Ch. 4: Isomers Flashcards
What are the two types of isomers? Explain.
Isomers are compounds with the same molecular formula but different structures. The two kinds of isomers are constitutional isomers and stereoisomers. A constitutional isomer differs in the way their atoms are connected, like ethanol and dimethyl ether. The atoms in stereoisomers are connected in the same way but differ in their atoms arranged in space.
What are the two kinds of stereoisomers? Explain.
Conformational isomers and configurational isomers. Conformational isomers are stereoisomers that rapidly interconvert at room temperature, and because they interconvert, they can’t be separated. Configurational isomers are stereoisomers that cannot interconvert unless covalent bonds are broken. Because they cannot interconvert, configurational isomers can be separated.
What are the two types of conformational isomers? Explain.
1) conformers due to rotation about C–C single bonds: staggered, eclipsed, chair, boat, etc.
2) those due to amine inversion
What are the two types of configurational isomers? Explain.
1) cis-trans isomers: result from restricted rotation
2) isomers that contain asymmetric centers
What causes restricted rotation in cis-trans isomers?
cause by either a cyclic structure or a double bond
Can compounds with C–C double bonds have cis and trans isomers?
Yes. But rotation about a double-bond can only occur if the pi bond breaks–that is ,only if the pi bonds are no longer parallel. Consequently, the energy barrier to rotation about a C–C double bond is much greater than the energy barrier to rotation about a C–C single bond: 62 kcal/mol vs. 2.9 kcal/mol
What does “E” represent in the E,Z system?
the E isomer has the high-priority groups on opposite sides
What does “Z” represent in the E,Z system?
the Z isomer has the high-priority groups on the same side
What’s the first step in determining the relative properties of E,Z isomer groups?
1) the relative properties depend on the atomic numbers of the atoms bonded directly to the sp2 carbon. The greater the atomic number, the higher the priority.
What’s the second step in determining the relative properties of E,Z isomer groups?
2) If the two atoms attached to an sp2 carbon are the same (there is a tie) then consider the atomic numbers of the atoms that are attached to the “tied” atoms.
What’s the third step in determining the relative properties of E,Z isomer groups?
3) If an atom is doubly bonded to another atom, the priority system treats it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, it is treated as if it were singly bonded to three of those atoms
What’s the fourth step in determining the relative properties of E,Z isomer groups?
4) If two isotopes (atoms with the same atomic number but different mass numbers) are being compared, the mass number is used to determine the relative properties
What is the cause of chirality in a molecule? Explain.
An asymmetric center. An asymmetric center is an atom bonded to four different groups
How many stereoisomers can exist for a compound with one asymmetric center?
two stereoisomers
What is an enantiomer?
molecules that are non superimposable mirror images of each other, is chiral
What is a stereocenter?
kind of like an asymmetric center, it is a atom at which the interchange of two groups produces a stereoisomer. It includes both (1) asymmetric centers, where the interchange of two groups produces an enantiomer, and (2) the sp2 carbons of an alkene or the sp3 carbons of a cyclic compound. (all asymmetric centers are stereocenters, but not all stereocenters are asymmetric centers)
stereocenters are three or more groups, and asymmetric centers are four
How do you draw an enantiomer?
1) show two of the bonds to the asymmetric center in the plane of the paper (make sure the two bonds are adjacent to each other)
2) show one bond as a solid wedge protruding forward out of the paper
3) show the fourth bond as a hatched wedge extending behind the paper (the solid and hatched wedges must be adjacent to each other and the solid wedge must be below the hatched wedge.)
What is a Fischer projection?
- horizontal lines represent the bonds that project out of the plane of the paper toward the viewer
- vertical lines represent the bonds that extend back from the plane of the paper away from the viewer
- the carbon chain is usually drawn vertically, with C-1 at the top
What are the two letters used to name enantiomers?
R and S
How do you name an enantiomer?
How do you name a compound drawn as a Fischer Projection?
How do you recognize a pair of enantiomers?
What does it mean for a compound to be optically active?
a compound that rotates the plane of polarization of a plane-polarized light, a chiral compound
What does it mean for a compound to be optically inactive?
a compound that does’t rotate the plane of polarization of a plane-polarized light, an achiral compound
What does it mean for a compound to be dextrorotatory?
when an optically active compounds rotates the plane of polarization clockwise, is indicated by the prefix +
What does it mean for a compound to be levorotatory?
when an optically active compound rotates the plane of polarization counterclockwise, indicated by the prefix -
What is the only way to tell if a compound is dextrorotatory(+) or levorotatory(-)?
to put the compound in a polarimeter
What is a polarimeter?
an instrument that measures the direction and amount an optically active compound rotates the plane of polarization of plane-polarized light
How do you calculate the specific rotation of an optically active compound?
What does it mean for a compound to enantiomerically pure and what is its observed specific rotation?
only one enantiomer is present, +23.1
If a sample is a racemic mixture, what will its observed specific rotation be?
0
What does it mean if a sample has an observed specific rotation that’s positive but less than +23.1?
it means the sample is a mixture of enantiomers but contains more S enantiomer than R enantiomer, because the S enantiomer is dextrorotatory
What is enantiomeric excess and how do you calculate it?
also called optical purity, tells us how much of an excess of one enantiomer is in the mixture
How do you calculate the number of stereoisomers a compound can have?
a compound can have a maximum number of 2^n stereoisomers, where n equals the number of asymmetric centers (provided it doesn’t also have stereoisomers that would cause it to have cis-trans isomers)
Classify the stereoisomers of a compound with the same four atoms or groups bonded to two different asymmetric centers.
there will be three stereoisomers, one will be a meso compound, and the other two will be enantiomers
How do you draw a perspective formula for a compound with two asymmetric centers?
What is a receptor?
a protein that binds a particular molecule. and because proteins are chiral, a receptor binds one enantiomer better than the other.
What is a racemic mixture?
a mixture of equal amounts of two enantiomers
What are diastereomers?
Stereoisomers that aren’t enantiomers. They aren’t identical and are non-superimposable on each other. Diastereomers occur when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters.
What are erythro enantiomers?
When Fischer projections are drawn for stereoisomers with two adjacent asymmetric centers, the enantiomers with the hydrogens on the same side of the carbon chain are erythro enantiomers
What are threo enantiomers?
When Fischer projections are drawn for stereoisomers with two adjacent asymmetric centers, the enantiomers with the hydrogens on opposite sides are called the threo enantiomers
What is a mesocompound?
When a stereoisomer has two asymmetric centers but the same groups on each so is achiral.
