Is Web Scraping Legal In Malaysia, Van Kirk Funeral Home Corbin, Ky Obituaries, Articles I

Amino acids are classified using their specific R groups. a) p-Nitroaniline, methyl p-aminobenzoate, p-chloroaniline To subscribe to this RSS feed, copy and paste this URL into your RSS reader. In other words, conjugate acid of $\ce{NH3}$ is more stable than that of $\ce{H2N-NH2}$. To clarify the first part, I am not saying that the electrons will jump to the protonated nitrogen. endobj You'll get a detailed solution from a subject matter expert that helps you learn core concepts. First, the groups exert a similar effect on NH acids (and the activating sequence is the same: RSO2 > RC=O > Ph). 11. You will hear a lot about bulky bases, which are nucleophilic but too darn big to be a nucleophile and can only be a base. The addition of substituents onto the aromatic ring can can make arylamines more or less basic. A piece of aluminum of mass 6.24kg6.24 \mathrm{~kg}6.24kg displaces water that fills a container 12.0cm12.0cm16.0cm12.0 \mathrm{~cm} \times 12.0 \mathrm{~cm} \times 16.0 \mathrm{~cm}12.0cm12.0cm16.0cm. It is nonpolar and does not exert a significant field-inductive effect, and it is incapable of delocalizing charge. The IUPAC name of (CH3)3CSH is 2-methyl-2-propanethiol, commonly called tert-butyl mercaptan. Ammonia (NH 3) acts as a weak base in aqueous solution. In the case of para-methoxyaniline, the lone pair on the methoxy group donates electron density to the aromatic system, and a resonance contributor can be drawn in which a negative charge is placed on the carbon adjacent to the nitrogen, which makes the substituted arylamine more basic than aniline. Why? The remaining steps are eliminations, similar in nature to those proposed for other alcohol oxidations. The resonance stabilization in these two cases is very different. 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. The effect of delocalization can be seen when viewing the electrostatic potential maps of aniline an methyl amine. 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). It is akin to saying that just because Sulphuric acid has two acidic hydrogens, it is a stronger acid than Perchloro-acid, which is untrue. b) p-Bromoaniline, p-Aminobenzonitrile, p-ethylaniline Organic chemistry is all about reactions. This means that O and N must have the same formal charge (item #1) and must be bonded to the same activating group (item #2). Bruce Edward Bursten, Catherine J. Murphy, H. Eugene Lemay, Matthew E. Stoltzfus, Patrick Woodward, Theodore E. Brown, Quiz #4 - States of Consciousness and Drugs. positive charge present on its sides , to an extent, should neutralize the intensity of the lone pair, making it somewhat stable. A cylindrical piece of copper is 9.009.009.00 in. Make certain that you can define, and use in context, the key term below. Each amino acid is attached to another amino acid by covalent bond, known as a peptide bond, which is formed by a dehydration reaction. Thus if the Ka for an ammonium ion is know the Kb for the corresponding amine can be calculated using the equation Kb = Kw / Ka. 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. 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. Are there tables of wastage rates for different fruit and veg? Legal. Negatively charged acids are rarely acidic. 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. The only neutral acids that are stronger than ROH2+ are H2SO4 and certain other RSO3H. << /Type /Page /Parent 8 0 R /Resources 3 0 R /Contents 2 0 R /MediaBox ), Virtual Textbook ofOrganicChemistry, Organic Chemistry With a Biological Emphasis byTim Soderberg(University of Minnesota, Morris). Describe how the structure of the R group of His at pH 7,4 and its properties. x"8NoWG0:ahvtYSU>eUg5Uyy/:s\2Qj0tB?4lTz^,|{uuv 2MCG l*mt! What's the difference between a power rail and a signal line? endobj The alkoxides are stronger bases that are often used in the corresponding alcohol as solvent, or for greater reactivity in DMSO. 5 0 obj Legal. The reasons for this different behavior are not hard to identify. Here are a couple of good rules to remember: 2. For example, C2H5SC3H7 is ethyl propyl sulfide and C2H5SCH2SC3H7 may be named 3,5-dithiaoctane. Aniline is substantially less basic than methylamine, as is evident by looking at the pKa values for their respective ammonium conjugate acids (remember that the lower the pKa of the conjugate acid, the weaker the base). Three examples of these DMSO oxidations are given in the following diagram. The alcohol cyclohexanol is shown for reference at the top left. Formulas illustrating this electron delocalization will be displayed when the "Resonance Structures" button beneath the previous diagram is clicked. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. in radius. I am quite confused I ampretty sure in an SN2reaction I- would be a good electrophile not nucelophile? RS() Na(+) + (CH3)2CHBr (CH3)2CHSR + Na(+) Br(). Basicity of common amines (pKa of the conjugate ammonium ions). For amines one can take advantage of their basicity by forming the protonated salt (RNH2+Cl), which is soluble in water. The aqueous layer is then treated with a base (NaOH) to regenerate the amine and NaCl. By providing an oxygen source to fix the product hydrogen as water, the endothermic dehydrogenation process may be converted to a more favorable exothermic one. 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. So instead, $\ce{-NH2}$ will pull electrons from it making it unstable. How is that? Map: Organic Chemistry (Vollhardt and Schore), { "21.01:_Naming__the_Amines" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.02:_Structural_and__Physical__Properties_of_Amines" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.03:_Spectroscopy_of__the_Amine__Group" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.04:_Acidity__and__Basicity__of_Amines" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.05:_Synthesis_of_Amines__by_Alkylation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.06:_Synthesis_of_Amines__by_Reductive_Amination" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.07:_Synthesis_of_Amines__from__Carboxylic_Amides" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.08:_Quaternary_Ammonium_Salts:__Hofmann_Elimination" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.09:_Mannich___Reaction:_Alkylation_of_Enols__by__Iminium__Ions" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21.10:_Nitrosation_of_Amines" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { "00:_Front_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "01._Structure_and_Bonding_in_Organic_Molecules" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02._Structure_and_Reactivity:_Acids_and_Bases_Polar_and_Nonpolar_Molecules" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03._Reactions_of_Alkanes:_Bond-Dissociation_Energies_Radical_Halogenation_and_Relative_Reactivity" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04._Cycloalkanes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05._Stereoisomers" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06._Properties_and_Reactions_of_Haloalkanes:_Bimolecular_Nucleophilic_Substitution" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07._Further_Reactions_of_Haloalkanes:_Unimolecular_Substitution_and_Pathways_of_Elimination" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "08._Hydroxy_of_Functional_Group:_Alcohols:_Properties_Preparation_and_Strategy_of_Synthesis" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "09._Further_Reactions_of_Alcohols_and_the_Chemistry_of_Ethers" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10:_Using_Nuclear_Magnetic_Resonance_Spectroscopy_to_Deduce_Structure" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "11:_Alkenes:_Infrared_Spectroscopy_and_Mass_Spectrometry" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "12:_Reactions_to_Alkenes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "13:_Alkynes:_The_Carbon" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "14:_Delocalized_Pi_Systems:_Investigation_by_Ultraviolet_and_Visible_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "15:_Benzene_and_Aromaticity:_Electrophilic_Aromatic_Substitution" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "16:_Electrophilic_Attack_on_Derivatives_of_Benzene:_Substituents_Control_Regioselectivity" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "17:_Aldehydes_and_Ketones_-_The_Carbonyl_Group" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "18:_Enols_Enolates_and_the_Aldol_Condensation:_ab-Unsaturated_Aldehydes_and_Ketones" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "19:_Carboxylic_Acids" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "20:_Carboxylic_Acid_Derivatives" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "21:_Amines_and_Their_Derivatives" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "22:_Chemistry_of_the_Benzene_Substituents:_Alkylbenzenes_Phenols_and_Benzenamines" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "23:_Ester_Enolates_and_the_Claisen_Condensation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "24:_Carbohydrates:_Polyfunctional_Compounds_in_Nature" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "25:_Heterocycles:_Heteroatoms_in_Cyclic_Organic_Compounds" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "26:_Amino_Acids_Peptides_Proteins_and_Nucleic_Acids:_Nitrogen-Containing_Polymers_in_Nature" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "zz:_Back_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic", "showtoc:no", "license:ccbyncsa", "licenseversion:40" ], https://chem.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fchem.libretexts.org%2FBookshelves%2FOrganic_Chemistry%2FMap%253A_Organic_Chemistry_(Vollhardt_and_Schore)%2F21%253A_Amines_and_Their_Derivatives%2F21.04%253A_Acidity__and__Basicity__of_Amines, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), Comparing the Basicity of Alkylamines to Amides, Organic Chemistry With a Biological Emphasis, status page at https://status.libretexts.org. 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. Most simple alkyl amines have pKa's in the range 9.5 to 11.0, and their aqueous solutions are basic (have a pH of 11 to 12, depending on concentration). Electronegative substituents usually enhance the acidity of a functional group through a combination of field and inductive effects. Heres another way to think about it: the lone pair on an amide nitrogen is not as available for bonding with a proton these two electrons are too stable being part of the delocalized pi-bonding system. account for the basicity and nucleophilicity of amines. The common base sodium hydroxide is not soluble in many organic solvents, and is therefore not widely used as a reagent in organic reactions. Thus RS- will be weaker base and consequently RSH will be stronger base. $$\ce{H2N-NH2 + H3O+ <=> H3N^+-NH2 + H2O} \tag2$$. 3) Polarizability The more polarizable an atom is, the more nucleophilic it will be. 2003-2023 Chegg Inc. All rights reserved. << /Length 14 0 R /Filter /FlateDecode >> Gly is more flexible than other residues. tall and 1.401.401.40 in. 4 0 obj << /ProcSet [ /PDF /Text ] /ColorSpace << /Cs1 5 0 R >> /Font << /F1.0 c) p-(Trifluoromethyl)aniline, p-methoxyaniline, p-methylaniline, 1) NH NH Compound A Compound B Options: orbital principle less acidic resonance principle induction principle more acidic atom. We didnt consider the M effect of NH2 here because its not possible to donate right @MathewMahindaratne ? Extraction is often employed in organic chemistry to purify compounds. Liquid-liquid extractions take advantage of the difference in solubility of a substance in two immiscible liquids (e.g. Thus, -SH is a thiol and C=S a thione. Nucleophilicity of Sulfur Compounds To answer this question we must evaluate the manner in which an oxygen substituent interacts with the benzene ring. An equivalent oxidation of alcohols to peroxides is not normally observed. Mild oxidation of disufides with chlorine gives alkylsulfenyl chlorides, but more vigorous oxidation forms sulfonic acids (2nd example). We really need to know what is nucleophilic and what is not so that we can determine what is going to react at the electrophilic site. NH2- is a strong base because it is unstable with its negative charge in a solution so that it wants to take the edge off with a negative charge by accepting a proton and acting as a base. Oxidation of thiols and other sulfur compounds changes the oxidation state of sulfur rather than carbon. Aromatic herterocyclic amines (such as pyrimidine, pyridine, imidazole, pyrrole) are significantly weaker bases as a consequence of three factors. This destabilizes the unprotonated form. 10 0 obj Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. NH2 - OH -F-SH - Cl-Br-I- The second lone pair is not involved in the acid-base reaction, it does not point towards the -NH4+ group. This is a major consideration when looking at SN vs E reactions. It only takes a minute to sign up. 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 ? Correspondingly, primary, secondary, and tertiary alkyl amines are more basic than ammonia. This principle can be very useful if used properly. Try drawing Lewis-structures for the sulfur atoms in these compounds. 7) Gly Gly . In p-methoxyaninline the electron donating methoxy group donates electron density into the ring. We've added a "Necessary cookies only" option to the cookie consent popup, Comparing basic strength of nitrogen heterocycles. 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. Scan a molecule for known acidic functional groups. 12 0 obj For the second point you made, more number of nucleophilic sites would mean more chances of attack of an $H^+$, which adds to the basicity of Hydrazine. The pka of the conjugate base of acid is 4.5, and not that of aniline. Substitution of the hydroxyl hydrogen atom is even more facile with phenols, which are roughly a million times more acidic than equivalent alcohols. Im thinking it would be weaker than NH3 because of the oxygen, but Im not sure. However, these simple amines are all more basic (i.e., have a higher pKa) than ammonia. #4 Importance - within a functional group category, use substituent effects to compare acids. Adding these two chemical equations together yields the equation for the autoionization for water: \[\cancel{\ce{RNH3+}(aq)}+\ce{H2O}(l)+\cancel{\ce{RNH2}(aq)}+\ce{H2O}(l)\ce{H3O+}(aq)+\cancel{\ce{RNH2}(aq)}+\ce{OH-}(aq)+\cancel{\ce{RNH3+}(aq)}\], \[\ce{2H2O}(l)\ce{H3O+}(aq)+\ce{OH-}(aq)\]. explain why primary and secondary (but not tertiary) amines may be regarded as very weak acids, and illustrate the synthetic usefulness of the strong bases that can be formed from these weak acids. My concern is that you understand what is meant by "all things being equal." Please visit our recent post on this topic> Electrophilic addition. Pyridine is commonly used as an acid scavenger in reactions that produce mineral acid co-products. The salt will extract into the aqueous phase leaving behind neutral compounds in the non-aqueous phase. Consider the reactions for a conjugate acid-base pair, RNH3+ RNH2: \[\ce{RNH3+}(aq)+\ce{H2O}(l)\ce{RNH2}(aq)+\ce{H3O+}(aq) \hspace{20px} K_\ce{a}=\ce{\dfrac{[RNH2][H3O]}{[RNH3+]}}\], \[\ce{RNH2}(aq)+\ce{H2O}(l)\ce{RNH3+}(aq)+\ce{OH-}(aq) \hspace{20px} K_\ce{b}=\ce{\dfrac{[RNH3+][OH-]}{[RNH2]}}\]. c) p-Methoxyaniline, p-methylaniline, p-(trifluoromethyl)aniline. ;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 The reaction is operationally easy: a DMSO solution of the alcohol is treated with one of several electrophilic dehydrating reagents (E). The electrostatic potential map shows the effect of resonance on the basicity of an amide. Therefore, $\ce{-NH2}$ group in $\ce{H3N^+-NH2}$ destabilizes the positive charge more than $\ce{-H}$ group in $\ce{H3N^+-H}$. 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). How many 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. NH3 pKa = 38 H2O pKa = 15.7 NH3 is a weaker acid than H2O. 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 . Hi, Despite their similarity, they are stronger acids and more powerful nucleophiles than alcohols. ~:5, *8@*k| $Do! 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. 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. The following compounds have similar pKa values because the activating groups are not bonded directly to OH: CH3C(=O)CH2OH, PhCH2OH, and CH3CH2OH. According to the Bronsted-Lowry acid-base definition, molecules that accept protons are bases and those which are donated protons are acids. 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. 2) Electronegativity The more electronegative an atom is, the less nucleophilic it will be. As a consequence, forward reaction of equation $(1)$ is favor than that in equation $(2)$. The carboxyl group of one amino acid and the amino group of the incoming amino acid combine, releasing a molecule of water. At pH 7,4 the surrounding will be more acidic than Histidine pI.It takes up a hydrogen atom at the R-group.