Optical Activity_Short Answer_Review_Young

What are the two broad classes of isomers?

Answer: Constitutional isomers and stereoisomers.

How do constitutional isomers differ?

Answer: They differ in their bonding sequence; their atoms are connected differently.

What distinguishes stereoisomers from constitutional isomers?

Answer: Stereoisomers have the same bonding sequence but differ in the orientation of their atoms in space.

Fill in the blank: Stereochemistry is the study of the ________ structure of molecules in space.

Answer: Three-dimensional.

True or False: Stereoisomers always have identical chemical properties.

Answer: False. Stereoisomers often have different physical, chemical, and biological properties.

Which type of isomer has atoms connected differently?

Answer: Constitutional isomers.

What is an example of a stereoisomer that has different biological properties?

Answer: Cis and trans isomers of butenedioic acid.

True or False: Stereoisomers have different bonding sequences.

Answer: False. They have the same bonding sequence but differ in spatial orientation.

What is another name for cis-trans isomers?

Answer: Geometric isomers.

What is the chemical formula for both maleic acid and fumaric acid?

Answer: HOOC-CH=CH-COOH.

Which isomer of butenedioic acid is toxic and irritating?

Answer: Maleic acid (cis isomer).

Why is fumaric acid biologically important?

Answer: It is an essential metabolic intermediate in plants and animals.

Fill in the blank: The cis isomer of butenedioic acid is called ________, while the trans isomer is called ________.

  Answer: Maleic acid; fumaric acid.

True or False: Cis-trans isomers have the same physical and biological properties.

Answer: False. They have different physical and biological properties.

Which isomer of butenedioic acid is a trans isomer?

Answer: Fumaric acid.

What does the term "chirality" mean?

Answer: Handedness.

What happens if an object cannot be superimposed on its mirror image?

Answer: The object is chiral.

Fill in the blank: A chiral molecule and its mirror image are ________.

Answer: Non-superimposable.

True or False: A pair of gloves is an example of achiral objects.

Answer: False. Gloves are chiral.

What is the relationship between two chiral objects?

Answer: They are non-superimposable mirror images.

Give three examples of chiral objects.

Answer: Hands, feet, screws.

What is the definition of an achiral object?

Answer: An object with a mirror image that is identical to itself.

True or False: Achiral objects have mirror images that are identical to the object.

Answer: True.

If a molecule’s mirror image is superimposable on the molecule, is it chiral or achiral?

Answer: Achiral.

What is an example of a chiral object often found in mechanical tools?

Answer: Screws or corkscrews.

True or False: All molecules have superimposable mirror images.

Answer: False. Some molecules have non-superimposable mirror images.

What is a stereogenic center?

Answer: A carbon atom bonded to four different groups, making it a chiral center.

True or False: A molecule without a chiral carbon is always chiral.

Answer: False. It is usually achiral.

Fill in the blank: A molecule with two or more chiral carbons ________ be chiral.

Answer: May or may not.

What is an example of a molecule with one chiral carbon?

Answer: 2-bromobutane.

Fill in the blank: A molecule with no chiral carbons is ________.

Answer: Usually achiral.

Why is cis-1,2-dichlorocyclopentane achiral?

Answer: Because its mirror image is superimposable.

What type of isomer is the trans isomer of 1,2-dichlorocyclopentane?

Answer: Chiral (non-superimposable mirror image).

What are enantiomers?

Answer: Pairs of non-superimposable mirror images of a molecule.

True or False: Achiral compounds have enantiomers.

Answer: False.

What is the chirality of a molecule with one stereogenic center?

Answer: Chiral.

What is another term for a chiral carbon atom?

Answer: Asymmetric carbon or stereogenic center.

What is the designation for a chiral carbon atom in structural formulas?

Answer: An asterisk (*).

What are the rules to determine whether a molecule is chiral?

Answer:

    1. If there is no chiral carbon, the molecule is achiral.

    2. If there is one chiral carbon, the molecule is usually chiral.

    3. If there are two or more chiral carbons, the molecule may or may not be chiral.

True or False: Rings are essential for chirality.

Answer: False.

What is the significance of 2-bromobutane in chirality studies?

Answer: It is chiral and exists in two enantiomeric forms.

Fill in the blank: A chiral molecule has a carbon atom with ________ different groups attached to it.

Answer: Four.

What does the presence of an internal mirror plane of symmetry indicate?

Answer: The molecule is achiral.

True or False: The absence of a mirror plane always means the molecule is chiral.

Answer: False.

Why is cis-1,2-dichlorocyclopentane achiral?

Answer: It has an internal mirror plane of symmetry.

Does trans-1,2-dichlorocyclopentane have a mirror plane of symmetry?

Answer: No, it does not.

Fill in the blank: Any compound with an internal mirror plane of symmetry cannot be ________.

Answer: Chiral.

True or False: A molecule with an asymmetric carbon is always chiral.

Answer: False. It may have an internal mirror plane of symmetry.

What is optical activity?

Answer: The rotation of the plane of polarized light by a chiral compound.

What kind of compounds are optically active?

Answer: Chiral compounds.

What instrument is used to measure optical activity?

Answer: Polarimeter.

What is the observed rotation of light symbolized by?

Answer: α (alpha).

What are dextrorotatory compounds?

Answer: Compounds that rotate plane-polarized light clockwise.

What are levorotatory compounds?

Answer: Compounds that rotate plane-polarized light counterclockwise.

Fill in the blank: Dextrorotatory compounds are designated as ________, while levorotatory compounds are designated as ________.

Answer: (+) or d; (-) or l.

True or False: Enantiomers rotate light in the same direction.

Answer: False. They rotate light in equal but opposite directions.

What factors affect observed rotation in a polarimeter?

Answer: Concentration of the solution, length of the sample cell, and the optical activity of the compound.

What is specific rotation?

Answer: The rotation observed in a polarimeter standardized to a 10 cm sample cell and 1 g/mL concentration.

What is a racemic mixture?

Answer: A mixture containing equal amounts of two enantiomers.

Why are racemic mixtures optically inactive?

Answer: The optical rotations of the two enantiomers cancel each other out.

What symbol is used to denote a racemic mixture?

Answer: (+) or (d,l).

Why are racemic mixtures common?

Answer: Because it is difficult to produce exclusively one enantiomer.

What system is used for assigning (R) and (S) configurations?

Answer: Cahn-Ingold-Prelog system.

What is the first step in assigning (R) or (S)?

Answer: Assigning priority to the four groups attached to the chiral carbon.

What do the horizontal lines in a Fischer projection represent?

Answer: Bonds coming out of the plane of the paper.

What do the vertical lines represent?

Answer: Bonds going into the plane of the paper.

True or False: Rotating a Fischer projection by 180° gives the same molecule.

Answer: True.

True or False: Rotating a Fischer projection by 90° gives the same molecule.

Answer: False. It gives the enantiomer.

How is the carbon chain positioned in a Fischer projection?

Answer: Along the vertical line, with the most highly oxidized carbon substituent at the top.

Fill in the blank: A correctly drawn Fischer projection must have the ________ group at the top.

Answer: Most oxidized.

What happens when you flip a Fischer projection?

Answer: It gives an incorrect representation of the molecule.

How do you draw the mirror image of a Fischer projection?

Answer: Exchange the left and right positions while keeping the top and bottom unchanged.

In a Fischer projection, if the mirror image cannot be made to look like the original by a 180° rotation, is the molecule chiral or achiral?

Answer: Chiral.

What happens if the lowest priority group is placed on the horizontal axis?

Answer: The assignment is reversed (clockwise becomes S, and counterclockwise becomes R).

How do you determine the priority of substituents in a Fischer projection?

Answer: By looking at the atomic number of the atoms attached to the chiral carbon.

Fill in the blank: In Fischer projections, clockwise rotation with the lowest priority group on the horizontal axis is assigned as ________.

Answer: S.

What is the priority order for the groups in glyceraldehyde (CHO, OH, CH2OH, H)?

Answer: 1. OH > 2. CHO > 3. CH2OH > 4. H.

If the arrow from the highest to lowest priority groups is counterclockwise, what configuration does the molecule have?

Answer: S configuration.

In Fischer projections, what is the result of interchanging two groups around the stereogenic center?

Answer: It results in the enantiomer of the original molecule.

What are diastereomers?

Answer: Stereoisomers that are not mirror images of each other.

Fill in the blank: Enantiomers are ________ images, while diastereomers are ________.

Answer: Mirror; not mirror images.

True or False: Diastereomers have identical physical properties.

Answer: False. They have different physical properties.

What are two types of diastereomers?

Answer: Geometric isomers (cis-trans) and molecules with two or more chiral centers.

How many stereoisomers are possible for a molecule with 2 stereogenic centers?

Answer: 4 stereoisomers (2²).

What is the formula to calculate the maximum number of stereoisomers for a molecule with n stereogenic centers?

Answer: 2ⁿ.

What distinguishes diastereomers from enantiomers?

Answer: Diastereomers are not mirror images, whereas enantiomers are.

What is an example of geometric diastereomers?

Answer: Cis and trans isomers of 2-butene.

True or False: Diastereomers can have different boiling and melting points.

  Answer: True.

How are diastereomers separated in the lab?

Answer: By normal techniques like distillation, recrystallization, or chromatography.

True or False: Enantiomers are easier to separate than diastereomers.

Answer: False. Diastereomers are easier to separate.

Fill in the blank: Diastereomers have ________ physical properties, while enantiomers have ________ physical properties except for optical activity.

Answer: Different; identical.

What is plane-polarized light?

Answer: Light that vibrates in only one plane.

What happens to plane-polarized light when it passes through a chiral compound?

Answer: It is rotated.

What is the rotation of plane-polarized light by a chiral compound called?

Answer: Optical activity.

What type of light source is used in a polarimeter?

Answer: Monochromatic light.

How does a polarimeter measure optical rotation?

Answer: By measuring the angle of rotation (α) of plane-polarized light after it passes through a chiral solution.

True or False: Enantiomers rotate light in the same direction.

Answer: False. They rotate light in equal but opposite directions.

Why are enzymes capable of chiral discrimination?

Answer: Because enzymes are chiral.

What happens if only one enantiomer fits into the chiral active site of an enzyme?

Answer: The enzyme can distinguish between the enantiomers.

True or False: The receptor sites for the sense of smell can distinguish between enantiomers.

Answer: True.