Alcohol Structure Hydroxyl Group: Structure, Nomenclature, Physical Properties, FAQs

The hydroxyl group (-OH ) is substituted for the hydrogen atom of aliphatic carbon in a particular class of organic molecules known as alcohols. So an alcohol molecule has two components, one with an alkyl group and the other with a hydroxyl group. R-OH , where R is the alkyl group, is used to represent it. Alkanes and alcohols have the same basic chemical formula, but alcohols have a functional group called the hydroxyl group, which has the chemical formula -OH . The most widely used alcohol is ethanol, which is also used as fuel (gasoline), a preservative for biological samples, a solvent for paints, and used in medicine.

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This Story also Contains

1. Structure Of Alcohol
2. Nomenclature Of Alcohol
3. Physical Properties Of Alcohol

The name -ol is added to the parent chain of the alkane name when naming alcohols. The name denotes the location of the functional group -OH . The parent chain is numbered at the end closest to the -OH .

The homologous series of alcohols have the general formula C_{n}H_{2n+1}OH, where n = 1, 2, 3.

Structure Of Alcohol

Alcohol may be seen as having a tetrahedral oxygen atom that is sp3 hybridised and has nonbonding pairs of electrons filling two of the four sp3 hybrid orbitals and there 2 sigma bonds one between Carbon and oxygen, and one between oxygen and hydrogen, similar to how water has four sp3 orbitals. However, because alkyl groups are often heavier than hydrogen atoms, the R-O-H bond angle in alcohols is typically greater than the 104.5° H-O-H bond angle in a water molecule. The 108.9° bond angle in methanol, for instance, demonstrates the impact of the methyl group, which is heavier than the hydrogen atom in water.

The carbon atom that is connected to the hydroxyl group in alcohol can be one method to classify them. The molecule is a primary alcohol if this carbon is primary (1°, bonded to just one other carbon atom). A secondary (2°) carbon atom that is linked to two other carbon atoms and possesses the hydroxyl group is what makes an alcohol secondary. In a similar way, tertiary alcohol has the hydroxyl group on a tertiary (3°) carbon atom that is linked to three other carbons. If the hydroxyl group is connected to an allylic carbon atom (next to a C=C double bond) or a benzylic carbon atom (next to a benzene ring), then the alcohol is said to be either allylic or benzylic alcohol.

Nomenclature Of Alcohol

Alcohols has formal and informal names, much like other kinds of organic molecules. The International Union of Pure and Applied Chemistry (IUPAC) meeting that took place in Paris in 1957 resulted in the adoption of the system that is the most broadly applicable. According to the IUPAC method, the name of alcohol incorporates the -ol suffix with the name of the parent alkane and a number to indicate where the hydroxyl group is located. The guidelines are broken down into three steps:

• Give the length of the longest carbon chain that includes the carbon atom with the OH group. The name of the alkane is modified by dropping the last -e and adding -ol.

• The longest carbon chain should be numbered, beginning at the end that is closest to the OH group. If required, use the appropriate number to indicate where the OH group is located.

• Give the substituents' names and numbers in the same manner as for an alkane or alkene.

Physical Properties Of Alcohol

• At room temperature, the majority of common alcohols are colourless liquids. Alcohols that flow freely and have a pleasant odour include methyl alcohol, ethyl alcohol, and isopropyl alcohol. The higher alcohols—those with four to ten carbon atoms—are a little oily or thick and have stronger fruity odours.

• Despite having comparable molecular weights, alcohols have substantially higher boiling points than alkanes. For instance, the boiling point of ethanol, which has a molecular weight of 46, is 78 °C, whereas the boiling point of propane, which has a molecular weight of 44, is 42 °C. The fact that their boiling temperatures differ so drastically shows that ethanol molecules are attracted to one another considerably more strongly than propane molecules. This difference is mostly caused by the intermolecular hydrogen bonds that ethanol and other alcohols are capable of forming.

• Small alcohols are entirely soluble in water; combining the two results in a single solution. But as the alcohol's hydrocarbon chain length increases, its solubility decreases. The reduction in solubility becomes apparent at four carbon atoms and above; mixing the two might result in the formation of a two-layered solution in a test tube.