Difference Between Hydroxyl and Alcohol

Main Difference – Hydroxyl vs Alcohol

Organic compounds are molecules that are essentially composed of C and H atoms. There can be other atoms such as O and N bonded to these molecules. Alcohol is such a molecule made out of C, H and O atoms. The characteristic feature of alcohols is the presence of a hydroxyl group (-OH) attached to an alkyl group. A functional group is a group of atoms that decide the properties and reactions of a particular molecule. Hydroxyl group is a functional group present in organic and inorganic compounds such as alcohols. The acidity or basicity of the molecule can vary depending on the location of the hydroxyl group in the molecule. But according to the IUPAC nomenclature (the method of naming compounds introduced by the International Union of Pure and Applied Chemistry – IUPAC), the term hydroxyl stands for the hydroxyl radical. However, we still use this term for both hydroxyl group and hydroxyl radical. The main difference between hydroxyl and alcohol is that a hydroxyl is a functional group whereas alcohol is an organic compound.

Key Areas Covered

1. What is Hydroxyl
      – Definition, Formation, and Properties
2. What is Alcohol
      – Definition, Properties, and Reactions
3. What is the Relationship Between Hydroxyl and Alcohol
      – Hydroxyl and Alcohol
4. What is the Difference Between Hydroxyl and Alcohol
      – Comparison of Key Differences

Key Terms: Alcohol, Hydroxyl, Hydroxyl Group, Hydroxyl Radical

What is Hydroxyl

According to the IUPAC nomenclature, the term hydroxyl is used to name ^.OH radical. But in common, we call every –OH group a hydroxyl group. Hydroxyl group is a functional group that can be found in both organic and inorganic compounds. The chemical formula is given as –OH. Therefore, hydroxyl group is composed of one hydrogen and one oxygen atom. Sometimes, the presence of a hydroxyl group indicates the acidity of a molecule; other times it indicates the basicity of a molecule.

The hydroxyl radical is highly reactive and can undergo chemical reactions quickly. Hydroxyl radical is the neutral form of the hydroxide ion. This hydroxyl radical has an unpaired electron that causes the high reactivity of the radical.

Figure 1: Reaction between Benzene and Hydroxyl Radical

In organic compounds such as alcohols and carboxylic acids, this –OH group can be found. When the hydroxyl group is in an alcohol, it is called alcohol group. When the hydroxyl group is found in a carboxylic acid, it is called carboxyl group. In these compounds, the hydroxyl group is bonded to the rest of the molecule through a covalent bond. In alcohols, the hydroxyl group decides the properties of the alcohol.

In inorganic compounds, the hydroxyl group can be found bonded to the rest of the molecule either through a covalent bond or an ionic bond. In molecules such as H₂O, H₂SO₄, etc. the hydroxyl group is bonded through a covalent bond. In compounds such as NaOH and other metal hydroxides, the hydroxyl group is bonded through an ionic bond.

What is Alcohol

Alcohol is an organic compound composed of a hydroxyl group attached to an alkyl group. The functional group of an alcohol is the hydroxyl group. An alcohol is composed of C, H and O atoms. According to the structure of alcohol, there are three major types of alcohols as follows.

Primary Alcohols

Primary alcohols are composed of a carbon atom that is attached to a hydroxyl group, two hydrogen atoms, and a single alkyl group.

Secondary Alcohols

Secondary alcohols are composed of a carbon atom that is bonded to a hydroxyl group, one hydrogen atom, and two alkyl groups.

Tertiary Alcohols

Tertiary alcohols are composed of a carbon atom that is bonded to a hydroxyl group and three alkyl groups.

Figure 02: Types of Alcohols

Alcohols are polar compounds due to the presence of the hydroxyl group. Therefore, they can be dissolved in polar solvents. Alcohols are also capable of forming hydrogen bonds. The presence of these hydrogen bonds causes the boiling point of alcohols to be increased than the corresponding alkane. Some examples for alcohols are methanol, ethanol, and butanol.

Alcohols are found in beverages as either the major component or the minor component. Apart from that, alcohols are included in some medicines. Alcohols having more than one hydroxyl group are named as polyols.

Relationship Between Hydroxyl and Alcohol

Alcohols are composed of hydroxyl groups. These hydroxyl group act as the functional group of alcohols. Therefore, the hydroxyl group determines the chemical and physical properties of alcohols. Given below are some examples that describe the properties of alcohols that arise due to the presence of hydroxyl group.

Alcohols are polar compounds due to the presence of hydroxyl group.
Alcohols dissolve in polar solvents.
Alcohols are able to form hydrogen bonds.
Alcohols can be dissolved in water.
Alcohols have higher boiling points than the corresponding alkanes due to the presence of these hydrogen bonds.

Definition

Hydroxyl: Hydroxyl is the IUPAC name used to name ^.OH radical.

Alcohol: Alcohol is an organic compound composed of a hydroxyl group attached to an alkyl group.

Nature

Hydroxyl: Hydroxyl group is a functional group.

Alcohol: Alcohol is an organic compound.

Atoms

Hydroxyl: Hydroxyl group is composed of H and O atoms.

Alcohol: Alcohols are composed of C, H and O atoms.

Reactivity

Hydroxyl: Hydroxyl radical is highly reactive.

Alcohol: Alcohols are less reactive compared to hydroxyl radical.

Conclusion

Although we use the term hydroxyl to name the hydroxyl group, it is the name for hydroxyl radical according to the IUPAC nomenclature. However, it is still the common name used to name a hydroxyl group. Alcohols are organic compounds that are composed of hydroxyl groups. The main difference between hydroxyl and alcohol is that hydroxyl is a functional group whereas alcohol is an organic compound.

References:

1.“Alcohol.” Encyclopædia Britannica, Encyclopædia Britannica, inc., Available here. Accessed 18 Aug. 2017.
2.“Hydroxyl Group: Definition & Structure.” Study.com, Study.com, Available here. Accessed 18 Aug. 2017.

Image Courtesy:

1. “Benzene hydroxyl reaction” By DMacks (talk) – Own work (Public Domain) via Commons Wikimedia
2. “1 2 3alcohols” By Chris Evans – D:\My Webs\1_2_3alcohols.htm (CC0) via Commons Wikimedia