Isomers Flashcards
Functional Isomer
Functional isomers are structural isomers with a different functional group
Tautomers
Structural isomers that interconvert with one another and exist in equilibrium like a keto enol tautomer
What would the following be classified as and why?
Tatomer- this is specifically a keto-enol tautomer. They are structural isomers that interconvert in equilibrium
What would the following be classifed as an why?
A tautomer- amine imine tautomer.
What are the different kinds of isomers in orgo?
Structural/Constitutional: share the same molecular formula but differ in CONNECTIVITY. They will differ in physical and chemical properties
Stereoisomers: these differ in their prganization around a bond. They have the same connectivity. You have conformational and configurational stereoisomers.
- Conformational: they differ in their rotation around a single bond. These are depicted by NEWMAN Projections
- Configurational: these can only be interchanged by breaking a bond. This is where your cis/trans, E/Z, and enantiomers/diasteromers come in
What is the major difference between cis/trans isomers and E/Z isomers
Cis/Trans describes a situation wher eyou have two IDENTICAL substituents across a doube bond whereas E/Z doesnt have to be identical
In an E/Z isomer, what does the E and Z stand for?
togeZer
Epart
How would you name this compound?
The stereochemistry of the alkene is E, as illustrated below: the alkene begins at carbon 2, and the higher priority group on carbon 2 is bromine, with atomic number 35 vs the atomic number of the aldehyde carbon, which is 6. Carbon 3, on the other end of the alkene, has a higher priority propyl group vs the methyl group. Therefore, the two higher priority groups are on “Epposite” sides. The parent chain, which begins at the aldehyde, has six carbons, making it a hexenal. E-2-bromo-3-methyl-hex-2-enal
What is the absolute configuration of the following molecule?
The priority order is set by the atomic number of the atoms bonded to the chiral carbon center. Highest priority is Br, then Cl, then OH, which results in a clockwise priority order, as shown below. If the lowest (fourth) priority hydrogen were at the back of the structure, clockwise would indicate the R configuration. However, since the hydrogen is in front, the absolute configuration must be opposite, which means the configuration is S.
A pure sample of (R)-limonene has a specific rotation of +125.6. If a mixture of (R)-limonene and (S)-limonene has a specific rotation of +62.8, what are the percentages of the R and S enantiomers in this mixture?
75% R, 25% S
Since the enantiomers in this case must be present in unequal proportions, a weighted average of the components should be used. In this case, the weighted average is given; therefore, we can set up an equation. If we designate x as the percentage of R and 1 - x as the percentage of S, we can solve for x using this equation:
(125. 6)(x) + (-125.6)(1-x) = 62.8
125. 6x - 125.6 + 125.6x = 62.8
251. 2x = 188.4
x = 0.75
1 - x = 0.25
Therefore, R makes up 75% of the solution, while S comprises the remaining 25%.
A particular biomolecule is known to have 30 diastereomers. It has only chiral centers, no alkenes, and does not exist in any meso forms. How many chiral centers must this molecule have?
A compound with 5 chiral centers will have 2n = 25 = 32 stereoisomers, including the original compound as well as its own enantiomer, neither of which count as diastereomers. This would leave 30 stereoisomers that are in fact diastereomers, making this the correct answer.
Epimer
Epimers are a particular type of diastereomer which differ at only one stereocenter.
Enantiomer
Enantiomers differ at every chiral center. Enantiomers have:
- same chemical formula
- same structural connectivity
- similar chemical and physical properties
- non-superimposable
So whereever one is R the other is S
True or false: The molecule shown below is a meso compound.
This statement is true. Achiral molecules that contain chiral centers and an internal plane of symmetry are known as meso compounds. As shown below, there is a plane of symmetry between the two chiral carbons, making this molecule a meso compound.
True or False: an equal 50/50 mixture of enantiomers will appear not optically active
true because the specific rotation of the enantiomers is equal and oppisote in magnitude
Is the following a stereocenter, chiral center, or both?
It is a stereocenter but not a chiral center. A stereocenter is an atom bound to at least three unique groups such that a new stereoisomer is formed if two of these groups are switched. Since stereoisomers can include cis/trans isomers, stereocenters are a broader category than chiral centers.
R or S? What molecule is this?
R, and its ALANINE
R or S?
S
A student determines the specific rotation of two sugars and reports them in the following table.
In a mixed solution of D-fructose and L-fructose, if the observed optical rotation is -75.2°, what is the enantiomeric excess of D-fructose? (Note: Measurements were obtained with a path length of 1.00 dm and concentration of 1.00 g/mL.)
82%.
To solve this problem, use the following equation: enantiomeric excess = (observed optical rotation x 100) / specific rotation. The specific rotation of D-fructose is given by the table (and that of its enantiomer, L-fructose, must have the same magnitude but an opposite sign, +92°).
How many stereoisomers?
2^4 or 16 possible stereoisomers
What kind of stereoisomer are these?
Enantiomers
What kind of stereoisomer?
Epimers
Anomer
Specific type of epimer with cyclic sugars that differs at the anomeric carbon in terms of the orientation of the hydroxyl group
Does this compound have an enantiomer?
THis compound is a meso compound so it does not have an enantiomer and it is not chiral
The sugars shown above can be described as:
Both anomer and epimers. Anomers are a subtype of epimer that differ only at the anomeric carbon, which is the carbon furthest to the right in both structures above. The two sugars shown are anomers, as their anomeric hydroxyl groups point downward (left structure with axial down OH) and upward (right structure with equatorial up OH), respectively. Epimers are molecules that differ at a single stereogenic center, which is true of the two sugars relevant to the question.
Which of the following is optically active?
The bromine containing compound. The other one is a meso compound.
What is the absolute configuration?
R
The concentration of a dilute solution of D-lactose is 0.300 g/mL. What must be the observed optical rotation of this solution when it is exposed to plane-polarized light with a sample path length of 1.00 dm?
What kind of isomers?
These molecules differ at every chiral center, which means they are enantiomers.
What isomer?
Diastereomers differ at at least one, but not all, chiral centers. These molecules differ at two of the three chiral centers and are therefore diastereomers.
