4)Label the following Lewis acid and Lewis base and draw Lewis structures showing the mechanism using curved arrows. a) F + BF3 ? BF4 b) NH3 + HBr ? NH4+ Br-
The Correct Answer and Explanation is :
Question:
Label the Lewis acid and Lewis base in the following reactions, and use Lewis structures with curved arrows to show the mechanism.
- ( F^- + BF_3 \rightarrow BF_4^- )
- ( NH_3 + HBr \rightarrow NH_4^+ + Br^- )
Answer and Explanation
1. ( F^- + BF_3 \rightarrow BF_4^- ):
- Lewis Acid: ( BF_3 )
Boron in ( BF_3 ) has an incomplete octet and can accept an electron pair. - Lewis Base: ( F^- )
The fluoride ion (( F^- )) has a lone pair of electrons that it can donate.
Mechanism:
- The lone pair of ( F^- ) is donated to the empty ( p )-orbital of boron in ( BF_3 ).
- This results in the formation of ( BF_4^- ), where boron now has a complete octet.
Curved Arrow Notation:
Draw an arrow from the lone pair on ( F^- ) to the boron atom in ( BF_3 ).
2. ( NH_3 + HBr \rightarrow NH_4^+ + Br^- ):
- Lewis Acid: ( HBr )
The hydrogen atom in ( HBr ) can accept a lone pair from ( NH_3 ) as it is partially positively charged. - Lewis Base: ( NH_3 )
Ammonia (( NH_3 )) has a lone pair of electrons that can be donated.
Mechanism:
- The lone pair of electrons on the nitrogen atom in ( NH_3 ) attacks the hydrogen atom in ( HBr ).
- This breaks the ( H-Br ) bond, transferring the electrons to the bromine atom, forming ( NH_4^+ ) and ( Br^- ).
Curved Arrow Notation:
- Draw an arrow from the lone pair on nitrogen in ( NH_3 ) to the hydrogen atom in ( HBr ).
- Draw another arrow from the ( H-Br ) bond to bromine.
Explanation (300 Words):
A Lewis acid is defined as an electron-pair acceptor, and a Lewis base is an electron-pair donor. In the first reaction, ( BF_3 ) acts as the Lewis acid because boron has an incomplete octet and can accept electrons. The ( F^- ) ion, being rich in electrons, donates a lone pair, forming a coordinate covalent bond with boron. The final product, ( BF_4^- ), is stable as boron achieves an octet configuration.
In the second reaction, ( NH_3 ) acts as the Lewis base due to the lone pair on nitrogen, which can donate electrons. ( HBr ), on the other hand, acts as the Lewis acid as the hydrogen atom is electron-deficient due to its bond with electronegative bromine. When ( NH_3 ) donates its lone pair to hydrogen, the bond between ( H ) and ( Br ) breaks, resulting in ( NH_4^+ ) and ( Br^- ). This is an acid-base reaction where ( HBr ) also behaves as a Brønsted-Lowry acid by donating a proton.
The use of curved arrows is crucial in representing electron movement during these reactions. Arrows originate from the lone pair or bond being donated and point toward the atom accepting electrons. This formalism highlights the flow of electrons, helping in understanding the bonding changes. These reactions emphasize the versatility of Lewis acid-base theory, extending beyond traditional hydrogen-ion exchanges.