What is Basicity: Definition and 27 Discussions

In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances which react with acids as originally proposed by G.-F. Rouelle in the mid-18th century.
Svante Arrhenius proposed in 1884 that a base is a substance which dissociates in aqueous solution to form hydroxide ions OH−. These ions can react with hydrogen ions (H+ according to Arrhenius) from the dissociation of acids to form water in an acid–base reaction. A base was therefore a metal hydroxide such as NaOH or Ca(OH)2. Such aqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can taste bitter and change the color of pH indicators (e.g., turn red litmus paper blue).
In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 at standard conditions. A soluble base is called an alkali if it contains and releases OH− ions quantitatively. Metal oxides, hydroxides, and especially alkoxides are basic, and conjugate bases of weak acids are weak bases.
Bases and acids are seen as chemical opposites because the effect of an acid is to increase the hydronium (H3O+) concentration in water, whereas bases reduce this concentration. A reaction between aqueous solutions of an acid and a base is called neutralization, producing a solution of water and a salt in which the salt separates into its component ions. If the aqueous solution is saturated with a given salt solute, any additional such salt precipitates out of the solution.
In the more general Brønsted–Lowry acid–base theory (1923), a base is a substance that can accept hydrogen cations (H+)—otherwise known as protons. This does include aqueous hydroxides since OH− does react with H+ to form water, so that Arrhenius bases are a subset of Brønsted bases. However, there are also other Brønsted bases which accept protons, such as aqueous solutions of ammonia (NH3) or its organic derivatives (amines). These bases do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of hydroxide ion. Also, some non-aqueous solvents contain Brønsted bases which react with solvated protons. For example in liquid ammonia, NH2− is the basic ion species which accepts protons from NH4+, the acidic species in this solvent.
G. N. Lewis realized that water, ammonia, and other bases can form a bond with a proton due to the unshared pair of electrons that the bases possess. In the Lewis theory, a base is an electron pair donor which can share a pair of electrons with an electron acceptor which is described as a Lewis acid. The Lewis theory is more general than the Brønsted model because the Lewis acid is not necessarily a proton, but can be another molecule (or ion) with a vacant low-lying orbital which can accept a pair of electrons. One notable example is boron trifluoride (BF3).
Some other definitions of both bases and acids have been proposed in the past, but are not commonly used today.

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  1. P

    Acidity and basicity of some compounds

    Why do some compounds, such as amides, ketones, aldehydes, ethers, behave neither as acids nor as bases in water? Does it depend on the difference between the pKa/pKb of water and the pKa/pKb of the compounds themselves?
  2. SpectraPhy09

    Chemistry Basicity of organic compounds

    I think the correct ans should be c (i.e C>D>A>B ) but its given d in my textbook( There can be a error also but I'm sure) I think C should have the highest basic strength since Hybridisation of N in this is Sp3 so it has the least electronegativity so it can donate its lone pair easily Then it...
  3. Vishesh Jain

    Steric inhibition of protonation in o-substituted anilines

    Homework Statement According to my textbook (Organic Chemistry by Solomon & Fryhle) "When an o-substituted aniline is protonated (at the N), increase of H-N-H bond angle leads to steric hindrance with the group at the o-position. This makes this protonated conjugate acid unstable and makes the...
  4. N

    Does resonance always affect acidity and basicity?

    Does resonance affect the acidity of an acid (or the basicity of its conjugate base) if the resonance isn't delocalizing the charge on the atom that is directly attached to the atom with the proton, hydrogen itself. For example a benzene ring with a carboxylic acid group attached to one side...
  5. baldbrain

    Basicity of Nitrogen Heterocycles

    Homework Statement Select the correct order of basicity of the following compounds: The Attempt at a Solution Obviously, (i) is more basic than (iv) because of the 2 acidic phenyl rings in (iv). Also, I know that amides are much less basic than amines. So, that leaves me with either (B) or (C)...
  6. KarolinaPL

    What is the basicity of carbon: primary, secondary, tertiary

    what is the basicity of carbon: primary, secondary, tertiary, e.g. in 1-adamantylamine (C connected with N is connected also with 3 other C, 2-adamantylamine (with two other C), 1-adamantymmethylamine (with just one next carbon)? more carbons connected to this first carbon: ...-C-NH2 makes it...
  7. M

    Order of basicity and lipophilicity in organic compounds

    Hello, I want to rank the following compounds in order of basicity: trimethylamine, tetramethylammonium chloride, 4-nitroaniline, aniline, acetanilide, 4-methylaniline. I think this is the order, but I'm not sure for the last ones. trimethylamine > 4-methylamine > aniline > 4-nitroaniline >...
  8. AdityaDev

    Does the basicity of hydrides increase with larger atom size?

    I am confused with the statement given by a website (http://www.chem.ucalgary.ca/courses/351/Carey5th/useful/acidbase.html "Size. When comparing atoms within the same group of the periodic table, the larger the atom the weaker the H-X bond and the lower the electron density making it a weaker...
  9. AdityaDev

    Basicity of hydrides if group 15 elements

    For group 15 elements the order of basicity given is NH3 > PH3 > AsH3 > SbH3 > BiH3 And order of reducing strength is BiH3 > sbH3 > AsH3 > PH3 > NH3 Why are they in opposite order? Reducing nature means tendency to donate electrons. Basicity means strength of bases and hence as basicity...
  10. Q

    Steric Effects on Basicity

    Homework Statement How do sterics affect basicity? Homework Equations Sterics has to do with kinetics. Basicity is a thermodynamic property. Nucleophilicity is a kinetic property. The Attempt at a Solution So is the answer "sterics don't affect basicity"? I can't see why sterics...
  11. R

    Acidity and basicity in amino acids

    Hello! I'm having a hard time understanding the acidity and basicity of amino acids. I read this: the acidic group in an amino acid is NOT the -COOH but the NH3+ and the basic group is COO- and not the NH2... why? Thanks in advance!
  12. Saitama

    Why is Diethylamine a Stronger Base Than Dimethylamine?

    This is no homework question, i came across this when i was checking out the pKb values for some amines. The pKb value of diethylamine is less that of dimethylamine in aqueous phase according to my textbook. This indicates that the former is more basic. My question is why diethylamine is a...
  13. F

    Basicity of Halogen Ions - Point Out My Error

    This isn't technically a homework problem; I'm reading about Sn2 reactions and the basicity of halogen ions came up and for some reason I can't get my head around this Chem I concept. Please point out the flaw in this thinking: 1) Fluorine (F) is a small, electronegative atom 2) F will...
  14. N

    Basicity of t-butoxide vs ethoxide

    why is t-butoxide a stronger base than ethoxide?
  15. Saitama

    Basicity of H3PO2: 1 or 3?

    Basicity is 1?? I was studying about Volumetric Analysis. In the chapter, there is a table which shows basicity of some acids. One is H3PO2. Its basicity is given as 1. But basicity of an acid is number of replacable hydrogen atoms present in one molecule of acid. So, bascitiy of H3PO2 should...
  16. C

    Neucleophilicity and Basicity

    I always have been confused about neucleophilicity and Basicity order of I- Br- Cl- F- whenever i think of this ... 2 things come in my mind: 1. As F has smallest size so it will hold e- pair strolgly and thus will provide a very strong center for attack by some + charge ... so it must be...
  17. S

    Basicity Order Questions: Exploring the Ortho Effect and Substituent Influence

    Hey folks. The orders given are stated in my book.I'm here to ask why. basicity: p-methyl aniline>m-()>aniline>o-() p exerts hyperconjugation and +I,m only +I ,but ortho exerts hyperconjugation and higher +I. Shouldn't o-methyl aniline have high basicity? basicity...
  18. P

    Nucleophilicity and basicity relationship

    Homework Statement This isn't so much a homework question as a question to help me undertstand how to do my homework. The book says that nucleophilicty roughly parallels basicity. Which makes sense since bases are electron pair donors, and an electron pair makes a nucleophile. but then...
  19. P

    Nucleophilicity and basicity

    I have read that basicity is compared by equlibrium and nucleophilicity by kinetics , then how do we compare basicity with nucleophilicity ?? That is if there is a negatively charged molecule and H+ and a carbocation , how do we come to know which will be attacked by the negatively charged...
  20. L

    Nucleophilic acyl substitution and basicity of groups

    I'm studying the reaction mechanisms for carboxylic acid and its derivatives and here it says whether a compound with a C=O bond undergoes nucleophilic addition (as in aldehydes and ketones) or nucleophilic acyl substitution depends on the relative basicities of the substituent group. For...
  21. H

    Basicity of histamine (chemistry)

    Homework Statement Arrange the nitrogen atoms of histamine in order of their decreasing basic strength. http://img134.imageshack.us/img134/7339/histamine.th.jpg [Broken] Homework Equations The more ready the nitrogen atoms donate lone pair, the greater the basicity; The Attempt at a Solution...
  22. K

    Phenylamine ammonia basicity

    propylamine ammonia basicity Homework Statement suggest which one of ammonia or propyl amine is more basic. Homework Equations The Attempt at a Solution i know that a base is a proton acceptor. i am trying to figure out which one of them would accept a proton more readily.
  23. P

    Increasing acidity n basicity

    i face difficulty in Q where we hav to arrange compounds/aromatic compounds in increasing order of acidity r basicity,please tell me easy way to solve such arranging sequences Q quickly.:confused:
  24. G

    Basicity of NaSH compared to NaOH?

    I am using NaSH to make a mercaptan by refluxing a bromo pyridine in ethanol. The only thing is that my pyridine also has an amide moiety, and I am afraid of the basicity of NaSH (since S is 1 below O on the periodic table). If I reflux this reaction I am afraid I might hydrolyze off the...
  25. A

    NH2- vs. H-: Electronegativity and Basicity

    Why is NH2- a stronger base than H-. I would think that due to N's greater electronegatavity, it would want to hold on to its electrons more, making it less basic.
  26. A

    Basicity of certain sites

    I had a question given, regarding the basicity of certain sites on a molecule. For example, glycine: How would you determine whether the OH end or the NH2 end would be more "basic", or the first to get protonated?
  27. K

    Relative basicity of two molecules.

    Hi can someone explain to me why is N-Acetylpiperidine is a weaker base than 1-aza-2-2oxobicyclo[2.2.2]octane ? Is it something about the ability to protonate the molecules ? thank you