Structure of Alcohol: Functional Group, Classification, Formula

Structure of Alcohol: Do you know what the main component of a hand sanitiser is? What solution do the doctors use to clean the area before giving a jab? It is alcohol. But what does alcohol look like? What is the functional group structure of alcohol? Alcohols are organic compounds attached to a saturated carbon by a hydroxyl group (-OH); that is, a carbon attached to four atoms by simple bonds (without double or triple bonds).

The structural formula for alcohol — the vast and versatile family of compounds — is ROH. To be considered alcohol in the strictly chemical sense, the OH group should be the most reactive molecular structure. Among the many molecules with OH groups, it is essential to be able to confirm which one is alcohol. In this article, let us learn everything about the structure of alcohol, alcohol formula and alcohol functional groups.

What is Alcohol?

Alcohols are a special class of organic compounds in which the hydrogen atom of an aliphatic carbon is replaced with the hydroxyl group \(\left({ – {\text{OH}}} \right).\) Thus, an alcohol molecule consists of two parts; one containing the alkyl group and the other containing the hydroxyl group. It is represented as \({\text{R}} – {\text{OH}},\) where \({\text{R}}\) is the alkyl group.

Alcohols have the same general formula as alkanes but the structure of alcohol functional group is \( – {\text{OH,}}\) called the hydroxyl group. The most common alcohol, known as ethanol, is used in alcoholic drinks, fuel (gasoline), a preservative for biological specimens, and a solvent for paints and drugs.

In naming alcohols, the suffix of the name -\({\text{ol}},\) is added to the parent chain of the alkane name. The position of the \({\text{-OH}}\) functional group is indicated in the name. The numbering of the parent chain is done at the end closest to where the \({\text{-OH}}\) is located.

The general formula of alcohols homologous series is\({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 1}}{\text{OH}},\) where \({\text{n}} = 1,2,3..\)

Structural Formula for Alcohol

In the table below we have mentioned the structural formula for alcohol.

The example of homologous series in alcohol can be methanol, ethanol, propanol, and butanol with chemical formulas of \({\text{C}}{{\text{H}}_3}{\text{OH}},{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}},{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OH}},\) and \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OH}},\) respectively, where each successive compound differs from the previous one by a \( – {\text{C}}{{\text{H}}_2}\) group.

Structure of Alcohol

Similar to water, alcohol can be pictured as having an \({\text{s}}{{\text{p}}^3}\) hybridized oxygen atom with two nonbonding pairs of electrons bonded to an \({\text{s}}{{\text{p}}^3}\) hybridised carbon atom of an alkyl group.

Alkyl groups are generally bulkier than hydrogen atoms; hence the \({\text{R}} – {\text{O}} – {\text{H}}\) bond angle in alcohols is generally larger than the \({\rm{104}}.{\rm{5}}^\circ \,{\rm{H}} – {\rm{O}} – {\rm{H}}\) bond angle in water.

For example, the bond angle in methanol \(\left( {{\text{C}}{{\text{H}}_3}{\text{OH}}} \right)\) is \({108.9^ \circ }\) shows the effect of the methyl group \(\left({ – {\text{C}}{{\text{H}}_3}} \right),\) which is larger than the hydrogen atom of water

There are three types of bonds in alcohol. \(\left( {{\rm{s}}{{\rm{p}}^3}} \right){\rm{C}} – {\rm{H}}\left( {1{\rm{s}}} \right)\) sigma bond, \(\left( {{\rm{s}}{{\rm{p}}^3}} \right){\rm{C}} – {\rm{O}}\left( {{\rm{s}}{{\rm{p}}^3}} \right)\) sigma bond and \(\left( {{\rm{s}}{{\rm{p}}^3}} \right){\rm{O}} – {\rm{H}}\left( {1\,{\rm{s}}} \right)\) sigma bond. This is diagrammatically represented as below.

Alcohols fall into different classes depending on how the \( – {\text{OH}}\) group is positioned on the chain of carbon atoms. One way of classifying alcohols is based on which carbon atom is bonded to the hydroxyl group. If the carbon is primary (\({1^ \circ },\) bonded to only one other carbon atom), the compound is a primary alcohol. Secondary alcohol has the hydroxyl group on a secondary \(\left({{2^ \circ }}\right)\) carbon atom, bonded to two other carbon atoms. Similarly, tertiary alcohol has the hydroxyl group on a tertiary \(\left({{3^ \circ }}\right)\) carbon atom, bonded to three other carbons.

In all these types, the carbon atom and the oxygen atom is \({\text{s}}{{\text{p}}^3}\) hybridised.

Primary Alcohols: Ethyl Alcohol Structure

In a primary \(\left({{1^ \circ }} \right)\) alcohol (for example, ethyl alcohol structure), the carbon atom that carries the \( – {\text{OH}}\) group is only attached to one alkyl group. Some examples of primary alcohols are shown below:

Notice that the complexity of the attached alkyl group is irrelevant. There is only one linkage to an alkyl group from the \({\rm{C}}{{\rm{H}}_2}\) group holding the \( – {\text{OH}}\) group in each case. There is an exception to this. Methanol, \({\text{C}}{{\text{H}}_3}{\text{OH,}}\) is counted as primary alcohol even though no alkyl groups are attached to the \( – {\text{OH}}\) carbon atom.

Secondary Alcohols: Isobutyl Alcohol

In a secondary \(\left({{2^ \circ }}\right)\) alcohol, the carbon atom with the \( – {\text{OH}}\) group attached is joined directly to two alkyl groups, which may be same or different. Examples include the following:

Tertiary Alcohols

In a tertiary \(\left({{3^ \circ }}\right)\) alcohol, the carbon atom holding the \( – {\text{OH}}\) group is attached directly to three alkyl groups, which may be any combination of the same or different groups. Examples of tertiary alcohols are given below:

Alcohols are referred to as allylic or benzylic if the hydroxyl group is bonded to an allylic carbon atom (adjacent to a \({\text{C}} = {\text{C}}\) double bond) or a benzylic carbon atom (next to a benzene ring), respectively.

Allylic Alcohols

In these alcohols, the \( – {\text{OH}}\) group is attached to a \({\text{s}}{{\text{p}}^3}\) hybridized carbon next to the carbon-carbon double bond, i.e., to an allylic carbon. Allylic alcohols may be primary, secondary, or tertiary. For example:

Benzyl Alcohol Structure

The alcohol in which \( – {\text{OH}}\) groups are present in the side chain attached to a benzene (aromatic ring) are called benzylic alcohols. In the benzyl alcohol structure, the \( – {\text{OH}}\) group is attached to a \([{\rm{s}}{{\rm{p}}^3}]\) the hybridised carbon atom next to an aromatic ring.

Vinylic Alcohol

Monohydric alcohol in which \( – {\text{OH}}\) group is attached to \({\text{s}}{{\text{p}}^2}\) Hybridized carbon-carbon double bonds are classified as vinylic alcohol. For example:

What is Carboxylic Acid?

Carboxylic acids are the special class of organic compounds that incorporate a carboxyl functional group \( – {\text{COOH}}.\) The name carboxyl comes from the fact that a carbonyl group \(\left({ – {\text{C}} = {\text{O}}} \right)\) and a hydroxyl group \(\left({ – {\text{OH}}} \right)\) are attached to the same carbon atom.

\({\text{Carbonyl}}\,{\text{group}}\,\left({ – {\text{C}} = {\text{O}}} \right) + {\text{Hydroxyl}}\,{\text{group}}\,\left( { – {\text{OH}}} \right) \to {\text{Carboxyl}}\,{\text{group}}\left({ – {\text{COOH}}} \right)\)

When naming organic acids, use the parent chain name of the alkane, and add the suffix ‘–oic acid’ by replacing the ‘e’ of the ‘–ane’.

For example, the organic compound with the chemical formula of \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH}}\) is propanoic acid, and \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COOH}}\) is butanoic acid. The first compound contains a three-carbon chain; therefore, the parent chain name of the alkane is propane. Since it is a carboxylic acid, the ‘e’ of ‘–ane’ is replaced by ‘–oic acid’, making the name of the compound propanoic acid. Similarly, the name of the carboxylic acid with the four-carbon chain is butanoic acid.

Carboxylic acid	Carbon number	Formula	Structure
Methanoic acid	\(1\)	\({\text{HCOOH}}\)
Ethanoic acid	\(2\)	\({\text{C}}{{\text{H}}_3}{\text{COOH}}\)
Propanoic acid	\(3\)	\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH}}\)
Butanoic acid	\(4\)	\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COOH}}\)

The general formula of the carboxylic acid homologous series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}}}}{{\text{O}}_2},\) where \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}}}}{{\text{O}}_2},\)

A homologous series of carboxylic acids can include pentanoic acid, hexanoic acid, and heptanoic acid with chemical formulas of \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COOH}},{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COOH}}\) and \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COOH}}\) respectively, where each successive compound differs from the previous one by \({\text{a}} – {\text{C}}{{\text{H}}_2}\) group.

Structure of Carboxylic Acid

The general structural formula of carboxylic acid is \({\text{RCOOH}},\) where \({\text{R}}\) stands for any side group comprising \({\text{H}}\) or an alkyl group or another \( – {\text{C}}\) bonded to a certain chain.

Hybridisation of the carboxylic carbon atom \( = \) Number of atoms attached \( + \) Lone pairs

\( = 3 + 0 = 3\)

The carbon atom of the carboxylic group is \({\text{s}}{{\text{p}}^2}\) hybridised \(\left({1{\text{s}} + 2{\text{p}} = 3{\text{s}}{{\text{p}}^2}} \right).\) This means the atomic orbitals of the carbon atom undergo intermixing to form \(3{\text{s}}{{\text{p}}^2}\) hybridised orbitals.

This means that the \({\text{s}}{{\text{p}}^2}\) hybridized carbon of the carbonyl group forms \(3\) sigma bonds. The formation of \(3\) sigma bonds gives the carbonyl group a basic trigonal shape with bond angles of \(120\) degrees. Only two out of three \({\text{p}}\) orbitals participate in hybridization; hence, one \({\text{p}}\) orbital is unhybridized.

This \({\text{p}}\) orbital forms a pi bond with the unhybridized \({\text{p}}\) orbital of the carbonyl oxygen atom. This \({\text{p}}\) orbital is directed above and below the plane of the paper.

The oxygen atom of the carbonyl group and that of the \({\rm{ – OR}}\) group have two lone pairs each. The \({\rm{ – OR}}\) oxygen allows one of its lone pair electrons to conjugate with the pi system of the carbonyl group. This makes the carboxyl group planar and can be represented with the following resonance structure.

Carboxylic acids can donate hydrogen to produce a carboxylate ion. The carboxylate ion so formed is resonantly stabilized and has equal \({\text{C}} – {\text{O}}\) bond length.

What is an Ester?

An ester is a carboxylic acid derivative in which the hydrogen atom of the hydroxyl group has been replaced with an alkyl group. The structure is the product of a carboxylic acid (the \({\text{R}}\)-portion) and an alcohol (the \({\text{R’}}\)-portion). The general formula for an ester is \({\text{RCOOR’}}\) or \({\text{RC}}{{\text{O}}_2}{\text{R’,}}\) which is shown below.

The \({\text{R}}\) group can either be a hydrogen atom or a carbon chain. The \({\text{R’}}\) group must be a carbon chain since a hydrogen atom would make the molecule a carboxylic acid.

Esters are named as if the alkyl chain from the alcohol is a substituent. No number is assigned to this alkyl chain. This is followed by the name of the parent chain from the carboxylic acid part of the ester with an \( – {\text{e}}\) removed and replaced with the ending \( – {\text{oate}}{\text{.}}\)

Ester	Formula	Structure
Methyl acetate	\({\text{C}}{{\text{H}}_3}{\text{COOC}}{{\text{H}}_3}\)
Ethyl acetate	\({\text{C}}{{\text{H}}_3}{\text{COOC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}\)
Ethyl propionate	\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}\)

The general formula of the homologous ester series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}}}}{{\text{O}}_2},\) where \({\text{n}} = 1,2,3…\)

In the table given above, ethyl acetate differs from methyl acetate by a \({\text{a}} – {\text{C}}{{\text{H}}_2}\) unit. Hence, they belong to the same homologous series.

Structure of Ester

To make an ester, a hydrogen atom from the hydroxyl group \(\left({ – {\text{OH}}} \right)\) of the alcohol and the \( – {\text{OH}}\) portion of the acid’s carboxyl group must be removed. The hydrogen atom and the \( – {\text{OH}}\) combine to form a water molecule \(\left({{{\text{H}}_2}{\text{O}}} \right).\)

Propanol \( + \) Ethanoic acid \( \rightleftharpoons \) Propyl Ethanoate \( + \) water

This same change can be represented using shortened structural formulae:

\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OH}} + {\text{C}}{{\text{H}}_3}{\text{COOH}} \rightleftharpoons {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OOCC}}{{\text{H}}_3} + {{\text{H}}_2}{\text{O}}\)

The ester link \(\left({ – {\text{COO}}} \right)\) separates the two parts of the molecule as shown below. The \({\text{C}} = {\text{O}}\) part of the molecule belongs to the acid.

Since the \({\text{C}} = {\text{O}}\) came from the parent acid, there were four carbon atoms in the acid molecule (butanoic acid) and two carbon atoms in the parent alcohol (ethanol). The ester so formed is called ethyl butanoate.

As the esters are derivatives of carboxylic acid and alcohol, the hybridisation of the carbonyl carbon atom is the same as that of the carboxylic acid. It is \({\text{s}}{{\text{p}}^2}\) hybridized with an unhybridized \({\text{Pz}}\) orbital projected above and below the plane of the paper. The \({\text{s}}{{\text{p}}^2}\) hybridized carbon of the carbonyl group forms \(3\) sigma bonds, and the unhybridized \({\text{Pz}}\) orbital laterally overlaps with the \(2{\text{p}}\) orbital of the carbonyl oxygen atom to form a pi bond.

The oxygen atom of the carbonyl group and that of the hydroxyl group has two lone pairs each. The hydroxyl oxygen allows one of its lone pair electrons to conjugate with the pi system of the carbonyl group. The formation of \(3\) sigma bonds gives the carbonyl group a basic trigonal shape with bond angles of \(120\) degrees. The following diagram represents the resonance structure of an ester.

The ester balances between both structures. These two structures influence the stability and reactivity of an ester.

What is Phenol?

Phenol is an aromatic hydroxy compound in which one \({\text{H}}\) atom from a benzene ring is replaced by the same number of \({\text{-OH}}\) groups. The chemical formula of Phenol is \({{\text{C}}_6}{{\text{H}}_5}{\text{OH}}.\) Phenol is a mildly acidic, toxic, white crystalline solid obtained as a byproduct in coal tar distillation. It is highly hygroscopic and volatile. It has a sickly sweet smell and a sharp burning taste. On a large scale, Phenol is obtained from petroleum-derived feedstocks. It is the precursor to producing many essential commodities such as plastics and antiseptics.

Structure of Phenol

1. All carbon atoms forming the aromatic ring of Phenol are \({\text{s}}{{\text{p}}^2}\) hybridized. Therefore, phenyl has a hexagonal planar structure with all bond angles \({120^ \circ }\) and delocalized \(\pi \)-electrons distributed over the ring.
2. The hybridization of the \({\text{C}} -{\text{O}}\) bond is \({\text{s}}{{\text{p}}^2} – {\text{s}}{{\text{p}}^3},\) respectively, and the \({\text{O}} – {\text{H}}\) bond is formed from \({\text{O}}\left({{\text{s}}{{\text{p}}^2}} \right) – {\text{H}}\left({1{\text{s}}} \right)\) hybridization.
3. Wo nonbonded electron pairs occupy the other two orbitals of the oxygen atom. It is due to this reason, hydroxyl functional group \({\text{C}} – {\text{O}} – {\text{H}}\) has a bent shape with a bond angle of \(109^\circ {\rm{ }}.\)
4. As oxygen is more electronegative than both carbon and hydrogen atoms, both the \({\rm{C – O}}\) and \({\rm{O – K}}\) bonds are polar.

5. In Phenol, the non-bonded electron pairs of oxygen atoms are in conjugation with the aromatic ring.
6. The delocalisation of pi electrons causes partial negative charge transfer from the oxygen atom to the aromatic ring system.
7. The delocalization of pi electrons strengthens the polarization of the \({\text{O}} – {\text{H}}\) bond. This results in the acidic character of Phenol.
8. Phenol is a weak acid. This is because it readily loses the hydrogen atom to form a phenoxide ion (phenolate) ion.

9. Both Phenol and its conjugate base are resonance stabilised. Dispersion of the negative charge over the molecule can be illustrated with the resonance structures or as a resonance hybrid as below:

Summary

This article taught us the general structure of alcohols, carboxylic acids, Esters and Phenol, and the fundamental difference between them. We also learned the cause behind the relative stability of these compounds.

FAQs on Structure of Alcohol

Q.1. What are the four types of alcohol?
Ans: The four types of alcohol are ethyl alcohol, denatured alcohol, isopropyl alcohol, and rubbing alcohol. One of the most commonly used alcohol is ethyl alcohol, also called ethanol or grain alcohol. It is made by fermenting sugar and yeast and is used in beer, wine, and liquor.

Q.2.What is the general structure of alcohol?
Ans: The generic structure of alcohols is \({\text{R-OH,}}\) where \({\text{R}}\) represents the alkyl group.

Q.3.What is the general structural representation of alcohol?
Ans: The general structural representation of an alcohol is \({\text{R}} – {\text{OH}},\) where \({\text{R}}\) represents an alkyl group.

Q.4. What is the structure of an alcohol functional group?
Ans: An alcohol is an organic molecule having an aliphatic carbon atom bearing the hydroxyl \(\left({{\text{OH}}} \right)\) functional group. We can represent alcohols by the general formula \({\text{ROH}},\) where \({\text{R}},\) is an alkyl group, and \({\rm{ – OH}}\) is the functional group of all alcohols.

Q.5. How do you classify alcohols?
Ans: One way of classifying alcohols is based on which carbon atom is bonded to the hydroxyl group. If the carbon is primary (\({1^ \circ },\) bonded to only one other carbon atom), the compound is a primary alcohol. Secondary alcohol has the hydroxyl group on a secondary \(\left({{2^ \circ }} \right)\) carbon atom bonded to two other carbon atoms. Similarly, tertiary alcohol has the hydroxyl group on a tertiary \(\left({{3^ \circ }} \right)\) carbon atom bonded to three other carbons.

Study Chemical Reactions of Alcohol

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