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MCAT Chemistry > Isomers > Flashcards

Isomers Flashcards

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1
Q

What are isomers?

A

Molecules with the same formula but different chemical structure

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2
Q

What is another name for structural isomers?

A

Constitutional isomers

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3
Q

What are physical properties? Examples?

A

No change in composition or matter

Ex: boiling point, melting point, solubility, odor, color, density

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4
Q

What are chemical properties? What are these generally attributable to?

A

Reactivity of molecule resulting in change in composition

To functional groups in the molecule

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5
Q

What do stereoisomers share?

A

The same structural backbone

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6
Q

By what do stereoisomers differ?

A

How their atoms are arranged in space

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7
Q

What are the 2 types of stereoisomers?

A
  1. Conformational

2. Configurational

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8
Q

Out of all the isomers, which are the most similar?

A

Conformation stereoisomers

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9
Q

What is another word for conformational isomers?

A

Conformers

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10
Q

By what do conformers differ?

A

In rotation around single sigma bonds

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11
Q

What is the antistaggered conformation of conformers?

A

The two largest groups are antiperiplanar (in the same plane but on opposite sidest

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12
Q

What is the gauche conformation of conformers?

A

The two largest groups are 60 degrees apart

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13
Q

What is the eclipsed conformation of conformers? What is the angle?

A

The two largest groups are eclipsed with other groups (120)

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14
Q

What is the totally eclipsed conformation of conformers?

A

The two largest groups are eclipsed

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15
Q

What is the highest energy state of conformers?

A

Totally eclipsed conformation

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16
Q

What does the stability of cycloalkanes depend on?

A

Ring strain

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17
Q

In cycloalkanes, what does ring strain arise from?

A
  1. Angle strain
  2. Torsional strain
  3. Nonbonded strain
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18
Q

In cycloalkanes, when does angle strain occur?

A

When bond angles deviate from their ideal values by being stretched or compressed

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19
Q

In cycloalkanes, when does torsional strain occur?

A

When cyclic molecules must assume gauche or eclipsed conformations

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20
Q

What is another name for nonbonded strain?

A

Van der Waals repulsion

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21
Q

In cycloalkanes, when does nonbonded strain occur?

A

When nonadjacent groups compete for the same space

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22
Q

What are the 3 main conformations of cyclohexane?

A
  1. Chair
  2. Boat
  3. Twist/Skewboat
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23
Q

What is the most stable conformation of cyclohexane?

A

Chair

24
Q

In the chair conformation of cyclohexane, what are the 2 types of H atoms?

A
  1. Axial: perpendicular to the plane of the chair

2. Equatorial: parallel to the plane of the chair

25
Q

What happens during a chair flip of cyclohexane?

A

All axial groups become equatorial and vice versa

26
Q

On substituded rings, what is the preferred position of bulky groups?

A

The equatorial position

27
Q

What are the cis and trans forms of rings?

A

Cis: bulky groups on same side of the ring
Trans: bulky groups on opposite sides of the ring

28
Q

What do configurational isomers require to interconvert?

A

Bond breaking

29
Q

What are the 2 categories of configurational isomers?

A
  1. Enantiomers

2. Diastereomers

30
Q

What are optical isomers? Why?

A

Enantiomers and diastereomers because the different spatial arrangement of groups affects the rotation of plane-polarized light

31
Q

When is an object considered chiral?

A

When its mirror image cannot be superimposed on the original object

32
Q

What is the best example of a chiral molecule?

A

A C with 4 different substituents

33
Q

What are enantiomers?

A

Nonsuperimposable mirror images with opposite stereochemistry at each chiral center

34
Q

What do enantiomers have in common? How do they differ?

A

Identical physical and chemical properties with two exceptions:

  1. Optical activity
  2. Reactivity in chiral environments
35
Q

When is a compound said to be optically active?

A

If it has the ability to rotate plane polarized light

36
Q

What is a levorotatory compound? How is it labelled?

A

Optically active compound that rotates plane polarized light counterclockwise
Labelled - of l-

37
Q

What is a dextrorotatory compound? How is it labelled?

A

Optically active compounds that rotates plane polarized light clockwise
Labelled + or d-

38
Q

How is the direction of the rotation of an optically active compound be determine?

A

Experimentally

39
Q

What is the equation to calculate the amount of rotation of polarized light?

A 
alpha = alpha observed / (c . l)
alpha = specific rotation in degrees
c = concentration in g/ml
l = path length in dm
40
Q

What is a racemic mixture?

A

A mixture in which + and - enantiomers are present in equal concentrations

41
Q

Does a racemic mixture display optical activity? Why?

A

It does not, because the rotations cancel each other out

42
Q

What are diastereomers? What does this mean they are required to have?

A

Non-mirror image configurational isomers, required to have more than one chiral center and they do not differ by all chiral centers

43
Q

What is the maximum number of optically active isomers/stereoisomers?

A

2^n

n = number of chiral centers

44
Q

What do diastereomers have in common? How do they differ?

A

They have different chemical properties, but might sometimes behave similarly in particular reactions because they have the same functional groups.
They have different physical properties

45
Q

What were cis-trans isomers used to be called?

A

Geometric isomers

46
Q

What are cis-trans isomers?

A

A sub group of diastereomers in which substituents differ in their position around an immovable bond or around a ring

47
Q

What is a meso compound?

A

A molecule with chiral centers with a plane of symmetry

48
Q

Is a meso compound optically active? Why?

A

No, because it has a plane of symmetry

49
Q

What is the relative configuration of chiral molecule?

A

Its configuration in relation to another chiral molecule

50
Q

When is the (E) and (Z) nomenclature used?

A

Compounds with polysubstituted double bonds

51
Q

How do you determine if the molecule is (E) or (Z)?

A
  1. Find the highest priority atom on each C of the double bond (highest atomic number)
  2. If the two highest priority substituents of each C are on the same side of the double bond: Z
  3. If the two highest priority substituents of each C are on different sides of the double bond: E
52
Q

How do you determine the absolute configuration of a chiral center?

A
  1. Assign priority by 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
  4. Clockwise = R / Counterclockwise = S
53
Q

How do you identify a chiral center?

A

Attached to 4 different substituents

54
Q

How can you tell if a molecule retains absolute configuration?

A

The bonds on the stereocenters are not broken

55
Q

How can you tell if a molecule retains relative configuration?

A

The reactant and product are both R or both S

MCAT Chemistry (17 decks)

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