# If SOCl2 reacts with alcohols via SNi, why doesn't POCl3?

A bit of a follow-up to Why is thionyl chloride preferred for preparing alkyl chlorides from alcohols?.

The reaction with $\ce{SOCl2}$ is also used instead of $\ce{PCl3}$ and $\ce{PCl5}$ when retention of stereochemistry is required. Phosphorus trichloride and phosphorus pentachloride both lead to inversion.

The mechanism proceeds via a $\mathrm{SN_i}$ pathway, or internal nucleophilic substitution. This step has been highlighed in red; an intimate ion pair$^{[\text{see below}]}$ is given in square brackets. Adding a nucleophilic solvent such as pyridine considerably increases inversion with $\ce{SOCl2}$ via an attack at the sulfur atom. Green marks blocking of the intramolecular substitution; blue is for inversion.

• As far as I know, $\ce{POCl3}$ has not been seen following an $\mathrm{SN_i}$ mechanism (even without pyridine). Why?

If possible, please provide a qualitative mechanistic reason. Something that is readily explained by waving hands. Regardless of whether such an approach is achievable, quantitative (calculational) answers are also most welcome.

References discussing intimate ion pair formation

• F. A. Carey, R. J. Sundberg. $(2007)$. Advanced Organic Chemistry Part A: Structure and Mechanisms, $4$th edition, pp 269$-$276. ISBN: 0-306-46242-7
• W. A. Hughes, E. D. Cowdrey, C. K. Ingold, S. Masterman, A. D. Scott. 'The Mechanism of Elimination Reactions. Part 1. Unimolecular Olefin Formation from Alkyl Halides in Bulphur Dioxide and Formic Acid'. Journal of the Chemical Society, $(1937)$, 1271$-$1277. DOI: 10.1039/JR9370001271
• E. S. Lewis, C. E. Boozer. 'The Kinetics and Stereochemistry of the Decomposition of Secondary Alkyl Chlorosulfites'. Journal of the American Chemical Society, $(1952)$, 74, 308$-$311. DOI: 10.1021/ja01122a005
• D. J. Cram. 'Studies in Stereochemistry. XVI. Ionic Intermediates in the Decomposition of Certain Alkyl Chlorosulfites'. Journal of the American Chemical Society, $(1953)$, 75, 332$-$338. DOI: 10.1021/ja01098a024
• C. C. Lee, A. J. Finlayson. 'Rearrangement In The Reaction Between Thionyl Chloride And $3$-Methyl-$2$-Butanol. Canadian Journal of Chemistry, $(1961)$, 39(1): 260$-$261. DOI: 10.1139/v61-030
• C. C. Lee, J. W. Clayton, D. G. Lee, A. J. Finlayson. 'Rearrangement Studies With $\ce{^14C-XIII}$: The Thermal Decomposition Of $1$-$\ce{^14C}$-$2$-Butyl Chlorosulfite'. Tetrahedron, $(1962)$, 18 1395$-$1402. DOI: 10.1021/ja01098a024
• H. Patin, G. Mignani, C. Mahe, J-Y. Le Marouille, A. Benoit, D. Grandjean. 'Ferrocenyltrithiocarbonates: I. Direct access from α-ferrocenylcarbinols by a $\mathrm{SN_i}$ mechanism. Absolute x-ray structure determination of (R)-ferrocenylmethylmethane S-methyl-trithiocarbonate'. Journal of Organometallic Chemistry, $(1980)$, 193, 1, 93$-$103. DOI: 10.1016/S0022-328X(00)86079-9
• J. L. Kice, G. C. Hanson. 'Mechanisms of SNi reactions. Effect of aralkyl group structure on ion-pair return in the decomposition of aralkyl thiocarbonates'. Journal of the American Chemical Society, $(1973)$, 38 (7), 1410$-$1415. DOI: 10.1021/jo00947a037
• M. B. Smith. $(2013)$. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, $7$th edition, pp 311, 486$-$487, 490, 598, 1316. ISBN: 978-0-470-46259-1
• The $\mathrm{SN_i}$ (substitution nucleophilic internal) mechanism: retention of configuration. Powerpoint presentation, Università degli Studi di Napoli Federico II.
• James. $\ce{SOCl2}$ and the $\mathrm{SN_i}$ Mechanism. Master Organic Chemistry, Alcohols. webpage
• Hopefully, the subscript letter $\mathrm{i}$ in $\mathrm{SN_i}$ is clearly visible, and so is not to be confused with $\mathrm{SN_1}$. (I for one initially thought it was a typo when I learned about it in December.) – Linear Christmas Feb 22 '17 at 16:12
• I don’t like the mechanisms as drawn due to the $\ce{S=O}$ double bonds which should be $\ce{\overset{+}{S}-\overset{-}{O}}$, but still $+1$ for a good question. – Jan Feb 22 '17 at 23:29
• @Jan I'll see what I can do about it after school tomorrow ;) – Linear Christmas Feb 22 '17 at 23:34
• @Jan I have updated the mechanism to include more delocalisation and true ionic bond character (both with dashed lines). Sadly, this has made the mechanism a bit harder to follow but is still alright. If there's anything amiss, feel free to ping me. – Linear Christmas Feb 23 '17 at 16:39
• How is the reaction using thionyl chloride providing the product from retention? The chloride is attacking the activated alcohol from the back side, correct? Whether it's intramolecular or intermolecular is irrelevant. – jerepierre Feb 23 '17 at 19:04

• There's just no way for the intramolecular $\mathrm{S}_{N}2$ reaction as written to take place, from a stereoelectronic perspective. There's probably some give on how tight the ion pair has to be though it has to be tight enough to explain retention. But we're in a gray area where mechanistic probes are probably going to be quite difficult. – Zhe Feb 23 '17 at 22:52
• It's still probably not one-step. The proposed transition state for a front side $\mathrm{S}_{N}2$ reaction seems unlikely. Therefore, I would think that we have (at least) two steps though the barriers would be quite small... – Zhe Feb 24 '17 at 17:00