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CH-005 - Isomers Cheat Sheet by

this sheet is red, like my blood, because i don't like this topic

Personal Frustr­ations

Today I looked at my hands and I realized they are enanti­omers of each other and now I'm stuck in a nightmare world where I can't escape from these things.
Help!

Basic Structural Isomers

These structural isomers all have fancy names. We need to learn what these are.

HOW TO FIND ALL THE ISOMERS OF A HYDRO-­CARBON

This is probably not super useful, and I might be missing stuff. It's just a basic checklist for my sanity :S
1. Find all the structural isomers (lol enjoy T_T)

2. Check for any chiral carbons. If there are any, you need to find the S and R enanti­omers. There might be several chiral carbons per molecule, and if so, you'll need to find every permut­ation of S- and R- carbons!

3. Check for any double­-bonds. If there are any and you can assign priorities to the groups on each carbon, you need to find the E and Z isomers.
Then go through every single one until your hands fall off from writing so much :D
 

WHAT TYPES OF ISOMER ARE THERE?

Isomers are DIFF­ERENT MOLECU­LES that share the same chemical formula.

In the order they were introd­uced:
1. Structural isomers
2. Enanti­omers (S/R)
3. E/Z (trans­/cis) isomers

STRUCTURAL ISOMERS

Things can be 'struc­tural isomers', meaning they have the same chemical formula, but in a completely different structure.
This is the one where you just put the bits in uniquely different places.
butane -> 2-meth­ylp­rop­ane
(note: 2-meth­ylp­ropane is also known as iso-bu­tane! see the table to the left)

Enanti­omers (S/R)

Enanti­omers are mirror­-images of each other.
This is the case when one of the carbon atoms is chiral, or when that carbon is bonded to 4 unique groups.
An enantiomer is either (S) or (R).
HOW TO FIND (S/R):
1. Assign 'prior­ities' -- highest priority goes to the highest atomic number. If there are multiple atoms of the same atomic number, give higher priority to the group that has the most overall stuff in it.
2. Send the lowest priority atom to the back. This is usually hydrogen.
3. Count 1->­2->3, where #1 is the highest priority.
4. If counting 1->­2->3 takes you clockwise, then it's (R). If anti-c­loc­kwise, it's (S).
 

Confor­mat­ions?

These things are rotational confor­mations as viewed head-on. They can be either:

ECLI­PSED
-> Totally eclipsed
-> Partially eclipsed
STAG­GERED
-> Gauche
-> Anti

Confor­mations are literally just the molecule twisting around single­-bonds. They're not techni­cally isomers, just the same molecule but in different shapes.

E/Z (trans­/cis) Isomers

These only occur around a double­-bond. A hydroc­arbon with a double­-bond is known as an alke­ne.
They're relatively easy to figure out, at least.
HOW TO FIND (E/Z):
1. Look at the 2 groups on each of the 2 carbons.
2. Assign priorities on each side.
3. If both carbons have their higher priorities on the same side, it's a cis or (Z) isomer. If they're on different sides, it's a trans or (E) isomer.

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