Citric acid is reported to be a highly efficient and eco-friendly catalyst for the Beckmann rearrangement under solvent free conditions.

Naturally occurring organic acids are reported to be highly efficient promoters of the Beckmann rearrangement. Citric, oxalic, tartaric, malic, succinic, malonic, and fumaric acids efficiently promote the Beckmann rearrangement under solvent-free conditions and thermal and microwave irradiation. Tartaric acid was found to be the best promoter of the Beckmann rearrangement under conventional conditions as well as under microwave irradiation. Compared with conventional heating, microwave irradiation provides higher reaction rate and slightly higher yields.

Molecular electrostatic potential and its topographical featuresFor a given molecule with N atoms located at R i with nuclear charges Z i , MESP at a point r denoted by V(r) in atomic units (a.u.) is defined equation given below where ρ(r 1 ) represents its electron density function.The rich topographical features of this three dimensional scalar field are brought out in terms of its critical points (CPs) i.e. points where first order partial derivatives of the equation vanish i.e.CPs are then characterized by nature of three eigenvalues {λi} obtained from corresponding Hessian matrix H (matrix of second order partial derivatives with elements Hij = a 2 v/x i ax j ) evaluated at the CPs.If at least one of the eigenvalues is zero then, that CP is said to be degenerate. A non-degenerate CP is found to exist when all eigenvalues are non-zero. Such a CP is represented as (R, σ) where rank (R) is the number of non-zero eigenvalues.Signature (σ) is derived from algebraic sum of the signs of eigenvalues. Rank of a non-degenerate CP of MESP scalar field is 3 and possible four types of CPs are: (3, +3), (3, -3) which represent local minimum, local maximum and (3, +1), (3, -1) are different types of saddles. These non-degenerate CPs bring out chemically significant features: a (3, +3) CP signifies presence of lone pairs or π bonds in molecules 1 -4 . Each bonded pair of atoms is reflected by presence of (3, -1) saddle. A (3, +1) CP is found to connect two minima and indicate presence of a ring or cage in the nuclear framework. The absence of non-nuclear maxima has been proven by Gadre and Pathak 5 .Supporting information / TACL 9(4) 2019 pp 453 -462

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