Ch. 2: Isomers

Question 1

Structural isomers or constitutional isomers

Answer 1

Share only a molecular formula, diff physical and chem and properties

Question 2

Physical Properties

Answer 2

Characteristics of processes that dont change the composition of matter, such as melting point, boiling point, solubility, odor, color, and density

Question 3

Chemical Properties

Answer 3

Have to do with the reactivity of the molecule w other molecules and result in changes in chemical composition

Question 4

Stereoisomers

Answer 4

Same atomic connectivity– same structural backbone– differ in how atoms are arranged in space (wedge-and-dash pattern)

Question 5

Conformational isomers/conformers

Answer 5

Differ by rotation around a single sigma bond; double bonds dont allow for rotation

Question 6

Newman projection

Answer 6

Molecule is visualized along a line extending through a carbon-carbon bond axis

Question 7

Staggered conformation

Answer 7

Have groups 60* apart as seen in a newman projection

• anti staggered molecules: two largest groups are 180* apart, and strain is minimized
• gauche staggered molecules: the two largest groups are 60* apart

Question 8

Eclipsed conformations

Answer 8

Have groups directly in front of each other as seen in a newman projection

Question 9

Totally eclipsed conformations

Answer 9

Two largest groups are directly in front of each other and strain is maximized

Question 10

Sources of ring strain in cyclic molecules

Answer 10

Angle strain, torsional strain, and nonbonded strain

Question 11

Angle strain

Answer 11

Created by stretching or compressing angles from their normal size

Question 12

Torsional Strain

Answer 12

From eclipsing conformations

Question 13

Nonbonded strain

Answer 13

From interactions between substituents attached to nonadjacent carbons– cyclic molecules will usually adopt non-planar shapes to minimize this strain

Question 14

Axial vs equatorial

Answer 14

Substituents attached to cyclohexane can be classified as axial (sticking up or down from the plane of the molecule and create more nonbonded strain) or equatorial (in the plane of the molecule)– molecules usually take equatorial position to minimize strain

Question 15

Configurational isomers:

Answer 15

Can only be interchanged by breaking and reforming bonds

Question 16

Enantiomers

Answer 16

Nonsuperimposable mirror images and thus have opposite stereochemistry @ every chiral carbon. Have same chemical and physical properties except for rotation of plane-polarized light and reactions in a chiral environment

Question 17

Optical activity

Answer 17

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

Question 18

Racemic mixtures

Answer 18

With equal concentrations of two enantiomers, will not be optically active bc the two enantiomers’ rotations cancel each other out

Question 19

Meso compounds

Answer 19

With an internal plane of symmetry, will also be optically inactive bc the two sides of the molecule cancel each other out

Question 20

Diastereomers

Answer 20

Non-mirror-image stereoisomers. Differ @ some, but not all, chiral centers. Have diff chemical and physical properties
- cis-trans: isomers are a subtype of diastereomers in which groups differ in position about an immovable bond (such as a double bond or in a cycloalkane)

Question 21

Chiral Centers

Answer 21

4 diff groups attached to the central carbon

Question 22

Relative Configuration

Answer 22

Gives the stereochemistry of a compound in comparison to another molecule

Question 23

Absolute configuration

Answer 23

Gives the stereochemistry of a compound without having to compare to other molecules. Uses cahn-ingold-prelog priority rules

Question 24

cahn-ingold-prelog priority rules

Answer 24

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 25

Alkene Z vs E

Answer 25

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

Question 26

Stereocenter’s configuration

Answer 26

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 circle is clockwise, the stereocenter is R, if it is counterclockwise the stereocenter is S

Question 27

Fischer diagrams

Answer 27

Vertical lines in fischer diagrams go in to the plane of the page (dashes); horizontal lines come out of the plane of the page (wedges)

• switching from one pair of substituents in a Fischer diagram inverts the stereochemistry of the chiral center. Switching 2 pairs retains the stereochemistry
• rotating a fischer diagram 90* inverts the stereochemistry of the chiral center. Rotating 180* retains the stereochemistry.