--- title: "Hydrogen Bond Acceptor | Organic Chemistry" description: "Hydrogen bond acceptor in Organic Chemistry: an atom with a lone pair that can accept an H-bond, shaping carboxylic acid boiling point, solubility, and dimers." canonical: "https://fiveable.me/organic-chem/key-terms/hydrogen-bond-acceptor" type: "key-term" subject: "Organic Chemistry" unit: "Unit 20" --- # Hydrogen Bond Acceptor | Organic Chemistry ## Definition A hydrogen bond acceptor is an atom with a lone pair that can accept a hydrogen bond from an O-H or N-H group. In Organic Chemistry, this shows up in carboxylic acids, carbonyls, and solubility patterns. ## What It Is A hydrogen bond acceptor is the atom in a molecule that has a lone pair available to receive a hydrogen bond from a hydrogen attached to an electronegative atom, usually oxygen or nitrogen. In Organic Chemistry, you use this idea to predict how molecules stick to each other, dissolve, and boil. The simplest way to picture it is this: one molecule has an H attached to O or N, so that H is slightly positive. Another atom nearby has a lone pair, so it can attract that H. That receiving atom is the acceptor. The donor supplies the hydrogen, while the acceptor supplies the lone pair. This matters a lot for oxygen-containing functional groups. A carbonyl oxygen is a strong hydrogen bond acceptor because it has two lone pairs and a fairly concentrated negative region around the C=O bond. In a carboxylic acid, the hydroxyl oxygen can also accept a hydrogen bond, but it is usually a weaker acceptor because its lone pairs are less available. That difference shows up in how carboxylic acids interact with themselves and with water. Carboxylic acids are a good example because they can both donate and accept hydrogen bonds. Two molecules often pair up as cyclic dimers, where each acid donates one hydrogen bond and accepts one through its oxygen atoms. That dimer pattern helps explain why many carboxylic acids have higher boiling points than similar molecules that cannot hydrogen bond as strongly. You will also see acceptor behavior when comparing functional groups. Alcohols have oxygen atoms that can accept hydrogen bonds, but they do not behave exactly like carboxylic acids because the carbonyl changes electron distribution. Carboxylate ions are even stronger hydrogen bond acceptors than neutral carboxylic acids because the negative charge is spread over two oxygens. So when you are ranking intermolecular forces or predicting solubility, the acceptor side of the molecule gives you a big clue about its behavior. ## Why It Matters Hydrogen bond acceptor is one of the fastest ways to predict physical properties in Organic Chemistry. If a molecule can accept hydrogen bonds, it usually interacts more strongly with polar solvents and other molecules, which can raise boiling point and change water solubility. This term is especially useful in carboxylic acid problems. The carbonyl oxygen and hydroxyl oxygen do not contribute equally, so you can explain why carboxylic acids dimerize and why they have stronger intermolecular forces than many similar-sized compounds. That gives you a real mechanism instead of a memorized fact. It also helps when comparing acids and their conjugate bases. A neutral carboxylic acid is a hydrogen bond acceptor, but the carboxylate ion is even better at accepting because of charge delocalization. That difference shows up in acid-base behavior, extraction problems, and structure-property questions. When you know how to spot acceptors, you can read a structure and predict what will happen without guessing. That skill shows up in naming, functional group comparison, lab discussion, and any question that asks you to explain why one compound is more soluble, more volatile, or more strongly associated than another. ## Connections ### Hydrogen Bond A hydrogen bond acceptor is only half of the hydrogen-bonding pair. The donor provides the H attached to O or N, and the acceptor supplies the lone pair that attracts it. In Organic Chemistry, identifying both sides lets you predict intermolecular attraction, dimer formation, and why some compounds have much higher boiling points than others. ### Carboxylic Acid Carboxylic acids are the main setting where this term shows up in the unit on structure and properties. Their carbonyl oxygen and hydroxyl oxygen can both accept hydrogen bonds, and that is part of why they form strong intermolecular interactions. The carboxyl group gives you a built-in donor-acceptor pattern. ### Intermolecular Forces Hydrogen bond acceptors strengthen intermolecular forces by creating specific attractions between molecules. In organic problems, that usually affects boiling point, melting point, and solubility. If two compounds are similar in size, the one with better hydrogen-bonding ability often behaves very differently in the lab or on a worksheet. ### [Carboxylate Ion](/organic-chem/key-terms/carboxylate-ion) The carboxylate ion is the deprotonated form of a carboxylic acid, and it is a much stronger hydrogen bond acceptor than the neutral acid. The negative charge is spread over both oxygens, which makes the ion especially good at interacting with water and other polar species. That is why salt forms often behave differently from acids. ## On the AP Exam A quiz question may show several structures and ask you which one can accept hydrogen bonds, or why one acid has a higher boiling point than a similar molecule. On a structure-property problem, you identify the lone-pair-bearing atoms, then decide whether they are strong or weak acceptors. For carboxylic acids, you should recognize that the carbonyl oxygen is the stronger acceptor and use that to explain dimer formation and solubility trends. In a lab report or short-answer question, you might use this term to justify why a compound dissolves in water, why an IR or boiling-point comparison makes sense, or why the neutral acid and carboxylate salt behave differently. The move is not just naming the acceptor, but connecting it to the observable result. ## Hydrogen Bond Acceptor vs Hydrogen Bond Donor A donor is the part that supplies the hydrogen in an H-bond, usually an O-H or N-H group. An acceptor is the atom with the lone pair that receives that interaction. A molecule can do one, the other, or both, and carboxylic acids are a classic example of a compound that can do both. ## Key Takeaways - A hydrogen bond acceptor is an atom with a lone pair that can attract the hydrogen from an O-H or N-H group. - In Organic Chemistry, acceptors help you predict intermolecular forces, boiling point, and solubility. - Carboxylic acids have two oxygen atoms that can accept hydrogen bonds, but the carbonyl oxygen is the stronger acceptor. - Carboxylic acids often form cyclic dimers because each molecule can both donate and accept a hydrogen bond. - The carboxylate ion is a stronger hydrogen bond acceptor than the neutral carboxylic acid because the negative charge is spread over the oxygens. ## FAQs ### What is hydrogen bond acceptor in Organic Chemistry? It is the atom in a molecule that has a lone pair available to receive a hydrogen bond from a nearby O-H or N-H group. In Organic Chemistry, that usually means oxygen or nitrogen, especially in functional groups like carbonyls and carboxylic acids. The acceptor side helps explain why some molecules stick together more strongly than others. ### Is the oxygen in a carboxylic acid a hydrogen bond acceptor? Yes, both oxygens in a carboxylic acid can accept hydrogen bonds, but they do not do it equally well. The carbonyl oxygen is the stronger acceptor because its lone pairs are more available. The hydroxyl oxygen can accept too, but it is a weaker acceptor in this functional group. ### Why do carboxylic acids form dimers? Two carboxylic acid molecules can pair up through two hydrogen bonds, creating a cyclic dimer. Each molecule donates one hydrogen bond and accepts one, so the pair is held together very tightly. This strong association helps explain their high boiling points. ### How do I tell if a molecule is a hydrogen bond acceptor? Look for an atom with a lone pair that is not tied up in a positive charge or otherwise unavailable. Oxygen and nitrogen are common acceptors, but the exact strength depends on the functional group. In carboxylic acids, the carbonyl oxygen is a good acceptor, while the hydroxyl oxygen is less strong. ## Related Study Guides - [20.2 Structure and Properties of Carboxylic Acids](/organic-chem/unit-20/structure-properties-carboxylic-acids/study-guide/TH8LwDxTiAYIKMYb) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/organic-chem/key-terms/hydrogen-bond-acceptor#resource","name":"Hydrogen Bond Acceptor | Organic Chemistry","url":"https://fiveable.me/organic-chem/key-terms/hydrogen-bond-acceptor","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/organic-chem/key-terms/hydrogen-bond-acceptor#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:40.731Z","isPartOf":{"@type":"Collection","name":"Organic Chemistry Key Terms","url":"https://fiveable.me/organic-chem/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/organic-chem/key-terms/hydrogen-bond-acceptor#term","name":"Hydrogen Bond Acceptor","description":"A hydrogen bond acceptor is an atom with a lone pair that can accept a hydrogen bond from an O-H or N-H group. 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