protonation

Examples of protonation in the following topics:

• Polyprotic Acid Titrations

  • Polyprotic acid are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule.
  • Certain types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).
  • Oxalic acid is an example of an acid able to enter into a reaction with two available protons, having different Ka values for the dissociation (ionization) of each proton.
  • The diprotic acid has two associated values of Ka, one for each proton.
  • Triprotic acids can make three distinct proton donations, each with a unique Ka.

• Atomic Number and Mass Number

  • The atomic number is the number of protons in an element, while the mass number is the number of protons plus the number of neutrons.
  • Neutral atoms of an element contain an equal number of protons and electrons.
  • For example, carbon's atomic number (Z) is 6 because it has 6 protons.
  • Protons and neutrons both weigh about one atomic mass unit or amu.
  • For example, a lithium atom (Z=3, A=7 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 – 3 = 4).

• Nuclear Stability

  • The protons, which are both positively charged, repel one another through electrostatic force.
  • This force is offset by the nuclear force, which attracts protons and neutrons.
  • This is because, for any constant number of protons, the difference between nuclear force and electrostatic repulsion of protons increases with increasing neutron count.
  • An example is nickel-48, which has 28 protons and 20 neutrons, both of which are magic numbers.
  • Stability of isotopes is shown as a function of proton and neutron numbers.

• Brønsted Acids and Bases

  • A Brønsted acid is any species capable of donating a proton; a Brønsted base is any capable of accepting a proton.
  • To that end, if a compound is to behave as an acid by donating a proton, there must be a base to accept that proton; the Brønsted-Lowry concept is therefore defined by the reaction:
  • The conjugate base is the ion or molecule that remains after the acid has donated its proton, and the conjugate acid is the species created after the base accepts the proton.
  • The reaction can proceed either forward backward; in each case, the acid donates a proton to the base.
  • Here, H2O donates a proton to NH3.

• The Brønsted-Lowry Definition of Acids and Bases

  • A Brønsted-Lowry acid is any species capable of donating a proton; a Brønsted-Lowry base is any species capable of accepting a proton.
  • This is because if a compound is to behave as an acid, donating its proton, then there must necessarily be a base present to accept that proton.
  • Here, a conjugate base is the species that is left over after the Brønsted acid donates its proton.
  • The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid.
  • For instance, in the presence of ammonia, water will donate a proton and act as a Brønsted-Lowry acid:

• Diprotic and Polyprotic Acids

  • As their name suggests, polyprotic acids contain more than one acidic proton.
  • Two common examples are carbonic acid (H2CO3, which has two acidic protons and is therefore a diprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore a triprotic acid).
  • With any polyprotic acid, the first amd most strongly acidic proton dissociates completely before the second-most acidic proton even begins to dissociate.
  • Ka1 > Ka2); this is because the first proton to dissociate is always the most strongly acidic, followed in order by the next most strongly acidic proton.
  • For example, sulfuric acid (H2SO4) can donate two protons in solution:

• Salts that Produce Acidic Solutions

  • Salts with acidic protons in the cation are most commonly ammonium salts, or organic compounds that contain a protonated amine group.
  • Acid salts can also contain an acidic proton in the anion.
  • Examples of anions with an acidic proton include:
  • Each of these anions contains a proton that will weakly dissociate in water.
  • The NH3+ group contains an acidic proton capable of dissociating in solution; therefore, a solution of anilinium chloride in pure water will have a pH less than 7.

• Overview of Atomic Structure

  • Atoms consist of three basic particles: protons, electrons, and neutrons.
  • The hydrogen atom (H) contains only one proton, one electron, and no neutrons.
  • Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams.
  • Both protons and neutrons have a mass of 1 amu and are found in the nucleus.
  • However, protons have a charge of +1, and neutrons are uncharged.

• Proton NMR Spectroscopy

  • To begin with, the nmr spectrometer must be tuned to a specific nucleus, in this case the proton.
  • The diagram above shows a number of representative proton signals will be displayed over the same magnetic field range.
  • It is not possible, of course, to examine isolated protons in the spectrometer described above; but from independent measurement and calculation it has been determined that a naked proton would resonate at a lower field strength than the nuclei of covalently bonded hydrogens.
  • Why should the proton nuclei in different compounds behave differently in the nmr experiment?
  • The answer to this question lies with the electron(s) surrounding the proton in covalent compounds and ions.

• Average Atomic Mass

  • For example, the element hydrogen (the lightest element) will always have one proton in its nucleus.
  • The element helium will always have two protons in its nucleus.
  • This is because each proton and each neutron weigh one atomic mass unit (amu).
  • The atomic number of chlorine is 17 (it has 17 protons in its nucleus).
  • Stylized lithium-7 atom: 3 protons (red), 4 neutrons (blue), and 3 electrons (black).