{"id":6794,"date":"2012-05-24T15:07:02","date_gmt":"2012-05-24T14:07:02","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=6794"},"modified":"2012-05-24T15:58:06","modified_gmt":"2012-05-24T14:58:06","slug":"surprises-in-the-addition-of-hcl-to-a-carbonyl-group","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6794","title":{"rendered":"Surprises (?) in the addition of HCl to a carbonyl group."},"content":{"rendered":"
\n

HCl reacting with a carbonyl compound (say formaldehyde) sounds pretty simple. But often the simpler a thing looks, the more subtle it is under the skin. And this little reaction is actually my prelude to the next post.<\/p>\n

\"\"<\/a><\/p>\n

The mechanism is studied using \u03c9B97XD\/6-311G(d,p)<\/a> with a simulated solvent (acetic acid) included (but not explicit solvent setting up any hydrogen bonds).<\/p>\n\n\n\n
\n
\"\"

Transition state HCl + H2C=O. Click for 3D animation.<\/p><\/div><\/td>\n

\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The transition state itself does not convey what is happening, largely because the transition state normal mode is mass-weighted. This leads to the heavier Cl not moving much, and the formaldehyde conducting a bee-dance like wag. For more detail, indeed insight, we need the intrinsic reaction coordinate (IRC):<\/p>\n\n\n\n\n
\"\"<\/a><\/td>\n<\/tr>\n
\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    \n
  1. At IRC 4, the HCl starts by aligning itself into the plane of the formaldehyde, with the hydrogen targeting the in-plane lone pair on the carbonyl oxygen.<\/li>\n
  2. At IRC 2.2, the hydrogen atom starts to transfer from the Cl to the O. As usual, a hydrogen transfer takes place very rapidly, and by IRC 1.7 the transfer is largely complete.<\/li>\n
  3. At IRC 1.5, the chlorine, now shorn of its proton, starts to move out of the plane.<\/li>\n
  4. At the transition state (IRC = 0.0) the chlorine is now inclined at an angle of about 45\u00b0 with respect to the plane of the formaldehyde (which is still largely co-planar).<\/li>\n
  5. Between IRC 0.0 and -2.0, the Cl…C bond starts to form, and the rotation goes to about 73\u00b0. It is held at this position because of an anomeric effect operating between one of the lone pairs on the oxygen atom, and the axis of the C-Cl\u00a0\u03c3* bond.<\/li>\n
  6. The overall process is concerted, but quite asynchronous, in as much as the formation of the O…H bond distinctly precedes that of the C…Cl bond. These bonds form at a dihedral (torsional) angle of\u00a0\u00a073\u00b0 with respect to each other and the need to align the two bonds in this manner means that they cannot form at the same rate!<\/li>\n<\/ol>\n

    Is this model a realistic one? Well, the missing component is hydrogen bonds. Between a solvent (this is being done by the way in acetic acid as simulated solvent) and the chlorine, which must assume a large measure of being actually a chloride anion, countered by the oxenium cation. It is possible that the reaction may actually therefore not be concerted, but it might stop at the half-way stage of an ion-pair before continuing its journey. The calculated barrier (~20 kcal\/mol) is actually quite reasonable for a thermal reaction, but hydrogen-bond stabilisation might be expected to reduce this to what in effect would correspond to a very fast room-temperature reaction.<\/p>\n

    Well, HCl + H2<\/sub>C=O does not sound complicated. But you can trust this blog to take something simple and make it less so!\u00a0<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    HCl reacting with a carbonyl compound (say formaldehyde) sounds pretty simple. But often the simpler a thing looks, the more subtle it is under the skin. And this little reaction is actually my prelude to the next post. The mechanism is studied using \u03c9B97XD\/6-311G(d,p) with a simulated solvent (acetic acid) included (but not explicit solvent […]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[284,373],"ppma_author":[2661],"class_list":["post-6794","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-acetic-acid","tag-tutorial-material"],"yoast_head":"\nSurprises (?) in the addition of HCl to a carbonyl group. - 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=6794\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Surprises (?) in the addition of HCl to a carbonyl group. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"HCl reacting with a carbonyl compound (say formaldehyde) sounds pretty simple. 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or ?","author":"Henry Rzepa","date":"March 4, 2011","format":false,"excerpt":"Nucleophilic addition of cyanide to a ketone or aldehyde is a standard reaction for introductory organic chemistry. But is all as it seems? The reaction is often represented as below, and this seems simple enough. But attention to detail suggests that, HCN being a weak acid, there will be only\u2026","rel":"","context":"In \"acidic solutions\"","block_context":{"text":"acidic solutions","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=acidic-solutions"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/03\/cyano1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6816,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6816","url_meta":{"origin":6794,"position":1},"title":"The mechanism (in 4D) of the reaction between thionyl chloride and a carboxylic acid.","author":"Henry Rzepa","date":"May 25, 2012","format":false,"excerpt":"If you have not previously visited, take a look at Nick Greeves' ChemTube3D\u00a0, an\u00a0ever-expanding gallery of reactions and their mechanisms. The 3D is because all molecules are offered with X, Y and z coordinates. You also get arrow pushing\u2021\u00a0in 3D. Here, I argue that we should adopt Einstein, and go\u2026","rel":"","context":"In \"acetic acid\"","block_context":{"text":"acetic acid","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=acetic-acid"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/05\/thionyl.svg","width":350,"height":200},"classes":[]},{"id":19569,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19569","url_meta":{"origin":6794,"position":2},"title":"Aromaticity-induced basicity.","author":"Henry Rzepa","date":"April 18, 2018","format":false,"excerpt":"The molecules below were discussed in the previous post as examples of highly polar but formally neutral molecules, a property induced by aromatisation of up to three rings. Since e.g. compound 3 is known only in its protonated phenolic form, here I take a look at the basicity of the\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":15395,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15395","url_meta":{"origin":6794,"position":3},"title":"I\u2019ve started so I\u2019ll finish. Kinetic isotope effect models for a general acid as a catalyst in the protiodecarboxylation of indoles.","author":"Henry Rzepa","date":"January 10, 2016","format":false,"excerpt":"Earlier I explored models for the heteroaromatic electrophilic protiodecarboxylation of an 3-substituted indole, focusing on the role of water as the proton transfer and delivery agent. Next, came\u00a0models for both water and the general base catalysed\u00a0ionization of indolinones. Here I\u00a0explore\u00a0general acid\u00a0catalysis by evaluating the properties of two possible models for\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":7100,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7100","url_meta":{"origin":6794,"position":4},"title":"Dynamic effects in nucleophilic substitution at trigonal carbon.","author":"Henry Rzepa","date":"July 16, 2012","format":false,"excerpt":"Singleton and co-workers have produced some wonderful work showing how dynamic effects and not just transition states can control the outcome of reactions. Steve Bachrach's blog contains many examples, including this recent one. This shows that tolyl thiolate (X=Na)\u00a0reacts with the dichlorobutenone to give two substitution products in a 81:19\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/singleton.svg","width":350,"height":200},"classes":[]},{"id":7027,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7027","url_meta":{"origin":6794,"position":5},"title":"Joining up the pieces. Peroxidation of ethyne.","author":"Henry Rzepa","date":"July 9, 2012","format":false,"excerpt":"Sometimes, connections between different areas of chemistry just pop out (without the help of semantic web tools, this is called serendipity). So here, I will try to join up some threads which emerge from previous posts. I had noted that antiaromaticity in cyclopropenium anion is lessened by the system adopting\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"","width":0,"height":0},"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\/6794","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=6794"}],"version-history":[{"count":17,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6794\/revisions"}],"predecessor-version":[{"id":6821,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6794\/revisions\/6821"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6794"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6794"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6794"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=6794"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}