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aa Topics in organic chemistry 1 > Fundamentals 1 > Flashcards

Fundamentals 1 Flashcards

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1
Q

Define chirality

A
  1. The geometric property of a molecule (or rigid object) of being non-superimposable on it’s mirror image
  2. Contain no rotation-reflection axis- Sn
  3. Including mirror planes- S1 and centre of inversion S2
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2
Q

What are the 4 Molecular symmetry types

A
  1. Proper rotation
  2. Plane of symmetry
  3. Center of inversion symmetry
  4. Improper roation
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3
Q

Give the symbol, symmetry element and symmetry operation of proper rotation

A
  1. symbol = Cn (n= degree of rotation 360/n)
  2. Element = Proper rotation axis
  3. Operation= rotation by 360/n degrees
  4. Principle axis - highest order proper rotation axis
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4
Q

Give the symbol, symmetry element and symmetry operation of plane of symmetry

A
  1. Symbol= sigma
  2. Element = reflection plane
  3. Operation = reflection in the plane - not the same as reflection in external mirror as that would have undergone translation
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5
Q

Give the symbol, symmetry element and symmetry operation of Center of inversion symmetry

A
  1. Symbol= i
  2. Element = inversion centre - always at centre of molecule
  3. Operation = inversion of point x,y,z to -x,-y-z
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6
Q

Give the symbol, symmetry element and symmetry operation of Improper Rotation

A
  1. Sn
  2. ELement- improper rotation axis
  3. Operation- rotation by 360/n then reflection perpendicular to reflection axis
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7
Q

What is S1 and S2 equivalent to

A
  1. S1= reflection- as rotating 360 degrees so just reflection
  2. S2= inversion
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8
Q

Define assymetric

A
  1. Used for a molecule devoid or any symmetry elements - such as tetrahedral carbon atom surrounded by 4 different substituents
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9
Q

Is asymmetric and chiral synonymous

A
  1. No
  2. Chiral molecules can still possess Cn axes even when they are non-superimposable on their mirror image
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10
Q

What does dissymmetric mean

A
  1. Lacking one particular symmetry element
  2. For chiral dissymmetric molecules this is the Sn axis
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11
Q

What is difference between chiral molecule and enantiomerically pure

A
  1. Property of chirality is to do with the molecule and is unrelated to the enantiomeric composition- which is make up of collection of molecules
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12
Q

Define enantiomer

A
  1. Either of a pair of chemical compounds whose molecular structures have a nonsuperimposable mirror-image relationship to each other
  2. A molecule that possesses the property of chirality with have two possible mirror-image, non-superimposable stereoisomers known as enantiomers
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13
Q

Define stereoisomer

A
  1. Is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space.
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14
Q

What are optical isomers

A
  1. Sometimes used as a synonym of enantiomers but it is a redundant term and strongly discouraged
  2. Not all chiral, non-racemic compounds have measurable optical rotations
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15
Q

What is a stereogenic centre

A
  1. An atom for which permutation of any two ligands generates a stereoisomer
  2. Its substituent groups give rise to chirality
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16
Q

Describe physical properties of enantiomers

A
  1. Show identical scalar physical properties in an achiral environment
  2. In a chiral environment (enzyme interior or chiral stationary phase) individual enantiomers may show different scalar physical properties
17
Q

Why should you not use chiral centre

A
  1. Refers to stereogenic centre in a chiral molecule
  2. Chirality is property of molecule as a whole not the atoms in it
  3. May have stereo centres but not be a chiral molecule
18
Q

Can you get chiral conformations of achiral compounds

A
  1. Yes- racemic distributions of chiral conformations lead to macroscopic achirality
19
Q

give example of an achiral compound with chiral conformers

A
  1. Butane - achiral compound but has chiral conformers
  2. Two achiral co-planar conformations - syn and anti
  3. But, the enantiomeric chiral conformations being rapidly interconvertible and equal in energy are always present in a 50:50 ratio- racemic
  4. So from a macroscopic perspective butane is an achiral compound because its enantiomeric forms are far too short-lived for separation of its enantiomeric conformers to be possible.
20
Q

Can a chiral molecule have achiral conformations

A
  1. No
  2. If it did it would forget which enantiomer it came from and have a 50:50 chance of then converting to the opposite enantiomeric conformation
21
Q

What is used to specify the absolute configuration of a particular stereocentre

A
  1. R/S stereodescriptors
  2. Rank groups in order of priority
  3. Lowest group pointing away
  4. Highest to lowest priority- clockwise= R and anticlockwise= S
22
Q

How do you assign R or S to a stereogenic axis

A
  1. Look down from one end of axis
  2. Substituents that are further away are lower priority
  3. Clockwise= R etc
23
Q

What determines a substituent priority

A
  1. Cahn-Ingold-Prelog (CIP) priority rules
  2. Based on atomic weights
24
Q

What happens when assigning priorities when there are multiply bonded groups e.g. -CH=O

A
  1. Each atom in the multiple bond is regarded as being associated with a phantom (duplicate atom) at the other end of the double bond
  2. e.g. C=O is treated as though the carbon has two single bonds to two oxygens
25
Q

How are priorities assigned to cyclic compounds

A
  1. The structure is converted to an acyclic tree diagram by turning the ring into two separate chains that each terminate in a phantom atom- represents the other ring atom attached to the stereocentre
26
Q

Are melting points of a racemate or an pure enantiomer higher or lowe

A
  1. Lower as mixture
27
Q

Describe optical activity of chiral molecules

A
  1. When a beam of polarised light passes through a solution of chiral organic molecules the plane of polarisation is rotated
  2. The amount of rotation can be measured with a polarimeter
  3. Amount of rotation, alpha, in degrees is measured
28
Q

What are the names for enantiomers which rotate plane of polarised light left or right

A
  1. Rotate plane of polarised light left/ anticlockwise = laevorotatory
  2. Opposite enantiomer will rotate it to the right = dextrorotatory
29
Q

How do we make the expression of optical rotations meaningful

A
  1. Specify standard conditions
  2. THe specific rotation [alpha]D of a compound is defined as the observed rotation when
  3. The sample pathway, l, is equal to 1dm (10cm)
  4. The sample concentration c, is equal to 1 g per 100 mL
  5. Light of 589 nm wavelenggth is used
  6. The temperature is specified in superscript
30
Q

What is the equation for the specific rotation

A
  1. [alpha]TD = alpha/c*l
  2. Units= 10^-1 deg cm^2 g^-1
  3. Alpha in formula = angle
  4. [alpha]TD = amount of rotation
31
Q

Define enantiomerically pure

A
  1. A sample of chiral compounds with only one enantiomer
32
Q

Define Enantiomerically-enriched

A
  1. A sample of chiral compounds which contains both enantiomers but one predominates
33
Q

What happens if there is a racemic mixture

A
  1. If a 50;50 mixture of enantiomers is present then the rotation effects will cancel out and the mixture will appear to have no optical activity
34
Q

What are psuedoscalar properties (chiroptical properties) and give examples

A
  1. Changes sign but not magnitude on reflection
  2. Optical rotary dispersion-ORD
  3. Circular dichroism (CD) - absorption spectroscopy method based on differential absorption of left and right circularly-polarised light - Optically active molecules will preferentially absorb one direction of the circularly polarised light
  4. Vibrational circular dichroism (VCD)

aa Topics in organic chemistry 1 (11 decks)

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