Term
| When does Geometric Isomerism arise? |
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Definition
| Due to a lack of free rotation about the pi bond in a double bond |
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Term
| What system is used when distinguishing between geometric isomers? how does the system work to differentiate? |
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Definition
Cis/Trans
Cis when the groups are both below the C=C bond
Trans when the groups are above and below the C=C
Z/E may also be used (Z-same side) (E-opposite) |
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Term
What does it mean if 2 objects are chiral? which objects are usually chiral?
What must the carbon atom have around it to be chiral? |
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Definition
2 objects are mirror images of eachother and are not superimposable on eachother
Asymetrical objects are chiral
4 different groups around a carbon atom |
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Term
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Definition
| Isomers that are mirror images of eachother |
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Term
| What similarities do Enantiomers share? |
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Definition
| Similar chemical & physical properties |
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Term
What difference does allow us to distinguish between different enantiomers?
What are these optical isomers said to be? (2 words) |
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Definition
They have different effects on plane polarised light (rotate clockwise/anticlockwise)
Compounds that have these different effects on plane polarised light are said to be optically active |
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Term
FACT: The body will respond to some molecules but will not always respond to their optical isomers because they are of a different shape.
It may respond but have a different effect to the desired effect
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Definition
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Term
| How would you identify a particular carbon atom as being chiral? |
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Definition
| It has 4 different groups around it |
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Term
What effect do enantiomers have that allows D- and L- enantiomers do be distinguished from each other?
(their effect on what) |
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Definition
Enantiomers will have opposite effects on the rotation of plane polarised light
One enantiomer will rotate the plane clockwise
One enantiomer will rotate the plane anticlockwise
+ : Clockwise
- : Anti-Clockwise |
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Term
| A '+' form of an enantiomer rotates plane polarised light in which direction? |
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Definition
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Term
| What is plane polarised light? |
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Definition
| Light in which the oscilaltions occur in one plane only, it is said to be plane polarised in that direction |
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Term
| If normal light passes through a polaroid filter it becomes polarised, if another polaroid filter is then rotated 90 degrees to the 1st filter, what will happen to the plane polarised light? |
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Definition
| It will not pass through the 2nd filter as the light is plane polarised to oscillate in the plane of the 1st filter so hence will not be able to pass through the 2nd filter |
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Term
What instrument is used to measure the angle of rotation of light caused by a sample?
How is optical activity measured using this instrument (clue given above) |
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Definition
A polarimeter
Measured by the number of degrees by which plane polarised ligh is rotated |
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Term
| What is a racemic mixture? |
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Definition
| A mixture of 1:1 ratio of each enantiomer, therefore the clockwise rotation caused by one enantiomer is cancelled by the anticlockwise rotation caused by the other |
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Term
| How many species are involved in the rate determing steps of SN1 & SN2? |
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Definition
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Term
| Why is a Racemic mixture produced when the SN1 mechanism takes place? |
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Definition
Nucleophillic attack can be on either side of the carbocation.
It is likely to occur in a 50:50 ratio.
So no overall racemic activity occurs |
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Term
| When a sample undergoes mainly SN1 will the resulting mixture be optically active or not? |
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Definition
No, SN1 is not optically active
1:1 ratio of nucleophillic attack on either side of the carbocation so equal number of enantiomers produced
No overall rotation |
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Term
| By which mechanism (nucleophillic) does inversion of the geometry of the molecule occur and why? |
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Definition
The nucleophile attacks and always joins on opposite side to halogen during the 5 co-ordinate transition state
Therefore when the halogen leaves, the OH is left attached on the opposite side to where the halogen was.
The optical activity has hence been reversed so will cause a different effect now on the rotation of plane polarised light (It will rotate it the other way) |
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Term
| Why might a racemic mixture not be produced during an SN1 reaction assuming the starting mixture was racemic? |
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Definition
After the heterolytic fission the halide ion leaves the molecule and this can prevent the approach of the nucleophile on the side that the halide ion is leaving on.
Therefore the nucleophile is more likely to attack on the opposite side to the leaving group so the resulting mixture could be slightly inversed as opposed to racemic |
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