Stereochemistry-Isomers-Chirality-Fischer Flashcards
Isomers
Draw the isomer flowchart without yes / no or the questions, this card is to get the structure
Isomer flow chart
What question separates isomer / not isomer?
Do the compounds have the same molecular formula
Yes = isomer
No = not isomer
Isomer flow chart
What question separates configurational and conformational isomers?
What type of isomer are they both?
can the compounds be interconverted by rotation about a single bond
Yes = conformational (rotational) isomer (conformer)
No = configurational isomer
They are both stereoisomers
Isomer flow chart
What question separates constitutional and stereoisomers?
Do the compounds have the same connectivity
Yes = stereoisomer
No = Constitutional (structural) isomer
Isomer flow chart
What question separates geometric and optical isomers?
What type of isomers are they both?
Is the isomerism at a double bond
Yes = geometric isomer
No = Optical isomer
They are both configurational isomers, which is a type of stereoisomer
Isomer flow chart
What question separates enantiomers and diastereomers ?
What type of isomers are they both?
Are the compounds non-superimposable mirror images
Yes = enantiomers (an ant in the mirror)
No = diastereomers (not mirror images)
They are both optical isomers, which are a type of configurational isomer, which is a type of stereoisomer
Isomer flow chart questions
Do the compounds have the same connectivity
Yes =
No =
same connectivity = steroeisomers
No = constitutional isomer
Isomer flow chart questions
Do the compounds have the same molecular formula
Yes =
No =
same molecular formula = isomer
No = not isomer
Isomer flow chart questions
Is the isomerism at a double bond?
Yes =
Geometric
Isomer flow chart questions
Are the compounds non-superimposable mirror images?
Yes =
No =
Yes = enantiomers
No = diastereomers
Chirality
- What makes a molecule chiral?
- What is a chirality centre?
- What is the most common way to locate a chirality centre?
- What values can a chirality centre have?
- What does it mean if there is one OR more than one chiral centre in a molecule
- An object or a system is chiral if it is distinguishable from its mirror image; that is, it cannot be superposed onto it. Conversely, a mirror image of an achiral object, such as a sphere, cannot be distinguished from the object. A chiral object and its mirror image are called enantiomorphs (Greek opposite forms) or, when referring to molecules, enantiomers. A non-chiral object is called achiral (sometimes also amphichiral) and can be superposed on its mirror image.
- Chiral centers are tetrahedral atoms (usually carbons) that have four different substituents.
- Each chiral center in a molecule will be either R or S.
- As noted above, molecules with a single chiral center are chiral. Molecules with more than one chiral center are usually chiral.
Stereochemistry - absolute configuration
- Define
- What values can it take?
- For what type of isomers is absolute configuration needed?
- An absolute configuration in stereochemistry refers to the spatial arrangement of the atoms of a chiral molecular entity (or group) and its stereochemical description
- e.g. R or S, referring to Rectus (clockwise), or Sinister (<strong>anti</strong>-clockwise), respectively.
- In order to be able to exist as a pair of enantiomers, a molecule requires the presence of a chirality center
Stereochemistry - absolute configuration
What are the rules for deciding if a chirality centre is R or S?
- Assign priority (by atomic number, highest =1) to the four attached atoms (only those directly attached to the sterecentre)
- if there is a tie write out the 3 atoms that the tying atoms are attached to (highest of each three first). Then use rule 1 to break the tie (use the first point of difference, not all three).
- If 4 is pointing into the page (ie on top or bottom)
123 clockwise=Rectus, 123 anticlockwise=Sinister
- If 4 is not pointing into the page then apply the single swap rule so that it is. Then apply step 3. The correct configuration is opposite to that shown in step 3 (the single swap rule reverses Rectus and Sinister)
Here’s how it the single swap rule works.
Take a molecule with a stereocenter, like (S)-2-butanol (drawn below).
If you swap ANY TWO substituents, you will invert the configuration of the stereocenter.
That is, switching any two substituents will give you (R)-2-butanol
Stereochemistry - absolute configuration
What is the single swap rule ?
Here’s how it works.
Take a molecule with a stereocenter, like (S)-2-butanol (drawn below).
If you swap ANY TWO substituents, you will invert the configuration of the stereocenter.
That is, switching any two substituents will give you (R)-2-butanol
Observed rotation - stereochemistry
- What is it?
- What is it used for?
- What is a racemic mixture?
- Pure enantiomers will always rotate plane-polarized light to an equal but opposite degree.
- To establish the mix of R and S
- a racemic mixture is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule
If a stereochemically pure sample of (S)-carvone, for example, rotates plane-polarized light by +10°(clockwise), then a sample of (R)-carvone (in the exact same concentration and under the same experimental conditions) will rotate light by (–)10° (counterclockwise).
Chiral molecules have a property known as optical activity: what this means is that a beam of plane-polarized light, when passed through a sample of a chiral substance, will interact with the substance in such a way that the angle of the plane of oscillation will rotate.
Stereochemistry cis trans
- What type of molecules can be cis or trans?
- What do cis and trans mean?
- What electrical difference is there between cis and trans?
- What is the difference in boiling point between cis and trans? and why?
- What is the difference in melting point between cis and trans? and why?
- What is steric hindrance? and what effect does it have?
- Geometric isomers (diastereometer with double bond) can be cis or trans
- cis = same side substituents trans = opposite side substituents
- cis has a dipole moment
- cis = higher boiling point (dipole moment gives stronger intermolecular forces)
- cis = lower melting point (poorer symmetry -> poorer crystal forming)
- cis subtituents crowd each other(higher energy levels -> higher heat of combustion)
Optically active isomers
- If an isomer has n chiral centres - how many optically active isomers can it have?
2n
Stereochemistry - meso
- What are meso compounds?
- Are they optically active?
- a meso compound is a molecule that has chiral centers but also has an internal plane of symmetry.
- This renders the molecule achiral: it does not have an enantiomer, and it does not rotate plane polarized light
Newman projections - stereo chemistry
- Draw the Newman projections for the conformers of Butane and name them
