1. What does a molecule need to have to be considered chiral? (8 points) 2. Whic
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Question
1. What does a molecule need to have to be considered chiral? (8 points) 2. Which of the following molecules could be chiral? (8 points) OH CI H2 The specific rotation of [Rt] camphor is +41 degrees. (a) What is the specific rotation of [S -] camphor? (6 points) (b) What would be the rotation of a 50/50 mixture of R and S camphor? Circle the chiral centers for the following compounds.(12 points) 3· 4. Br H NH CH " OH CH3 Br OH 5. What instrument is used to determine the optical rotation of a sample? (8 points) 6. Give the absolute rotation (R or S) of the following compounds. (10 points) H. C H3C-Cc-CH3 H2Explanation / Answer
1. For a molecule to be considered chiral it should possess two more chiral centers or asymmetric carbons and be devoid of any symmetry elements. A chiral center refers to any carbon in a molecule that is attached to four different substituents. When the substituents directly attached to the asymmetric or chiral carbon are the same, then the molecule is traced on atom-by-atom from the chiral carbon till the first point of difference.
Chirality arises due to the non-superimposable mirror image like orientation of two molecule of similar linkage and molecular formula existing with just different orientation of the attached group in space. Depending upon the orientation of the substituents and their atomic numbers, based upon the Cahn-Ingold-Prelog (CIP) Rules, the absolute configuration of each chiral center is given as R and S for right and left-handed isomers respectively.
In certain molecules having more than one chiral center, at times the chiral centers are aligned such that there exists a plane of symmetry in the molecule. Such compounds are called meso-compounds and are considered achiral on the whole.
Chiral molecules that are non-superimposable mirror images of each other are called enantiomers. In case of more than chiral center in a molecule and a pair not being enantiomers are termed a diastereomers.
2. Of the four molecules given, benzene is identical throughout and 2-propanol has two methyl groups attached to the same carbon thus rendering them achiral. The alkene however does not have four different groups and just three and the tetrahedral arrangement of carbon responsible for chirality is also absent making that too achiral. Finally, the first molecule on the left - 2-chlorobutane is chiral, with the chiral center on the carbon containing the chlorine atom. This is due to that carbon containing four different groups namely a hydrogen, a methyl group, an ethyl group and a chloride.
3. Chiral compounds have the ability to rotate plane polarized light in a certain direction depending upon their configuration at their chiral center(s). Usually R enantiomers turn the light right and S enantiomers turn them left. Since they are both exactly the same chemically and vary only with th alignment in space, when an enantiomer rotates plane polarized light in one direction to a certain magnitude, the other enantiomer will rotate the light in exactly the opposite direction to the same extent only. Thus, if the R enantiomer has +41, then the S enantiomer will rotate it by -41 degrees.
A racemic mixture contains 50/50 ratio of both enantiomers. Therefore, it will have equal quantity of enantiomers trying to rotate the plane polarized light in completely opposite directions to the same extent. This gives racemic mixtures no optical activity and have a rotation of 0 degrees.
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