Unit-11 Alcohols, Phenols and Ethers Alcohols, phenols and ethers are the basic compounds for the formation of detergents, antiseptics and fragrances, respectively. Objectives
After studying this Unit students will be able to name alcohols according to the IUPAC system of nomenclature; discuss the reactions involved in the preparation of alcohols from (i)alkenes (ii) aldehydes, ketones and carboxylic acids; correlate physical properties of alcohols with their structures; discuss chemical reactions of the alcohols on the basis of their functional groups.
Alcohols are formed when a hydrogen atom in a hydrocarbon, aliphatic is replaced by OH group. Alcohols find wide applications in industry as well as in day-to-day life. For instance, Spirit used for polishing wooden furniture is Ethanol. The sugar we eat, the cotton used for fabrics, the
paper we use for writing, are all made up of compounds containing OH groups. An alcohol contains one or more hydroxyl (OH)group(s) directly attached to carbon atom(s), of an aliphatic system , For eg. CH3OH(Alcohol) , HO-CH2-CH2-OH(Glycol),
HO-CH2-CH(OH)-CH2-OH(Glycerol) Classification The classification of compounds makes their study systematic and hence simpler. Alcohols may be classified as mono, di, tri- or polyhydric compounds depending on whether they contain one, two, three or many hydroxyl groups respectively in their structures as given below:
CH3OH(Alcohol) , HO-CH2-CH2-OH(Glycol), HO-CH2-CH(OH)-CH2-OH(Glycerol) Monohydric alcohols may be further classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached. (i) Compounds containing Csp3 - OH bond: In this class of alcohols,
the OH group is attached to an sp3 hybridised carbon atom of an alkyl group. They are further classified as follows: Primary, secondary and tertiary alcohols: In these three types of alcohols, the OH group is attached to primary, secondary and tertiary carbon atom, respectively as depicted below: CH2OH Primary(10) CH-OH
Secondary(20) C-OH Tertiary(30) Allylic alcohols: In these alcohols, the OH group is attached to a sp3 hybridised carbon next to the carbon-carbon double bond, that is to an allylic carbon. For example CH2=CH-CH2-OH
Tertiary(30) Benzylic alcohols: In these alcohols, the OH group is attached to a sp3hybridised carbon atom next to an aromatic ring. For example Primary(10) CH2OH Secondary(20)
CH3-CH-OH Tertiary(30) CH 3 CH3-C-OH (ii) Compounds containing Csp2- OH bond: These alcohols contain OH group bonded to a carboncarbon double bond i.e., to a vinylic carbon.
These alcohols are also known as vinylic alcohols, eg. CH2 = CH OH Classify as primary, secondary and tertiary alcohols and also identify allylic alcohols in the following examples.: CH3 (i) CH3-C-OH CH3
CH3-CH-OH (ii) CH2=CH-CH2-OH OH CH2-CH-CH3 (iii) CH3-CH2-CH2-OH
CH3 CH=CH-C-OH CH3 Nomenclature The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. eg. CH3OH is methyl alcohol. According to IUPAC system,the name of an alcohol is derived from the name of the
alkane from which the alcohol is derived, by substituting e of alkane with the suffix ol. The position of substituents are indicated by numerals. For this, the longest carbon chain (parent chain) is numbered starting at the end nearest to the hydroxyl group. The positions of the OH group and other substituents are indicated by using the numbers of carbon atoms to which these are attached. For naming polyhydric alcohols, the e of alkane is retained and the ending ol is added. The number of OH groups is indicated by adding the multiplicative prefix, di, tri, etc., before ol. The positions of OH groups are indicated by appropriate locants
e.g., HOCH2CH2OH is named as ethane1, 2-diol. Common and IUPAC Names of Some Alcohols Compound Common name CH3 OH
Propane -1, 2, 3-triol Cyclic alcohols are named using the prefix cyclo and considering IUPAC name Cyclic alcohols are named using the prefix cyclo and considering the OH group attached to C1.
OH OH CH3 Cyclohexanol 2-Methylcyclopentanol
Give IUPAC names of the following compounds: Structure of Functional Group In alcohols, the oxygen of the OH group is attached to carbon by a sigma () bond ) bond formed by the overlap of a sp3 hybridised orbital of carbon with a sp3 hybridised orbital of oxygen. Fig. depicts structural aspects of methanol. The bond angle in alcohols is slightly less than the
tetrahedral angle (109.28). It is due to the repulsion between the unshared electron pairs of oxygen . Preparation of Alcohols Alcohols are prepared by the following methods: 1. From alkenes (i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikovs rule.
2. From carbonyl compounds (i) By reduction of aldehydes and ketones: Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation). The usual catalyst is a finely divided metal such as Pt,Pd &Ni. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH 4). Aldehydes yield primary alcohols whereas ketones give
secondary alcohols. (ii) By reduction of carboxylic acids and esters: Carboxylic acids are reduced to primary alcohols in excellent yields by LiAlH4, a strong reducing agent. However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. ++
+ H2O Commercially, acids are reduced to alcohols by converting them to the esters followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation). 3. From Grignard reagents By the reaction of Grignard reagents with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an
adduct followed by acidic Hydrolysis of the adduct yields an alcohol. The overall reactions using different aldehydes and ketones are as follows: You will notice that the reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones Question- Give the structures and IUPAC names of the products expected from the following reactions:
(a) Catalytic reduction of butanal. (b) Hydration of propene in the presence of dilute sulphuric acid. (c) Reaction of propanone with methylmagnesium bromide followed by hydrolysis. Solution- Question-1- Question-2-
Physical Properties:Alcohols consist of two parts, an alkyl group and a hydroxyl group. The properties of alcohols are chiefly due to the hydroxyl group. The nature of alkyl group simply modify these properties. Boiling Points The boiling points of alcohols increases with increase in the number of carbon atoms (increase in van der Waals forces) and decreases with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area). The OH group in alcohols is involved in intermolecular hydrogen bonding as shown
below: It is interesting to note that boiling points of alcohols are higher in comparison to hydrocarbons and ethers of comparable molecular masses. This is due to the presence of intermolecular hydrogen bonding in alcohols which is lacking in ethers and hydrocarbons.
Solubility:Solubility of alcohols in water is due to their ability to form hydrogen bonds with water molecules as shown. The solubility decreases with increase in size of alkyl (hydrophobic) groups. Several of the lower molecular mass alcohols are miscible with water in all proportions. Question:Arrange the following sets of compounds in order of their increasing boiling points: (a) Pentan-1-ol, butan-1-ol, butan-2-ol, ethanol, propan-1-ol, methanol.
(b) Pentan-1-ol, n-butane, pentanal, ethoxyethane. (a) Methanol, ethanol, propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol. (b) n-Butane, ethoxyethane, pentanal and pentan-1-ol. Chemical Reactions Alcohols are versatile compounds. They react both as nucleophiles and electrophiles. The bond between OH is broken when alcohols react as nucleophiles.
(ii) The bond between CO is broken when they react as electrophiles. Protonated alcohols react in this manner. Based on the cleavage of OH and CO bonds, the reactions of alcohols and phenols may be divided into two groups: (a) Reactions involving cleavage of OH bond 1. Acidity of alcohols Reaction with metals: Alcohols react with active metals such as sodium, potassium and
aluminium to yield corresponding alkoxides and hydrogen. The above reaction shows that alcohols are acidic in nature. In fact, alcohols are Brnsted acids i.e., they can donate a proton to a stronger base (B:). Question:- Solution:-
Esterification Alcohols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. The reaction with carboxylic acid and acid anhydride is carried out in the presence of a small amount of concentrated sulphuric acid. The reaction is reversible, and therefore, water is removed as soon as it is formed. The reaction with acid chloride is carried out in the presence of a base (pyridine) so as to neutralise HCl which is formed during the reaction. It shifts the equilibrium to the right hand side.
The introduction of acetyl (CH3CO) group in alcohols is known as acetylation. (b) Reactions involving cleavage of carbon oxygen (CO) bond in alcohols The reactions involving cleavage of CO bond take place only in alcohols 1. Reaction with hydrogen halides: Alcohols react with hydrogen halides to form alkyl halides . ROH + HX RX + H2O The order of reactivity of alcohols with a HX is 3>2>1.
Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl2) while their halides are immiscible and produce turbidity in solution. In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily. Primary alcohols do not produce turbidity at room temperature. 2. Reaction with PCl3 & PCl5 : Alcohols are converted to alkyl bromides by reaction with PCl3 & PCl5 .
3. Dehydration: Alcohols undergo dehydration to form alkenes on treating with a protic acid e.g., conc. H2SO4 or H3PO4, or catalysts such as anhyd. ZnCl2 or Al2O3. Ethanol undergoes dehydration by heating it with concentrated H2SO4 at 443 and 413K to give an alkene and an ether. 4. Oxidation: Oxidation of alcohols involves the formation of a carbon oxygen double bond with
cleavage of an O-H and C-H bonds. Such a cleavage and formation of bonds occur in oxidation reactions. These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. Strong oxidising agents such as acidified potassium permanganate are used for
getting carboxylic acids from alcohols directly. Anhydrous CrO3 is used as the oxidising agent for the isolation of aldehydes. A better reagent for oxidation of primary alcohols to aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl. Secondary alcohols are oxidised to ketones by chromic anhydride Tertiary alcohols do not undergo oxidation reaction. Under strong reaction conditions such as strong oxidising
agents (KMnO4) and elevated temperatures, cleavage of various C-C bonds takes place and a mixture of carboxylic acids containing lesser number of carbon atoms is formed. When the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed respectively. while tertiary alcohols undergo dehydration gives an alkene.
Question:- Question:- Biological oxidation of methanol and ethanol in the body produces the corresponding aldehyde followed by the acid. At times the alcoholics, by mistake, drink ethanol, mixed with methanol also called denatured alcohol.
In the body, methanol is oxidized first to methanal and then to methanoic acid, which may cause blindness and death. A methanol poisoned patient is treated by giving intravenous infusions of diluted ethanol. The enzyme responsible for oxidation of aldehyde (HCHO) to acid is swamped allowing time for kidneys to excrete methanol. Some Commercially Important Alcohols
Methanol and ethanol are among the two commercially important alcohols. 1. Methanol:Methanol, CH3OH, also known as wood spirit, was produced by destructive distillation of wood. Methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO Cr2O3 catalyst. Methanol is a colourless liquid and boils at 337 K.
It is highly poisonous in nature. Ingestion of even small quantities of methanol can cause blindness and large quantities causes even death. Methanol is used as a solvent in paints, varnishes and chiefly for making formaldehyde. 2. Ethanol Ethanol, C2H5OH, is obtained commercially by fermentation,
The oldest method is from molasses(sugarcane) or grapes converted to glucose and fructose, (C6H12O6), in the presence of an enzyme, invertase. Glucose and fructose undergo fermentation in the presence of another enzyme, zymase, which is found in yeast. In wine making, grapes are the source of sugars and yeast. As grapes ripen, the quantity of sugar increases and yeast grows on the outer skin. When grapes are crushed, sugar and the enzyme come in contact
and fermentation starts. Fermentation takes place in anaerobic conditions i.e. in absence of air. Carbon dioxide is released during fermentation. The action of zymase is inhibited once the percentage of alcohol formed exceeds 14 percent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.
Ethanol is a colourless liquid with boiling point 351 K. It is used as a solvent in paint industry and in the preparation of a number of carbon compounds. The commercial alcohol is made unfit for drinking by mixing in it some copper sulphate (to give it a colour) and pyridine(a foul smelling liquid). It is known as denaturation of alcohol. THANKS
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