Difference Between Primary and Secondary Alcohol

Main Difference – Primary vs Secondary Alcohol

Monohydric alcohols are classified into three types: primary alcohols, secondary alcohols, and tertiary alcohols. This article focuses on the difference between primary and secondary alcohol. The main difference between primary and secondary alcohols is that the hydroxyl group of a primary alcohol is attached to a primary carbon, whereas the hydroxyl group of a secondary alcohol is attached to a secondary carbon atom. There are various tests to distinguish between primary and secondary alcohol. Such methods include oxidation test, reaction with hot reduced copper, Lucas test, and Victor Meyer’s method.

This article examines,

1. Classification of Alcohols
2. What is Primary Alcohol
      – Characteristics, Structure, Properties
3. What is Secondary Alcohol
      – Characteristics, Structure, Properties
4. What is the difference between Primary and Secondary Alcohol

Classification of Alcohols

Alcohols are a type of hydrocarbon in which the hydroxyl (-OH) functional group is bonded to a sp³ hybridized carbon of an alkyl group. As per the definition of alcohols, the hydroxyl group must be bonded to a carbon atom of the main chain, which is indicated by the position number. In general, the simple alcohols are named by their alkyl group, followed by the word alcohol; for example, ethyl alcohol. These hydrocarbons are polar compounds due to the presence of C-O and O-H bonds. Because of the unshared electrons on the oxygen atom, alcohols can form oxonium ion (ROH₂⁺) by reacting with a strong acid. Hence, alcohols are known as weak Lewis bases. An alcohol can form hydrogen bonds with water and another alcohol molecule. Alcohols are weak acids with 16 to 18 pK_a values. Alcohols react with halogen acids such as HCl, HBr, and HI and convert into alkyl halides. Monohydric alcohols consist of one hydroxyl group, e.g., CH₃OH, C₂H₅OH, etc. Alcohols with two hydroxyl groups are called dihydric alcohols, e.g., CH₂OH-CH₂OH (ethylene glycol). Based on the type of carbon atom to which the hydroxyl group is attached, monohydric alcohols are classified into three types, namely; primary alcohols, secondary alcohols, and tertiary alcohols.

What is Primary Alcohol?

Primary alcohols are the alcohols in which hydroxyl group is attached to the primary carbon of the alcohol molecule. This is the simplest form of alcohol. Primary alcohols can be oxidized to form aldehydes (RCHO) under mild conditions. However, when stronger oxidants are used, primary alcohols first transform into aldehydes and then rapidly transform into carboxylic acids (RCOOH). The reactivity of primary alcohol is less than that of a secondary alcohol. In Lucas test, primary alcohols do not form oily layers, unlike secondary and tertiary alcohols. However, they form volatile alkyl halides upon heating. Examples of primary alcohol include methanol, ethanol, propanol, etc.

Figure: Methanol, an example of primary alcohol.

What is Secondary Alcohol?

Secondary alcohols are the alcohols in which hydroxyl group is attached to the secondary carbon atom of the alcohol molecule. Secondary alcohols are converted into ketones (R₂CO) upon oxidation under mild condition. Stronger oxidants, however, lead to the formation of acids or mixture of acids. With Lucas test, secondary alcohols form oily layers within about five minutes. Secondary alcohols are more stable and more reactive than primary alcohols. However, secondary alcohols are less acidic than primary alcohols. Some examples of secondary alcohol include isopropyl alcohol and butyl alcohol.

Figure 02: Secondary Alcohol

Difference Between Primary and Secondary Alcohol

Definition

Primary Alcohols: Primary alcohol is an alcohol which has the hydroxyl group connected to a primary carbon atom.

Secondary Alcohols: Secondary alcohol is an alcohol which has the hydroxyl group on a secondary carbon atom.

Reactivity

Primary Alcohols: Primary alcohols are less reactive than secondary alcohols.

Secondary Alcohols: Secondary alcohols are more reactive than primary alcohols.

Esterification

Primary Alcohols: Rate of esterification is highest in primary alcohols.

Secondary Alcohols: Rate of esterification is less in secondary alcohols.

Easiness of Dehydration

Primary Alcohols: Primary alcohols are difficult to dehydrate than secondary alcohols.

Secondary Alcohols: Secondary alcohols are easier to dehydrate.

Dehydrogenation

Primary Alcohols: Primary alcohols result in aldehydes and hydrogen.

Secondary Alcohols: Secondary alcohols results in ketones and hydrogen.

Lucas Test

Primary Alcohols: Primary alcohols do not form oily layers.

Secondary Alcohols: Primary alcohols form oily layers within about five minutes.

Stability

Primary Alcohols: Primary alcohols are less stable than secondary alcohols.

Secondary Alcohols: Secondary alcohols are more stable.

Victor Meyer’s Test

Primary Alcohols:  Primary alcohols give a blood red color.

Secondary Alcohols: Secondary alcohols give a blue color.

Acidity

Primary Alcohols: Primary alcohols are more acidic than secondary alcohols.

Secondary Alcohols: Secondary alcohols are less acidic.

Summary

Primary and secondary alcohols are two types of alcohols coming under monohydric alcohols. In primary alcohols, -C-OH group is attached one carbon atom, whereas, in secondary alcohols, -C-OH group is attached to another two carbon atoms. Based on this main difference between primary and secondary alcohols, their properties vary. Lucas test and Victor Meyer’s test are used to identifying primary and secondary alcohols.

Reference:
1.Johnson, A. W. (1999). Invitation to organic chemistry. Jones & Bartlett Learning.
2.Madan, G. (2005). S. Chands Success Guide (Q&A) Inorganic Chemistry. S. Chand Publishing.
3.Competition Science Vision, 2003, Vol.5, Pratiyogita Darpan Group, India

Image Courtesy:
1. “Methanol flat structure” By Cacycle – Own work (Public Domain) via Commons Wikimedia
2. “Secondary alcohol” By Skend – Own work (Public Domain) via Commons Wikimedia