Alcohols PDF

**Alcohols** Alcohols are the basic compounds for the formation of detergents. Alcohols when a hydrogen atom in a hydrocarbon, aliphatic, is replaced by --OH group. These classes of compounds find wide applications in industry as well as in day-to-day life. For instance, have you ever noticed that...

**Alcohols** Alcohols are the basic compounds for the formation of detergents. Alcohols when a hydrogen atom in a hydrocarbon, aliphatic, is replaced by --OH group. These classes of compounds find wide applications in industry as well as in day-to-day life. For instance, have you ever noticed that ordinary spirit used for polishing wooden furniture is chiefly a compound containing hydroxyl group, 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. Just think of life without paper; no note-books, books, newspapers, currency notes, cheques, certificates, etc. The magazines carrying beautiful photographs and interesting stories would disappear from our life. It would have been really a different world. An alcohol contains one or more hydroxyl (OH) group(s) directly attached to carbon atom(s), of an aliphatic system (CH~3~OH). **Classification -Mono, Di, Tri or Polyhydric Compounds** The classification of compounds makes their study systematic and hence simpler. Therefore, let us first learn how are alcohols classified? 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: Monohydric alcohols may be further classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached. **(i) Compounds containing C sp^3^ -OH bond:** In this class of alcohols, the --OH group is attached to an sp^3^ 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:  **Allylic alcohols:** In these alcohols, the ---OH group is attached to a sp^3^ hybridised carbon next to the carbon-carbon double bond, that is to an allylic carbon. For example **Benzylic alcohols:** In these alcohols, the ---OH group is attached to a sp^3^---hybridised carbon atom next to an aromatic ring. For example  Allylic and benzylic alcohols may be primary, secondary or tertiary. **(ii) Compounds containing C sp^2^ -OH bond:** These alcohols contain ---OH group bonded to a carbon-carbon double bond i.e., to a vinylic carbon or to an aryl carbon. These alcohols are also known as vinylic alcohols. Vinylic alcohol: CH~2~ = CH -- OH **Nomenclature** a. **Alcohols:** The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. For example, CH~3~OH 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., HO--CH~2~--CH~2~--OH is named as ethane--1, 2-diol. Table below gives common and IUPAC names of a few alcohols as examples. Cyclic alcohols are named using the prefix cyclo and considering the ---OH group attached to C--1.  **Structures of Functional Groups** In alcohols, the oxygen of the --OH group is attached to carbon by a sigma (σ) bond formed by the overlap of a sp^3^ hybridised orbital of carbon with a sp^3^ hybridised orbital of oxygen. Figure below depicts structural aspects of methanol, phenol and methoxymethane. 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 Markovnikov's rule.  \(ii) By hydroboration--oxidation: Diborane (BH~3~)~2~ reacts with alkenes to give trialkyl boranes as addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide. The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp^2^ carbon carrying greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov's rule. In this reaction, alcohol is obtained in excellent yield. **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 platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH~4~) 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 lithium aluminium hydride, a strong reducing agent. However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. 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).  1. **From Grignard reagents** Alcohols are produced 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. 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. **Physical Properties** Alcohols consist of two parts, an alkyl and a hydroxyl group. The properties of alcohols are chiefly due to the hydroxyl group. The nature of alkyl group simply modifies these properties. **Boiling Points:** The boiling points of alcohols increase with increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease 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 other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses. For example, ethanol and propane have comparable molecular masses but their boiling points differ widely. The boiling point of methoxymethane is intermediate of the two boiling points.  The high boiling points of alcohols are mainly due to the presence of intermolecular hydrogen bonding in them 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 (hydro-phobic) groups. Several of the lower molecular mass alcohols are miscible with water in all proportions. **Chemical Reactions** Alcohols are versatile compounds. They react both as nucleophiles and electrophiles. The bond between O--H is broken when alcohols react as nucleophiles. Alcohols as nucleophiles (i) \(ii) The bond between C--O is broken when they react as electrophiles. Protonated alcohols react in this manner. Protonated alcohols as electrophiles  Based on the cleavage of O--H and C--O bonds, the reactions of alcohols may be divided into two groups: **(a) Reactions involving cleavage of O--H bond** 1\. Acidity of alcohols \(i) Reaction with metals: Alcohols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides and hydrogen. The above reactions show that alcohols are acidic in nature. In fact, alcohols are Brönsted acids i.e., they can donate a proton to a stronger base (B:). \(ii) Acidity of alcohols: The acidic character of alcohols is due to the polar nature of O--H bond. An electron-releasing group (--CH~3~, --C~2~H~5~) increases electron density on oxygen tending to decrease the polarity of O-H bond. This decreases the acid strength. For this reason, the acid strength of alcohols decreases in the following order:  Alcohols are, however, weaker acids than water. Water is a better proton donor (i.e., stronger acid) than alcohol. Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors. 1. **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. The introduction of acetyl (CH~3~CO) group in alcohols is known as acetylation. Acetylation of salicylic acid produces aspirin. Aspirin possesses analgesic, anti-inflammatory and antipyretic properties.  \(b) Reactions involving cleavage of carbon -- oxygen (C--O) bond in alcohols. The reactions involving cleavage of C--O bond take place only in alcohols. 1\. Reaction with hydrogen halides: Alcohols react with hydrogen halides to form alkyl halides. ROH + HX → R--X + H~2~O The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another **(Lucas test).** Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl~2~) 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 phosphorus trihalides: Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide. 3\. Dehydration: Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a protic acid e.g., concentrated H~2~SO~4~ or H~3~PO~4~, or catalysts such as anhydrous zinc chloride or alumina. Ethanol undergoes dehydration by heating it with concentrated H~2~SO~4~ at 443 K.  Secondary and tertiary alcohols are dehydrated under milder conditions. For example Thus, the relative ease of dehydration of alcohols follows the following order: Tertiary \> Secondary \> Primary Tertiary carbocations are more stable and therefore are easier to form than secondary and primary carbocations; tertiary alcohols are the easiest to dehydrate. 4\. Oxidation: This 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. CrO~3~ in anhydrous medium is used as the oxidising agent for the isolation of aldehydes.  A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl. Secondary alcohols are oxidised to ketones by chromic anhyride (CrO~3~).  Tertiary alcohols do not undergo oxidation reaction. Under strong reaction conditions such as strong oxidising agents (KMnO~4~) 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. 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 oxidised 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, CH~3~OH, also known as 'wood spirit', was produced by destructive distillation of wood. Today, most of the methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO -- Cr~2~O~3~ 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, C~2~H~5~OH, is obtained commercially by fermentation, the oldest method is from sugars. The sugar in molasses, sugarcane or fruits such as grapes is converted to glucose and fructose, (both of which have the formula C~6~H~12~O~6~), 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. Nowadays, large quantities of ethanol are obtained by hydration of ethene. Ingestion of ethanol acts on the central nervous system. In moderate amounts, it affects judgment and lowers inhibitions. Higher concentrations cause nausea and loss of consciousness. Even at higher concentrations, it interferes with spontaneous respiration and can be fatal.