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L1 exams > Conformation > Flashcards

Conformation Flashcards

1
Q

What is a conformation

A

3D arrangement of atoms
from rotation around a single bond

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2
Q

What is an eclipsed conformation

A

When the same atoms are in a line

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3
Q

What is the effect of an eclipsed conformation

A

Experience more repulsion

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4
Q

What is the gauche conformation

A

Same atoms 60° apart

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5
Q

What is an antiperiplanar conformation

A

Same atoms 180° apart

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6
Q

What is the third staggered conformation

A

Same atom 120° apart

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7
Q

What is the order of stability of the staggered conformations

A

Antiperiplanar
Gauche
120°
Unstaggered

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8
Q

Why are configuration changes unfavourable

A

Interconvert stereogenic centre to enantiomer
typically involves bond breaking

Need to pass through
square planar transition state

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9
Q

What are atropisomers

A

Stereoisomers form from hindered rotation
around single bond
where steric strain barrier to rotation
high enough to allow for
isolation of the conformers

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10
Q

What is the conformation of a 3 membered ring

A

Adjacent CH2 always eclipsed

Bond angle 60°
sever angular strain

~115 kJmol-1 strain energy

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11
Q

What is the conformation of a 4 membered ring

A

90° bond angle

All CH2 eclipsed
puckers to avoid

~110 kJmol-1 strain energy

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12
Q

What is the conformation of a 5 membered ring

A

108° bond angle

All CH2 eclipsed
puckers to avoid

~26 kJmol-1 strain energy

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13
Q

What is the conformation of a 6 membered ring

A

All CH2 staggered
torsional strain avoided

2x gauche interactions

~7.6 kJmol-1 strain energy

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14
Q

What is the conformation of a 7 membered ring

A

~26 kJmol-1 strain energy

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15
Q

Why does total strain increase

A

Due to trans annular steric strain

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16
Q

What is the ranked stability of chair conformations

A

Chair
Twisted boat
eclipsed H off set
True boat
eclipsing H across boat
Half-chair

17
Q

What are 1,3-diaxial interactions

A

Steric interactions between
axial atom on C1
& H atoms on C3/C5

18
Q

What is the most stable position of Me

A

Equatorial

19
Q

What role can tBu have

A

Often used to ‘lock’ conformation

Has strong preference for
equatorial position

20
Q

Where would the largest group sit

A

In chain conformation
largest group prefers equatorial

21
Q

What position will groups react in

A

Both need to be axial
∵ they are closer than equatorial

L1 exams (7 decks)

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