Chiral center (also called stereogenic center, chiral carbon and stereo center) is a carbon that is attached to four different groups. In order to identify whether there are four different groups, we must go atom by atom until we find a difference. If no difference is found, the groups are identical.
I suggest first showing all the hydrogens that were not shown on a bond angle formula. Remember, a carbon wants to have four bonds so a carbon with two bonds, for example, must have two hydrogens that were not shown. Carbons with two or three hydrogens can not be chiral centers since they can not have four different groups (they already have two or three of the same groups: hydrogens). Carbons that are on double or triple bond, likewise, can not be chiral centers either.
Let's take a look at some examples:
In molecule a, let's look at the carbon with an asterisk. This carbon is directly attached to a bromine, a hydrogen, carbon on the left and another carbon on the right. Bromine and hydrogen are clearly different but what about the carbons? The carbon on the left is attached to hydrogens only (not shown), while the carbon on the right is attached to another carbon. Therefore all four groups are different.
In molecule b, the carbon with an asterisk is attached to a hydrogen, a methyl group, a carbon on the left and another carbon on the right. On the left the carbon is attached to another carbon while on the right, the carbon is attached to another carbon through a double bond. That's a difference! Therefore, there are four different groups here as well.
Let's now take a look at some of the videos to understand chiral centers more.
How to find chiral centers when a name is given?
How to find chiral centers in cholesterol?
How to find chiral centers and determine the number of stereoisomers possible.
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