+2 UNIT 13 PAGE- 2
Effect of substituents on the acidic strength of phenol
Phenol shows acidic character simply because phenate ion is more stable than phenol. Any substituent which further stabilises the phenate ion will enhance the acidic strength. At the same time, any substituent which tends to destabilise the ion will diminish the acid strength. Let us discuss the effect of both the electron withdrawing and electron releasing substituents on the acidic strength of phenol.
(a) Effect of electron withdrawing substituents or groups : These substituents or groups ( e.g., -NO2, -CN, -X etc) stabilize the phenate ion by dispersing the negative charge on it because of their electron withdrawing nature.
It may be noted that electron withdrawing groups are more effective in increasing the acidic strength at para position relative to ortho position because of greater dispersal of charge on oxygen atom. However, their effect at meta position is comparatively very little. This is quite evident from the data given below :
Thus the order of decreasing acidic strength is :
p-Nitrophenol > o-Nitrophenol > m-Nitrophenol > phenol
(b) Effect of electron releasing substituents or groups
These substituents or groups (e.g., -OH , -NH2, -R etc.) destabilise the phenate ion by increasing the charge density due to their electron releasing nature.
The electron releasing groups are also more effective in increasing electron density on the oxygen atom of the phenate ion when present at para position than at ortho position. However, their effect when present at the meta position is less pronounced. This is quite evident from the data listed below.
The decreasing order of acidic strength is :
Phenol > m-Cresol > o-Cresol > p-Cresol
Comparison of relative acid strength of phenols and aliphatic alcohols
Phenols , in general , are strong acids ( Ka » 10-10) than the aliphatic alcohols ( Ka » 10-18). This is further supported by the fact that phenols turn blue litmus red and neutralise caustic alkalis such as NaOH and KOH while alcohols do not. The difference in the relative acidic strengths in the members of two families may be attributed to resonance or conjugation which is present in phenols but not in alcohols.
Problem
01. Arrange the following compounds in increasing order of their acid strengths :
Propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol.
(ii) Esterification
Alcohols and phenols react with carboxylic acids , acid chlorides and acid anhydrides to form esters.
The reaction with carboxylic acid and an acid anhydride is carried out in 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 presence of a base (pyridine) so as to remove HCl. It also shifts the equilibrium to the right hand side. The introduction of acetyl (CH3CO-) group in alcohols or phenols is known as acetylation.
Phenol forms acyl derivatives when reacted with either acid chloride or acid anhydride
Aspirin possesses analgesic , anti-inflammatory and antipyretic properties.
Reaction with Zinc dust
Upon heating with zinc dust , phenol is reduced to benzene. For example,
Reaction with benzoyl chloride
Phenol reacts with benzoyl chloride in the presence of NaOH to form phenyl benzoate (ester). This reaction is known as Schotten Bauman reaction.
Reaction with Grignard reagents
Phenols react with Grignard reagents just like alcohols to form the corresponding hydrocarbons.
Reaction with Ferric chloride
Phenols give violet colouration with neutral ferric chloride solution. In some cases , the colour may be green or red. This is because of the formation of ferric salt of phenol.
2. Reaction involving cleavage of carbon-oxygen ( C-O) bond.
Reaction with hydrogen halides
Alcohols react with hydrogen halides according to the following equation :
ROH + HX ® R-X + H2O
The alcohol may be primary, secondary or tertiary and hydrogen halide may be HCl, HBr, or HI. For example:
In this reaction, tertiary alcohols are the most and primary alcohols , the lessr reactive .
Reactivity increases from 1° to 3°. ®
The alkyl group R has + I effect. It increases the electron density towards the carbon atom as well as the oxygen atom of C-OH bond. As a result, the bond cleavage becomes easy. Greater the number of alkyl groups present, more will be the reactivity of alcohol. Thus the relative order of reactivity of the alcohols is justified.
Among hydrogen halides, HI is most and HCl , the least reactive.
For a given alcohol : Order of reactivity of HX : HI > HBr > HCl
For a given halogen acid : Order of reactivity of alcohols :
Tertiary > Secondary > Primary
The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another (Lucas test).
Lucas test
This test is based on the relative reactivities of primary, secondary and tertiary alcohols with hydrochloric acid. The given alcohol is treated with Lucas reagent which is an equimolar mixture of concentrated HCl and anhydrous zinc chloride (dehydrating agent). The product is alkyl chloride or chloroalkane accompanied by white turbidity or cloudiness.
The time taken for the appearance of turbidity is different in three types of alcohols and affords a method for their distinction.
· If turbidity appears immediately, alcohol is tertiary.
· If turbidity appears after sometime, alcohol is secondary.
· In case turbidity appears on heating, alcohol is primary.
Reaction with phosphorus halides
Alcohols can be easily converted into haloalkanes by treating with phosphorus halides under suitable conditions.
(a) Alkyl chlorides or chloroalkanes are prepared by reacting a particular alcohol with phosphorus pentachloride (PCl5) or phosphorus trichloride (PCl3).
(b) Alkyl bromides or bromoalkanes are obtained by the action of phosphorus tribromide (PBr3) on a specific alcohol. Since the compound is not stable, it is prepared in the reaction mixture ( in situ) by the action of red phosphorus on bromine.
Phosphorus pentabromide (PBr5) is not used in the reaction since it does not exist.
(c) Alkyl iodides or iodoalkanes are prepared by the action of phosphorus tri-iodide (PI3) also obtained in situ from red phosphorus and iodine.
Reaction with thionyl chloride
Alkyl chlorides can be prepared by refluxing alcohols with thionyl chloride in the presence of pyridine. This is called Darzen’s method.
The reaction with thionyl chloride is preferred as the byproducts (SO2 and HCl ) formed are gases and are easily removed from the reaction mixture.
(iii) Dehydration
Acidic dehydration of alcohols is an elimination reaction and is carried with concentrated H2SO4 on heating. The dehydration can also be carried with aluminium oxide Al2O3 or phosphoric acid (H3PO4) but at high temperatures. A molecule of water is eliminated and alkene is formed as the product. For elimination reaction, the presence of b-hydrogen on alcohol is necessary.
The relative ease of dehydration of alcohols follows the order :
Tertiary alcohol > Secondary alcohol > Primary alcohol
Mechanism of dehydration
The mechanism of dehydration of ethanol involves the following steps :
Step 1 : Formation of protonated alcohol :
Step 2 : Formation of carbocation : It is the slowest step and hence the rate determining step of the reaction.
Step 3 : Formation of ethene
The acid used in step (I) is finally released after the reaction.
(iv) Oxidation
Oxidation of alcohols involves the formation of a carbon oxygen double bond with a cleavage of an O-H and C-H bonds.
Such a cleavage and formation of bonds occurs in oxidation reactions. These are also known as dehydrogenation reactions as these involve loss of hydrogen from alcohol molecule. The products of oxidation reactions depend on the nature of alcohols, primary, secondary or tertiary nature of carbon and also the oxidising agent used. A primary alcohol may be oxidised to an aldehyde or to a carboxylic acid.
Strong oxidising agents such as acidified potassium permanganate , are used for getting carboxylic acids directly. Cr(VI) in anhydrous medium is used as oxidising agent for the isolation of aldehydes.
Secondary alcohols are oxidised to ketones by chromic anhydride (CrO3).
Tertiary alcohols having no hydrogen on carbon bearing –OH group do not undergo oxidation reaction. Under strong reaction conditions such as strong oxidising agents and at elevated temperatures, cleavage of various C-C bonds takes place.
(v) Dehydrogenation
When vapours of a primary or secondary alcohol are passed over copper heated at 573 K, an aldehyde or a ketone is formed. Tertiary alcohols undergo dehydration to form alkene.
REACTIONS OF PHENOL
1. Electrophilic aromatic substitution
The -OH group attached to the benzene ring in the phenol activates it towards electrophilic substitution. It also directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to their electronic effect ( plus mesomeric effect).
The common electrophilic aromatic substitution reactions taking place in phenol are as follows:
(i) Nitration : With dilute nitric acid at low temperature (298 K) , phenol yields a mixture of ortho(15%) and para(30-40%) nitrophenols.
The ortho isomers can be separated by steam distillation. o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding, while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.
With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol(picric acid). In this , nitration is accompanied by oxidation of phenol.
Industrially, picric acid is prepared by treating phenol , first with con. H2SO4 which converts it to phenol-2,4-disulphonic acid. This yields picric acid on treating with con. HNO3.
Note : Picric acid is a very strong acid inspite of the absence of any carboxyl (-COOH) group. This is because of the presence of three electron withdrawing –NO2 groups in the ring which accelerate the release of H+ ion from O–H group in phenol.
(ii) Halogenation
In aqueous medium, halogenation takes place at the available ortho and para positions resulting in a tri-substituted product. For example,
However, in a non-polar solvent such as CCl4 or CS2, the halogenation becomes slow and only one halogen atom is introduced either at the ortho or para position. Thus a mixture of isomeric products is obtained. For example,
In aqueous medium phenol ionises to give phenate ion. The presence of negative charge on the oxygen atom activates the ring to a large extent towards electrophilic substitution resulting in trisubstitution. However, in non-polar solvent, phenate ion is not formed. This means that the activation of the ring gets suppressed since the oxygen has no negative charge on it. As a result, -OH group donates the electron pair to the ring only to a small extent and mono-substitution takes place.
(iii) Sulphonation
It is done by heating phenol with concentrated H2SO4 when hydrogen atom in the ring gets replaced by a sulphonic acid (-SO3H) group to form a mixture of ortho and para sulphonic acid derivatives as follows :
At low temperature (298 K) , ortho isomer is the major product , while at higher temperature (373 K) , para isomer is formed in excess.
Problem
02. Write the structures of the major products expected from the following reacttions :
(a) mononitration of 3-methyl phenol.
(b) Dinitration of 3-methylphenol
(c) Mononitration of phenyl ethanoate
(iv) Alkylation
Alkylation i.e., replacement of a hydrogen atom in the ring by alkyl group is normally done in the presence of anhydrous AlCl3 (Friedel Craft’s reaction). However, in this case AlCl3 co-ordinates with the lone electron pairs on oxygen atom since it is electron deficient in nature (Lewis acid). As a result, the activating influence of phenolic group is considerably decreased. Alkylation of phenol is carried in the presence of hydrofluoric acid(HF).
(v) Hydrogenation
When hydrogen is passed through phenol in the presence of finely divided nickel at 533 K , it gets completely hydrogenated to form cyclohexanol as follows :
Some Special Reactions of Phenol
1. KOLBE’S REACTION
On reacting sodium salt of phenol with carbondioxide gas, ortho hydroxybenzoic acid is formed as the main product.
From salicylic acid, aspirin, salol and methyl salicylate are prepared.
Aspirin
Aspirin is obtained by acetylating salicylic acid with acetic anhydride in presence of conc. H2SO4 under completely anhydrous conditions.
Aspirin is a very useful medicinal compound. It is used as an analgesic to relieve the body from the pains and also as an antipyretic to lower body temperature in case of fever.
Salol
Salol is prepared by heating salicylic acid with phenol in presence of phosphorus oxychloride. It is a useful intestinal antiseptic.
Methyl salicylate
Methyl salicylate is an Important constituent of oil of winter green and is formed by refluxing salicylic acid and methyl alcohol in presence of HCl.
Methyl salicylate is a constituent of iodex and is also used in mouth wash.
2. REIMER-TIEMANN REACTION
On treating phenol with chloroform in presence of sodium hydroxide, a –CHO group is introduced at the ortho position of benzene ring. The reaction is known as Reimer Tiemann reaction.
The intermediate substituted benzal chloride is hydrolysed in presence of alkali to produce salicylaldehyde.
It is an electrophilic substitution reaction ; the electrophile dichlorocarbene , is formed by reaction of NaOH with CHCl3 or CCl4.
Dichlorocarbene contains a sextet of electrons and therefore , it is a strong electrophile.
3. FRIES REARRANGEMENT
Esters of phenols yield phenolic ketones on treatment with anhydrous aluminium chloride. For example, phenyl ethanoate yields ortho and para hydroxyacetophenones. It involves migration of an acyl group from phenolic oxygen to ortho and para positions of the aromatic ring.
GATTERMANN’S REACTION
When vapours of hydrogen chloride gas along with liquid hydrogen cyanide are passed into phenol in presence of anhydrous AlCl3 catalyst , p-hydroxy benzaldehyde is formed as the main product. This reaction is also known as formylation reaction.
HCl + H-CºN ® ClCH =NH
Coupling reaction
In slightly alkaline medium (pH = 9 – 10) an ice cold solution of phenol reacts with ice cold solution of benzene diazonium chloride to form p-hydroxy azobenzene which is a coloured azo dye.
Condensation with phthalic anhydride
When phenol is heated with pthalic anhydride in the presence of a few drops of concentrated sulphuric acid, it forms phenolphthalein. The reaction is called phthalein reaction.Phenolphthalein is a very useful indicator in acid-alkali titration. In acidic medium , it is colourless , but becomes pink in the alkaline medium.
Condensation with Formaldehyde ( formation of Bakelite)
Phenols condense with formaldehyde in the presence of base (or acid) to form a polymer known as bakelite.
ETHERS
Preparation
1. By dehydration of alcohols
Alcohols undergo dehydration in presence of protonic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether depends on reaction conditions. For example ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K and at 410 K ethoxyethane is the main product.
The dehydration of secondary and tertiary alcohols to get corresponding ethers is unsuccessful as alkenes are formed easily in these reactions.
Dehydration of alcohols can also be carried with Al2O3 at 523 K. For example,
2. Williamson Synthesis
It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide.
Ethers containing substituted alkyl groups (secondary and teriary) may also be prepared by this method. The reaction involves a nucleophilic substitution of halide ion by alkoxide ion.
Good results are obtained if alkyl halide is primary. If a tertiary alkyl halide is used , an alkene is the only reaction product and no ether is formed. For example, the reaction of CH3ONa with (CH3)3C-Br gives exclusively 2-methylpropene.
It is because alkoxides are not only nucleophiles but also strong bases as well. They react with alkyl halides leading to elimination reaction.
Phenols are also converted to ethers by this method.
Physical properties
1. Physical state
Dimethyl ether and ethylmethyl ether are gases at room temperature, while the other members are generally liquids with characteristic smell called ethereal smell.
2. Dipolar nature
The dipolar nature of ethers is linked to their structures. These are bent molecules and their structures is similar to that of water. In fact ethers are dialkyl or diaryl derivatives of water and the oxygen atom is sp3 hybridised. Ethers have therefore , tetrahedral geometry as shown in Fig.
However, the C-O-C bond angle is nearly 110° while H-O-H bond angle is 104.5°. This is because of greater size of the alkyl (or aryl) groups resulting in greater forces of repulsion. Due to the bent structures, the polarities of the C-O bonds do not cancel. Thus ethers have net dipole moment values varying from 1.15 D to 1.30 D. For example, dipole moment of dimethyl ether is 1.30 D while the value for diethyl ether is 1.18 D.
3. Boiling points
The C-O bonds in ether are polar. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular mass but are much lower than the boiling points of alcohols.
Solubility
Ethers containing upto three carbon atoms are soluble in water due to their hydrogen bond formation with water molecules.
The solubility decreases with increase in the number of carbon atoms.
Chemical properties
Ethers are very little reactive chemically and under normal conditions do not combine with acids, bases or reducing agents. The low reactivity or inert character is due to the reason that the divalent oxygen is linked to carbon atoms on both sides (C-O-C) and has no active site just as in the case of alcohols/phenols(-OH) , aldehydes /ketones (>C=O). However, under drastic conditions, they do react mainly because of lone pair on the oxygen atom or of cleavage of C-O bonds. The important chemical characteristics of ethers are as follows :
Reactions due to ethereal oxygen
The ethereal oxygen atom has two lone pairs which can be partially or completely donated to electron deficient species resulting in the following chemical reactions.
i. Formation of oxonium salts
Due to the presence of lone pair of electrons on oxygen atom which can be easily donated, ethers form salts with strong mineral acids such as conc. H2SO4 or conc. HCl at very low temperature ( in cold conditions). For example,
ii) Formation of co-ordinate complexes
Ethers form co-ordinate complexes with Lewis acids such as BF3, AlCl3 and Grignard reagent. For example,
Since ethers form complexes with Grignard reagents, these are used as solvents for them. Grignard reagents are often prepared and used in presence of ethers.
Reactions involving cleavage of Carbon-Oxygen Bond
The typical reactions of this type are as follows :
1. Reaction with halogen acids
Ethers are cleaved by concentrated hydroiodic acid (HI) or hydrobromic acid (HBr) at 373 K and the reaction is completed in two stages.
(a) If halogen acid is in limited amount, a mixture of alcohol and alkyl halide is formed.
(b) If excess of halogen acid is available, then alcohol further reacts with halogen acid to form alkyl halide and water.
The net reaction may be written as :
Alkyl aryl ethers are cleaved at the alkyl –oxygen bond due to the low reactivity of aryl-oxygen bond. The reaction yields phenol and alkyl halide.
Ethers with two different alkyl groups are also cleaved in the same manner.
R-O-R’ + H X ® R - X + R’ -OH
The order of reactivity of halogen acids is : HI > HBr > HCl. This is based on bond dissociation energy and HCl is found to be very little reactive.
In the cleavage of mixed ethers with two different alkyl groups, the alcohol and alkyl iodide that form depend on the nature of alkyl groups. When primary or secondary alkyl groups are present , it is the lower alkyl group that forms alkyl iodide. For example,
When one of alkyl group is tertiary group, the halide formed is tertiary halide.
It is because the attack by I- takes place at that carbon of the alkyl group which has a greater electron pushing inductive effect and lower electron density.
Analytical importance of the reaction
The reaction of ethers with hydroiodic forms the basis of Zeisel’s Method for detection and estimation of alkoxy groups in a compound. A known weight of the ether is heated with approximately 57% HI. The alkyl halide formed is volatile and is absorbed in an alcoholic solution of AgNO3. Reaction of alkyl halide with silver nitrate gives a precipitate of silver iodide, which is filtered washed dried and weighed. Thus AgI º RI
Problem
03. Give the major products that are formed by heating each of the following ethers with HI.
2. Reaction with sulphuric acid (Hydrolysis)
Ethers are hydrolysed to alcohols when heated with dilute H2SO4 under pressure.
In case concentrated acid is used a mixture of alcohol and alkyl hydrogen sulphate is formed.
3. Reaction with phosphorus pentachloride
Ethers are cleaved by PCl5 to form alkyl chlorides as follows.
Reactions involving alkyl groups
In these reactions , the C-H bonds of the alkyl groups participate.
1. Action of air and light
When ethers are exposed to air and light for a long time, these are oxidised to form peroxides to small extent.
Explosive nature of peroxides .The peroxides have higher boiling points than corresponding ethers. These are quite dangerous as due to the presence of peroxide linkage, they decompose violently when distilled. Keeping this view, ethers should never be evaporated to dryness as the peroxides left will decompose with explosion. It is therefore necessary to remove the peroxide from a particular ether (generally old samples) before it is subjected to distillation.
2. Halogenation
When ethers are treated with chlorine or bromine in dark, one hydrogen atom linked to carbon atoms attached to oxygen atom (a-hydrogen) are readily substituted by halogen atoms as follows :
However, in the presence light and excess of chlorine, all the hydrogen atoms in ether molecule are substituted. For example,
Electrophilic substitution in alkyl aryl ethers
The alkoxy group (-OR) is ortho-para directing and acivates the aromatic ring towards electrophilic substitution in the same way as in phenol.
(i) Halogenation
Phenyl alkyl ethers undergo usual halogenation in benzene ring, e.g., anisole undergoes bromination with bromine in ethanoic acid even in absence of iron(III) bromide catalyst. It is due to the activation of benzene ring by the methoxy group. Para isomer is obtained in 90% yield.
(ii) Friedel Craft’s reaction
Anisole undergoes Friedel Craft’s reaction i.e., alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in presence of aluminium chloride (a Lewis acid) as catalyst.
(iii) Nitration
Anisole reacts with a mixture of conc. H2SO4 to yield a mixture of ortho and para nitroanisole.
Crown Ethers
The polar nature of C-O bonds and the presence of unshared electron pairs on oxygen atom allow ethers to form complexes with metal ions.
The strength of this oxygen-metal bond depends on the structure of ether. A class of polyethers, also known as crown-ethers are known to form more stable complexes with metal ions than simple ethers. Crown ethers are cyclic polyethers containing four or more ether linkages in a ring of twelve or more atoms. Crown ethers were given this name because their molecular models resembles crowns.
A general abbrreation for crown ethers is xCy, where x is total number of atoms in the crown (ether ring) , y is the number of oxygen atoms and C is used for crown. For example, 18-crown-6 having 18 membered ring with 6 oxygen atoms.
A crown ether binds certain metal ions depending on the size of cavity.
In this reaction the crown ether is the ‘host’ and species it binds is the ‘guest’ . The crown-guest complex is called an inclusion compound. The crown ethers allow inorganic salts to dissolve in non-polar solvents. For example, potassium ion of potassium permanganate forms complex with the crown ether, thereby making KMnO4 soluble.