base (E2)
SECONDARY alkyl halide reacts with
strong base and nuc (SN2 + E2)
strong bulky base (E2)
weak base and nuc (SN1+E1)
TERTIARY alkyl halide reacts w/
weak base and nuc (SN1 + E1)
strong base (E2)
hydride shift (1,2-H shift):
less stable carbocations rearrange to more stable carbocations by the shift of an H atom
alkyl shift
LESS stable carbocations rearrange to more stable carbocations by the shift of an alkyl group
