{"id":10073,"date":"2013-04-02T15:20:56","date_gmt":"2013-04-02T14:20:56","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=10073"},"modified":"2013-04-02T15:49:16","modified_gmt":"2013-04-02T14:49:16","slug":"the-mechanism-of-ester-hydrolysis-via-alkyl-oxygen-cleavage-under-a-quantum-microscope","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10073","title":{"rendered":"The mechanism of ester hydrolysis via alkyl oxygen cleavage under a quantum microscope"},"content":{"rendered":"
\n

My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond<\/span><\/a>). There is a much rarer[1]<\/a><\/span> alternative: alkyl-oxygen cleavage (green bond<\/span>) which I now place under the microscope.<\/p>\n

\"alkyl-ester\"<\/p>\n

Here, guanidine is used as a general acid\/base, which results in a reasonable activation barrier for the hydrolysis (using pure water as the catalyst led to high barriers). What I will call the classical stepwise route is shown above, with charge-separated structures in abundance (particularly at the allyl group, where the possibility of forming a carbocation at this centre is central to the mechanism). My philosophy here is to allow quantum mechanics to decide whether to separate charge or not (in effect, only it is allowed decisions about where electrons are). So one can start with a concerted mechanism in which no formal charges are separated, and by subjecting them to wB97XD\/6-311G(d,p)\/SCRF=water calculation, decide where and if charges develop.<\/p>\n

There are two distinct possibilities; hydrolysis with either retention or inversion of configuration at the alkyl group. The results for the transition states are shown below, with the analogous energy for acyl-oxygen cleavage shown for comparison.<\/p>\n\n\n\n\n\n\n\n\n
Relative energies for hydrolysis of Alkyl acetate<\/th>\n<\/tr>\n
R<\/td>\nAcyl-oxygen<\/td>\nAlkyl O,inversion<\/td>\nAlkyl-O,retention<\/td>\n<\/tr>\n
all H<\/td>\n0.0 <\/a><\/td>\n15.3<\/a><\/td>\n42.5<\/a><\/td>\n<\/tr>\n
Me<\/td>\n0.0 <\/a><\/td>\n16.6<\/a><\/td>\n35.0<\/a><\/td>\n<\/tr>\n
Me,Me<\/td>\n0.0 <\/a><\/td>\n16.3<\/a><\/td>\n18.2<\/a><\/td>\n<\/tr>\n
Me,Me,Me<\/td>\n0.0 <\/a><\/td>\n16.4 (14.4)<\/a><\/td>\n\u00a0?<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

For R1=R2=R3=H and R1=Me,R2=R3=H proceeding with retention of configuration. T<\/strong>he IRCs are as below, which reveal a “hidden intermediate” feature (visible as a dip in the gradient norm), which corresponds to a charge-separated zwitterionic intermediate immediately preceding the proton transfer. In other words, the non-charge-separated cyclic\/concerted mechanism shown above is “interrupted” by charge separation in a hidden way during<\/em><\/strong>, and in an explicit way at<\/em><\/strong> the final stage, preferring finally to form the ionic ion-pair rather than neutral acetic acid and guanidine.<\/p>\n\n\n\n\n\n
\"alkylg\"[2]<\/a><\/span><\/td>\n\"alkylg\"<\/td>\n<\/tr>\n
\"alkylMe\"[3]<\/a><\/span><\/td>\n\"alkylMe\"<\/td>\n<\/tr>\n
\"alkylMeG\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

For R1=R2=Me, R3=H, we have a change. The C-O bond lengths at the solvolysing methyl increase as the substitution at this carbon increases, e.g. 2.2\u00c5 (R=H) \u2192 2.4\u00c5 (R1=Me) as the transition state becomes more carbocation like. With increasing carbocationic character, the acidity of the adjacent C-H group increases, until with\u00a0R1=R2=Me, R3=H it has become acidic enough to be abstracted by any close-by base (in this instance, guanidine). Experimentally, the aqueous hydrolysis of t-butyl acetate is known to proceed with alkyl-oxygen cleavage[1]<\/a><\/span>. In the computational model, the solvolysis mechanism has been intercepted by an elimination mechanism: the two potential surfaces under these circumstances are very close and they merge to ensure a different outcome of the reaction. You can see this effect below;<\/p>\n

\"alkylG-Me2\"

Click for 3D.<\/p><\/div>\n

The reaction barrier also drops as the degree of substitution at the migrating carbon increases. At time of writing, no TS had been located for R1=R2=R3 (? in table above) but as you can see the trend could easily take it below the energy for acyl oxygen hydrolysis.<\/p>\n

A much lower energy route however is apparently available for the alkyl-oxygen solvolysis route. For R1=R2=R3=H, it proceeds much more favourably with inversion of configuration, an intramolecular Sn2 solvolysis in fact.<\/p>\n\n\n\n\n
\"alkylg-inva\"<\/td>\n\"alkylg-inva\"<\/td>\n<\/tr>\n
\"alkylg-invg\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

That for R1=R2=R3 shows a qualitative difference, in resembling the mechanism for Sn1 solvolysis of t-butyl chloride <\/a>in water. In this case the bond O-C bond labelled 2.3 is cleaving, whilst the C-O bond labelled 3.1 has not yet started to form; an apparently classical Sn1 solvolysis. But take a look at the two atoms labelled [1] and [2]; this C-H bond is also set up to be abstracted by an adjacent base (the carboxylate), and indeed an IRC shows the formation of butene (not solvolysis) to be the final outcome.\u00a0<\/p>\n

\"Click

Click for 3D.<\/p><\/div>\n

Unlike the mechanism involving retention of configuration, the barrier for the inversion route does not change much as the substitution at the carbon increases, remaining above the acyl-oxygen solvolysis for even the t-butyl ester (R1=R2=R3=Me).\u00a0<\/p>\n

To summarise what we might have learnt. Firstly, the mechanism of the apparently simple hydrolysis of alkyl esters of ethanoic (acetic) acid suddenly got much more complicated. It might seem that solvolysis of the O-alkyl bond can proceed with either inversion or retention of configuration at the alkyl carbon; if the latter then the barrier seems to decrease as the stabilisation of the carbocation at this carbon increases. But for both retention and inversion, the mechanistic pathway can easily be subverted by a different reaction involving the formation of an alkene.<\/p>\n

One starts to suspect that the model I am using here to study this reaction may be either the wrong kind, or certainly incomplete. In the absence of any explicit water (merely a continuum model acting on its behalf), it seems more basic molecules bound in by hydrogen bonds (guanidine or carboxylate) can take over by acting as bases and abstracting hydrogens from a H-C bond adjacent to the carbocationic centre. In order to redirect the mechanism onto the solvolysis pathway, one probably needs to have a few more explicit water molecules hanging around (so to speak) so as to quickly intercept the forming carbocation, before it can release its proton to the base. In other words, one needs to set up a more statistical model, in which the probability of the desired outcome is in part determined by the probability of having a favourable molecule adjacent to the reacting centre. Who would have thought such a basic prototype for organic chemistry could be so tricky to pin down in a computational model!\u00a0<\/p>\n

References<\/h2>\n
  1. C.A. Bunton, and J.L. Wood, \"Tracer studies on ester hydrolysis. Part II. The acid hydrolysis of tert.-butyl acetate\", Journal of the Chemical Society (Resumed)<\/i>, pp. 1522, 1955. https:\/\/doi.org\/10.1039\/jr9550001522<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C4H13N3O3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.663603<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for C5H15N3O3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.663619<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond). There is a much rarer alternative: alkyl-oxygen cleavage (green bond) which I now place under the microscope. Here, guanidine is used as a general acid\/base, which results in a reasonable activation barrier for the hydrolysis (using pure water […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[],"tags":[284,1037,24,1036,843,373],"ppma_author":[2661],"class_list":["post-10073","post","type-post","status-publish","format-standard","hentry","tag-acetic-acid","tag-analogous-energy","tag-energy","tag-lower-energy-route","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nThe mechanism of ester hydrolysis via alkyl oxygen cleavage under a quantum microscope - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10073\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The mechanism of ester hydrolysis via alkyl oxygen cleavage under a quantum microscope - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond). 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There is a much rarer alternative: alkyl-oxygen cleavage (green bond) which I now place under the microscope. 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To get a good grade, one might have to reproduce something like the below. Here, I subject that answer to a reality check. In this scheme, HA is a general acid, R=Me, and the net result\u2026","rel":"","context":"In \"ALSO\"","block_context":{"text":"ALSO","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=also"},"img":{"alt_text":"acyl-ester","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/acyl-ester.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":23522,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23522","url_meta":{"origin":10073,"position":1},"title":"A computational mechanism for the aqueous hydrolysis of a ketal to a ketone and alcohol.","author":"Henry Rzepa","date":"April 1, 2021","format":false,"excerpt":"The previous post was about an insecticide and made a point that the persistence of both insecticides and herbicides is an important aspect of their environmental properties. Water hydrolysis will degrade them, a typical residency time being in the order of a few days. I noted in passing a dioxepin-based\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/03\/R-1024x699.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":6618,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6618","url_meta":{"origin":10073,"position":2},"title":"The mechanism of the Baeyer-Villiger rearrangement.","author":"Henry Rzepa","date":"May 7, 2012","format":false,"excerpt":"The Baeyer-Villiger rearrangement was named after its discoverers, who in\u00a01899\u00a0described the transformation of menthone into the corresponding lactone using Caro's acid (peroxysulfuric acid). The mechanism is described in all text books of organic chemistry as involving an alkyl migration. Here I take a look at the scheme described by\u00a0Alvarez-Idaboy, Reyes\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/05\/bv1.svg","width":350,"height":200},"classes":[]},{"id":14823,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14823","url_meta":{"origin":10073,"position":3},"title":"The roles of water in the hydrolysis of an acetal.","author":"Henry Rzepa","date":"November 18, 2015","format":false,"excerpt":"In the previous post, I pondered how a substituent (X below) might act to slow down the hydrolysis of an acetal. Here I extend that by probing\u00a0the role of water molecules\u00a0in the mechanism of acetal hydrolysis. Water molecules can participate in three ways: One water acts as a nucleophile to\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":14740,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14740","url_meta":{"origin":10073,"position":4},"title":"How to stop (some) acetals hydrolysing.","author":"Henry Rzepa","date":"November 12, 2015","format":false,"excerpt":"Derek Lowe has a recent post entitled \"Another Funny-Looking Structure Comes Through\". He cites a recent medchem article in which the following acetal sub-structure appears in a promising drug candidate (blue component below). His point is that orally taken drugs have to survive acid (green below) encountered in the stomach,\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":23562,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23562","url_meta":{"origin":10073,"position":5},"title":"Dimethyl ketal hydrolysis catalysed by hydroxide and hydronium ions","author":"Henry Rzepa","date":"April 7, 2021","format":false,"excerpt":"In the preceding post, I looked at a computed mechanism for the hydrolysis of a ketal by water. Of course, pure water consists of three potential catalysts, water itself or [H2O], and the products of autoionisation, [OH-] and\u00a0[H3O+]. The latter are in much smaller concentration, equivalent to a penalty of\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/04\/ketyl-hydroxyl.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10073","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=10073"}],"version-history":[{"count":57,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10073\/revisions"}],"predecessor-version":[{"id":10143,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10073\/revisions\/10143"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10073"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10073"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10073"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=10073"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}