An alternative to making the halide: ROH ROTs p-toluenesulfonyl chloride Tosyl chloride TsCl...
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Transcript of An alternative to making the halide: ROH ROTs p-toluenesulfonyl chloride Tosyl chloride TsCl...
An alternative to making the halide: ROH ROTs
p-toluenesulfonyl chloride
Tosyl chloride
TsCl
ROH +S OO
Cl
S OO
O
CH3 CH3
R Tosylate group, -OTs, good leaving
group, including the
oxygen.
The configuration of the R group is unchanged.
Preparation from alcohols.
Example
CH3
H OH
C2H5
C3H7 CH3
TsCl
CH3
H OTs
C2H5
C3H7 CH3
Preparation of tosylate.
Retention of configuration
Substitution on a tosylate
The –OTs group is an excellent leaving group
Acid Catalyzed Dehydration of an Alcohol, discussed earlier as reverse of hydration
Protonation, establishing of good leaving group.
Elimination of water to yield carbocation in rate determining step.
Expect tertiary faster than secondary.
Rearrangements can occur.
Elimination of H+ from carbocation to yield alkene.
Zaitsev Rule followed.
Secondary and tertiary alcohols, carbocations
Primary alcohols
Problem: primary carbocations are not observed. Need a modified, non-carbocation mechanism.
Recall these concepts:
1. Nucleophilic substitution on tertiary halides invokes the carbocation but nucleophilic substitution on primary RX avoids the carbocation by requiring the nucleophile to become involved immediately.
2. The E2 reaction requires the strong base to become involved immediately.
Note that secondary and tertiary protonated alcohols eliminate the water to yield a carbocation because the carbocation is relatively stable. The carbocation then undergoes a second step: removal of the H+.
The primary carbocation is too unstable for our liking so we combine the departure of the water with the removal of the H+.
What would the mechanism be???
Here is the mechanism for acid catalyzed dehydration of Primary alcohols
1. protonation
2. The carbocation is avoided by removing the H at the same time as H2O departs (like E2).
As before, rearrangements can be done while avoiding the primary carbocation.
Principle of Microscopic Reversibility
Same mechanism in either direction.
Pinacol Rearrangement: an example of stabilization of a carbocation by an adjacent lone
pair.
Overall:
MechanismReversible protonation.
Elimination of water to yield tertiary carbocation.
1,2 rearrangement to yield resonance stabilized cation.
Deprotonation.
This is a protonated
ketone!
Oxidation
Primary alcohol
RCH2OH RCH=O RCO2H
Na2Cr2O7
Na2Cr2O7
Na2Cr2O7 (orange) Cr3+ (green) Actual reagent is H2CrO4, chromic acid.
Secondary
R2CHOH R2C=O
Tertiary
R3COH NR
KMnO4 (basic) can also be used. MnO2 is produced.
The failure of an attempted oxidation (no color change) is evidence for a tertiary alcohol.
Na2Cr2O7
OH
CH2OH
HO
Na2Cr2O7
acid
OH
CO2H
O
Example…
Oxidation using PCC
Primary alcohol
RCH2OH RCH=OPCC
PCCSecondary
R2CHOH R2C=O
Stops here, is not oxidized to carboxylic acid
Periodic Acid Oxidation
OH
OH
glycol
HIO4
O
O
two aldehydes
+ HIO3
OH
O HIO4
O
O
aldehydes
HO
carboxylic acid
+ HIO3
O
O HIO4
O
OHO
carboxylic acidcarboxylic acid
OH + HIO3
OH
O 2 HIO4
OH
O
OH
OHO
O
+ 2 HIO3
Mechanistic Notes
Cyclic structure is formed during the reaction.
Evidence of cyclic intermediate.
Sulfur Analogs, Thiols
Preparation
RI + HS- RSH
SN2 reaction. Best for primary, ok secondary, not tertiary (E2 instead)
Acidity
H2S pKa = 7.0
RSH pKa = 8.5
Oxidation
Ethers, Sulfides, Epoxides
Variety of ethers, ROR
Aprotic solvent
Reactions of ethers
Ethers are inert to (do not react with)
•Common oxidizing reagents (dichromate, permanganate)
•Strong bases
•Weak acids. But see below.Ethers do react with conc. HBr and HI. Recall how HX reacted with ROH.
Characterize this reaction:
Fragmentation
Substitution
Regard as leaving group.
Compare to OH, needs protonation.
Expectations for mechanism
Protonation of oxygen to establish leaving group
For 1o alcohols: attack of halide, SN2
For 2o, 3o: formation of carbocation, SN1
HX protonates ROH, set-up leaving group followed by SN2 (10) or SN1 (20 or 30).
Look at this reaction and attempt to predict the mechanism…
Mechanism
R-O-R
H+
RO
R
H
primary R
X-
RO
R
H
X
Inversion of this R group
This alcohol will now be
protonated and reacted with
halide ion to yield RX. Inversion will
occur.
Secondary, Tertiary R
RO
R
H
X-
R X
This alcohol is protonated, becomes
carbocation and reacts with halide.
Loss of chirality at reacting carbon. Possible rearrangement.
Properties of ethers
Aprotic Solvent, cannot supply the H in H-bonding, no ether to ether hydrogen bonding
Ethers are polar and have boiling points close to the alkanes.
propane (bp: -42)
dimethyl ether (-24)
ethanol (78)
Hydrogen Bonding
R
O
H
R
OH
H acceptor H donor
protic
Ethers are not protic, no ether to ether H bonding
However, ethers can function as H acceptors and can engage in H bonding with protic compounds. Small ethers have appreciable water solubility.
Requirements of Hydrogen Bonding: Need both H acceptor and donor.
Synthesis of ethers
Williamson ether synthesis
RO- + R’X ROR’
Characteristics
• RO-, an alkoxide ion, is both a strong nucleophile (unless bulky and hindered) and a strong base. Both SN2 (desired) and E2 (undesired side product) can occur.
• Choose nucleophile and electrophile carefully. Maximize SN2 and minimize E2 reaction by choosing the R’X to have least substituted carbon undergoing substitution (electrophile). Methyl best, then primary, secondary marginal, tertiary never (get E2 instead).
• Stereochemistry: the reacting carbon in R’, the electrophile which undergoes substitution, experiences inversion. The alkoxide undergoes no change of configuration.
nucleophile electrophile
C2H5
H3C H
O
D H
H CH3
C2H5
H CH3
Provide a synthesis starting with alcohols.
Analysis (devise reactants and be mindful of stereochemistry)
Use Williamson ether synthesis.
•Which part should be the nucleophile?
•Which is the electrophile, the compound undergoing substitution?
Electrophile should ideally be 1o. Maximizes subsitution and minimizes elimination.
C2H5
H3C H
O
D H
H CH3
C2H5
H CH3
Electrophile, RX undergoing substitution
Nucleophile or
C2H5
H3C H
O
D H
H CH3
C2H5
H CH3
Electrophile, RX undergoing substitution
Nucleophile
1o
2o
1o
2o
We can set it up in two different ways:
Remember: the electrophile (RX) will experience inversion. Must allow for that!
C2H5
H3C H
O
D H
H CH3
C2H5
H CH3
Electrophile (RX)
Nucleophile
1o
2o
C2H5
H3C H
X
H D
O
H CH3
C2H5
H CH3
SN2Note allowance
for inversion
Preferably use tosylate as the leaving group, X. Thus….
C2H5
H3C H
O
D H
H CH3
C2H5
H CH3
SN2
C2H5
H3C H
OTs
H D
C2H5
H3C H
OH
H D
TsCl
retention
inversion{ O
H CH3
C2H5
H CH3
OH
H CH3
C2H5
H CH3retention
K
Done!
Acid catalyzed dehydration of alcohols to yield ethers.
2 ROH ROR + H2OH
Key ideas:
•Acid will protonate alcohol, setting up good leaving group.
•A second alcohol molecule can act as a nucleophile. The nucleophile (ROH) is weak but the leaving group (ROH) is good.
Mechanism is totally as expected:
•Protonation of alcohol (setting up good leaving group)
•For 2o and 3o ionization to yield a carbocation with alkene formation as side product. Attack of nucleophile (second alcohol molecule) on carbocation.
• For 1o attack of nucleophile (second alcohol molecule) on the protonated alcohol.
Mechanism
RCH2OH RCH2 - OH2
RCH2OH
RCH2OCH2R
H
RCH2OCH2R
primaryalcohols
ether
For secondary or tertiary alcohols.
E1 eliminationSN1 substitution
H-O-H leaves, R-O-H attached.
For primary alcohols.
ROH ROH2 H2O + carbocation
ether
ROH
alkene
- H+
Use of Mechanistic Principles to Predict Products
OH
acid
C10H22O
OH
protonate
H+
OH2+
Have set-up leaving group which would yield secondary carbocation.
H
Check for rearrangements. 1,2 shift of H. None further.
H
Carbocation reacts with nucleophile, another alcohol.
OH
H
O
H
deprotonate H
O
Acid catalyzed addition of alcohol to alkene
Recall addition of water to an alkene (hydration). Acid catalyzed, yielded Markovnikov orientation.
Using an alcohol instead of water is really the same thing!!
Characteristics
Markovnikov
Alcohol should be primary to avoid carbocations being formed from the alcohol.
Expect mechanism to be protonation of alkene to yield more stable carbocation followed by reaction with the weakly nucleophilic alcohol. Not presented.
HOH
acid
OH
alcohol
ROH
acid
OR
ether