Chemistry · Alcohols, Phenols And Ethers · NEET
Both start from phenol (as sodium phenoxide) and both add a group at the ortho position. The difference is the reagent and the product. Kolbe uses CO2 (carbon dioxide) under pressure and adds a -COOH group, so the product is salicylic acid (2-hydroxybenzoic acid). Reimer-Tiemann uses CHCl3 (chloroform) with NaOH and adds a -CHO group, so the product is salicylaldehyde (2-hydroxybenzaldehyde). Simple split: Kolbe -> acid (-COOH), Reimer-Tiemann -> aldehyde (-CHO).
Plain phenol is not reactive enough. When phenol reacts with NaOH it forms the phenoxide ion (C6H5O-). The negative charge on oxygen pushes electron density into the benzene ring, making the ring much richer in electrons. This makes it easy for the ring to attack a weak electrophile like CO2 or dichlorocarbene. So the phenoxide step is what makes both reactions work. This matters for NEET because questions often show 'NaOH' as the first reagent for a reason.
The electrophile is dichlorocarbene, written :CCl2. It is made in two steps: CHCl3 + OH- gives -CCl3 (loses a proton), then -CCl3 loses Cl- to give :CCl2. This neutral carbene is electron-poor at carbon, so it attacks the electron-rich ortho carbon of the phenoxide ring. This exact point was asked directly in NEET 2018, so remember it: the electrophile is NOT a cation like +CHO, it is the carbene :CCl2.
In both reactions the -OH (as -O-) is an ortho/para director, so both ortho and para products can form. But the ortho product is the major one. In Kolbe, the sodium ion holds CO2 close to the -O- group, favouring the ortho carbon next to oxygen. So the main products are salicylic acid (Kolbe) and salicylaldehyde (Reimer-Tiemann), both ortho. NEET usually expects the ortho product as the answer.
Salicylic acid (2-hydroxybenzoic acid) is the product of Kolbe's reaction. When you react it with acetic anhydride you get aspirin (acetylsalicylic acid), a common painkiller. NEET does not ask you to draw aspirin, but linking Kolbe -> salicylic acid -> aspirin helps you remember which reaction gives the acid.
It is just another name for salicylaldehyde. 'Ortho' means the two groups are on neighbouring carbons. 'Hydroxy' is the -OH from phenol. 'Benzaldehyde' means a benzene ring with a -CHO group. So ortho-hydroxybenzaldehyde = a benzene ring carrying -OH and -CHO on adjacent carbons = salicylaldehyde.
In the reaction C6H5OH + CHCl3 + NaOH -> salicylaldehyde, the electrophile involved is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Salicylic acid (2-hydroxybenzoic acid). Sodium phenoxide reacts with CO2 under pressure, then acid work-up gives salicylic acid with the -COOH group at the ortho position.
Salicylaldehyde (2-hydroxybenzaldehyde). Sodium phenoxide reacts with CHCl3 and NaOH, and the -CHO group is added at the ortho position.
Kolbe's reaction: NaOH (to form phenoxide) then CO2 under pressure, then H+. Reimer-Tiemann reaction: CHCl3 (chloroform) with aqueous NaOH, then H+ hydrolysis.
Kolbe's reaction. It gives salicylic acid, and salicylic acid plus acetic anhydride gives aspirin. Reimer-Tiemann gives an aldehyde, not the acid needed for aspirin.
Both can form, but the ortho product is major in both reactions. So the standard NEET answers are salicylic acid and salicylaldehyde, both ortho-substituted.
