Which of the following is a Lewis base? A) HBr, B) CH₄, C) NH₃, D) NH₄⁺ or E) CH₃CH₂CH₃.
A Lewis base is a special type of base, a substance that does or can donate a lone pair of electrons to another substance. So we’re not dealing with Bronsted–Lowry acids and bases which have to do with the transfer of protons or hydrogen ions. We have five candidates here. So we can start our filtering these by checking whether they have a lone pair at all.
HBr or hydrogen bromide has this Lewis structure. Bromine is in group 17, sometimes known as group seven. So it has seven valence electrons. While hydrogen is in group one, giving us eight valence electrons to play with. We can fill the outer shell of hydrogen, which contains a maximum of two electrons, with a single bond between the hydrogen and the bromine. And we use the six electrons left over to complete bromine’s octet. So HBr passes this test with flying colors. We have lots of lone pairs on the bromine. That’s not to say it’s definitely a good Lewis base. There are other tests to follow. But let’s pass all the components through this test first.
Methane, with symbol CH₄, has this structure. Carbon from group 14 has four valence electrons, which it uses completely to form single bonds to the hydrogens. So CH₄ fails our first test. There are no lone pairs. Nitrogen is in group 15, so it has five valence electrons. Three of those go to bonds with hydrogen leaving two for a lone pair. So NH₃, otherwise known as ammonia, has lone pairs. And it’s a decent candidate to be a Lewis base.
On NH₄, ammonium, on the other hand, we’ve used up that lone pair bonding to a proton, leaving the positive ammonium group. So ammonium has all bonding pairs and no lone pears. And lastly, we have propane, CH₃CH₂CH₃. All the valence electrons of those carbon atoms are fully occupied in single bonds to hydrogens or carbons. So we have no lone pairs to play with and propane cannot be a Lewis base. Our two contenders for the final round are HBr, hydrogen bromide, and NH₃, ammonia.
The next test, are they Lewis Basic? The test for whether something is Lewis basic is to see whether it reacts with a Lewis acid. A Lewis acid is simply a substance that receives a lone pair. A good Lewis acid is BH₃. Borane, the boron in the middle of the molecule is electron deficient. Having only three valence electrons to begin with and six valence electrons once bonded to the hydrogens. This means it’s too short of having a full valence shell. So it’s favorable for borane, BH₃, to receive a lone pair from another substance fulfilling its octet.
When mixed with borane, there’s no direct reaction between hydrogen bromide and borane. This is because the lone pairs in hydrogen bromide are not very basic. The story is quite different with the bromide ion, where the negative charge makes that lone pair more basic. But that’s not the case here. But when we use ammonia in place of hydrogen bromide, we get a reaction. The more basic lone pair of ammonia fills up the octet for boron producing H3NBH₃. So of the five candidates given, the one that is a Lewis base is NH₃, ammonia.