identification of primary alcohol

Because of their many applications and functions in our everyday lives, alcohols are the most well-known chemical molecules. Alcohols are chemical compounds that have a hydroxyl group linked to either aryl or an alkyl group (ROH). The hydroxyl carbon atom is only connected to one R group in a primary alcohol. It is defined as a secondary alcohol if it has two R groups and tertiary alcohol if it contains three R groups. Because of the existence of a benzene ring, alcohols, like many other chemical compounds, may be aromatic. Phenol is the most basic aromatic alcohol.

Primary alcohol examples

The carbon atom of the hydroxyl group (OH) is linked to just one alkyl group in primary alcohols. Methanol (propanol), ethanol, and other primary alcohols are primary alcohol examples.

Primary alcohol structure

The main feature of any alcohol has nothing to do with the intricacy of this alkyl chain. The presence of a single link between a –OH group and an alkyl group defines the primary alcohol.

Secondary alcohol

Secondary alcohols have the hydroxyl group’s carbon atom linked to two alkyl groups on either side. The two alkyl groups present may be structurally similar or dissimilar. The following are some instances of secondary alcohols.

Tertiary alcohol

Tertiary alcohols are made up of three alkyl groups and a hydroxyl group connected to a carbon atom. The structure of these alcohols has a considerable impact on their physical characteristics. Because of the presence of the -OH group, alcohols may establish hydrogen bonds with their neighbouring atoms. Alcohols have a higher boiling point than alkanes due to the weak hydrogen bonds created. 

Alcohol identification

There are many methods for determining primary, secondary, and tertiary alcohols. Inorganic chemistry, alcohol identification may be performed by evaluating the unique features of distinct alcohol kinds. A variety of approaches, including nuclear magnetic resonance (NMR) and qualitative testing, are used to analyse the devices. Alcohol may be detected similarly to ketones and aldehydes.

Primary Alcohol detection method

• Test using ferric chloride

One method for distinguishing aromatic and aliphatic alcohols is to use iron (III) chloride. The iron chloride component gives the solution a reddish-orange colour. When aromatic alcohol, such as phenol, is present, the chlorine atoms are replaced by the aromatic alcohol due to a change in the coordination behaviour of the core iron atom. As a consequence, the solution becomes purple. The solution keeps its brilliant red-orange colour because aliphatic alcohols do not react with iron (III) chloride.

• Jones test

Another approach for detecting the presence of alcohol is the Jones test, which uses chromium trioxide as an oxidising agent in the presence of sulphuric acid. A primary alcohol is first transformed to an aldehyde in the presence of Jones reagent. The primary alcohol is then turned into a carboxylic acid, while the secondary alcohol is converted to a ketone. In this test, the chromium oxidation state is essential. In Jones reagent, chromium is in the oxidation state +6. The presence of Cr(VI) complexes gives the reagent a vivid red and orange colour.

Chromium is chemically reduced from Cr (VI) to oxidation state +3 — Cr (III). To begin, chromate ester is formed when chromic acid and alcohol acid combine. When H2O, a base, cleaves the C-H alcohol bond, the carbonyl group is formed, converting Cr(VI) to Cr (IV). The Cr(IV) loses two electrons, whereas the alcohol’s carbon loses two electrons. As a result, the reduction-oxidation phase is known as this phase.

Furthermore, Cr(IV) participates in further oxidation processes before being reduced to Cr (III). Hexa aqua chromium (III) ions — [Cr(H2O)6]3+ — and Cr(III) complexes with one or more sulphate ions — [Cr(H2O)5(SO4)]+ — are the most prevalent forms of chromium (III) (III). Each of these complexes includes Cr(III), which gives them their distinctive green colour.

Because tertiary alcohols do not react with chromium, no precipitate occurs, preserving the solution’s orange colour. As a result, the Jones test may be used to detect whether primary, secondary, or tertiary alcohols are present.

The difference between elementary, secondary, and tertiary alcohol

Alcohols should have a single or a single carbon atom linked straight since they are made up of carbon(C) atoms attached to a hydroxyl (OH) group. These alcohols include a – CH2OH group, with the OH group often situated towards the terminal of the carbon chain. A two-carbon straight bond should be present in alcohols with a carbon atom and a hydroxyl (OH) group. These alcohols have a -CHOH group with an OH group in the middle of the carbon chain.

Direct linkages to the three carbon atoms should be present in alcohols with a carbon(C) atom bound to a hydroxyl (OH) group. The -COOH group is present in all of these alcohols, and the OH is often found at the branched carbon chain junction. To assess the degree of alcohol, count the number of carbon atoms that are directly connected to the carbon bound to the OH group.

Schiff reagent

Schiff’s reagent may be used to differentiate between primary and secondary alcohols. It is best recognised as a fuchsia dye that has been decolourised. It becomes bright pink in the presence of aldehyde, even in trace concentrations. It must, however, be used extremely cold since ketones react very slowly with it to produce the same colour. Furthermore, although heat causes a fast colour shift, the competing ketone reaction may be deceiving. While the reaction mixture is being warmed in a hot water bath, pass some of the Schiff’s reagent vapours through it. Thus primary alcohols on oxidation to carbonyl give this test but secondary alcohols dont.

Oxidation test

Sodium dichromate is used to oxidise alcohol. The oxidation rate of various alcohols varies, and the oxidation rate may be used to recognise and identify alcohols. Primary alcohols are normally quickly oxidised to form aldehyde, which is then further oxidised to form carboxylic acid, depending on the circumstances. Aldehyde is produced when an excessive quantity of alcohol is consumed, and it is rapidly distilled away. Alcohols are oxidised to ketones and aldehydes. When ethanol is oxidised, acetaldehyde is produced, which may be detected using Schiff’s test, which rapidly produces a pink colour.

Lucas test

It is one of the most important tests to distinguish between primary, secondary and tertiary alcohols. The reaction used is:

R-OH + HCl → R-Cl+ H20

The reaction takes place in the presence of ZnCl2.

The reaction is extremely fast for tertiary alcohols since they follow SN1 mechanism. It takes 3-4 minutes for secondary alcohols and does not occur for primary alcohols without heating. The change in solution is noticed by formation of two phases of liquids in the solution.

Conclusion

Alcohols are considered to be water derivatives because one of the hydrogen atoms has been replaced by an alkyl group, which is an organic structure commonly denoted by the letter R. This we have discovered. Additionally, several varieties of alcohol are available. 

Primary alcohols differ from other alcohols in that they have just one connection between the -OH and the alkyl groups. In secondary alcohol, two alkyl groups are joined together on each side of the carbon atom to form a ring.