40 Questions
What is a molecule that is superimposable on its mirror image called?
Achiral
What is the term for isomers that differ only in the three-dimensional arrangement of substituents?
Stereoisomers
What is a carbon atom bonded to four different groups called?
Tetrahedral stereogenic center
What is the result of interchanging two groups on a tetrahedral stereogenic center?
The conversion of one enantiomer into another
What is a characteristic of achiral molecules?
The presence of a plane of symmetry
What is the definition of a chiral molecule?
A molecule that is not superimposable on its mirror image
What is the term for a mirror image that is not superimposable?
Enantiomer
What is the primary focus of stereochemistry in organic chemistry?
The structure of molecules in three dimensions
What is the main difference between constitutional isomers and stereoisomers?
The orientation of atoms in space
What do stereoisomers have in common?
Identical IUPAC names and functional groups
Which of the following is a characteristic of chiral molecules?
They are not superimposable on their mirror images
What is the difference between a chiral molecule and its mirror image?
They are not identical
What is the characteristic of achiral molecules?
They are superimposable on their mirror images
Why do stereoisomers have the same physical properties?
Because they differ only in their three-dimensional arrangement
What is the characteristic of a carbon atom that is a tetrahedral stereogenic center?
It is bonded to four different groups
Which of the following molecules has a plane of symmetry?
CH2BrCl
What should you omit from consideration when looking for stereogenic centers?
All C atoms that can’t be tetrahedral stereogenic centers
How many stereogenic centers does 2-butanol have?
One
What is the characteristic of a molecule with one tetrahedral stereogenic center?
It exists as a pair of enantiomers
Why is C3 a stereogenic center in 3-methylcyclohexene?
It is bonded to four different groups
What is the result of 3-methylcyclohexene having one tetrahedral stereogenic center?
It exists as a pair of enantiomers
What is the main component of a polarimeter that holds the substance being examined for optical activity?
A tube
What is the purpose of the two lenses in a polarimeter?
To pass light vibrating in the same plane
What happens when a substance does not affect the plane of polarization?
Light transmission is at a maximum
What is the term for a substance that rotates the plane of polarization to the right?
Dextrorotatory
What symbol is used to indicate a rotation to the left?
What is the purpose of adjusting the lenses in a polarimeter?
To allow a maximum amount of light to pass
What happens when a substance is optically active?
The lens nearer our eye must be rotated
What is the term for a substance that rotates the plane of polarization to the left?
Levorotatory
What is the function of the light source in a polarimeter?
To provide light for the polarimeter
A polarimeter consists of a light source, one lens, and a tube to hold the substance being examined.
False
The maximum amount of light reaches our eye when the two lenses are arranged to pass light vibrating in perpendicular planes.
False
A substance is said to be optically active if it does not affect the plane of polarization.
False
If the rotation of the plane of polarization is to the right, the substance is levorotatory.
False
The symbol (-) is used to indicate a rotation to the right.
False
A polarimeter is used to examine the optical activity of a substance.
True
A substance is optically inactive if it rotates the plane of polarization.
False
The lenses in a polarimeter are arranged to pass light vibrating in perpendicular planes when the tube is empty.
False
The rotation of the plane of polarization is measured by rotating the lens nearer the eye.
True
A polarimeter is used to measure the physical properties of a substance.
False
Study Notes
Stereochemistry
- Stereochemistry is the part of organic chemistry that deals with structure in three dimensions.
- Isomers are different compounds that have the same molecular formula.
Constitutional Isomers
- Constitutional (or structural) isomers differ in the way the atoms are connected to each other.
- Constitutional isomers have:
- Different IUPAC names.
- The same or different functional groups.
- Different physical properties.
- Different chemical properties.
Stereoisomers
- Stereoisomers differ only in the way atoms are oriented in space.
- Stereoisomers have identical IUPAC names (except for a prefix like cis or trans).
- Stereoisomers always have the same functional group(s) because they differ only in the three-dimensional arrangement of atoms.
Chiral and Achiral Molecules
- A molecule (or object) that is not superimposable on its mirror image is said to be chiral.
- Chiral molecules contain one or more stereogenic centers.
- A carbon atom bonded to four different groups is a tetrahedral stereogenic center.
- A molecule that is superimposable on its mirror image is said to be achiral.
Plane of Symmetry
- A plane of symmetry is a mirror plane that cuts a molecule in half, so that one half of the molecule is a reflection of the other half.
- Achiral molecules usually contain a plane of symmetry but chiral molecules do not.
Enantiomers
- Enantiomers are mirror images that are not superimposable.
- CHBrClF is a chiral molecule, and its mirror image is a different compound.
- Enantiomers are isomers differing only in the three-dimensional arrangement of substituents.
Polarimeter
- A polarimeter is used to examine the optical activity of a substance.
- If a substance does not affect the plane of polarization, it is optically inactive.
- If a substance rotates the plane of polarization, it is optically active.
Tetrahedral Stereogenic Centers
- Any carbon atom bonded to four different groups is a tetrahedral stereogenic center.
- To locate a stereogenic center, examine each tetrahedral carbon atom in a molecule and look at the four groups (not the four atoms) bonded to it.
- Always omit from consideration all C atoms that can’t be tetrahedral stereogenic centers, including CH2 and CH3 groups, and sp or sp2 hybridized C.
Drawing a Pair of Enantiomers
- Any molecule with one tetrahedral stereogenic center is a chiral compound and exists as a pair of enantiomers.
- To draw both enantiomers, use the typical convention for depicting a tetrahedron: place two bonds in the plane, one in front of the plane on a wedge, and one behind the plane on a dash.
Test your understanding of stereochemistry, a fundamental concept in organic chemistry that deals with the three-dimensional structure of molecules. This quiz covers the types of isomers, including constitutional and stereoisomers, and their differences in molecular formula and physical properties.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
