Alcohols, Phenols And Ether

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Alcohols and phenols introduction

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Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by O-H group. An alcohol contains one or more hydroxyl (O-H) groups directly attached to carbon atoms of an aliphatic system (CH3OH) while a phenol contains -OH groups directly attached to carbon atom of an aromatic system (C6H5OH).

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Ethers introduction

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The substitution of a hydrogen atom in hydrocarbon by an alkoxy or aryloxy group (R-O/ Ar-O) yields another class of compounds known as ethers, for example, (CH3OCH3) dimethyl Ether. Ethers are compounds formed by substituting the hydrogen atom of hydroxyl group of an alcohol or phenol by an alkyl and aryl group.

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Classification of alcohols and phenols

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Alcohols and phenols maybe 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 in the figure.

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Further classification of monohydric alcohols

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These are classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached.

2nd part is in the HINT.

Hint 4

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Classification of ethers

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Nomenclature of alcohols

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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, 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 alcohol, 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-CH2-CH2-OH is named as ethane-1, 2-diol. Cyclic alcohols are named using the prefix cyclo and considering the -OH group attached to C-1.

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Nomenclature of phenols

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Simplest hydroxy derivatives of benzene is phenol. It is its common name and also an accepted IUPAC name. As structure of phenol involves a benzene ring, in its substituted compounds the terms Ortho (1,2- disubstituted), meta(1,3- disubstituted) and para (1,4- disubstituted) are often used in the common names.

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Nomenclature of ethers

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Common names of ethers are derived from the names of alkyl or aryl groups written as separate words in alphabetical order and adding the word 'ether' at the end. For example, CH3OC2H5 is ethyl methyl ether. If both the alkyl groups are the same, the prefix 'di' is added before the alkyl group. For example, C2H5OC2H5 is diethyl ether. According to IUPAC system of nomenclature, ethers are regarded as hydrocarbon derivatives in which a hydrogen atom is replaced by an -OR or -OAr group, where R and Ar represent alkyl and aryl group, respectively. The larger group is chosen as the parent hydrocarbon.

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Structure of alcohols

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In alcohols, the oxygen of the -OH group is attached to carbon by the sigma bond formed by the overlap of SP3 hybridised orbitals of carbon with SP3 hybridised orbitals of oxygen. The bond angle C-O-H in alcohols is slightly less than the tetrahedral angle (109 degrees 28 minutes). It is due to the repulsion between the unshared electron pairs of oxygen.

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Structure of phenol

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In phenols, the -OH group is attached to SP2 hybridised carbon of an aromatic ring. The carbon-oxygen bond length (136 picometre) in phenol is slightly less than that in methanol. This is due to (1) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring and (2) SP2 hybridised state of carbon to which oxygen is attached.

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Structure of ethers

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In ethers, the 4 electron pairs, i.e., the two bond pairs and two lone pairs of electrons in oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky groups. The C-O bond length (141 picometres) is almost the same as in alcohols.