Isomers ***

Question 1

Structural isomers

Answer 1

share only a molecular formula. They have different physical and chemical properties.

Question 2

Conformational isomers

Answer 2

differ by rotation around a singe sigma bond.

Question 3

staggered conformations

Answer 3

have groups 60* apart, as seen in a . Newman projection. In anti staggered molecules, the two largest groups are 180* apart, and strain is minimized. In gauche staggered molecules, the two larges groups are 60* apart.

Question 4

Eclipsed conformations .

Answer 4

have groups directly in front of each other as seen in Newman projection. In totally eclipsed conformations, the two largest groups are directly in front of each other and strain is maximized.

Question 5

Angle strain

Answer 5

the strain in cnyclic molecules created by stretching or compressing angles from their normal size.

Question 6

torsional strain

Answer 6

from eclipsing conformations

Question 7

nonbonded strains

Answer 7

from interactions between substituents attached to nonadjacent carbons

Question 8

Cyclic molecules usually adopt

Answer 8

nonplanar shapes to minimize strain

Question 9

axial

Answer 9

sticking up or down form the plane of the molecule. Create more nonbonded strain.

Question 10

equatorial

Answer 10

in the plane of the molecule

Question 11

configurational isomers

Answer 11

can only be interchanged by breaking and reforming bonds

Question 12

enantiomers

Answer 12

are non-superimposable mirror images and thus have opposite stereochemistry at every chiral carbon. They have the same chemical and physical properties except for rotation of plane-polarized light and reaction in a chiral environment.

Question 13

Optical activity

Answer 13

refers to the ability of a molecule to rotate plane-polarized light: d- or (+) molecules rotate light to the right; l- or (-) molecules rotate light to the left.

Question 14

racemic mixtures

Answer 14

with equal concentrations of two enantiomers, will not be optically active because the two enantiomers’ rotations cancel each other out.

Question 15

meso compound

Answer 15

with an internal plane of symmetry, will also be optically inactive because the two sides of the molecule cancel each other out. They are not optically active.

Question 16

diasterioisomers

Answer 16

are non-mirror-image stereoisomers. They differ at some, but not all, chiral centers. they have different chemical and physical properties.

Question 17

cis-trans isomers

Answer 17

are . subtype of diastereomers in which groups differ in position about an immovable bond (such as a double bond or in a cycloalkane.

Question 18

chiral centers

Answer 18

have four different groups attached to the central carbon

Question 19

relative configuration

Answer 19

gives the stereochemistry of a compound in comparison to another molecule

Question 20

absolute configuration

Answer 20

gives the stereochemistry of a compound without having to compare to other molecules.

Question 21

Cahn-Ingold-Prelog priority rules

Answer 21

in which priority is given by looking at the atoms connected to the chiral carbon or double bonded carbons; whichever has the highest atomic number gets highest priority. If there is a tie, one moves outward from the chiral carbon or double-bonded carbon until the tie is broken.

Question 22

alkene

Answer 22

is Z if the highest priority substituents are on the same side of the double bond and E if on the opposite sides.

Question 23

stereocenter’s configuration

Answer 23

determined by putting the lowest priority group in the back and drawing a circle from group 1 to 2 to 3 in descending priority. If this circle is clockwise, the stereocenter is R; if it is counter clock wise, the stereo center is S.

Question 24

Fischer diagrams

Answer 24

vertical lines go into the plane of the page (dashes); horizontal lines come out of the plane of the page (wedges). switching one pair of substituents in a Fischer diagram inverts the stereochemistry of the chiral center. Switching two pairs retains the stereochemistry. Rotating a Fischer diagram 90* inverts the stereochemistry of the chiral center. Rotating 180* retains the stereochemistry.