Organic Chemistry Chapter 5

What makes an object chiral?

when its mirror image is different from the original object.

What makes an object achiral?

when its mirror image can be superposed

What is an enantiomer?

compounds that are nonsuperimposable mirror images

Do all chiral compounds have an enantiomer?

yes

What is chirality in a carbon atom

where it is an asymmetric carbon (bonded to 4 different groups)

Are carbons from a double bond or triple bond chiral?

no, they are always achiral

How do you have to rotate an achiral carbon to see that it is the same?

180 degrees

WHat is the chirality of a molecule with a plane of symmetry?

achiral

What refers to a symmetry plane?

σ

Are trans/cis molecules chiral?

ultimately depends on the structure, however cis cannot ever be chiral because they have a plane of symmetry.

What is the R and S configuration?

enantiomers have different spatial arrangements of the 4 groups attached to the asymmetric carbon. R and S are those two different arrangements

What is the first step in identifying R and S?

1. Assign priority

How do you assign priority>

1. Assign based on atomic #

2. I > Br > Cl > S > F > O > N > 13C > 12 C > 2 H > 1 H

WHat to do if there is a tie when assigning priority?

use the next atoms on the chain of each group as tie breakers

How are multiple bonds treated when assigning R and S priority?

only in tie breakers, as if each were a bond to a separate atom

After assigning priority, how must the molecule be rotated?

so that the lowest priority group is in the back, then draw an arrow from the highest to lowest priority (1 to 2 to 3)

What is a clockwise arrow rotation mean?

R

What is a counterclockwise arrow rotation?

S

What are the similarities between enantiomers in terms of properties?

they have the same boiling point, melting point, and density. 

They have the same refraction index

They rotate the plane of polarized light in the same magnitude, but in opposite directions

They are optically active. 

What does a polarimeter measure

the optical rotation of a substance

What is it called when light is rotated clockwise?

Dextrorotary (+)

What is it called when light is rotated counterclockwise?

Levorotatory (-)

What is a racemic mixture?

a mixture that contains equal quantities of d - and l- enantiomers

How are racemic mixtures notated?

(±)

How are racemic mixtures optically?

they are not optically active

What direction are the horizontal lines in a Fischer projection pointing?

out at the viewer

What direction are the vertical lines in a Fischer projection pointing?

behind the plane

Where in the fischer projection is the highest oxidized carbon?

at the top

How are R and S configurations determined with Fischer projections?

apply the normal rules backward. Assign priority and then rotate through. The direction you rotate corresponds to the configuration, except assign the opposite one.

What are diastereomers?

molecules with two or more chiral carbons, they are stereoisomers that are not mirror images of each other.

How do diastereomers work with cis and trans?

trans are chiral and have enantiomers of each other, but cis is achiral.

Cis and Trans are stereoisomers of each other that are not mirror images of each other and they both can have two or more chiral carbons, making them diastereomers of each other.

What are meso compounds?

compounds with chiral carbons and a plane of symmetry (if one image was rotated 180 degrees, then it could be superimposed on the other image)

Are meso compounds chiral?

no, the plane of symmetry results in the overall molecule to be achiral.

How are meso compounds named?

meso-2,3-dibromobutane. 

How is R and S denotated in naming?

(2S,3S)-dibromobutane

What is the maximum number of isomers a compound can have?

2ⁿ, where n = # of chiral carbons. (have to double check to make sure that none of them are meso and have a plane of symmetry).