Stereochemistry - Conformational Flashcards

1
Q

STEREOCHEMISTRY

  • branch of chemistry that take into account the ____ ____ ____ of _____ in ____
  • concerned with the _____ structure of compounds, specifically the _____ geometry of CARBON with _____ attached groups
  • 2 types
A
  • relative spatial arrangement
  • atoms
  • molecules
  • 3D
  • tetrahedral
  • 4
  1. Conformational
  2. Configurational
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2
Q

CONFORMATIONAL vs CONFIGURATIONAL

A

differ in spatial arrangement due to:

CONFORMATIONAL
- rotation around single bonds

CONFIGURATIONAL
- requires bond breaking and and reformation

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3
Q

What are the POSSIBLE individual isolations in CONFORMATIONAL STEREOCHEMISTRY?

+ reason

A

NONE

  • the molecules INTERCONVERT too RAPIDLY that individual isolations are impossible
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4
Q

3 KINDS of strains present in STEREOCHEMISTRY
+ where found
+ occurs due to what reason?

A
  1. Angle strain
    - found ONLY in rings
    - due to expansion/compression of bond angles
  2. Torsional strain
    - found in any alkanes, alkenes, alkynes (eclipsing formations)
    - due to repulsion between electron clouds in eclipsing bonds
  3. Steric strain
    - found in any alkanes, alkenes, alkynes (eclipsing and gauche formations)
    - due to repulsion between atoms/groups of atoms that are close to each other
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5
Q

Other term for STERIC STRAIN

A

Van der Waal’s strain

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6
Q

2 representations of CONFORMATIONAL STEREOCHEMISTRY

+ their 2 conformations

+ WHICH of the 2 conformations have lower energy and more unstable

A
  1. Sawhorse Projection
  2. Newman Projection

a. Staggered (low PE; more stable)
b. Eclipsed (high PE: less stable)

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7
Q

PALATANDAAN in drawing the 2 conformations in CONFORMATIONAL STEREOCHEMISTRY

A

STAGGERED
- 60 degrees yung bonds LAGI in both newman and sawhorse

ECLIPSED
- 0 degrees yung bonds LAGI in both newman and sawhorse

+ consistent yung itsura nya regardless of what alkane it is

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8
Q

CONFORMATIONAL STEREOCHEMISTRY

To rotate on a single bond, _____ _____ ____ are overcome

A

small energy barriers

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9
Q

The most stable conformation in alkanes

A

zig-zag shape

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10
Q

CONFORMATIONAL STEREOCHEMISTRY

  1. draw the energy diagram per angle conformations for the ff:
  • ethane
  • propane
  1. draw the 6 butane (newman) conformations
    + which is the most stable
    + difference in energy from MOST STABLE CONFORMATION

*and draw its example staggered conformation

A
  • ethane (additional 12 kJ/mol from staggered to eclipsed)
  • propane (additional 14 kJ/mol from staggered to eclipsed)
  • most stable/anti-confirmation: dihedral angle (angle between the 2 methyl grps) is 180 degrees*
  • least stable: eclipsing bonds (higher than ^* by 19 kJ/mol in energy)
  • gauche conformation: steric staggered bonds ((higher than ^* by 3.8 kJ/mol in energy)
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11
Q

Determine the strain energy in kJ/mol + strain components for the ff interactions:

  1. H - H eclipsing interaction
  2. H - CH3 eclipsing interaction
  3. CH3 - CH3 eclipsing interaction
  4. CH3 - CH3 gauche interaction
A
  1. 4.0 kJ/mol (torsional)
  2. 6.0 kJ/mol (torsional & steric)
  3. 11.0 kJ/mol (torsional & steric)
  4. 3.8 kJ/mol (steric)
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12
Q

Why does H-H eclipsing interaction only contain torsional strain?

A

no steric strain bc hydrogen atoms are very small

steric strain only applies to bulky groups getting closed

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13
Q

BAEYER STRAIN THEORY

states that

  • what ring sizes are too strained to exist?
  • sp3 carbons have bond angles of _____
  • ___ ____ or ____ ____ bond angles are ____ assuming ____ ring polygon angles

however… what was proven?

A
  • other than 5 or 6 membered rings (so butane and propane)
  • 109.5 degrees
  • too large or too small
  • impossible
  • flat

however, since cycloalkanes in real life are not “planar” (flat all throughout) at all (contains bends), they still exist yet are unstable

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14
Q

5 membered-rings and above relieve angle strain through?

A

puckered conformations

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15
Q

3 IMPORTANT points of the BAEYER STRAIN THEORY in terms of:

  1. ideal shape to minimize strains
  2. cause for greater strains in a ring
  3. very cause of angle strain + other term for this strain
A
  1. tetrahedral
  2. angle value becoming farther from 109.5 = greater strain
  3. cyclic compounds forces CARBON ATOMS into FIXED ANGLES that may deviate from 109.5, causing BAEYER (angle) STRAIN
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16
Q

CYCLOPROPANE

  • ____ bonds
  • 2 types of strains + level of intensity + due to what?
  • structure

+ draw newman

A
  • banana bonds
  • HIGH angle and HIGH torsional strain due to C - H eclipsing bonds
  • very planar
17
Q

CYCLOBUTANE

  • 2 types of strains + level of intensity
  • structure

+ draw newman

A
  • BALANCE between angle and torsional strain
  • slight bending
18
Q

CYCLOPENTANE

  • 2 conformation types
  • 1;2 types of strains + level of intensity + due to what? + how

+ structure of second conformation type

+ draw newman

A
  1. Planar (envelope)
    - high torsional strain
    - no angle strain
  2. Non-Planar / Puckered (butterfly)
    - BALANCE between angle and torsional strain due to puckering
    - slight bending which reduces TS but still not deviating from 109.5 thus limited changes to angle strain
19
Q

CYCLOHEXANE

  • 2 conformation types (both are ______)
  • which of the 2 is more stable?
  • difference in energy value between the 2
  • 2 types of bonds + how many in a cyclohexane + projection + sequence

+ draw newman for each conformation

A
  • non-planar
    1. Chair - more stable
    2. Boat - 23 kJ/mol higher in energy than chair

a. 6 axial bonds (alternate upwards/downwards per carbon atom)
b. 6 equatorial (alternate upwards/downwards diagonally per carbon atom)

*basically each carbon atom has 1 axial and 1 carbon

20
Q

Are axial and equatorial bonds only present in CYCLOHEXANES?

21
Q

If the axial bond of a carbon atom is projected upwards, its partner equatorial bond will be projected _____ side-ways

22
Q

WHAT do you call the interconversion from 1 chair conformation to a ____ conformation

A

RING FLIPPING
- 1 chair conformation to 1 chair conformation

*boat conformation is IN THE PROCESS

23
Q

In RING FLIPPING, what are the 2 types of bonds that are also interconverted?

A

AXIAL -> EQUATORIAL

EQUATORIAL -> AXIAL

  • take note that despite the change in type of bonds, their projections is the same (upward -> upward diagonally)
  • ALTHOUGH remember that the specific carbon atom that you are observing will change upon perspective
24
Q

ENERGY BARRIER in ring flipping is?

25
Q

MONOSUBSTITUED CYCLOHEXANE

  • 2 compound examples? + drawing
  • difference in energy + why?
  • what interaction is formed in what compound?
A
  1. a*-methylcyclohexane = 7.8 kJ/mol higher in energy than e
  2. e*-methylcyclohexane

1,3 - (CH3-H) - diaxial interaction (3.8 kJ/mol each)

*italicized

26
Q

DISUBSTITUTED CYCLOHEXANE

  • 3 compound examples? + drawing
  • energy value for each
  • most stable -> least stable + why
A

MOST STABLE
1. trans* - 1,2 - dimethylcyclohexane (equatorial) - 3.8 kJ/mol : no eclipsing
2. cis* - 1,2 - dimethylcyclohexane - 11.4 kJ/mol : gauche interaction
3. trans* - 1,2 - dimethylcyclohexane (axial) - 15.2 kJ/mol : w/ eclipsing

+ draw the ring flipping for cis*

27
Q

DIAXIAL interaction VS GAUCHE interaction

  • type of strain
  • cause of strain
  • energy of strain
A
  • BOTH ARE STERIC
  1. DIAXIAL
    - axial bulky group + axial H atom
    - 3.8 kJ/mol for 1 axial CH3 - axial H interaction
  2. GAUCHE
    - CH3 - CH3 interaction
    - 3.8 kJ/mol for 1 CH3 - CH3 interaction
28
Q

In GAUCHE INTERACTION, is the energy value always 3.8 kJ/mol?

A

no, the 3.8 value is ONLY for CH3 - CH3 interactions

the bulkier the group, the higher the energy value is

+ it ISNT necessary that the bulky groups are EQUAL to exhibit gauche interaction

29
Q

CELLULOSE VS STARCH

  • which is more stable?
  • why ^^?
  • which is more and less resistant to ________
  • which does and does not breakdown by the enzyme ______
A

CELLULOSE
- more stable
- equatorial bulky groups
- more resistant to hydrolysis
- does not breakdown w amylase

STARCH
- less stable
- axial bulky groups
- less resistant to hydrolysis
- breakdowns w amylase

30
Q

Do molecules exhibit more stable conformations if the bulkier groups is axial or equatorial?

A

equatorial