Chemistry 240, Organic Chemistry 1 R, S stereoisomers: Rules for assigning R, S stereochemistry 1. Build the following molecule using your model kit. Prioritize the 4 atoms bonded to chirality center. CH3 H3CH2C 2. Practice redrawing the molecule such that the lowest priority group points towards the BACK. Determine whether this stereoisomer is R or S. 3. Repeat 1 and 2 with the following molecule: CH3 H2N' H. 4. Assign R and S stereochemistry to the chirality centers in the following molecules: OH Br CH3 CH3 5. Instead of redrawing a molecule to make the lowest priority group point towards the BACK, there is a trick you can use to determine R, S stereochemistry: a. Prioritize the four groups present b. Switch the lowest priority group with the group pointing back. By switching the two groups you've made the other enantiomer. Prove this to yourself! Once lowest priority group is pointing back, determine the stereochemistry. The original molecule has the stereochemistry that is the opposite of what you've just determined! C. Try trick from # 5 with the following molecule: CH3
Reactive Intermediates
In chemistry, reactive intermediates are termed as short-lived, highly reactive atoms with high energy. They rapidly transform into stable particles during a chemical reaction. In specific cases, by means of matrix isolation and at low-temperature reactive intermediates can be isolated.
Hydride Shift
A hydride shift is a rearrangement of a hydrogen atom in a carbocation that occurs to make the molecule more stable. In organic chemistry, rearrangement of the carbocation is very easily seen. This rearrangement can be because of the movement of a carbocation to attain stability in the compound. Such structural reorganization movement is called a shift within molecules. After the shifting of carbocation over the different carbon then they form structural isomers of the previous existing molecule.
Vinylic Carbocation
A carbocation where the positive charge is on the alkene carbon is known as the vinyl carbocation or vinyl cation. The empirical formula for vinyl cation is C2H3+. In the vinyl carbocation, the positive charge is on the carbon atom with the double bond therefore it is sp hybridized. It is known to be a part of various reactions, for example, electrophilic addition of alkynes and solvolysis as well. It plays the role of a reactive intermediate in these reactions.
Cycloheptatrienyl Cation
It is an aromatic carbocation having a general formula, [C7 H7]+. It is also known as the aromatic tropylium ion. Its name is derived from the molecule tropine, which is a seven membered carbon atom ring. Cycloheptatriene or tropylidene was first synthesized from tropine.
Stability of Vinyl Carbocation
Carbocations are positively charged carbon atoms. It is also known as a carbonium ion.
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