UNIT 13 ORGANIC COMPOUNDS WITH FUNCTIONAL GROUPS CONTAINING OXYGEN-I ALCOHOLS, PHENOLS AND ETHERS
Syllabus
· Classification
· Nomenclature
· Structures of Functional Groups
· Alcohols and Phenols
· Ethers
· Some commercially important compounds
The important classes of organic compounds with functional groups containing oxygen are alcohols, phenols, ethers, aldehydes, ketones , carboxylic acids and their derivatives. In this unit , we shall study the chemistry of three classes of compounds, viz., (i) alcohols (ii) phenols and (iii) ethers.
An alcohol contains one or more hydroxyl (OH) group(s) directly attached to aliphatic carbon atom(s), while phenol contains -OH group(s) directly attached to an aryl carbon atom(s). In an ether , an oxygen atom is attached to two carbon atoms of two alkyl or an alkyl and an aryl or two aryl groups. The simplest examples of an alcohol, phenol and ether are methanol (CH3-OH) , phenol (C6H5-OH) and methoxymethane (CH3-O-CH3) respectively.
CLASSIFICATION
Alcohols are classified as primary, secondary or tertiary according to the carbon that bears the -OH group.
In primary alcohols, only one carbon atom is attached to carbon carrying the –OH group. e.g. ethanol (CH3CH2OH).
In secondary alcohols, two carbon atoms are attached to carbon carrying the –OH group, e.g. propan-2-ol (CH3–CHOH–CH3).
In tertiary alcohols , three carbon atoms are attached to carbon carrying the –OH group, e.g. 2-methylpropan-2-ol (CH3–C(CH3)(OH) –CH3).
Alcohols and phenol can also classified as mono, di, trihydric as per one, two or three –OH groups are present in a molecule.
Ethers are known as simple or symmetrical, if two alkyl or aryl groups attached to oxygen atom are same and mixed or unsymmetrical, if the two groups are different. CH3OCH3 or C2H5OC2H5 is simple or symmetrical ether and C2H5OCH3 or C2H5OC6H5 is mixed or unsymmetrical ether.
Nomenclature of alcohols.
In the common system, alcohols are named as alkyl alcohols. In IUPAC system alcohols are derived from the corresponding alkanes by replacing 'e' of alkanes by 'ol'. The position of the carbon atom carrying the hydroxyl group gets the lowest number. For example,
The alcohols with two -OH groups are named as diols(common name glycol) and those with three -OH groups are named as triols(common name glycerol). For example,
The common and IUPAC names of some alcohols are given below:
Phenols
The simplest hydroxyl derivative of benzene is phenol which is also its accepted IUPAC name. The hydroxyl derivatives of toluene are o- , m- and p-cresol.
Dihydroxy derivatives of benzene are known as 1,2-, 1,3- and 1,4-benzenediol.
ETHERS
Nomenclature
According to Common System, ethers are named according to alkyl groups attached to oxygen atom. The names of two alkyl or aryl groups linked to oxygen are written alphabetically followed by the word 'ether'.
For example,
Some aromatic ethers have special common names. For example,
According to IUPAC system, ethers are named as alkoxyalkanes. The larger alkyl group forms the parent chain while lower alkyl group is taken with ethereal oxygen and forms part of the alkoxy group.
For example,
The common and IUPAC names of a few ethers are given below:
Problem
01. Give the IUPAC names of the following compounds.
02. Write the structural formulae and IUPAC names of all the isomeric alcohols with the molecular formula C5H12O.
03. Write the structures and IUPAC names of all the cyclic isomers (alcohols) with molecular formula C4H7OH.
04. Write the IUPAC names of the following compounds .
STRUCTURES OF FUNCTIONAL GROUPS
In alcohols, the oxygen of the –OH is attached to sp3 hybrised carbon by a sigma(s) bond formed by the overlap of sp3 hybrid orbital of carbon with an sp3 hybrid orbital of oxygen. The following Fig illustrates bonding in methanol.
(b)
Methanol is formed by the formation of a bond resulting due to the overlap of an sp3 orbital of carbon of –CH3 and an sp3 orbital of oxygen of –OH group.
The bond angle C–O–H in alcohols is slightly less than the tetrahedral angle (109°28¢) . It is due the repulsion between the unshared electron pairs of oxygen. In phenols, the –OH group is attached to sp2 hybrid carbon of an aromatic ring. The bond angle C–O–H in phenol is 109°. The carbon-oxygen bond length (136 pm) in phenol is slightly less than that in methanol. This is due to partial double bond character on account of the conjugation of unshared electron pair of oxygen with aromatic ring.
In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons around oxygen are arranged approximately in a tetrahedral arrangement. The C–O–C angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (-R) groups. The C–O bond length (141 pm) in ethers is almost the same as in alcohols.
ALCOHOLS
Preparation
Alcohols are synthesised by a number of methods.
1. By the Hydrolysis of Haloalkanes
Haloalkanes when boiled with aqueous solution of an alkali hydroxide or moist silver oxide furnish alcohols.
Primary alkyl halides give good yield of alcohols. However, tertiary alkyl halides , in this reaction give mainly alkene due to dehydrohalogenation.
Secondary alkyl halides give a mixture of alcohol and alkene.
This method is not very useful for preparing alcohols, because haloalkanes are themselves obtained from alcohols. However, the preparative utility of this method lies in the preparation of aromatic alcohols. For example,
2. From Aldehydes and Ketones
The aldehydes and ketones are converted into alcohol by the following methods:
Reduction : Aldehydes and ketones are reduced to primary and secondary alcohols respectively by reducing agents such as Lithium aluminium hydride (lithium tetrahydrido-aluminate(III),LiAlH4), Sodium borohydride(sodium tetrahydridoborate (III), NaBH4) or hydrogen gas in the presence of nickel or platinum as catalyst. For example,
3. From alkenes
(i) Hydration :The hydration of alkenes can be achieved indirectly by addition of H2SO4 to furnish alkyl hydrogen sulphate which on hydrolysis with hot water gives alcohol. For example,
Some reactive alkenes undergo direct hydration in the presence of mineral acids which act as catalysts.
Since alkenes are readily obtained by the cracking of petroleum , this method can be used for the manufacture of alcohols.
(ii) Direct addition of water at low temperature and high pressure in the presence of Al2O3(catalyst).
(ii) Oxymercuration –demercuration
Alkenes react with mercuric acetate in presence of water to yield hydroxymercurial compounds. These are reduced to alcohol by sodium borohydride.
The reaction is quite fast and produces alcohol in high yield. The alcohol obtained corresponds to Markownikov’s addition of water to an alkene.
(iii) Hydroboration
Diborane , B2H6, is an electron deficient molecule. It acts as an electrophile and reacts with alkene to yield alkylboranes, R3B. These are oxidised to alcohol on reaction with hydrogen peroxide in presence of alkali.
In each addition step, the boron atom is attached to the sp2 carbon atom that is bonded to greater number of hydrogen atoms. The hydrogen atom is transferred from boron atom to the other carbon atom of the double bond. Thus, it is an anti-Markovnikov’s addition. During oxidation of trialkyl borane , boron is replaced by –OH group. The yield of alcohol is excellent and the product is easy to isolate.
4. From carboxylic acids and esters
Carboxylic acids are reduced to primary alcohols in presence of strong reducing agent, lithium aluminium hydride.
The alcohol is obtained in excellent yield. However, LiAlH4 being an expensive reagent, is used for preparing special chemicals only. Commercially , acids are reduced to alcohols by converting them to the esters followed by their reduction using (i) hydrogen in presence of a catalyst (catalytic hydrogenation), and (ii) sodium and alcohol.
5. From Grignard reagents
Grignard reagent (RMgX) are alkyl or aryl magnesium halides. The Mg➛C bond in Grignard reagent is highly polar bond as carbon is electronegative relative to electropositive magnesium. Due to this polar nature of C-Mg bond, Grignard reagents are very versatile reagents in organic synthesis.
The magnesium salt is convereted into alcohol by treating with water. The overall result is to bind the alkyl group of Grignard reagent to the carbon of carbonyl group and hydrogen to oxygen. In this method, all types of alcohols are prepared as given below :
(i) Formaldehyde gives a primary alcohol.
(ii) Aldehydes other than formaldehyde give secondary alcohols.
(iii) Ketones give tertiary alcohols.
6. Oxo Process
Alkenes react with carbon monoxide and hydrogen in the presence of [Co(CO)4] as catalyst at high temperature and pressure to give aldehydes. The catalytic hydrogenation of aldehydes gives primary alcohols.
7. Fermentation of carbohydrates
Ethanol is manufactured by fermentation of starch or sugar. Fermentation is a process in which complex organic compounds are broken down into simpler molecules by the action of biological catalysts known as enzymes. Enzymes are complex organic compounds which act as catalysts in reaction taking place in living organisms. These are called bio-catalysts.
i) Ethanol from sugar solution(molasses). Molasses is the mother liquor left after the removal of sugar from sugar cane juice. It still contains 50% sugar. This is then diluted to about 10% concentration of sugar. Then, calculated amount of yeast is added at about 298 K. Yeast supplies the enzymes invertase and zymase which are essential for fermentation. These enzymes convert sugar ultimately into ethanol. In this process about 8 -10% solution of ethanol, called ‘wash’ is obtained. Further dehydration with quick lime and distilling with sodium or calcium gives a 99.8% ethanol sample.
ii) Ethanol From starch : Ethanol is also prepared industrially from starchy materials like potato, yam, etc. Starchy materials are made into a paste by heating with super heated steam at about 335 K and malt is added. The malt contains the enzyme diastase which converts starch into maltose.
The product obtained is cooled to about 305 K and yeast is added. It gives enzyme maltase which converts maltose to glucose.
The enzyme zymase also provided by yeast converts glucose into ethanol.
8. From water gas
Water gas is mixed with half its volume of hydrogen and passed over oxides of zinc, chromium and copper(catalyst) under 200 atm. at 623-670 K.
Methanol can also be prepared as bye-product during destructive distillation of wood.
Problems
05. Give the structures and IUPAC names of products expected from the following reactions :
a. catalytic reduction of butanal.
b. Hydroboration of 1-butene
c. Hydration of propene in the presence of dil. H2SO4.
d. Reaction of propanone with methyl magnesium bromide followed by hydrolysis of the adduct.
06. Give the structures of the compounds whose IUPAC names are as follows :
2-Methylbutan-2-ol
1-Phenyl propan-2-ol
3,5-Dimethylhexane-1,3,5-triol
2,3-Diethyl phenol
1-Ethoxypropane
3-Methyl-2-ethoxypentane
Cyclohexylmethanol
07. Draw the structures of all isomeric alcohols of molecular formula C5H12O and give their IUPAC names. Classify them as primary, secondary and tertiary alcohols.
08. Write equations for the preparation of propan-2-ol from (i) an alkene and (ii) a Grignard reagent.
09. Give the structures and IUPAC names of monohydric phenols of molecular formula C7H8O.
Phenols
Phenols are prepared by the following methods :
1. Alkali fusion of sodium benzene sulphonates When sodium benzene sulphonate is fused with alkali (NaOH) sodium phenoxide is formed. This on acidification gives phenol.
2. From diazonium salts
A diazomium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO2 + HCl ) at low temperature. Diazonium salts are hydrolysed to phenols on treating with dilute acids.
3. From aryl halides (Dow's process)
Phenol is obtained on a large scale by heating chlorobenzene with 10% NaOH solution at about 623 K and under a pressure of 320 atmospheres in the presence of copper catalyst. Phenol is obtained by the acidification of sodium phenoxide.
4. By the decarboxylation of sodium salt of salicylic acid
Phenol can also be obtained by the decarboxylation of sodium salicylate with soda lime( an equimolar mixture of NaOH and CaO).
5. From Grignard reagent
When oxygen is bubbled through the solution of phenyl magnesium bromide in ether, it forms an addition product which on acidification with dilute acid gives phenol.
Physical properties
Alcohols and phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are due to the –OH group. The alkyl and aryl groups modify these properties.
The boiling points of alcohols and phenols increase with increase in number of carbon atoms (increase in van der Waal’s forces). In alcohols, the boiling points decrease with increase in branching (decrease in van der Waal’s forces due to decrease in surface area).
The –OH group in alcohols and phenols contain a hydrogen bonded to an electronegative oxygen atom. Therefore, it is capable of forming hydrogen bond as shown below :
It is due to the presence of intermolecular hydrogen bonding that alcohols and phenols have higher boiling points corresponding to other classes of compounds, namely, hydrocarbons, ethers and haloalkanes/haloarenes of comparable molecular masses. The solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules. The solubility decreases with increase in size of hydrophobic group R.
Problem
10. Arrange the following set of compounds in the order of their increasing boiling points.
(i) Pentan-1-ol, butan-2-ol , butan-1-ol, methanol, ethanol
(ii) Pentan-1-ol , ethoxyethane, pentanal, n-butane
CHEMICAL PROPERTIES
The reactions of –OH groups in alcohols and phenols may be divided into two classes.
1. Reactions involving clevage of O-H bond
(i) Reactions with metals
Alcohols and phenols react with metals such as sodium , potassium and aluminium to yield corresponding alkoxides and hydrogen.
In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides.
The above reactions show that alcohols and phenols are acidic in nature. In fact , alcohols and phenols are Bronsted acids i.e., they can donate a proton to a strong base (B:).
On treating an alkoxide ion with water , the starting alcohol is obtained.
This reaction shows that water is a better proton donor than alcohol. In other words alcohols are weaker acids than water. Also in the above reaction , we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which shows that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide).
The acidic character of alcohols is due to the polar O–H bond. An electron – releasing group (–CH3, –C2H5) increases electron density over the oxygen atom tending to decrease the polarity of the O–H bond. This decreases the acid strength. For this reason acid strength of alcohols decreases in the following order :
Obviously, the basic strength of their alkoxides follows the reverse order.
Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs over oxygen, which makes alcohols proton acceptors.
Acidity of phenols
The reaction of phenols with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water. Let us examine how hydroxyl group attached to an aromatic ring is more acidic than the hydroxyl group attached to an alkyl group. The ionisation of an alcohol and phenol takes place as follows:
The greater acidity of phenol is due to the stability of the phenoxide ion , which is resonance stabilized as shown below.
Let us compare the relative stabilities of hybrids in phenol and phenoxide ion.
In the hybrid for phenol, the presence of partial positive charge (d+ ) on the electronegative oxygen is likely to make it less stable compared with hybrid for phenate ion in which the electronegative oxygen atom has partial negative charge (d-).
In addition to this, there is charge separation (partial positive and partial negative) in case of hybrid for phenol while there is no such charge separation in hybrid for phenate ion. The separation of charge is likely to make the hybrid for phenol less stable than for phenate ion.
Thus, we conclude that the less stable hybrid for phenol changes to more stable phenate ion (also called phenoxide ion) by the release of H+ . This accounts for the acidity of phenol.