AS Chemistry - Alcohols

Oct 28, 2019

[latexpage]

The alcohols form a homologous series with the general formula $\ce{C_{n}H_{2n + 1}OH}$. They involve hydroxy groups, or the $\ce{-OH}$ groups. They occupy a central position in organic functional group chemistry: They can be readily converted to adehydes and carboxylic acids by oxidation, and can be formed from them by reduction; And they can be converted to and from halogenoalkanes by nucleophilic substitution.

Because the polar hydroxy groups, alcohols form hydrogen bonds between each other.

The lower alcohols (1, 2, 3 and some isomers of 4 carbon alcohols) are completely miscible with water, owing to hydrogen bonding between the alcohol molecules and water molecules.

The boiling points of alcohols are much higher than their isoelectric alkanes, oweing to the presence of hydrogen bonds.

This enhancement of boiling point decreases as the number of carbon atoms in the molecule increases.

Isomerism and nomenclature

The suffix of nomenclature of alcohols is ‘-ol’.

The locational indicators should come right before the suffix, like ‘prop-2-ol’

Similar to halogenoalkanes, alcohols can also be classified into primary, secondary or tertiary alcohols.

Ethers are isomers of alcohols, but they do not contain the hydroxy group and so has none of the reactions of alcohols, and do not form intermolecular hydrogen bonds. So it is easy to deduce that they have relatively lower boiling points than alcohols.

Reactions of alcohols

Reactions are classified into four groups:

1. Combustion

All alcohols burn well in air, nut only the combustion of ethanol is of everyday importance:
$$
\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}
$$
Combustion reactions are highly exothermic therefore we can use alcohols as fuels.

2. Reactions involving the breaking of the O-H bond

With sodium metal

The hydroxyl hydrogen in alcohols is slightly acidic, just like the hydrogen atoms in water, but the alcohols ionize to a lesser extent. So they are less acidic than water.

Alcohols liberate hydrogen gas when treated with sodium metal:
$$
\ce{CH3CH2OH + Na -> CH3CH2O- Na+ + 1/2H2 (g)}
$$
The reaction is less vigorous than that between sodium and water. The product is called sodium ethoxide. This is a white solid, which is soluble in ethanol and water (which gives a strongly alkaline solution).

All compounds containing hydroxide groups liberate hydrogen gas when treated with sodium. The fizzing that ensues is a good test for the presence of a hydroxy group, as long as water is absent.

Esterification

Esters are compounds that contain the $\ce{R-COO -R’}$ group. They can be obtained by reacting alcohols with carboxylic acids or acyl chlorides.
$$
\text{alcohol} + \text{carboxylic acid} \longrightarrow \text{ester} + \text{water}\
\text{alcohol} + \text{acyl halide} \longrightarrow \text{ester} + \text{hydrogen halide}
$$

3. Reactions involving the breaking fo the C-O bond

Although the $\ce{C-O}$ bond is polarized, it is a strong bond, and does not easily break heterolytically.

If, however, the oxygen is protonated, or bonded to a sulfur or phosphorys atom, the $\ce{C-O}$ bond is much bore easily broken. As a result, alcohols undergo several nucleophilic substitution reactions.

Reaction with hydrochloric acid

Tertiary alcohols react easily with concentrated hydrochloric acid on shaking at room temperature. The reaction proceeds via the $S_N1$ mechanism:

image-20190512145249993.png

Secondary and primary aochols also react with concentrated hydrochloric acid, but at a slower rate. So anhydrous zinc chloride needs to be added as a catalyst, and the mixture requires heating:
$$
\ce{CH3CH2OH + HCl ->[\ce{ZnCl2}\text{, heat}] CH3CH2Cl + H2O}
$$
This reaction is the bases of the Lucas test, to distinguish whether an alcohol is primary, secondary or tertiary. It relies on the fact that alcohol is soluble in the reagent, while halogenoalkanes are not:

Type of alcohol Observation on adding conc. $\ce{HCl + ZnCl2}$
$\ce{R3COH}$ (Tertiary) immediate cloudiness appears in the solution
$\ce{R2CHOH}$ (Secondary) cloudiness apparent within 5 minutes
$\ce{RCH2OH}$ (Primary) no cloudiness apparent unless warmed
Reaction with phosphorus(V) chloride or sulfur dichloride oxide

Both of these reagents convert alcohols into chloroalkanes, and in both cases hydrogen chloride is evolved This fizzing with phosphorus(V) chloride and the production of misty fumes, can be used as a test for the presence of an alcohol (as long as water is absent). Both reactions occur on gently warming the reagents.
$$
\ce{CH3CH2OH + PCl5 -> CH3CH2Cl + POCl3 + HCl}\
\ce{CH3CH2OH + SOCl2 -> CH3CH2Cl + SO2 + HCl}
$$

Reaction with hydrogen bromide

Alcohols can be converted into bromoalkanes by reaction with concentrated hydrobromic acid:
$$
\ce{CH3CH2OH + H+ ->[+\ce{Br-}] CH3CH2OH2+ + Br- ->[\text{Nucleophilic substitution}] CH3CH2Br + H2O}
$$

Reaction with red phosphorus and bromine or iodine

These halogens react with phosphoris to give the phosphorus trihalides:
$$
\ce{2P + 3Br2 -> 2PBr3}
\ce{2P + 3I2 -> 2PI3}
$$
Then the phosphorus trihalides react with alcohols:
$$
\ce{3CH3CH2OH + PI3 -> 3CH2CH2I + P(OH)3}
$$

Elimination reaction with concentrated sulfuric acid

This is a reversible reaction:
$$
\ce{CH3CH2OH + H2SO4 <=> CH3CH2OSO3H + H2O}
$$
A compound called ethyl hydrogensulfate forms. The ethyl hydrogen sulfate can then undergo an elimination reaction to form ethene:

image-20190512150902518.png

Overall, the sulfuric acid acts as a catalyst to catalyze the elimination of ethanol:
$$
\ce{CH3CH2OH ->[\ce{H2SO4}] CH2=CH2 + H2O}
$$
Concentrated phosphoric acid also acts as a catalys for the reaction. This is often preferred because it is not a strong oxidizing agent as sulfuric acid. So the formation of by-products is minimized.

A third method of dehydration is to pass the vapor of alcohol over strongly heated aluminium oxide.

Tertiary alcohols dehydrate very easily on warming with an acid. The reaction goes vbia the tertiary carbocation, which then loses a proton to form an alkene.

4. Reactions of the >CH(OH) group

Oxidation

Primary and secondary alcohols are easily oxidised by heating with an acidified solution of potassium dichromate(VI). The orange dichromate ions are reduced to green chromium(III) ions. Tertiary alcohols, however, are not easily oxidised.

Secondary alcohols are oxidised to ketones:

image-20190512181833884.png

Primary alcohols are oxidized to aldehydes, which, in turn, are even more easily oxidised to carboxylic acids:

image-20190512152400200.png

Using this reaction, we can once again identify the type of alcohol is:

Type of alcohol Observation on warming with reagent Effect of distillate on universal indicator
$\ce{R3COH}$ (Tertiary) stays orange Neutral – only water is produced
$\ce{R2CHOH}$ (Secondary) turns green Neutral – ketone is produced
$\ce{RCH2OH}$ (Primary) turns green Acidic – carboxylic acid is produced

5. The triiodomethane (iodoform) reaction

Alcohols that caontain the group $\ce{CH3CH(OH) -}$, that is, those that have a methyl group and a hydrogen atom on the same carbon atom that bears the OH group, can be oxidised by alkaline aqueous iodine to the corresponding carbonyl compound $\ce{CH3C(O)-}$. This can then undergo the tri=iodomethane reaction:

image-20190512153842339.png

All other alcohols that reacts with iodine to give iodoform are secondary alcohols. Ethanol is the only exception.

Iodoform is a pale yellow precipitate.

Preparing alcohols

1. From halogenoalkanes, by nucleophilic substitution

An hydroxide anion acts as a nucleophile, and substitutes the halide atom from the halogenoalkane, giving an alcohol.

2. From alkenes, by hydration

This electrophilic addition of water to alkenes can be done by adding concentrated sulfuric acid first, then diluting it with water. Industrially, one passes vapours over a catalyst of phosphuric acid at 300 ºC and 70 atm.

3. From aldehydes or ketones, by reduction

  • By hydrogen on a nickel catalyst:

$$
\ce{CH3CH2CHO ->[\ce{H2 + Ni}] CH3CH2CH2OH}
$$

  • By sodium tetrahydridoborate(III) (sodium borohydride) in alkaline methanol:

$$
\ce{CH3CH2COCH3 ->[\ce{NaBH4 + OH-}][\text{in methanol}]CH3CH2CH(OH)CH3}
$$

  • BY lithium tetrahydridoaluminate(III) (lithium aluminium hydride) in dry ether:

$$
\ce{CH3COCH3 ->[\ce{LiAlH4}\text{ in dry ether}]CH3CH(OH)CH3}
$$

4. Preparing ethanol by fermentation

Under the absence of air, yeasts undergo an anaerobnic fermentation that ferments glucose into alcohol:
$$
\ce{C6H12O6 ->[yeast]2C2H5OH + 2CO2}
$$
The conditions required for successful fermentation are:

  • Yeast
  • Water
  • Yeast nutrients (ammonium phosphate is often used)
  • Warmth (30 ºC is ideal)
  • Absense of air
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