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MCAT Organic Chemistry Review > Isomers > Flashcards

Isomers Flashcards

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1
Q

What are isomers?

A

Isomers have the same molecular formula but different structures.

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2
Q

What are physical properties of compounds? Provide examples.

A

Physical properties are properties that do not deal with the change in composition of matter. Some examples include: boiling pt, melting pt, solubility, odor, color and density.

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3
Q

What are chemical properties of compounds?

A

Chemical properties are properties that have to do with the reactivity of a molecule with other molecules. These change the composition of matter. These are attributable by the functional groups within a molecule.

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4
Q

What do structural isomers share in common?

A

Their molecular formula.

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5
Q

What do constitutional isomers share in common?

A

Their molecular formula. Constitutional isomers are the same as structural isomers.

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6
Q

What are stereoisomers?

A

Stereoisomers have the same molecular formula and have the same atomic connectivity, but differ in the ways that atoms are arranged in space.

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7
Q

What is the largest distinction between stereoisomers?

A

Conformational isomers (conformers) and configurational isomers.

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8
Q

How are conformational isomers similar? How do they differ?

A

Conformational isomers have the same molecular formula but represent different points in the natural rotation around single bonds. Varying degrees of rotation cause varying degrees of strain.

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9
Q

All isomers that are not structural isomers are:

A

Stereoisomers

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10
Q

What do Newman projections do?

A

Showcase molecules on a carbon-bond axis

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11
Q

What is the most stable butane conformation?

A

Where its two methane groups are 180 degrees apart

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12
Q

When are atoms in their lowest-energy state in a Newman projection?

A

When atoms are as far apart as possible, minimizing steric repulsion between their election clouds

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13
Q

What does staggered conformation mean?

A

When there is no atom overlap along the line of sight

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13
Q

What is the most energetically favorable (lowest-energy state) conformation?

A

Anti staggered

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13
Q

What is anti-conformation?

A

When a molecule’s two largest groups are antiperiplanar

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13
Q

What does it mean for two groups in a molecule to be antiperiplanar?

A

For two groups to be in the same plane but on opposite sides

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13
Q

What is gauche-conformation?

A

For a molecule’s two largest groups to be 60 degrees apart

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14
Q

What conformation must a molecule pass through to get from anti to gauche?

A

Eclipsed

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15
Q

What is an eclipsed conformation?

A

Two substituents are 120 degrees apart; and overlap with the hydrogen atoms on the adjacent carbon

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16
Q

What is a totally eclipsed conformation?

A

The largest groups in a molecule directly overlap each other with 0 degrees of separation

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17
Q

When is a molecule in its highest energy state and why?

A

Totally eclipsed conformation; the two largest groups in the molecule are synperiplanar

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18
Q

What does it mean for a molecule’s groups to be synperiplanar?

A

Same plane, same side

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19
Q

What are the three types of ring strain?

A

Angle strain, torsional strain, and nonbonded strain

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20
Q

What is angle strain?

A

Results when bond angles are different from their ideal values from being stretched

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21
What is torsional strain?
Results when cyclic molecules are forced to be in eclipsed/gauche conformations
22
What is nonbonded strain?
Results when nonadjacent atoms compete for the same space
23
What conformations does cyclohexane mainly exist in?
Chair, boat, or twist-boat conformations
24
What is cyclohexane's most stable conformation?
Chair conformation
25
What does it mean for hydrogen atoms to be axial?
When hydrogen atoms are perpendicular to the plane of the ring
26
What does it mean for hydrogen atoms to be equatorial?
When hydrogen atoms are parallel to the plane of the ring
27
What happens when cyclohexane undergoes a chair flip?
All axial groups turn equatorial and all equatorial groups turn axial
28
What is cyclohexane's least stable conformation?
Twist-boat
29
What is main source of strain for cyclohexane's twist-boat conformation?
Nonbonded strain
30
What position do bulky groups (like tert-butyl) on substituted rings favor? Why?
equatorial position; reduce nonbonded strain w/ axial groups in the molecule
31
In rings with more than one substituent, how is the preferred chair conformation determined?
Determined by the largest group in the molecule that will ultimately prefer the equatorial position
32
If substituents are located on the same side of the molecule (both wedge bonds), are they cis or trans?
cis
33
If substituents are located on different sides of the molecule (wedge bond and dash bond), are they cis or trans?
trans
34
What are configurational isomers?
Isomers that must break and reform covalent bonds in order to change from one form to another
35
What are the two types of configurational isomers?
Enantiomers and diastereomers
36
What kinds of isomers are enantiomers and diastereomers?
Configurational isomers and optical isomers
37
What are optical isomers?
Configurational isomers whose different arrangement affects the rotation of plane-polarized light
38
Are all configurational isomers optical isomers?
No, but all optical isomers are configurational isomers
39
When is an object considered chiral?
An object is considered chiral when its mirror image cannot be superimposed on the original object
40
What does it mean for a molecule to be chiral?
A molecule is not superimposable on its mirror image; it lacks an internal plane of symmetry
41
When is an object considered achiral?
An object is considered achiral when its mirror image can be superimposed on the original object
42
How do you identify if a molecule has a chiral center?
When a carbon atom has four different substituents, it has a chiral center; the molecule lacks a plane of symmetry
43
What are enantiomers?
Enantiomers are molecules that are nonsuperimposable images of each other; chiral
44
What are diastereomers?
Diastereomers are molecules that are chiral and have the same connectivity but are not mirror images of each other
45
What is the difference between enantiomers and diastereomers?
Enantiomers are molecules that are mirror images of each other and nonsuperimposable while diastereomers are nonsuperimposable but not mirror images of each other
46
How can you remember the differences between enantiomers and diastereomers?
Enantiomers are like hands, they are mirror images but hands are nonsuperimposable when you lay them on each other. Diastereomers are different shapes altogether, even if they share the same elements.
47
Can a molecule with a carbon atom that has three different substituents be chiral?
No, the carbon atom must have four different substituents to be a chiral center and thus the molecule to be chiral
48
Explain what an enantiomer is in terms of connectivity.
Enantiomers have the same connectivity but opposite configurations at every chiral center in the molecule.
49
What properties do enantiomers share with each other? Are there any exceptions?
Enantiomers have the same physical/chemical properties of each other, but differ in the way they rotate plane-polarized light (opposite directions) and react in chiral environments.
50
What does optical activity refer to?
Refers to the rotation of plane-polarized light by a chiral molecule
51
How do you refer to a compound that rotates the plane of polarized light to the right (clockwise)?
Dextrorotatory (d-), labeled (+)
52
How do you refer to a compound that rotates the plane of polarized light to the left (counter-clockwise)?
Levorotatory (l-), labeled (-)
53
How is optical activity determined?
Experimentally
54
How is rotation of plane-polarized light measured?
[α] = αobs/c*l
55
What does each variable symbolize in the following equation to determine the rotation of plane-polarized light: [α] = αobs/c*l ?
[α] = specific rotation in degrees; αobs= observed rotation in degrees; c= concentration in g/mL; l= path length in dm
56
What mixture forms when both (+) and (-) enantiomers are present in equal concentrations?
A racemic mixture
57
Does a racemic mixture observe any optical activity? Why or why not?
No, the different rotations between the (+) and (-) enantiomers cancel each other out
58
How can you separate a racemic mixture?
By reacting two enantiomers with a single enantiomer of another compound leading to two diastereomers and then separating the diastereomers by a separation process like (crystallization, filtration, or distillation) and then reacting the diastereomer to regenerate the original enantiomer
59
How can you separate diastereomers?
Crystallization, filtration, distillation, and others
60
Explain what a diastereomer is in terms of connectivity.
Diastereomers are molecules that have two or more chiral centers and differ at some, but not all of those chiral centers.
61
What formula do you use to determine the amount of possible stereoisomers?
For any molecule with n chiral centers, there are 2^n possible stereoisomers
62
In what do diastereomers differ in?
Diastereomers differ in their chemical properties even if they may react similarly in certain reactions due to having the same functional groups. They also have different physical properties because each isomer has different arrangements in space.
63
What are cis-trans isomers also known as?
Geometric isomers
64
What family of isomers do cis-trans isomers fall under?
Diastereomers
65
What is characteristic of cis-trans isomers?
The substituents of the isomers differ in their position around an immovable bond like a double bond or a ring structure.
66
What are meso compounds?
Meso compounds are molecules with chiral centers that have an internal plane of symmetry, and thus superimposable onto their mirror image, optically inactive, and do not rotate polarized light. These molecules are achiral.
67
Are meso compounds optically active?
No, because meso compounds have an internal plane of symmetry.
68
What is the difference between conformational and configurational isomers?
Conformational and configurational isomers are both stereoisomers, but conformational isomers have the same molecular connectivity while configurational isomers do not.
69
When can you use cis/trans notation?
When trying to describe the arrangement of substituents around a double bond when there are exactly 2 substituents and 2 hydrogens on each carbon of the double bond.
70
What is the configuration of a stereoisomer based on?
The spatial arrangement of atoms within a molecule
71
What is the relative configuration of a chiral molecule based on?
Its configuration in relation to another chiral molecule
72
What is the absolute configuration of a chiral molecule based on?
Its exact spatial arrangement of its atoms completely independent of other molecules
73
When can you use E and Z nomenaclature?
Used for compounds with polysubstituted double bonds (more than 2 substituents on each carbon atom in a double bond)
74
How do you identify priority of a substituent when determining E or Z designation?
Based on the priority of the atom bonded to each double-bonded carbon, the higher the atomic number the atom has, the greater its priority
75
What does E assignation stand for?
Highest priority groups are on the different sides of the molecule
76
What does Z assignation stand for?
Highest priority groups are on same side of the molecule
77
How do you determine the absolute configuration at a chiral center using R/S nomenaclature?
1. Assign priority based on highest atomic number 2. Arrange the molecule with the lowest priority substituent in the back 3. Draw a circle around the molecule from highest to lowest priority (1-2-3) 4. Determine if circle is Clockwise (R) / Counterclockwise (S)
78
What does R nomenclature indicate?
Indicates a clockwise arrangement of substituents around a chiral center
79
What does S nomenclature indicate?
Indicates a counter-clockwise arrangement of substituents around a chiral center
80
What is a Fischer projection?
A Fischer projection uses horizontal lines to indicate bonds that project out from the page (wedge) and bonds that project into the page (dash); points of intersection indicate a carbon atom
81
How do you determine R/S designation using a Fischer projection?
1. Organize groups by priority ensuring that the lowest priority atom lies projecting into the page 2. If the lowest priority atom lies projecting out of the page, assign priority as normal but know that R/S designation will be opposite as to what is determined
82
For each of the Fischer projection manipulations listed below: is stereochemistry retained or inverted? 1. switching a pair of substituents 2. switching two pairs of substituents 3. rotating the molecule 90 degrees 4. rotating the molecule 180 degrees
1. inverts it 2. retains it 3. inverts it 4. retains it
83
How do you know if the relative configuration of a molecule is retained?
If the bonds of the stereocenter are not broken

MCAT Organic Chemistry Review (4 decks)

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