Introduction
Organic chemistry has always been a wider research subject among science enthusiasts. The basic organic chemistry's idea is to propagate the elementary information about the organic compounds, exist around us and provide a solid foundation concerning the further exploration of organic compounds and factors that govern the properties of these compounds.
The organic compounds form a series, called homologs series, where the successive compounds contain similar functional groups and vary from one another by a –CH2 group. Alcohol is one of the various functional groups that are found in organic compounds. Let us discuss more on the structure of alcohol, phenol, and others.
What is Alcohol?
Alcohols are organic compounds, where an aliphatic carbon or a hydrogen atom is replaced with the hydroxyl group. Therefore, an alcohol molecule consists of two parts; one containing the alkyl group and another containing the hydroxyl group, and they have a sweet odor. They exhibit a unique set of both physical properties and chemical properties. The both physical and chemical properties of alcohol are primarily due to the hydroxyl group presence. The alcohol structure depends on different factors.
Alcohols are classified into various groups based on where the hydroxyl group is placed in the molecule. This results in a few differences in the chemical properties. The classification of alcohol is given as follows:
Primary Alcohols
In primary alcohol, the carbon with the hydroxyl group will only be attached to a single or one alkyl group.
Some examples of primary alcohols are given below in the diagrammatic representation.
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Methanol is an exception to the above representation. Still, it is the primary alcohol though the carbon, having the hydroxyl group attached does not have any other alkyl group attached to it.
Secondary Alcohols
In secondary alcohol, the carbon with the hydroxyl group will be attached to the two alkyl groups.
Some examples of secondary alcohols are represented below:
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Tertiary Alcohols
The carbon present with the hydroxyl group gets attached to three other alkyl groups in the tertiary alcohol.
Some examples of tertiary alcohols are depicted below.
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Alcohol Structure
The alcohol atomic structure is primarily attributed to the presence of the hydroxyl group alcohol. In alcohols, the main chain's carbon atom gets bonded to the oxygen atom of the hydroxyl group alcohol by a sigma (σ) bond.
This sigma bond is formed because of the overlap of an sp3 hybridized orbital of carbon with an oxygen atom's sp3 hybridized orbital. Because of the repulsion between the unshared electron oxygen pairs, the bond angle of the C-O-H bonds present in alcohol is slightly less than that of the tetrahedral angle (109°-28′).
The structure of Alcohol can be represented as follows:
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Structure of Phenol
Phenol structure is primarily attributed to two main factors, as listed below:
The partial double bond character because of the resonance occurs in the aromatic ring due to a conjugated electron pair of oxygen.
Hybridization is the carbon to which the oxygen atom of the hydroxyl group is. The carbon atom attached to the oxygen is sp2 hybridized in phenol.
Hence, the C-O bond length in phenol is slightly less than that of methanol.
The structure of phenol can be represented as below:
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Ether Structure
An ether molecule contains a tetrahedral structure.
Because of the repulsive interaction between the two bulky (–R) groups, the bond angle (R-O-R) is a bit greater than the tetrahedral angle.
The C–O bond length present in ether is almost similar to that as in alcohol.
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Nomenclature of Alcohol
Etymology
The term "alcohol" is introduced from an Arabic word, Arabic kohl, which is a powder used as an eyeliner. Al- is the definite article of Arabic that is equivalent to the in English. Alcohol was originally used for the fine powder, which is formed by the natural mineral stibnite sublimation to produce the antimony trisulfide Sb2S3.
It was also considered to be either as "spirit" or "essence" of this mineral. Moreover, it was used as an eyeliner, cosmetic, and antiseptic. Generally, the meaning of alcohol was extended to the distilled substances and then narrowed to ethanol, when a synonym, "spirits" was for hard liquor.
In the translation of John of Vigo, in 1543, Bartholomew Traheron introduces the word "barbarous" as a term used by authors for "fine powder." He wrote, "the barbarous authors use alcohol, or (sometimes I find it as written) alcohol, for the finest powder."
Lexicon Chymicum, in 1657, by William Johnson, glosses the word as "antimonium sive stibium." By extension, the same word had come to refer to any fluid come by distillation, including "alcohol of wine," the distilled essence of wine. Also, in 1594, Libavius in Alchymia referred to as, "vini alcohol vel vinum alcalisatum."
Ethanol was invented in 1892 by combining the word ethane with "-ol" and ending up with "alcohol".
FAQs on Alcohol Hydroxyl Group
1. What is the hydroxyl group in the context of alcohols?
In organic chemistry, the hydroxyl group is the functional group responsible for the characteristic properties of alcohols. It consists of an oxygen atom covalently bonded to a hydrogen atom (-OH). In an alcohol, this hydroxyl group is attached to a saturated, sp³-hybridized carbon atom of an alkyl or substituted alkyl chain.
2. How are alcohols classified based on the carbon atom attached to the hydroxyl (-OH) group?
Alcohols are classified into three main types based on the number of other carbon atoms attached to the carbon bearing the -OH group:
- Primary (1°) Alcohol: The carbon atom with the -OH group is attached to only one other alkyl group. Example: Ethanol (CH₃CH₂OH).
- Secondary (2°) Alcohol: The carbon atom with the -OH group is attached to two other alkyl groups. Example: Propan-2-ol (CH₃CH(OH)CH₃).
- Tertiary (3°) Alcohol: The carbon atom with the -OH group is attached to three other alkyl groups. Example: 2-methylpropan-2-ol ((CH₃)₃COH).
3. What is the main difference between the hydroxyl group in an alcohol and a phenol?
The primary difference lies in the type of carbon atom the hydroxyl group is attached to. In alcohols, the -OH group is bonded to an sp³-hybridized carbon of an alkyl group. In phenols, the -OH group is directly bonded to an sp²-hybridized carbon of an aromatic ring. This difference significantly impacts their chemical properties, making phenols more acidic than alcohols due to resonance stabilization of the phenoxide ion.
4. Why is the C-O-H bond angle in alcohols slightly smaller than the ideal tetrahedral angle of 109.5°?
Although the oxygen atom in the hydroxyl group is sp³ hybridized, the C-O-H bond angle is slightly compressed. This is due to the presence of two lone pairs of electrons on the oxygen atom. The repulsion between these lone pairs is stronger than the repulsion between the bonding pairs of electrons. This stronger lone pair-lone pair repulsion pushes the C-O and O-H bonds closer together, resulting in a bond angle slightly less than 109.5°, such as approximately 108.9° in methanol.
5. How does the hydroxyl group influence the physical properties of alcohols, like boiling point and solubility?
The polar hydroxyl group is responsible for two key physical properties of alcohols:
- Higher Boiling Points: The -OH group allows alcohol molecules to form strong intermolecular hydrogen bonds with each other. More energy is required to break these bonds, giving alcohols significantly higher boiling points than alkanes or ethers of comparable molecular mass.
- Water Solubility: The polar -OH group can also form hydrogen bonds with water molecules. This allows lower-molecular-weight alcohols (like methanol and ethanol) to be highly soluble in water. Solubility decreases as the nonpolar alkyl chain gets longer.
6. How can one distinguish between an alcohol's hydroxyl group (-OH) and a carboxylic acid's carboxyl group (-COOH)?
While both functional groups contain an -OH moiety, their structures and chemical reactivity are distinct. In an alcohol, the -OH is attached to a simple alkyl group's carbon. In a carboxylic acid, the hydroxyl group is part of a larger carboxyl group (-COOH), where its carbon atom is also double-bonded to another oxygen atom. This structure makes the proton of the -OH group in a carboxylic acid much more acidic and readily donated compared to the proton in an alcohol's hydroxyl group.
7. What are some important real-world applications of alcohols based on the hydroxyl group's role?
The reactivity and polarity of the hydroxyl group make alcohols versatile for many applications:
- Methanol: Used as an industrial solvent and a raw material for producing formaldehyde, which is used in making polymers.
- Ethanol: Widely used as a solvent in perfumes and medicines, as an antiseptic, in alcoholic beverages, and as a clean-burning fuel additive.
- Isopropyl Alcohol (Propan-2-ol): Commonly used as rubbing alcohol for disinfection.
- Glycerol (Propane-1,2,3-triol): A polyhydric alcohol used as a humectant (moisture-retaining agent) in cosmetics and food products.
