Organic Reaction Mechanisms


The mechanism of a reaction is a pathway by which the atoms travel from their initial positions in the reactants to the final ones in the products; of all the possible pathways, the one followed by most of the molecules in a sample is that which requires the smallest rise in free energy along the way. Reactions are called stepwise if they involve intermediate species with a finite lifetime, or concerted if they do not.

Keywords: chemical kinetics; reactive intermediates; resonance; stereochemistry; transition states; tunnelling

Figure 1.

(a) The free energy profile of a concerted reaction. (b) The free energy profile of a stepwise reaction. The first maximum need not be the lowest one.

Figure 2.

(a) Quantum‐mechanical tunnelling into a wide barrier. (b) Tunnelling through a low and narrow barrier.

Figure 3.

(a) A Diels–Alder reaction occurring in concerted and stereospecific fashion as allowed by the symmetries of the highest occupied molecular orbital of the diene and the lowest unoccupied orbital of the dienophile. (b) The stepwise mechanism would produce a diradical, which presumably could undergo a bond rotation before closure to give a stereoisomeric product.

Figure 6.

The dotted curves show how the energy varies with the length of the C–X and C–Y bonds in Scheme 2; the solid curve is the energy profile for a concerted displacement of Y by X.

Figure 10.

Upon heating tetramethyllead, a lead deposit forms and ethane emerges; a new film forms and the old one vanishes when the flame is moved to the left, but not if it is moved far to the left, suggesting that the decomposition produces methyl, which quickly dimerizes.

Figure 18.

A concerted 1,2‐shift in a carbocation.

Figure 4.

Examples of pericyclic reactions. In order: electrocyclization, a cycloaddition and a sigmatropic shift.

Figure 5.

A concerted displacement reaction. The configuration of the carbon atom is inverted in the process.

Figure 7.

A stepwise displacement reaction. A tight ion‐pair functions as the intermediate.

Figure 8.

The triphenylmethyl radical. Two of the many possible resonance structures are shown.

Figure 9.

Tri‐β‐naphthyl. This radical is a stable substance that does not dimerize.

Figure 11.

The spin trapping process. A transient radical is converted into a detectable one.

Figure 12.

Competing, concerted and stepwise reactions. The [2+4] cycloadditions are stereospecific; the [2+2] process is not.

Figure 13.

Competing stepwise and concerted reactions. The [2+4] cycloadditions have more compact transition states.

Figure 14.

The Reimer–Tiemann reaction. Dichlorocarbene is an intermediate in this process.

Figure 15.

The trapping of an intermediate. Olefins react with dihalocarbenes in synthetically useful reactions.

Figure 16.

Benzyne as an intermediate. The scattering of a 14C label shows that this apparently simple nucleophilic displacement is not concerted.

Figure 17.

A typical rearrangement. Ionization followed by a 1,2‐shift and re‐neutralization produces an isomer of the starting compound.

Figure 19.

Competing views of the 2‐norbornyl cation. One of these held it to be a rapidly equilibrating pair, the other saw it as a single ‘nonclassical’ structure.

Figure 20.

Examples of carbanions and of a Grignard reagent. They are commonly used in synthetic chemistry.



Brown HC and Rei M‐H (1964) Comparison of the effect of substituents at the 2‐position of the norbornyl system with their effect in representative secondary and alicyclic derivatives. Evidence for the absence of nonclassical stabilization of the norbornyl cation. Journal of the American Chemical Society 86(22): 5008–5010.

Doering WvE and Hoffmann AK (1954) The addition of dichlorocarbene to olefins. Journal of the American Chemical Society 76(23): 6162–6165.

Gomberg M (1900) Triphenylmethyl, ein Fall von dreiwerthigem Kohlenstoff. Berichte der Deutschen Chemischen Gesellschaft 33(17): 3150–3163.

Gonikberg MG and Vereshchagin LF (1949) K voprosu o zavisimosti skorosti khimicheskikh reaktsii ot davleniya. Zhurnal Fizicheskoi Khimi 23(12): 1447–1448.

Hantzsch A (1921) Die Konstitution der Carboniumsalze. Berichte der Deutschen Chemischen Gesellschaft 54(10): 2573–2612.

Hine J (1950) Carbon dichloride as an intermediate in the basic hydrolysis of chloroform. A mechanism for substitution reactions at a saturated carbon atom. Journal of the American Chemical Society 72(6): 2438–2445.

Meerwein H and van Emster K (1922) Über die Gleichgewichts‐Isomerie Zwischen Bornylchlorid, Isobornylchlorid und Camphen‐chlorhydrat. Berichte der Deutschen Chemischen Gesellschaft 55(8): 2500–2528.

Myhre PC, Webb GG and Yannoni CS (1990) Magic angle spinning nuclear magnetic resonance near liquid helium temperatures. Variable‐temperature CP MAS spectra of the 2‐norbornyl cation to 6 K. Journal of the American Chemical Society 112(24): 8991–8992.

Paneth F and Hofeditz W (1929) Über die Darstellung von freiem Methyl. Berichte der Deutschen Chemischen Gesellschaft 62(5): 1335–1347.

Roberts JD, Simmons HE, Carlsmith LA and Vaughan CW (1953) Rearrangement in the reaction of chlorobenzene‐1‐C14 with potassium amide. Journal of the American Chemical Society 75(13): 3290–3291.

Seltzer S (1965) The mechanism of the Diels‐Alder reaction of 2‐methylfuran with maleic anhydride. Journal of the American Chemical Society 87(7): 1534–1540.

Shine HJ, Waters JA and Hoffman DM (1963) The decomposition of acetyl peroxide in solution. III. Kinetics and use of radical traps. Journal of the American Chemical Society 85(22): 3613–3621.

Sneen RA and Bradley WA (1972) Substitution at a saturated carbon atom. XIV. The case for discrete, distinct allylically related ion pairs. Journal of the American Chemical Society 94(20): 6975–6982.

Stewart CA (1972) Competing diradical and electrocyclic reactions. Difference in activation volumes. Journal of the American Chemical Society 94(2): 635–637.

Wheland R and Bartlett PD (1970) Simultaneous biradical 1,2 and concerted 1,4 cycloaddition of cis‐ and trans‐1,2‐dichloro‐1,2‐difluoroethylene to cyclopentadiene. Journal of the American Chemical Society 92(12): 3822–3824.

Whitmore FC (1932) The common basis of intramolecular rearrangements. Journal of the American Chemical Society 54(8): 3274–3283.

Winstein S and Trifan D (1952) Neighbouring carbon and hydrogen. XI. Solvolysis of exo‐norbornyl‐p‐bromobenzenesulfonate. Journal of the American Chemical Society 74(5): 1154–1160.

Wittig G (1942) Phenyl‐lithium, der Schlüssel einer neuen Chemie metallorganischer Verbindungen. Die Naturwissenschaften 30(46/47): 696–703.

Woodward RB and Hoffmann R (1969) The conservation of orbital symmetry. Angewandte Chemie, International Edition 8(11): 781–853.

Further Reading

Ingold CK (1969) Structure and Mechanism in Organic Chemistry, 2nd edn. Ithaca, NY: Cornell University Press.

le Noble WJ (1974) Highlights of Organic Chemistry. New York: Marcel Dekker.

March J (1992) Advanced Organic Chemistry, 4th edn. New York: Wiley‐Interscience.

Wheland GW (1960) Advanced Organic Chemistry, 3rd edn. New York: Wiley.

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le Noble, William J(Apr 2001) Organic Reaction Mechanisms. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0000614]