Now i got a question
So in the Examples, why does the numbering start where it starts? Like why in 3 does it start closer to the methyl but in 4 it starts at the ethyl instead?
Thanks
Firstly, the ethyl is written before the methyl because we want to maintain alphabetical order. E lies before M in the English the alphabet. But it seems like you already knew that.
We would like to choose, so that we always minimise the sum of the locant numbers in the molecule.For Q3, if we count from left to right we will have
3-ethyl-5-methyl-hex-3-ene
3 + 5 = 8
If we count right to left we will have
4-ethyl-2-methyl-hex-3-ene
4 + 2 = 6
So we favour the latter.
For Q4, firstly keep in mind that there is an ethyl group on the right, because the double bond is what gives pentadiene it's name.
Now, using the same process
If we count from left to right we will have
4-ethyl-2,3-dimethyl-1,4-pentadiene
If we count from right to left we will have
2-ethyl-3,4-dimethyl-1,4-pentadiene
And here we have a problem. From maths, we know that 4+2+3=2+3+4. Oh dear!
Which is exactly the point I call you out for being a selective school student (or attempt to) and having to do harder questions.
The reason why the second one is favourable is that in the event that your sum of locants are the same
(very unlikely to occur in the HSC), we again treat alphabetical order as priority. E lies before M, so the ethyl will get the lower number here.