What Would Be the Molecularity of A Reaction Having Two Rate Determining Steps? A Conceptual Analysis

Sanjeev Rachuru^1, , Jagannadham Vandanapu^2, , David Geelan³, Adam A Skelton⁴, Malleli Murali⁵, Sateesh Kuna⁵ and J. Bhargavilakshmi⁵

¹Department of Chemistry, St. Ann’s College for Women, Mehdipatnam, Hyderabad, 500028, India

²Department of Chemistry, Osmania University, Hyderabad 500007, India

³School of Education, University of Notre Dame, Australia

⁴Department of Pharmacy, School of Health Science, University of KwaZulu–Natal, Durban 4000, South Africa

⁵Department of Chemistry, Geethanjali College of Engineering and Technology, Hyderabad, India

Cite this paper:
Sanjeev Rachuru, Jagannadham Vandanapu, David Geelan, Adam A Skelton, Malleli Murali, Sateesh Kuna and J. Bhargavilakshmi. What Would Be the Molecularity of A Reaction Having Two Rate Determining Steps? A Conceptual Analysis. World Journal of Chemical Education. 2025; 13(2):30-33. doi: 10.12691/wjce-13-2-1

Abstract

One of the conclusions of C Stegelmann’s, JACS article titled "Degree of Rate Control: How Much the Energies of Intermediates and Transition States Control Rates” is that the ‘degree of control of reaction’ is a concept similar to the concept of the ‘rate determining step’ but more direct to apply and much more widely applicable (since there is rarely a single rate determining step). Thus, an important inference from this JACS article is that a reaction having two or more rate determining steps in its multistep mechanism is in the realm of possibility. Can we write the molecularity of such reactions (that is a reaction having two or more rate determining steps)? The answer to this question and its implication is probed in this conceptual analysis. An important spin-off, of the answer to this question is that, the age-old classic substitution nucleophilic unimolecular i.e., S_N¹ reaction between tertiary butyl bromide and water being termed as ‘unimolecular’ in many textbooks and the entry in the Wikipedia is incorrect. The authors are of the opinion that this article should be discussed when S_N¹ reaction is taught at undergraduate level.

Keywords:
Molecularity Rate determining Step Elementary reaction & SN1 reaction

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