ALCOHOLS

What are alcohols?

Alcohols are organic compounds containing the hydroxyl (-OH) group. Since -OH group is the

major functional group in alcohols, hence it is the site of chemical reactions.

Why alcohols?

Alcohols have wide range of applications worldwide, almost in every industry in one way or the

other. Ethylene glycol (HO–CH2CH2–OH) is used as an antifreeze in cold countries. They can be

used as preservatives in the fields of science and medicine. Phenol, which we use in our homes to

clean the floor, is also an alcohol which behaves as a disinfectant. They also act as polar solvents in

many chemical processes.

Structure of Alcohols

The structre of alcohol resembles that of water in which a H atom is replaced by an alkyl group.

Water (H2O) Alcohol (ROH)

Classification of Alcohols

In order to study a wide range of alcohols, it is naturally wise to classify them into four broad

categories:

● Primary AlcoholsIf the carbon bearing the -OH group is attached to only one carbon atom, then that alcohol is

called primary alcohol.

● Secondary AlcoholsIf the carbon bearing the -OH group is attached to two carbon atoms, then that alcohol is

called secondary alcohol.

● Tertiary AlcoholsIf the carbon bearing the -OH group is attached to three carbon atoms, then that alcohol is

called tertiary alcohol.

● PhenolsCompounds with a hydroxyl group bonded directly to an aromatic ring are called phenols. It

also includes alcohols which are the derivatives of phenols.

Nomenclature of Alcohols

The IUPAC name of alcohols carries the -ol suffix, together with a number to give the location of

the hydroxyl group.

The naming procedure contains the following simple steps:-

I. Identify the main chain containing the -OH group.

II. Write the root word and saturation/unstaturation suffix (ane, ene or yne).

III. Drop the final -e and add -ol.

IV. Give numbering to the compound such that the carbon atom bearing -OH group gets the

least possible number. Remember hydroxyl group dominates double bonds and triple bonds

in IUPAC nomenclature.

V. Write complete name of the compound.

Examples:-

fig 1

In fig 1, according to given numbering, we get -OH at position 3 but we get it at position 4 if we

follow the reverse of that shown. IUPAC name is 5-Methylhexan-3-ol or 5-Methyl-3-hexanol.

Fig 2

In fig 2, main chain is easy to identify. What is tricky is the numbering. Since -OH is the major

functional group, it must get the lowest number. It even dominates the double bond. Once position 1

is allotted to -OH, its simple to name. IUPAC name: 2-Bromo-7-chloro-oct-7-en-3,5-diyn-1-ol.

While naming substituted phenols, treat the -OH group as one substituent and then use numbering

accordingly.

Examples:

Physical Properties of Alcohols

Most of the alcohols, upto 11-12 carbon atoms, are liquid at room temperatures.

Boiling pointsTwo types of interactive forces are present in alcohols. Hydrogen bonding is intermolecular

whereas dipole-dipole interactions are both intermolecular and intramolecular.

Due to intermolecular hydrogen bonding, alcohols have higher boiling points than other

compounds with similar molecular weights.

Example:

Although diethyl ether and n-butanol have exactly same molecular weight, boiling point of

n-butanol is much higher than that of diethyl ether. This is due to the presence of

intermolecular hydrogen bonding in n-butanol which is absent in diethyl ether. The last

compound is ionic in nature, so it has much higher boiling point. We needn't deal with last

compound in this discussion.

Now a natural question arises, why boiling point of a molecule is affected by the presence of

hydrogen bond?

When any compound contains intermolecular hydrogen bonding, the interaction among

different molecules of same compound increases and so its boiling point also increases. Say

you have 10 n-butanols and 10 diethyl ethers. These 10 n-butanols will more strongly

interact with each other via intermolecular hydrogen bonding and hence it will be difficult to

break these strong intermolecular forces which is a primary prerequisite for boiling, ie, to

turn to the gaseous phase. On the other hand, 10 diethyl ethers will interact via Vander

Waal's forces which are much weaker than hydrogen bonding. So, it will be easy to break

weak intermolecular forces in diethyl ether.

If we compare inter- vs intra- molecular hydrogen bonding, we conclude that compounds

having intra H bond will less ineract among their own species but will interact more within

the molecule. So boiling point of alcohol containing intramolecular H bond will be lower

than that with intermolecular H bond.

Example:

ortho-nitrophenol para-nitrophenol

Solubility propertiesAlcohols are polar molecules due to electronegativity difference between C and O in C-O-H

bond. They form hydrogen bonds with water and lower weight alcohols are miscible in

water. When dissolved in water, they behave as 'hydrophobic' and 'hydrophilic' groups. The

hydroxyl group is hydrophilic because of its affinity for water while the alkyl group is

hydrophobic because it acts as non-polar alkane. It disrupts the network of hydrogen bonds

and dipole-dipole interactions of water. The bigger the alkyl group of alcohol, the lesser the

solubility of alcohol in water.

Acidity of alcohols

Like hydroxyl proton of H2O, -OH group of ROH is weakly acidic and loses a proton in polar

solvents to give alkoxide ion (RO-).

Ka of all alcohols is less than that of water which implies that alcohols are less acidic than water.

The acidity of alcohols decreases as the substitution on the alkyl group increases because the bulky

substituted alkyl groups stearically hinder the solvation of the alkoxide ion and hence the

equilibrium shifts to the left.

Since the electronegativity order of halogens is F>>Cl>Br>I, hence due to decrease in -I effect from

left to right in the above example, acidity of alcohol decreases because the alkoxide ion will be

more stabilised iff there is more -I effect available.

As the inductive effect is the distance dependent effect, so as F comes closer to the carbon

containing the -OH group, acidity increases because the alkoxide ion so formed is more stabilised.

Identification of 1o, 2o and 3o alcohols

1. LUCAS REAGENT TEST

Lucas Reagent is ZnCl2 + HCl.

3o alcohols react rapidly with Lucas Reagent to give white precipitate at room temperature.

2o alcohols react slowly on heating with Lucas Reagent to give white precipitate after 15-20

minutes.

1o alcohols doesn't react with Lucas Reagent at all even after heating.

2. VICTOR MEYER'S TEST

Treat ROH with following reagents stepwise as indicated,

1. Red P + I2

2. AgNO2

3. NaNO2 + dil HCl

4. NaOH

We get some precipitate at the end. Analyse the colour of ppt.

Blood red colour – Primary alcohol

Blue colour – Secondary alcohol

No Colour – Tertiary alcohol

Synthesis of Alcohols:-

1. Nucleophilic substitution on alkyl halideWhen an alkyl halide is treated with -OH group, nucleophilic substitution reaction takes

place either by SN1 or SN2 reaction to give the corresponding alcohol.

2. From alkenes(1) Acid catalysed hydration

In this reaction, H+ attacks H2O to give H3O+ which acts as an electrophile and

attacks on the double bond. After that, one H+ is added to the carbocation thus formed

to give an alcohol. In this reaction, Markovnikov and Anti-Markovnikov reactions

come into play.

(2) Oxymercuration – demercuration

In this reaction, Hg(OAc)2 generates the electrophile HgOAc+ which attacks on the

double bond. Subsequently, OH- attacks the carbocation thus formed to give an

alcohol. Remember this is a rapid reaction, so no reaction intermediate is formed and

no rearrangement of carbocation takes place. We get non-rearranged Anti-

Markovnikov's product.

(3) Hydroboration oxidation

In this reaction, BH3 acts as lewis acid because it is electron deficient, so its acts as a

neutral electrophile. As -C+-C-BH3+ is formed, H+ shifts from BH3

- to C+ to give intermediate

as shown in second figure. When hydrogen peroxide is added in the presence of a strong

base, oxidation of B-H bond takes place. Finally we get an alcohol as shown and BH3.

(4) Hydroxylation – synthesis of 1,2-diols from alkenes

The reagent used in the above reaction is called Bayer's reagent. Syn-diol is obtained as a

product. It is a characteristic named reaction which is important to be remembered.

Reactions of alcohols

Oxidation of alcohols

Alcohols can be oxidised to carbonyl compounds using suitable cataylsts.

• Alcohols to aldehydes or ketones

Alcohols when treated with PCC (Pyridinium Chloro Chromate) or PDC (Pyridinium

Dichloro Chromate) get oxidised to correponding carbonyl compounds, that is, 1O

alcohols give corresponding aldehydes, 2o alcohols give corresponding ketones and

3o alcohols doesn't undergo reaction. PCC and PDC are mild oxidising agents.

Primary alcohol

Secondary alcohol

Tertiary alcohol doesn't undergo reaction as it lacks alpha hygrogen atom.

• Alcohols to acids

When strong oxidising agents such as Na2Cr2O7, chromic acid in water, or KMnO4 in

H2SO4 are used, alcohols are oxidised to correponding acids.

Substitution reaction

-OH in alcohols can be replaced by nucleophiles.

It follows bimolecular substitution mechanism (SN2) in the above rection.

But with SOCl2, it follows intramolecular nucleophilic substitution mechanism.

Esterification reaction

Esterification reaction refers to a reaction between an alcohol and an acid derivative to give

a sweet smelling product known as ester.

In this reaction, the carboxylic acid behaves as a base, that is, it loses OH- and the alcohol behaves

as an acid, that is, it loses H+. This can be clearly understood from the above mechanism. First H+

present in the medium as a catalyst attacks the acid which generates a carbocation. Then O of

alcohol, being a nucleophile, attacks the carbocation. Then proton shift occurs to finally give the

product shown above.

It is a reversible reaction and ester, thus formed, is stable only when conc H+ is present in the

reaction medium; otherwise it undergoes backward reaction known as hydrolysis of ester.

Dehyration reaction

There are two types of dehydration reaction possible for alcohols. They are intramolecular which

gives an alkene from alcohol and the other is intermolecular which gives ether from alcohols.

The good dehydrating agents are conc H2SO4, H3PO4, ZnCl2, CaO, FeCl3, etc.

• Intramolecular dehydration of alcohols

When an alcohol is treated with conc H2SO4 at 170o C, then intramolecular dehydration

takes place to give corresponding alkene.

• Intermolecular dehydration of alcohols

When an alcohol is treated with conc H2SO4 at 140o C, then intermolecular dehydration

takes place to give corresponding ether.