[sharbeldam]

So if you look at the molecule on the left, you can see its counterclock wise (S) but since group priority 4 isn't going inside I should change it to R. but in this case it doesn't work.
I really thought this way always works, so to be sure, is it better to just always switch two groups instead?

Thanks! hope you get my question

[Hunter2]

You have to exchange the substituents in this way that substituent of lowest order is on the back. In this case its the hydrogen. This you do if you exchange ethyl with hydrogen and methyl with propyl. Important is you need two exchanges to get the same molecule, otherwise you change R to S or wiseversa.
The first molecule is S configuration and the second one R. 1. propyl, 2. ethyl 3. methyl, 4.hydrogen.

[sharbeldam]

Yes I get that always works, but the first way that i suggested, i thought it always worked, but in this case it doesnt? or am i doign something wrong?
Ive been taught many years ago:
1) do normal configuration, then check priority 4 , IF its not in back, then change configuration. if it is in back, then keep it like it is.

[Hunter2]

Yes that is what I described. Here H is not in the back so change configuration 2 times.

[sharbeldam]

thanks ! so weird that this first method is much faster and always worked until i got this example :/

[Hunter2]

I dont understand  what you mean. Give an example.

[sharbeldam]

Way was like this, i do it normally like the pic, i get S, **BUT** since priority 4 is not in the back then the molecule is R.

(obviously i realized it doesnt work...)

[Hunter2]

No, you cannot say if  the hydrogen is not in the back, the molecule is R or S. You have to Transfer it as I explaned and then use Rule of CIP.

[spirochete]

If the hydrogen is in the plane of the paper, then the direction of 1->3 is meaningless in the original picture. This is very different from the hydrogen going toward you, such as on a wedge.

In this case you have to either reorient the molecule mentally, or you could do a double swap: switch #4 with the dash, and also switch the other two groups. This gives a new picture of the same molecule, and you can assign the configuration to this new picture. This is technically just drawing the molecule from a new perspective, although it requires zero 3-dimensional aptitude. It just involves following a series of instructions that you could memorize.