6 Write a chemical equation showing the reaction between ammonia and boron trifluoride BF3 in a Lewis acid-base reaction Show how the covalent bond forms?

Answer 1

The reaction between ammonia and boron trifluoride, BF3

If you have recently read the page on covalent bonding, you may remember boron trifluoride as a compound which doesn't have a noble gas structure around the boron atom. The boron only has 3 pairs of electrons in its bonding level, whereas there would be room for 4 pairs. BF3 is described as being electron deficient.

The lone pair on the nitrogen of an ammonia molecule can be used to overcome that deficiency, and a compound is formed involving a co-ordinate bond.

Using lines to represent the bonds, this could be drawn more simply as:

The second diagram shows another way that you might find co-ordinate bonds drawn. The nitrogen end of the bond has become positive because the electron pair has moved away from the nitrogen towards the boron - which has therefore become negative. We shan't use this method again - it's more confusing than just using an arrow.

Answer 2

NH3 + BF3 → NH3-BF3

In the reaction between ammonia and boron trifluoride, the lone pair on the nitrogen atom in ammonia donates an electron pair to the boron atom in boron trifluoride, forming a covalent bond between them. This forms a stable molecule known as an adduct.