What are the Properties of Alcohols?
- Alcohols are organic compounds in which a hydrogen atom of an aliphatic carbon is replaced with a hydroxyl group. Thus an alcohol molecule consists of two parts; one containing the alkyl group and the other containing functional group hydroxyl group.
- They have a sweet odour. They exhibit a unique set of physical and chemical properties.
The physical and chemical properties of alcohols are mainly due to the presence of hydroxyl group.
1. Boiling Point
- Alcohols show an increase in boiling point with an increase in molecular weight amongst homologues.
- Alcohols have a higher boiling point than hydrocarbons of the same molecular weight. The reason for the higher boiling point is the intermolecular H-bonding present in alcohols.
Ethanol has a higher Boiling Point than Ethane
Hydrogen Bonding in Ethanol
2. Solubility in Water
- As molecular weight increases solubility in water decreases.
- The lower alcohols are miscible with water. This is due to intermolecular hydrogen bonding between alcohol and water molecules.
Intermolecular H bond between water & alcohol molecules
Tables showing Solubility of various Alcohols
Preparation of Alcohols1. From Alkenes(a) By acid-catalyzed hydration of alkenes
- Formation of carbocation intermediate (Markovnikov addition, rearrangement possible).
The reaction of Alkenes with Acids follow Markovnikov's Rule
(b) By Oxymercuration - Demercuration Process
- Oxymercuration involves an electrophilic attack on the double bond by the positively charged mercury species. The product is a mercurinium ion, an organometallic cation containing a three-membered ring.
- In the second step, water from the solvent attacks the mercurinium ion to give (after deprotonation) organomercurial alcohol.
- The third step is demercuration to remove the Hg. Sodium borohydride (NaBH4, a reducing agent) replaces the mercuric acetate fragment with hydrogen.
+ OH- + NaBH4
(c) By Hydroboration-oxidation process
(Forms anti-Markovnikov alcohol, no rearrangement).
General reaction: R-Ch=CH2R-CH2-CH2-OH
Question: Give the major product of the following reaction:
Ans. Major product is
because 3o carbocation is more stable.
2. From Alkyl Halides
By nucleophilic substitution reactions.
(a) By SN2 Mechanism (Second-Order Substitution)
- It is given by primary (and some secondary) halides.
General reaction: R-CH2-Br R-CH2-OH
e.g. (CH3)2CHCH2CH 2-Br (CH3)2CHCH2CH 2- OH
(b) By SN1 mechanism (first-order substitution)
- It is given by tertiary and some secondary halides.
General reaction :
3. From Grignard's Reagent
(a) From air
- A Grignard reagent may be used to synthesize an alcohol by treating it with dry oxygen and decomposing the product with acid :
RMgX RO2MgX 2ROMgX 2ROH
e.g. C2H5MgBrC2H5O2 MgX2C2H5OMgX 2C2H5OH MgBr(OH)
(b) From ethylene oxide
- The addition of Grignard reagent to ethylene oxide gives primary alcohol (with two carbon atoms added).
RCH2CH2OMgX RCH2CH2OH MgX(OH)
e.g. C2H5MgBr C2H5CH2CH2 OMgX MgBr(OH)
(c) From carbonyl compounds
- Nucleophilic addition to the carbonyl groups by Grignard's reagent.
(i) Addition of formaldehyde gives primary alcohol
= O RMgX RCH2-OH
(ii) Addition to an aldehyde (other than formaldehyde) gives secondary alcohol
(iii) Addition to a ketone gives tertiary alcohol.
e.g. CH3CH2MgCl +
(iv) Addition to an acid halide or an ester gives tertiary alcohol.
- Esters on treatment with Grignard's reagent first form ketones which then react with the second molecule of Grignard's reagent and form tertiary alcohol.
4. By Reduction of Carbonyl Compounds(a) Catalytic hydrogenation of aldehydes and ketones
e.g. CH3CHO + 2H CH3CH2OH
e.g. + 4H
(b) Lithium aluminium hydride reduction of aldehydes and ketones
(iii) RCOOH+ 4H RCH2OH + H2O
(iv) RCH2OH+ HCl
(v) R-CH2OH +R'OH
e.g. C2H5CH2OH +HCl
(c) By NaBH4 (sodium borohydride)
- It is insoluble in ether and is used in an aqueous ethanolic solution to reduce carbonyl compounds. It does not reduce esters and acids.
e.g. CH=CHCHO +4H CH3CH=CHCH2OH
(i) Reduction of a ketone gives secondary alcohol
(d) Bouveault-Blanc reduction
The reduction of aldehydes, ketones or esters by means of excess of sodium and ethanol or n-butanol as the reducing agent.
(i) Aldehyde RCHO RCH2OH
(ii) Esters R'CO2R'' R'CH2OH R''OH
(iii) Ketones R2CO R2CHOH
The Bouveault-Blanc reduction is believed to occur in steps involving transfer of one electron at a time.
e.g. CH3CHO +2H CH3CH2OH
e.g. CH3COOC2H5 +4H2CH3CH2OH
Ex. Identify (X) in the following reaction:
Ex. What are the product A, B, C, D and E in the following reactions?
Ans. A : (EtOH)
B : Ester part is not affected by NaBH4
C : Ester part and keto parts are affected by LiAlH4
5. By Reaction of Nitrous Acid on Aliphatic Primary Amines
General reaction: R-NH2 + HONO R-OH +N2 + H2O
R-NH2 (R) ROH +N2 +
e.g. (i) C2H5NH2 +HNO2 C2H5OH+ N2 + H2O
(ii) CH3-CH2- +HONO CH3-CH2-+ N2 +H2O
6. Hydroxylation: Forms vicinal diols (glycols)
- Converting an alkene to a glycol requires adding a hydroxy group to each end of the double bond. This addition is called hydroxylation of the double bond.
(a) Syn hydroxylation, using KMnO4 / NaOH or using OsO4/H2O2
General reaction :
(b) Anti hydroxylation, using peracids
Chemical Reactions of Alcohols
1. Reaction with Hydrogen Halides
R - OH + HX R - X + H2O (R may rearrange)
Reactivity of HX : Hl > HBr > HCl
Reactivity of ROH: allyl > benzyl > 3o > 2o > 1o
R - OH R- R-X
2. Reaction with Phosphorus Trihalides
- Several phosphorus halides are useful for converting alcohols to alkyl halides. PBr3, PCl3, & PCl5 work well and are commercially available.
- Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with ter. alcohols. The two phosphorus halides used most often are PBr3 and the P4 / I2 combination.
3R - OH + PX3 3R - X + H3PO3
The mechanism for the reaction involves attachment of the alcohol group on the phosphorus atom, displacing a bromide ion and forming a protonated alkyl dibromophosphite (see the following reaction).
In a second step a bromide ion acts as a nucleophile to displace HOPBr2, a good leaving group due to the electronegative atoms bonded to the phosphorus.
3. Reaction with Thionyl Chloride
R-Cl SO2 HCl
4. Dehydration of Alcohols
H2O (Rearrangement may occur)
Step 1 :
Step 2 :
Step 3 :
Reactivity of ROH : 3º > 2º > 1º
5. Reaction with Metals
6. Ester Formation
+ + H2O
e.g. CH3CH2O - H + + H2O
7. Oxidation Reactions
(a) Oxidation of primary alcohols
- Oxidation of primary alcohol initially forms an aldehyde. Obtaining the aldehyde is often difficult since most oxidizing agents are strong enough to oxidize the aldehydes formed. CrO3 acid generally oxidizes primary alcohol all the way upto the carboxylic acid
(b) Oxidation of secondary alcohols
- Sec. alcohols are easily oxidized to give excellent yields of ketones. The chromic acid reagent is often best for laboratory oxidations of secondary alcohols. The active species in the mixture is probably chromic acid, H2CrO4, or the acid chromate ion,
(c) Resistance of tertiary alcohols to oxidation
- Oxidation of tertiary alcohol is not an important reaction in organic chemistry. Ter-alcohols have hydrogen atoms on the carbonyl carbon atom, so oxidation must take place by breaking C-C bonds. These oxidations require severe conditions and result in mixtures of products.
Some Commercially Important Alcohols
Methanol and Ethanol
- Methanol (CH3OH) and ethanol (C2H5OH) are the simplest members of the primary alcohol family and have a wide range of applications in the fuel industry. Some important uses of methanol and ethanol are listed in this article.
Uses of Ethanol
- Owing to its antibacterial and antifungal properties, ethanol (also known as ethyl alcohol) is used in many hand sanitizers and medical wipes.
- Ethanol is also used as an antiseptic and as a disinfectant.
- In cases of ethylene glycol poisoning or methyl alcohol poisoning, ethanol is often administered as an antidote.
- Several medications that are insoluble in water are often dissolved in ethanol. For example, ethanol (in concentrations ranging from 1% to 25%) is used as a solvent for some analgesics and mouthwashes.
- Ethanol is the primary ingredient in many alcoholic drinks that are orally consumed for recreational purposes. It acts as a psychoactive drug by reducing anxiety and creating a feeling of euphoria in Humans. However, it also impairs cognitive and motor functions and acts as a central nervous system (CNS) depressant.
- Ethanol is used industrially in the production of ethyl esters, acetic acid, diethyl ether, and ethyl amines.
- This compound is widely used as a solvent due to its ability to dissolve both polar and nonpolar compounds.
- Since it has a melting point of -114.1oC, ethanol is used as an ingredient in cooling baths in several laboratories. It also serves as the active fluid in many spirit thermometers.
Uses of Ethanol as a Fuel
- Ethanol is widely used as a fuel additive and as an engine fuel. Some forms of gasoline are known to contain up to 25% ethanol.
- This compound has also been used as rocket fuel in some bipropellant rockets. When used in fuel, ethanol is believed to reduce carbon monoxide and nitrogen oxide emissions.
- Since it is widely available and has low toxicity and cost, ethanol is used in direct-ethanol fuel cells (or DEFCs). However, commercially used fuel cells generally use methanol, hydrogen, or natural gas.
Uses of Methanol
- Methanol is widely used in the production of acetic acid and formaldehyde.
- In order to discourage the recreational consumption of ethanol, methanol is often added to it as a denaturant.
- This compound is also used as an antifreeze (an additive that is used to lower the freezing point of a liquid) in many pipelines.
- It is also used in sewage treatment plants since it serves as a carbon-based food source for denitrifying bacteria.
- The polyacrylamide gel electrophoresis (PAGE) technique involves the use of methanol as a destaining agent.
- A mixture of water and methanol is used in high-performance engines in order to increase power.
- Methanol is used in the production of hydrocarbons, olefins, and some aromatic compounds.
- It is also used in the production of methyl esters and methylamines.
Uses of Methanol in Fuel
- Methanol can be used as a fuel in several internal combustion engines. The chemical equation for the burning of methanol is given by:
- 2CH3OH + 3O2 → 4H2O + 2CO2
- However, the primary disadvantage of methanol as a fuel is that it has a tendency to corrode aluminum and some other metals.
- Another shortcoming of methanol as a fuel is that its energy density is approximately half of the energy density offered by gasoline. An advantage of methanol as a fuel is that it is relatively easy to store.
- The storage of liquid methanol is much easier than the storage of hydrogen gas or natural gas. Other merits of this compound include its biodegradability and its short half-life in groundwater.