NOMENCLATURE
Tutorial 8.1
Give the IUPAC name for the alcohol CH3CH(OH)CH2CH2CH2CH3. Answer
What if you have a chlorine and a hydroxide in the hydrocarbon? This is where common sense is needed. For many a "chloro" is more easily understood than a "ol" so we prefer to name the compound as an alcohol rather than a chloride.
Example: CH3CH(Cl)CH2CH2CH(OH)CH3.
We would name it 5-chloro-2-hexanol (not 2-chloro-5-pentanol). 5-ol-2-hexyl chloride do not strike a bell to many.
PHYSICAL PROPERTIES
PREPARATION
Alcohols are prepared by;.
SN Reaction
CH3CH2CH2CH2Cl + Na+OH‾ H2O / 100˚C
�CH3CH2CH2CH2OH + Cl‾ Grignard Reaction These reactions not only give the alcohol but also increase the hydrocarbon chain length by one (if an aldehyde is used) or two (if an oxirane is used) carbon units. When a ketone is used we will get a tertiary alcohol.
Reduction of aldehyde, ketone and ester.
The industry prepares ethanol and t-butanol by the electrophilic addition of water onto alkenes. Ethene is passed into a 98% sulphuric acid to give the ethylbisulphate. This is then hydrolysed to give the ethanol.
Tutorial 8.2
Write the reaction equations for the preparation of ethanol from ethene and sulphuric acid. Answer
For the preparation of t-butanol the iso-butene gas is passed into a 60% sulphuric acid to achieve an equilibrium.
(CH3)2CH=CH2 + H2O |
H+ ⇆ |
(CH3)3C−OH |
The formation of t-butanol is favoured by low temperature and the presence of water. t-butanol was at one time used as a replacement of tetraethyl lead for unleaded gasoline, as its octane number is 108.
Methanol and ethanol can also be prepared from biomass.
REACTIONS
LIKE WATER
The smaller alcohol can be considered as a cousin of water. Alcohol is ROH and water HOH. Like water it should be able to dissociate;
| ROH + ROH | ⇆ | ROH2+ + RO‾ |
However such dissociate is insignificant. What is significant is that it can react with alkali metal like water.
| ROH + Na | ⇆ | RO‾Na+ + � H2 |
So we consider alcohol as a weak acid and the alkoxide, RO‾, a strong conjugate base. So alkoxide is very often used as a base for Elimination Reaction.
Tutorial 8.3
How did we come up with the term alkoxide? Answer
SN REACTION with HALIDES
OH‾ is a stronger nucleophile than halides, so the SN reactions have to be assisted. However to promote SN2 Reaction (and so avoiding the formation of carbonium ion the reactions are normally conducted for primary and secondary alcohols.
Tutorial 8.4
Why do we seek to avoid carbonium ion formation? Answer
- For bromides a protic acid is sufficient.
The alcohol is refluxed with a hydrobromic (HBr) solution with a little sulphuric acid added. n-Butyl alcohol will yield about 95% of n-butyl bromide.
Br‾ + CH3CH2CH2CH2−OH2+ � CH2CH2CH2CH2−Br
Water is a good leaving group.
- Chloride is a poorer nucleophile, as compared to bromide, so more extreme conditions are needed.
- Lucas Reagent: a mixture of ZnCl2 and conc. hydrochloric acid.
The alcohol is reflux with the Lucas reagent, and the ZnCl2 functions as a Lewis acid.
Cl‾ + CH3CH2CH2CH2−OHZnCl2+ � CH2CH2CH2CH2−Cl + [ZnCl2(OH)]‾
With tertiary alcohol the reaction will take place at room temperature to give about 80% of the alkyl chloride.
- Thionyl chloride
The alcohol reacts with the thionyl chloride to give a chlorosulphite, a good leaving group.
RCH2−OH + SOCl2 − HCl
�RCH2−OSOCl � RCH2−Cl + SO2 - Sulfonyl Chloride
Other cousins of thionyl chloride that can be used are benzenesulfonyl chloride and p-toluenesulfonyl chloride. The reaction is promoted by a tertiary amine. The aromatic sulfonate ion is a good leaving nucleophile (100 times better than chloride).
R−OH + Ph−SO2Cl � R−OSO2Ph + HCl R−OSO2Ph + NaX � R−X + PhSO3‾Na+ Tutorial 8.5
How can pyridine C5H5NH, promote the reaction between benzenesulfonate chloride and an alcohol? Answer- Phosphorus Tribromide
Phosphorus Tribromide is commercially available but it can also be prepared by reacting phosphorus with bromine. It is a dense liquid with B.Pt. = 173˚C.
Phosphorus bromide reacts with alcohol to give the phosphite. Experiment
ROH + PBr3 � (RO)3P + 3 HBr � 3 RBr + H3PO3
- Thionyl chloride
- Chloride is a poorer nucleophile, as compared to bromide, so more extreme conditions are needed.
It must also be mentioned that alcohol can also execute an SN Reaction upon itself. When ethanol is refluxed with conc. sulphuric acid with water been constantly removed, it gives diethyl ether.
E REACTION
When ethanol is heated with sulphuric acid or phosphoric acid at a high temperature of 170˚C the product is ethylene. Experiment.
CH3CH2OH � CH2=CH2 + H2O
OXIDATION
Alcohol can be oxidised to an aldehyde (or ketone).
| Primary alcohol | RCH2−OH | − 2H+ � |
RCHO |
| Secondary alcohol | (R)2CHOH | − 2H+ � |
(R)2C=O |
One common procedure is to warm the alcohol with a sodium dichromate/sulphuric acid mixture. The chromium(VI) is reduced to chromium(III).
2 [CrO4]‾� + 2H+ � [Cr2O7]‾� + H2O
For aldehydes the reaction generally proceeds rapidly to give the acid RCOOH. So it may not be possible to isolate the aldehyde. However, the ketones can be isolated. Experiment.
A very simplistic outline of the mechanism is;
           |
◄─► |  |
� |   |
The Cr(IV) then reacts with the Cr(VI) to give Cr(V). The Cr(V) can then proceed to oxidise the ketone in a similar manner to become Cr(III). Basically this mechanism explains that oxidation is not possible for tertiary alcohol as it has no β−H.
       |
  |
� |   |
For larger alcohols that are not so soluble in water, it will be difficult to use the sodium dichromate/sulphuric acid mixture. To solve the solubility problem we can use an equimolar mixture of pyridine / chromium trioxide / HCl. The pyridinium chlorochromate is soluble in organic solvents such as methylene chloride.
