In order to get the reactions of alkenes straight, it’s an extremely useful exercise to go through each one and figure out the bonds that form and the bonds that break. Like this:
Once you get enough practice with learning reactions of alkenes in the forward direction – paying attention to the bonds that form and the bonds that break – it’s an extremely useful exercise to try some “thought experiments” that put the reaction in the reverse direction.
The process is kind of like when you learned arithmetic in grade school. After a long time answering questions like 2+3 = ? , 4+5 = ? , 2+6 = ?, your teacher mixed things up a bit by introducing you the skill of subtraction:
2 + ? = 9
The same concept applies to reactions in organic chemistry. Once you know how the pattern works in the forward direction, you can apply the reverse of the pattern to see how the reaction would work in the reverse direction. We’re asking: “What starting material would give this product?”.
Important misconception – we’re not implying here that the reaction is reversible. It’s not. This is a mental exercise: we’re asking ourselves, “if this is the product of this reaction, what was the starting material?”.
The cool thing is, once you know the pattern of bonds that form and bonds that break for a given reaction, you can apply this pattern to work backwards, by switching the columns. So take the bonds that form, and move them to the “break” column, and take the bonds that break, and move them to the “form” column. Then apply this to the molecule (answer below).
Doesn’t this remind you just a little bit of subtraction, when you move a number over to the other side of the equals symbol and you change the sign? It’s the same principle.
When you’re able to do this for reactions of alkenes, you’ll be in good shape for synthesis questions.
Tomorrow: let’s finish alkenes with one last reaction.
Thanks for reading! James
P.S. Answer – 2-methyl-1-pentene