alcohols have seen many reactions to synthesize alcoholsbiewerm/11.pdfwith 1˚ alcohols the...
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Alcohols
Have seen many reactions to synthesize alcohols:
In this chapter we will study reactions of the alcohols
Oxidation
Need to understand the nomenclature of organic reduction/oxidation
In general chemistry learn that reduction is gain of electrons and oxidation is loss of electrons
Definition works well with transition metals, but it is hard to distinguish RED/OX reactions in organic reactions by this definition
It is very important that one recognizes whether two compounds are of the same oxidation level or whether one is “oxidized” relative to the other
For organic compounds want to compare oxidation level of different compounds
Anytime two compounds can be converted with only water, basic water (NaOH) or acidic water (H+) then the two compounds are at the same oxidation level
For example:
All compounds are at the same oxidation level
oxidized form reduced form
Using LAH, however, creates two compounds at different oxidation levels
Comparison of Oxidation States
We can use a bookkeeping technique to assign oxidation states
In this technique assume every bond is ionic and the electrons are closer to the more electronegative atom
This is obviously just for convenience – does not correspond to actual system with covalent bonds (where electrons are shared between two atoms)
Consider bonds to carbon:
C H
C Cl
C O
C H
C Cl
C O
Used only for bookkeeping - C-H bond is not ionic
Comparison of Oxidation States
With this bookkeeping technique, oxidation states can be compared
Oxidation state
compound
CnHx
CHnClx
CHnOx
CHnNx
-4 -2 0 +2 +4
alkane alkene alkyne CH4 C2H4 C2H2
alkyl halide dihalide trihalide tetrahalide CH3Cl CH2Cl2 CHCl3 CCl4
alcohol aldehyde acids carbon dioxide CH3OH CH2O HCO2H CO2
amine imine nitrile diimide CH3NH2 CH2NH HCN C(NH)2
Oxidation
Reduction
Oxidation States Relative to Reactions
Oxidation state
CH3CH3 H2C=CH2 C2H2
CH3CH2Cl CH3CHCl2 CH3CCl3 CCl4
CH3CH2OH CH3CHO CH3CO2H CO2
CH3CH2NH2 CH3CHNH CH3CN C(NH)2
Interconversions between same oxidation state can occur with water or acidic/basic conditions
HCl
Interconversions between different oxidation states need an oxidizing (if going to the right) or reducing agent (if going to the left)
LAH
Choosing Reagents for an Organic Reaction
The proper choice of reagents therefore involves deciding whether a reaction changes oxidation states
If a reaction is more oxidized, then an oxidizing agent is required If a reaction is more reduced, then a reducing agent is required
H3CCO
CH3H3CC CH3
HO H
Assign oxidation state to each carbon
-3 -3 -3 -3 0 +2
The middle carbon changes from 0 to +2 oxidation state, Therefore the carbon has been oxidized
Need oxidizing agent
Oxidation of Alcohols
Typical procedure to oxidize an alcohol is to use a chromium (VI) reagent e.g. CrO3, H2Cr2O7, H2CrO4
The alcohol reacts with Cr(VI) to form a chromate ester
OH HO CrO
OOH O Cr
O
OOH
HO
Cr(IV)
B
The chromate ester then reacts with base (usually water) to oxidize the alcohol [and reduce the chromium reagent]
With 1˚ alcohols the initially formed aldehyde reacts further
The aldehyde equilibrates to the geminal diol form, called acetal (which is further promoted in acidic conditions)
This acetal can behave like an alcohol to oxidize in a second step
1˚ alcohols therefore give almost exclusively carboxylic acids (especially in acidic conditions)
One way to avoid this overoxidation is to use a Cr(VI) reagent in nonacidic conditions
The pyridine acts as a base and therefore there is less acetal formation and hence oxidation of 1˚ alcohols stop at the aldehyde stage
Other Oxidants
Cr(VI) is not the only oxidant for alcohols
Swern Oxidation -another method to synthesize aldehydes from 1˚ alcohols
Advantages: mild method, easy to separate and purify products, 1˚ alcohol stops at aldehyde
In addition to oxidation, can also react alcohols as either nucleophiles or electrophiles
To make alcohols more nucleophilic, need to abstract the acidic hydrogen (remember pKa’s!)
With this method, can make nucleophilic oxygen that can react through any SN2 type reaction already studied
Esterification
To form esters the alcohol can react as a nucleophile without forming the alkoxides - generally need to activate the carbonyl first (through protonation)
to make it more electrophilic
Esters can also be formed with alcohols by reacting alcohol with more reactive carbonyl groups
The better leaving group (chloride) allow the alcohol to react at the carbonyl without first protonating
Formation of Inorganic Esters
- Same type of mechanisms already observed
Tosylates are typically formed by reacting alcohols and tosyl chlorides
Using common abbreviations:
Other Common Ester Types
Follow same mechanism as seen earlier for ester formation (alcohol reacts at carbonyl site {N=O or P=O in these examples} followed by loss of water)
OHH
HH
NUC
Alcohols Reacting as Electrophiles
In these reactions the alcohol is the leaving group (the C-O bond is broken during the reaction)
NUCH
HHOH
Usually the hydroxide, or alkoxide, is a BAD leaving group, therefore we need to convert the alcohol into a GOOD leaving group
One Method is Tosylate Formation
The tosylate, which was seen earlier, is commonly used as a way to make the alcoholic oxygen a good leaving group
OTsH
HH
NUCNUC
H
HHOTs
Another method we have already observed is protonation to form water as the leaving group
OH OH2 XH+
X
With either an SN1 or SN2 reaction the alcohol would need to be protonated first in order to make a better leaving group
For alcohol reactions what type of reaction that will occur (SN1 or SN2) depends upon the order of the attached carbon
3˚ alcohols must be SN1 (cannot undergo SN2) 2˚ alcohols mainly SN1 1˚ alcohols SN2 (1˚ carbocations are high in energy)
Lucas Reagent
Chloride reacts slower than bromide (less nucleophilic)
Therefore often need to add an additional Lewis acid to convert an alcohol to alkyl chloride (even with 2˚ and 3˚ alcohols that proceed through SN1)
-with Lucas reagent need to add ZnCl2 to increase reaction rate
While it is relatively easy to react alcohols with hydrobromic or hydrochloric acid, to generate alkyl bromides or alkyl chlorides,
both HF and HI are difficult to react in this manner
Need different routes to these compounds
Alkyl iodides can be prepared with phosphorous halides
An example with PCl3:
SN2 reaction – works well for 1˚ and 2˚ alcohols, not a choice for 3˚ alcohols
Method of choice for Iodides
PI3 can be used Often in practice PI3 is generated in situ from P/I2
Another method for conversion of alcohols to alkyl halides is thionyl chloride
Due to rapidity of the last step, these is retention of stereochemistry
In practice almost any alcohol can be converted into an alkyl halide
Type of alcohol and desired halide often determine which method is best
Best reagents for interconversions:
Type of alcohol chloride bromide iodide primary
secondary tertiary
SOCl2
SOCl2
HCl HBr HI
PBr3
PBr3
P/I2
P/I2
Stereochemistry, and possibility of rearrangements, depends on mechanism for each reaction
Diol Reactions
Some reactions are unique to vicinal diols (glycols)
Pinacol rearrangement:
O
H
HO
H
HO
H
HOHH OCH3
OH
methyl-!-D-glucopyranoside
Periodic Cleavage of Glycols
HIO4 will react with glycols
Very important reaction to determine sugar structures
Only vicinal diols react
O
H
OH
HH OCH3
OH
HIO4
O
H
O
OH