Ch 4: Structure and Stability (Princeton Review)

Question 1

What is an achiral object?

example?

Answer 1

An object that IS superimposable upon its mirror image (after being rotated). The best example is a coffee cup.

(Khan Academy video)

Question 2

What is a chiral object?

Answer 2

An object that is NOT superimposable upon its mirror image. (Remember that the word chiral comes from Greek kheir which means hand, and our hands are chiral objects.)

Question 3

What is a constitutional isomer?

Answer 3

Isomers that have the same molecular formula, but different connectivity (also known as structural isomers)
If the connectivity is the same, but the molecules have a different spatial arrangement, they are stereoisomers.

p. 68, 77

Question 4

What is a configurational isomer?

Answer 4

These are stereoisomers that cannot be interconverted by rotation about single (sigma) bonds.
If they CAN be interconverted, then they are conformational isomers.

Question 5

What is geometric isomerism?

Answer 5

Two configurational isomers are geometric isomers if the isomerism occurs at a double bond.

If this is not the case, they are optical isomers.

Question 6

What are enantiomers?

Answer 6

Two optical isomers are enantiomers if they are non-superimposable mirror images of one another. (comes from Greek for “opposite”)

If they non-superimposable non-mirror images of each other, they are diastereomers.

Question 7

Define stereoisomers.

Answer 7

molecules that differ in the 3-D arrangement of atoms

Question 8

What is a chirality center?

Answer 8

a tetrahedral carbon (sp3 hybridized) that has 4 different groups attached.
1 chiral center implies 2 different stereoisomers

2ⁿ is the formula used to calculate the number of possible stereoisomers, where n is the # of chirality centers present in a molecule

Question 9

In looking at 2 conformational isomers, it is important to recognize that there are an ________ number of conformations for a molecule that has free rotation around a C-C bond. Also, any change in the rotation of one carbon, relative to its adjacent neighbor, is a change in _________ ____________.

Answer 9

infinite

molecular conformation

p. 70

Question 10

Which is more stable? A staggered conformation or an eclipsed conformation and why?

Answer 10

The staggered conformation is more stable bc of electronic repulsion. This shape places the sigma bonds as far apart as possible.

p. 70

Question 11

The more unable the conformation, the higher the ________ ______.

Answer 11

potential energy

Question 12

The staggered conformation is referred to as the ____ __________ and arises when the 2 largest groups attached to adjacent carbons are 180 degrees apart. This produces the most sterically favorable, and hence the most energetically favorable (______ energy) conformation.

Answer 12

anti conformation

lowest

p. 71

Question 13

A ______ conformation arises when the two largest groups on adjacent carbon atoms are in a staggered conformation 60 degrees apart.

Answer 13

gauche

Question 14

When in the eclipsed conformation with the 2 methyl groups directly aligned behind one another and therefore in the most crowded position, we refer to this as the ___ __________.

Answer 14

syn conformation

p. 71

Question 15

It is possible for the gauche conformation to be more stable than the anti conformation because of intramolecular hydrogen bonds.

p. 72

Question 16

Considering cyclohexane, the most stable conformation is a very puckered molecule referred to as the ____ ____.

Answer 16

chair form

p. 73

Question 17

When analyzing the structure of cyclohexane, it is more favorable for large groups to occupy the __________ position rather than a crowded axial position.

Answer 17

equatorial

p. 75

Question 18

In determining absolute configuration (R, S) types, what are the 3 rules?

Answer 18

1. Prioritize the 4 groups attached to the chirality center; the higher the atomic #, the higher the priority.
2. Orient the groups so that the lowest priority group is projecting away from you.
3. Determine if the sequence 1-2-3 is going clockwise (R), or counterclockwise (S).

(remember R stands for “RIGHT”, as in going around the clock the right way)

Question 19

If you are determining the absolute configuration of a molecule that contains 2 isotopes, which should get higher priority?

Answer 19

the heavier isotope

p. 81

Question 20

Where do the abbreviations ‘R’ and ‘S’ come from and what do they mean?

Answer 20

R comes from Latin “rectus” meaning right

S comes from Latin “sinister” meaning left

p. 82

Question 21

In a Fischer projection, _______ lines are assumed to go back into the page, and _______ lines are assumed to come out of the page.

Answer 21

vertical

horizontal

(imagine the horizontal lines as arms, coming out to give you a hug)

p. 82

Question 22

How do two enantiomers differ from one another?

Answer 22

Most properties such as melting point, boiling point, polarity, and solubility are the same. One difference though, is their optical activity.

Question 23

In the eclipsed conformation (think Newman projection of butane), the “dihedral angle” or “torsion angle” is __ degrees.

Answer 23

0 degrees

Question 24

A compound that rotates plane-polarized light is said to be _________ ______.

Answer 24

optically active

p. 87

Question 25

If an optically active compound rotates light clockwise, it is said to be ____________.

Answer 25

dextrorotatory (d), also denoted by +

p. 87

Question 26

If an optically active compound rotates light counterclockwise, it is said to be ____________.

Answer 26

levorotatory (l, also denoted by -)

p. 87

Question 27

The magnitude of rotation of plane-polarized light for any compound is called its _______ _______.

What is the formula?

Answer 27

specific rotation

p. 87

Question 28

A pair of enantiomers will rotate plane-polarized light with equal magnitude, but in ________ __________.

Answer 28

opposite directions

p. 87

Question 29

The specific rotation of a 50/50 mixture of enantiomers –- a _______ mixture -– is _ degrees. Therefore, this mixture, known as a _______, is not optically active!

Answer 29

racemic

0 degrees

racemate

p. 87

Question 30

Remember that the R and S designation has nothing to do with (+) and (-). These two notations describe separate, unrelated properties.

Question 31

Diastereomers are stereoisomers that are not __________. That is, they are non-superimposable, non-mirror images.

Answer 31

enantiomers

p. 89

Question 32

True or False?

Diastereomers can have different physical properties, including melting and boiling points.

Answer 32

True

Question 33

In the chair conformation of cyclohexane, where will the larger groups “want” to be in order to prevent gauche interactions?

Answer 33

equatorial with the ring; bc of this, the hydrogens should be axial

Question 34

Enantiomers like (R)-carvone and (S)-carvone may have the same melting point, boiling point, and density, but they have very different ______.

Answer 34

scents

Question 35

A molecule is said to be saturated if it contains no…

Answer 35

…pi bonds and no rings.

p. 46

Question 36

A saturated compound with n carbon atoms has exactly….

Answer 36

….2n + 2 hydrogens.

p. 46

Question 37

If a molecule is optically active, it must be ______.

Answer 37

chiral

Question 38

To experimentally to determine if a molecule is chiral, you can….

Answer 38

….shine plane-polarized light through the sample.

Question 39

Nucleophiles are species that have ________ _____ of electrons or _______ and, frequently, a negative or partial negative charge. As the name nucleophile implies, they are “nucleus-seeking” or “nucleus-loving” molecules. Since nucleophiles are electron pair donors, they are also known as Lewis bases.

Answer 39

unshared pairs

pi bonds

p. 63

Question 40

Nucleophilicity increases as negative charge _______.

Answer 40

increases

p. 63

Question 41

Nucleophilicity ________ going down the periodic table within a particular group.

Answer 41

increases

This trend is directly related to polarizability, or how easy it is for the electrons surrounding an atom to be distorted. As you go down any group in the periodic table, atoms become larger and generally more polarizable and more nucleophilic.

p. 63

Question 42

Nucleophilicity increases going _____ in the periodic table across a particular period.

Answer 42

left

This trend is related to the electronegativity of the nucleophilic atom. The more electronegative the atom is, the better it is able to support its negative charge. Therefore, the less electronegative an atom is, the higher its nucleophilicity.

p. 63

Question 43

Nucleophilic substitution properly describes the nucleophilic attack of an -OH group onto a ________ to create an ester.

Answer 43

carbonyl

Question 44

The general ranking for organic compound acidity is:

1. ) strong acids
2. ) sulfonic acids
3. ) carboxylic acids
4. ) phenols
5. ) alcohols and water
6. ) aldehydes and ketones (α hydrogens)
7. ) sp hybridized C-H bonds
8. ) sp² hybridized C-H bonds
9. ) sp³ hybridized C-H bonds

Question 45

What is an epimer?

Answer 45

a subclass of diastereomers that differ in their absolute configuration at a single chiral center (only 1 stereocenter is inverted)

p. 94

Question 46

Just like R and S, D and L are entirely ________ to optical activity.

Answer 46

unrelated

p. 94