O Chem 1 Chapter 5 Things

Stereoisomerism

What are constituional isomers?

they have the same molecular formula but different connectivity

what are the two types of stereoisomers?

ring stereoisomers and chirality

What can single bonds freely do around sigma bonds?

single bonds can ROTATE

what are stereoisomers?

two molecules that have the same connectivity but are not identical because their are rotated differently

what is done to maintain orbital overlap in a pi bond?

The double bonds cannot rotate, making them stereoisomers, which are then cis or trans.

what are chiral molecules?

a chiral object is asymmetric, which means it is not the same as its mirror image (nonsuperimposable on its mirror image.

what is true about achiral objects?

that they are identical to mirror images and are superimposable

they possess a plane of symmetry.

what is true about chiral objects?

they are not identical to their mirror image and therefore not superimposable.

when does chirality most often result?

when a carbon atom is bonded to four unique atoms or groups

what is it called when an atom forms a tetrahedral center with 4 different groups?

it is called a chiral center

what can some stereoisomers ALSO be classified as?

enantiomers!

what are enantiomers?

they are two molecules that are mirror images but are NOT identical and not superimposable.

only chiral centers can have an entantiomer.

do enantiomers have identical names?

No, they do not, since they are different compounds.

They have opposite configurations at their chiral centers, and instead of drawing the mirror image, dashes can be inverted to wedges, and vice versa.

What is the Cahn-Ingold-Prelog system and what does it do?

It assigns each chiral center the R or S configuration.

How do you assign an R or and S to a chiral center?

1. Prioritize the four groups attached to the chiral center

2. Arrange the molecule in space so the lowest-priority groups face AWAY from you (on a dash)

3. Count the group priorities 1,2,3 to determine whether the order progresses in a clockwise or counterclockwise direction

4. clockwise= R and counterclockwise= S

What do you do when the groups attached to a chiral center are tricky? How do you prioritize them?

Analyze the atomic numbers one layer at a time (if there is a tie go to the next group)

When using the Cahn, Ingold and Prelog systems what do double bonds count as?

they count as two single bonds

What are fischer projections?

they can be used to represent molecules with chiral centers.

What do horizontal lines represent in fischer projections?

they represent wedges

what do the vertical lines represent?

they represent dashes

what aree fischer projections most useful for?

for drawing molecules that have multiple chiral centers (like SUGARS)

how do you assign R and S configurations to Fischer projections?

you use the same rules with the wedge/dash drawings, remember the lowest priority but be on a dash if NOT you must flip)

what do you do if the OH on a fischer projection is on the left?

you will get a S then FLIP IT

what do you if an OH in a fischer projection is on the right?

you will get a R then FLIP IT

do enantiomers have generally the same physical properties?

Yes, they do because they only differ in the same way your right hand differs from you left

How do enantiomers differ?

1. how they interact with other chiral compounds

2. their optical activity

what do enantiomers confirgurations (R and S) do with plane-polarized light?

They rotate them in opposite directions.

how do you get light waves that travel in only one plane?

The light must travel through a filter.

what happens when plane-polarized light is passed through a sample of chiral compound?

the plane that the light travels on will rotate.

what do you call compounds that can rotate plane-polarized light?

optically active (only chiral compounds are and they act as a filter)

what device can measure the rotation of plane-polarized light cause by optically active compounds?

polarimeter

how do you use a polarimeter?

you point it at a light source, and rotate the filter until light passes through and you can see light

do enantiomers rotate light in the exact opposite directions?

YES

what does the degree to which light is rotated depend on?

it depends on the sample concentration and the pathlength of light.

can temperature and wavelenght of light affect rotation also…?

yes they can… but shhhhh

what are positive rotations called?

dextrorotatory

what are negative rotations called?

levrorotatory

If R is negative then what was S be?

S will be positive.

can the magnitude and direction of optical rotation be predicted?

no it has to be measured experimentally

what is a racemic mixture?

a 50/50 mixture of 2 enantiomers

what happens if one enatiomer is present in excess relative to the other?

then the mixture will have an optical rotation but will be less than the pure enantiomer.

can polarimetry tell you if you have one enantiomer or a mixture or both?

yes it can.

how do you find the enantiomer excess?

oberved/ pure enantiomer.

what is true if there is only one enantiomer?

optically pure and is strong or rotation max

what is true if there is a 50/50 mixture?

it is optically inactvie and racemic

what happens if there is more of one enantiome than the other?

it is enantiomerically enriched. And it rotaes partially in direction of more prevelant enantiomer

what are the two sub-categories of stereoisomers?

enantiomers and diastereomers

what is an enantiomer?

stereoisomers that are mirror images

what is a diastereomer?

stereoisomers that are not mirror images.

do diastereomers have the same or different physical properties?

they have different physical properties

what is the equation to tell how many numbers of stereoismers are in a molecule?

2^n where n is the number of chiral centers

what must each chiral center be opposite for chiral centers to be enantiomers?

each chiral center must be opposite for ALL chiral centers to be enantiomers

What is a compound with chiral centers, but is achiral because of symmetry, called?

a meso compound!

what do meso compounds have less of?

the have less than the predicted number of stereoisomers and are optically inactive.

what are the E and Z notations used for?

they are useed instead of cis/trans diastereomers and are for C=C double bonds.

What happens when there are more than 2 non hydrogen substitutents?

cis/trans isn’t adequate to differentiate between 2 diastereomers

How do you assign an E or a Z group?

you prioritize the groups attached to the C=C double bond BASED on atomic number, if the 2 groups that are high priority on the same side then it is Z if they are opposite then it is E.