Ch. 5 - ChiralitySee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

One of the special features of chiral molecules is that they are able to rotate plane-polarized light. Unfortunately, this means that now professors have an excuse to ask you math problems. Let’s see how this works. 

The Concept of Optical Activity

Concept #1: Specific rotation vs. observed rotation.  

  • Clockwise rotation = dextrorotary (d) or (+)
  • Counterclockwise rotation = levororatory (l) or (-)

These random names/signs have nothing to do with the chirality of a molecule!

Additional Problems
Which of the following is true about any  R enantiomer?   (1) It has a (+) rotation in a polarimeter. (2) It has a (-) rotation in a polarimeter. (3) It is a mirror image of the S enantiomer.  (4) When mixed with the S enantiomer, it forms a meso compound.
Determine whether each statements is true or false: (a) A racemic mixture of enantiomers is optically inactive. 
A forensic chemist (chemist who deals with crime-related investigations) analyzed a sample of a poisonous alkaloid (generic term for a naturally occurring nitrogen-containing molecule) and determined that its specific rotation [α] at 25 °C using sodium D light was + 12 °. What can the chemist properly conclude?  a) there is one enantiomer in the sample b) the enantiomer present has the R configuration c) the sample is optically pure d) the butler did it e) none of the above
All S enantiomers rotate the plane of polarized light anti-clockwise ( True or False ) 
Which statement is true? a) Compounds with R stereocenters rotate plane-polarized light clockwise.  b) For equal concentrations and equal path lengths, solutions of (+) and (-) enantiomers rotate plane-polarized light equally, but in opposite directions.  c) Racemic mixtures can rotate plane-polarized light either clockwise or counterclockwise. d) Meso compounds can rotate plane-polarized light either clockwise or counterclockwise. 
One of the following isn't true about a racemic mixture. a) It is optically inactive b) It has equal amounts of enantiomers c) It doesn't rotate plane polarized light d) Its ee is 50% e) all are true
The specific rotation of (S)-carvone (at 20°C) is +61. A chemist prepared a mixture of (R)-carvone and its enantiomer, and this mixture had an observed rotation of –55°. What is the specific rotation of (R)-carvone at 20°C?
Which of the following molecules is optically active?
Pure (S)-2-butanol has a specific rotation of +13.5 degrees. You have made and purified a sample that has a calculated specific rotation of -6.76 degrees. What can you conclude about this sample?      
The following statements describe some important properties of molecules. Choose from among the following three types of isomers and in the space provided write the letter of the one or more types of isomers for which the statemetn is TRUE.A. A single enantiomer          B. A meso compound          C. A racemic mixtureA solution of this will rotate the plane of plane polarized light.   _________________A solution of this will NOT rotate the plane of plane polarized light.   _____________________Might be produced when an alkene with no chiral centers reacts with a non-chiral reagent such as H-Br or Br2.  ___________________The LEAST desirable option when developing a new drug  ___________________
Which of these tartaric acid isomers will have the same melting point?a) (+)-tartaric acid and (-)-tartaric acid onlyb) (+)-tartaric acid and (-)-tartaric acid and racemic (±)-tartaric acid onlyc) (+)-tartaric acid and (-)-tartaric acid and meso-tartaric acid onlyd) meso-tartaric acid and racemic (±)-tartaric acid only
Which compound, if any will be optically active?