A similar set of resonance structures for the phenolate anion conjugate base appears below the phenol structures. sulfoxides) or four (e.g. ;zP"$ O&o_b$AS(A\Be]/gWU_A(Pbpg/X-^O&cGA=+}"$!yFT9TQpzkxnW $A%UCV|^s!0nHd;qr![FiETZ>>2f>j;V2~3;TwY5{Z-_B:~Y(UF?wF4 Solved a) the stronger acid or SH NH2 or b) the stronger | Chegg.com In other words, conjugate acid of $\ce{NH3}$ is more stable than that of $\ce{H2N-NH2}$. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. For p-Nitroaniline virtually all of the electron density, shown as a red/yellow color. I looked it up and NH3 -> NH2- has a pka of 38 while H2 -> H- has a pka of 36 so they're both about the same strength, with NH2- being slightly more basic. SH NH2 Compound A Compound B Options: less acidic atom principle induction principle more acidic resonance principle orbital principle Even without reference to pkas, we can predict that compound A is than compound B by applying the The formulas written here neutralize this charge separation by double bonding that expands the valence octet of sulfur. To answer this question we must evaluate the manner in which an oxygen substituent interacts with the benzene ring. Acidity of Substituted Phenols - Chemistry LibreTexts Alternatively, a plausible general mechanism for this interesting and useful reaction is drawn below. Note that the arylammonium ion derived from aniline, PhNH3+, is commonly referred to as the anilinium ion. Aromatic herterocyclic amines (such as pyrimidine, pyridine, imidazole, pyrrole) are significantly weaker bases as a consequence of three factors. 11. Michael David Wiley Ph.D. in Organic Chemistry, University of Washington (Graduated 1969) Author has 188 answers and 231.1K answer views 4 y Related Is NH2- a stronger base than OH-? 14 years and about 60,000 students later, we are still helping students to learn organic chemistry one reaction at a time at https://www.aceorganicchem.com, thank you so much for the informations Evaluating Acid-Base Reactions SH . The alcohol cyclohexanol is shown for . Other names are noted in the table above. The keyword is "proton sponge". Read "The Protonation of Acetamide and Thioacetamide in Superacidic Solutions: Crystal Structures of [H3CC(OH)NH2]+AsF6- and [H3CC(SH)NH2]+AsF6-, Zeitschrift fr anorganische und allgemeine Chemie" on DeepDyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. The following chart shows how each group of atoms activates an OH acid (pKa values range from 16 to -2): CH3 is considered a spectator group wherever it appears in these molecules. This is because more electronegative atoms will hold electron density closer, and therefore will be less likely to let that electron density participate in a reaction. Three examples of such reactions are shown below, with the acidic hydrogen colored red in each case. In 2006, we started AceOrganicChem.com in order to make learning organic chemistry fast and easy. What do you call molecules with this property? 5 0 obj Sulfoxides have a fixed pyramidal shape (the sulfur non-bonding electron pair occupies one corner of a tetrahedron with sulfur at the center). Strong nucleophiles are VERY important throughout organic chemistry, but will be especially important when trying to determine the products of elimination and substitution (SN1 vs SN2) reactions. Now, since $\ce{N}$ is less electronegative than $\ce{O}$, it's lone pair is more readily available than that of $\ce{OH-}$. Oxygen assumes only two oxidation states in its organic compounds (1 in peroxides and 2 in other compounds). Describe the categorization of these amino acids, and which amino acids that belong to each group. [ /ICCBased 9 0 R ] Negatively charged acids are rarely acidic. Mild oxidation of disufides with chlorine gives alkylsulfenyl chlorides, but more vigorous oxidation forms sulfonic acids (2nd example). Consequently, it is possible to replace CH3 with other spectator groups (for example, H and other R) without affecting reactivity much. An energy diagram showing the effect of resonance on cyclohexanol and phenol acidities is shown on the right. These effects are enhanced when 1) the substituent is located closer to the acidic group, and 2) there are multiple substituents. Barton's base is a strong, poorly-nucleophilic, neutral base that serves in cases where electrophilic substitution of other amine bases is a problem. With reference to the discussion of base strength, the traditional explanation for the basestrengthening effect of electronreleasing (I) substituents is that such substituents help to stabilize the positive charge on an arylammonium ion more than they stabilize the unprotonated compound, thereby lowering G. The electrostatic potential map shows the effect of resonance on the basicity of an amide. As a third row element, sulfur has five empty 3d-orbitals that may be used for p-d bonding in a fashion similar to p-p () bonding. The common base sodium hydroxide is not soluble in many organic solvents, and is therefore not widely used as a reagent in organic reactions. the more EN the attached atom, the more acidic the molecule C < N < O < F relative electronegativity-C H 3< -N 2 < HO-< F-relative stability of conjugate bases CH 4< NH 3< H 2O < HF relative acidity 1. The larger the value of Kb and the smaller the value of pKb, the more favorable the proton-transfer equilibrium and the stronger the base. The inductive effect makes the electron density on the alkylamine's nitrogen greater than the nitrogen of ammonia. PDF Acids and Bases - San Diego Mesa College Whose hydrogen is more acidic, OH or NH2? - Quora The conversion of 1 and 2-alcohols to aldehydes and ketones is an important reaction which, in its simplest form, can be considered a dehydrogenation (loss of H2). How is that? The best answers are voted up and rise to the top, Not the answer you're looking for? Finally, oxidation of sulfides with hydrogen peroxide (or peracids) leads first to sulfoxides and then to sulfones. Prior to all of this, he was a chemist at Procter and Gamble. Indeed, we have seen in past chapters that amines react with electrophiles in several polar reactions (see for example the nucleophilic addition of amines in the formation of imines and enamines in Section 19.8). 2003-2023 Chegg Inc. All rights reserved. Why is ammonia more basic than acetonitrile. Remember, in any case, there will be only ONE protonation at a time. An equivalent oxidation of alcohols to peroxides is not normally observed. 3. The first compound is a typical 2-amine, and the three next to it are characterized by varying degrees of nitrogen electron pair delocalization. Which of the following will be more basic, hydrazine $\ce{H2N-NH2}$ or ammonia $\ce{NH3}$? Consequently, sulfoxides having two different alkyl or aryl substituents are chiral. So, would R-O-NH2 be a fair nucleophile or a weak nucleophile? 1,8-Bis(dimethylamino)naphthalene has a pKa of 12.3, it's one of the strongest known amine bases. 12 0 obj if i not mistaken. The reaction of oxalyl chloride with DMSO may generate chlorodimethylsulfonium chloride which then oxidizes the alcohol (Swern Oxidation). We've added a "Necessary cookies only" option to the cookie consent popup, Comparing basic strength of nitrogen heterocycles. In $\ce{H3N+-NH2}$, although the lone pair cannot be accommodated, but the positive charge present on its sides , to an extent, should neutralize the intensity of the lone pair, making it somewhat stable. I am quite confused I ampretty sure in an SN2reaction I- would be a good electrophile not nucelophile? In other words, how much does that lone pair want to break away from the nitrogen nucleus and form a new bond with a hydrogen. a. none, there are no acids in pure water b. H 2O c. NH 4 + d. However, as you locate OH and NH bonds, you will need to decide whether these bonded atoms should be lumped into a functional group with neighboring atoms. This is the best answer based on feedback and ratings. x}OHQ%Be&RNW`okn%B.A1XI:b]"(7373{@](mzy(;>7PA+Xf$vlqd}] UxiO:bM1Wg>q[ As explained earlier, although NH3 is a weak base and standard solution of ammonia has a pH 11 but still, it is amphoteric in nature which means it can act as both acid as well as a base under different conditions. The nucleophilic site of the nucleophile is the region of a molecule that is reactive and has the electron density. Acidic protons are usually bound to O or N. Therefore, the first step is to look for all OH and NH bonds. "Scan and rank" sounds simple, but it conceals several difficulties that are elaborated below. The ONLY convenient method for identifying a functional group is to already know some. It should be noted that the first four examples have the same order and degree of increased acidity as they exhibited decreased basicity in the previous table. Given these principles, we expect the acidity of these carboxylic acids to follow this trend. Like ammonia, most amines are Brnsted-Lowry and Lewis bases, but their base strength can be changed enormously by substituents. Most base reagents are alkoxide salts, amines or amide salts. Where does this (supposedly) Gibson quote come from? Experts are tested by Chegg as specialists in their subject area. For amines one can take advantage of their basicity by forming the protonated salt (RNH2+Cl), which is soluble in water. If the iodide ion is a stronger nucleophile than the hydroxide ion, why does the latter displace the former in a reaction involving aqueous Sodium hydroxide and alkyl iodide? The poor nucleophiles is more favor to Sn1 reaction than Sn2 reaction. a) p-Nitroaniline, methyl p-aminobenzoate, p-chloroaniline Thus, thermodynamics favors disulfide formation over peroxide. Find pI of His. Basicity of common amines (pKa of the conjugate ammonium ions). Great nucleophile, really poor base. While the electron density of aniline's nitrogen is delocalized in the aromatic ring making it less basic. This is expected, because the -NH 2 group is more electronegative than -H or -CH 3. For ammonia this is expressed by the following hypothetical equation: The same factors that decreased the basicity of amines increase their acidity. Fortunately, the Ka and Kb values for amines are directly related. This is not possible because $\ce{NH_3^+}$(no vacant orbital) doesn't have any space at all to get involved with the lone pair. Please visit our recent post on this topic> Electrophilic addition. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. endobj This relationship shows that as an ammonium ion becomes more acidic (Ka increases / pKa decreases) the correspond base becomes weaker (Kb decreases / pKb increases), Weaker Base = Larger Ka and Smaller pKa of the Ammonium ion, Stronger Base = Smaller Ka and Larger pKa of the Ammonium ion. Sn1 proceed faster in more polar solvent compare to Sn2. Bases accept protons, with a negative charge or lone pair. However, Kb values are often not used to discuss relative basicity of amines. Adding these two chemical equations together yields the equation for the autoionization for water: \[\cancel{\ce{RNH3+}(aq)}+\ce{H2O}(l)+\cancel{\ce{RNH2}(aq)}+\ce{H2O}(l)\ce{H3O+}(aq)+\cancel{\ce{RNH2}(aq)}+\ce{OH-}(aq)+\cancel{\ce{RNH3+}(aq)}\], \[\ce{2H2O}(l)\ce{H3O+}(aq)+\ce{OH-}(aq)\]. It was proposed that resonance delocalization of an oxygen non-bonded electron pair into the pi-electron system of the aromatic ring was responsible for this substituent effect. So instead, $\ce{-NH2}$ will pull electrons from it making it unstable. Ammonia is more basic than hydrazine, by about one order of magnitude. stream To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Think about it for a second.good nucleophiles (as shown above) can have a negative charge and will almost always have a lone pair. You can, however, force two lone pairs into close proximity. Sponsored by Grammarly Grammarly helps ensure your writing is mistake-free. << /Length 10 0 R /N 3 /Alternate /DeviceRGB /Filter /FlateDecode >> The SS single bond is nearly twice as strong as the OO bond in peroxides, and the OH bond is more than 25 kcal/mole stronger than an SH bond. To clarify the first part, I am not saying that the electrons will jump to the protonated nitrogen. endobj The most acidic functional group usually is holding the most acidic H in the entire molecule. $$\ce{H2N-NH2 + H3O+ <=> H3N^+-NH2 + H2O} \tag2$$. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Oxidation of 1 and 2-alcohols to aldehydes and ketones changes the oxidation state of carbon but not oxygen. Due to the exothermic nature of the reaction, it is usually run at -50 C or lower. Is it plausible for constructed languages to be used to affect thought and control or mold people towards desired outcomes? Bruce Edward Bursten, Catherine J. Murphy, H. Eugene Lemay, Matthew E. Stoltzfus, Patrick Woodward, Theodore E. Brown, Quiz #4 - States of Consciousness and Drugs. In addition to acting as a base, 1o and 2o amines can act as very weak acids. This R-group, or sidechain, gives each amino acid proteins specific characteristics, including size, polarity and pH. For more information on this and other topics of organic chemistry interest, please visit organic chemistry, Dr. Mike Pa got a bachelors degree in chemistry from Binghamton University, a masters degree in organic chemistry from the University of Arizona and a PhD in bio-organic chemistry from the University of Arizona. We all know that electran withdrawing ability ($-I$ effect) of $\ce{-NH2}$ group is higher than that of $\ce{-H}$ group. Increased Basicity of para-Methoxyaniline due to Electron-Donation. Liquid-liquid extractions take advantage of the difference in solubility of a substance in two immiscible liquids (e.g. Legal. Transcribed image text: SH NH2 Compound A Compound B Options: less acidic atom principle induction principle more acidic resonance principle orbital principle Even without reference to pkas, we can predict that compound A is than compound B by applying . The contributing structures to the phenol hybrid all suffer charge separation, resulting in very modest stabilization of this compound. Asking for help, clarification, or responding to other answers. A cylindrical piece of copper is 9.009.009.00 in. The prefix thio denotes replacement of a functional oxygen by sulfur. LDA is a very strong base and is commonly used to create enolate ions by deprotonating an alpha-hydrogen from carbonyl compounds (Section 22-7). My chemistry teacher said that $\ce{NH3}$ is more basic since after giving $\ce{H+}$ to hydrazine results in $\ce{H3N+-NH2}$, and accommodation of the lone pair of $\ce{-NH2}$ is not possible by $\ce{-N+H3}$, since it doesn't have the space. positive charge present on its sides , to an extent, should neutralize the intensity of the lone pair, making it somewhat stable. 4Ix#{zwAj}Q=8m The effect of delocalization can be seen when viewing the electrostatic potential maps of aniline an methyl amine. Since hydrogen sulfide (H2S) is a much stronger acid than water (by more than ten million fold), we expect, and find, thiols to be stronger acids than equivalent alcohols and phenols. We didnt consider the M effect of NH2 here because its not possible to donate right @MathewMahindaratne ? An aqueous solution of ammonium nitrite, NH4NO2NH_4NO_2NH4NO2, decomposes when heated to give off nitrogen, N2N_2N2. CCl3NH2 this is most basic amine. sulfones) electrons. c. the more concentrated the acid. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. (o{1cd5Ugtlai"\.5^8tph0k!~D Thd6:>f&mxA4L&%ki?Cqm&/By#%i'W:XlErr'=_)i7,F|N6rm^UHW5;?h How can I find out which sectors are used by files on NTFS? Nucleophiles will not be good bases if they are highly polarizable. Amino acids Flashcards | Quizlet The first of these is the hybridization of the nitrogen. I am not so pleased with this argument. A free amino acid can act both as an acid and a base in a solution. The region and polygon don't match. A limit involving the quotient of two sums, Redoing the align environment with a specific formatting. for (CH3)3C- > (CH3)2N->CH3O- Solved SH NH2 Compound A Compound B Options: less acidic - Chegg The second lone pair is not involved in the acid-base reaction, it does not point towards the -NH4+ group. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. It is noteworthy that the influence of a nitro substituent is over ten times stronger in the para-location than it is meta, despite the fact that the latter position is closer to the hydroxyl group. ROCO Acid-Base: Most acidic H - Reed College The equilibrium constant for this reaction is the base ionization constant (Kb), also called the base dissociation constant: \[K_b=\dfrac{[RNH3^+][OH^]}{[NH2]} \label{16.5.5}\]. $_____________________________$. In between, he did NOT compete at the 1996 Olympics, make the Atlanta Braves opening day roster, or become the head coach of the Indiana Pacers, as he had intended. (at pH 7). Correspondingly, primary, secondary, and tertiary alkyl amines are more basic than ammonia. The difference in pKa between H3O+ and H2O is 18 units, while the difference in pKa between NH4+ and NH3 is a gigantic 26 units. 21.4: Acidity and Basicity of Amines is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts. Abel already answered that at one time only one $\ce{-NH_2}$ takes part when we determine basicity and the second $\ce{-NH_2}$ plays no role. Supporting evidence that the phenolate negative charge is delocalized on the ortho and para carbons of the benzene ring comes from the influence of electron-withdrawing substituents at those sites. Acid with values less than one are considered weak. The small amount of extra negative charge built up on the nitrogen atom makes the lone pair even more attractive towards hydrogen ions. Remarkably, sulfoxides (equation # 2), sulfinate salts (# 3) and sulfite anion (# 4) also alkylate on sulfur, despite the partial negative formal charge on oxygen and partial positive charge on sulfur. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Substitution of the hydroxyl hydrogen atom is even more facile with phenols, which are roughly a million times more acidic than equivalent alcohols. explain why amines are more basic than amides, and better nucleophiles. The electrophilic character of the sulfur atom is enhanced by acylation. My concern is that you understand what is meant by "all things being equal." Each amino acid is attached to another amino acid by covalent bond, known as a peptide bond, which is formed by a dehydration reaction. *;xUg!@9=XKf"aP>ax/L6ER{*UVV&r r^(>GS;E!,uf:^8:wI/s5-q'GZ8TS3qgm}lE53_;)]Uq84?1S]~3Y!upVdSO*ZeN!K4Wb>tnSd[o*ojo We reviewed their content and use your feedback to keep the quality high. (The use of DCC as an acylation reagent was described elsewhere.) Learn more about Stack Overflow the company, and our products. When evaluating the basicity of a nitrogen-containing organic functional group, the central question we need to ask ourselves is: how reactive (and thus how basic and nucleophilic) is the lone pair on the nitrogen? Ammonia has no such problem so it must be more basic. Hnig's base is relatively non-nucleophilic (due to steric hindrance), and is often used as the base in E2 elimination reactions conducted in non-polar solvents. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Is NH2 or NH more acidic? - KnowledgeBurrow.com We see this in calculations and experiments that show nucleophilicity decreases as you get closer to fluorine on the periodic table (C > N > O > F). This means basicity of ammonia is greater compared to that of hydrazine. In the first case, mild oxidation converts thiols to disufides. Describe the general structure of a free amino acid. << /Length 4 0 R /Filter /FlateDecode >> View the full answer. tall and 1.401.401.40 in. When NH3 acts as a base, it will donate its lone pair to a proton H+ and form its conjugate acid NH4+ whereas when NH3 acts as an . xZMs7E&I\qrBHYZizco~z~q LDv .^-/w?ru On the other hand, the phenolate anion is already charged, and the canonical contributors act to disperse the charge, resulting in a substantial stabilization of this species. Co-solvents such as methylene chloride or THF are needed, since pure DMSO freezes at 18. a) p-Chloroaniline, methyl p-aminobenzoate, p-nitroaniline OH NH2 H3C CH CH COOH SH NH2 CH2 CH COOH . Jordan's line about intimate parties in The Great Gatsby? Bonding of sulfur to the alcohol oxygen atom then follows. Formulas illustrating this electron delocalization will be displayed when the "Resonance Structures" button beneath the previous diagram is clicked. What about nucleophilicity? [Organic Chemistry]SH more acidic than OH : r/HomeworkHelp - reddit The second lone pair is not involved in the acid-base reaction, it does not point towards the -NH 4+ group. I am not a huge fam of memorizing charts, but this might be a good one to know pretty well. What is this bound called? This is because when the proton leaves the compound, the negative charge on RSH is dispersed more on it as compared to ROH (due to larger size of S than O). Why does silver oxide form a coordination complex when treated with ammonia? You can, however, force two lone pairs into close proximity. Bases will not be good nucleophiles if they are really bulky or hindered. endstream Is NH3 (Ammonia) an Acid or a Base? - Techiescientist If this spring is cut in half, does the resulting half spring have a force constant that is greater than, less than, or equal to kkk ? Below is a table of relative nucleophilic strength. 2003-2023 Chegg Inc. All rights reserved. xKo@|9R{&CV{:%r;_PQ0flf7|;0E"$w] g(o6Mf=aVZ_v7b6QD9$0 5TFN>0d8K4[:KsW `0p'a`b>lxvlU7a8\!E^-\:,U b) p-Ethylaniline, p-Bromoaniline, p-aminobenzonitrile NH3 pKa = 38 H2O pKa = 15.7 NH3 is a weaker acid than H2O. Extraction is often employed in organic chemistry to purify compounds. The lone pair of electrons on the nitrogen atom of amines makes these compounds not only basic, but also good nucleophiles. According to the Bronsted-Lowry acid-base definition, molecules that accept protons are bases and those which are donated protons are acids. inorganic chemistry - Which is more basic, hydrazine or ammonia 4 0 obj How many Connect and share knowledge within a single location that is structured and easy to search. Table of Acid and Base Strength - University of Washington This reaction may be used to prepare pure nitrogen. The isoelectric point (pl) for histidine (His) is 7,6. Thanks for contributing an answer to Chemistry Stack Exchange! Euler: A baby on his lap, a cat on his back thats how he wrote his immortal works (origin?). Polarizability is defined as the ability to distort the electron cloud of an atom, which allows it interact with a reaction site more easily. { Acidity_of_Phenols : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Acidity_of_Substituted_Phenols : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Physical_Properties_of_Phenol : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { Properties_of_Phenols : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Reactivity_of_Phenols : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Synthesis_of_Phenols : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic-category", "authorname:wreusch", "showtoc:no", "license:ccbyncsa", "licenseversion:40" ], https://chem.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fchem.libretexts.org%2FBookshelves%2FOrganic_Chemistry%2FSupplemental_Modules_(Organic_Chemistry)%2FPhenols%2FProperties_of_Phenols%2FAcidity_of_Substituted_Phenols, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), status page at https://status.libretexts.org.