McMurry (Kap 12,11) Flashcards
What is the difference between the Lewis definition of an acid and base from the Brønsted-Lowry definition?
It is broader and more encompassing, because it is not only limited to substances that donate and accept H+(protons).
What is the definition of a Lewis Acid?
It is a substance that accepts an electron pair.
What is the definition of a Lewis base?
It is a substance that donated an electron pair.
(the donated electron pair is shared between the acid and the base in a covalent bond).
Under what premises does a Lewis acid accept an electron pair from the lewis base?
1) If it has a vacant orbital (for example BF3). Typically compounds from group 3A in the periodic table.
2) If it has a polar bond to Hydrogen, and can donate the hydrogen (proton H+) to a base and obtain the electrons from that bond to itself.
3) Various metal cations (for instance Mg2+) - because they accept a pair of electrons when they form a bond. Also (Li+).
4) Many transition metal compounds such as TiCl4, FeCl3, ZnCl2, SnCl4.
What is the definition of a Lewis base?
A compound with a pair of nonbinding electrons that can be used to bond to a lewis acid.
What atoms in organic compounds can act as a lewis base?
Nitrogen and Oxygen containing compounds, because they both have lone pairs.
Which compounds can both act as an acid and base?
Carboxylic acids and alcohols, because they have both a polar hydrogen bond and a lonepair on oxygen.
When they donate H+ they act as an acid, but when their Oxygen accept a H+/lewis acid, it is a base.
Some lewis bases have more than one atom with lonepairs (for example acetic acid), and can therefore react in more then one site.
What determines which site it chooses to react as a base?
Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed.
The protonation of the double bonded oxygen is preferred over the single bonded oxygen (in acetic acid), because that product is stabilized by two resonance forms, unlike the other one which will cause a 2+ charge on oxygen if resonance.
