Chapter 5 Introduction to Stereochemistry

Stereochemistry

____________________ - chemical studies that take into account the spatial aspects of molecules

Constitutional isomers

____________________ - isomers whose atoms have a different connectivity

Stereoisomers

_______________ - isomers that have the same connectivity but that differ in the arrangement of their atoms in space

Conformational stereoisomers

______________________ - stereoisomers differing in space only due to rotations about a single bond

Configurational stereoisomers

_______________________ - stereoisomers that differ in the 3D arrangement of groups around a stereocenter

conformational isomerism

__________________________ - occurs when two groups bonded by only a single bond can undergo rotation about that bond with respect to each other

conformation

_________________ - temporary molecular shape that results from rotation about a single bond

conformer

________________

- ‘conformational isomer’

- each possible structure

conformational analysis

_______________________ - analysis of the energy changes that occur as a molecule undergoes rotations about single bonds

angle strain

torsional strain

steric strain

________________ - strain due to expansion or compression of bond angles

________________ - strain due to eclipsing of bonds on neighboring atoms

________________ - strain due to repulsive interactions when atoms approach each other too closely

eclipsed

_____________ conformation

- the atoms bonded to carbon atoms at each end of a C-C are directly opposed to one another

- the dihedral angle between them is 0°

staggered

_______________ conformation

- the atoms bonded to carbon atoms at each end of a C-C bond are as far apart as possible

- the dihedral angle between them is 60° or multiples of it

gauche

Conformational Analysis of Butane

________________ conformation - staggered conformation with two larger groups 60° from each other

anti

Conformational Analysis of Butane

____________ conformation - a staggered conformation with two larger groups 180°nfrom each other

Baeyer strain theory

____________________ - the bond angle in a perfect tetrahedron is 109°28’, then the amount of strain in a small ring compound would be the half the difference between the tetrahedral value and the internal angle of a regular polygon

Between chair and boat conformation which is more stable? ____________-

• equatorial

• axial

• ______________ positions are located in the plane of the ring (around the equator)

• ______________ positions are located above and below the ring (along a perpendicular axis)

Ring flipping

Ring flipping turns axial/equatorial hydrogens into equatorial/axial hydrogens.

_____________

- cyclohexane does not remain in a single conformation

- the bonds twist and bend, resulting in new arrangements, but the movement is more restricted

Ring flipping turns __________ hydrogens into ____________ hydrogens.

chirality

superposability

chiral molecule

achiral molecule

As long as it has a plane of symmetry it is achiral

Configurational Stereoisomers

_____________

• property of having handedness

• cheir – Gr. ‘hand’
  

______________

• two objects are superposable if, when one object is placed on top of the other, all parts of each coincide

______________

• a molecule that is not superposable on its mirror image

• have handedness; capable of existing as a pair of enantiomers
  

________________

• a molecule that is superposable on its mirror image

• lack handedness; incapable of existing as a pair of enantiomers

______________ occur only with compounds whose molecules are chiral.

stereogenic center

________________ - any atom at which an interchange of groups produces a stereoisomer

Convert from 3d to Fischer

Absolute configuration

__________________ - the actual orientation in space of the groups around a stereocenter

Enantiomers

polarimeter

_______________ have identical melting and boiling points, index of refraction, solubility in common solvents, infrared spectra and rates of reactions with achiral reagents.

_________________ - device that is used for measuring the effect of optically active compounds on plane-polarized light

dextrorotatory, d, (+)

levorotatory, l, (–)

optically active compound

___________________

• a substance that rotates planepolarized light in a clockwise direction
  

___________________

• a substance that rotates planepolarized light in a counterclockwise direction

___________________-

• a compound that rotates the plane of polarization of planepolarized light

racemic mixture or racemate

____________________

• an equimolar mixture of two enantiomers

• causes no net rotation of plane polarized light

How to calculate total number of stereoisomers?

Enantiomers - are pairs of stereoisomers that are non-superimposable mirror images of each other, often described as having "handedness" (chirality).

Diastereomers - are stereoisomers that are non-mirror image, non-superimposable

Draw all the stereoisomers and label which are enantiomeric pairs and which are diastereomeric pairs.

Meso compound

_________________

- an achiral molecule that contains chirality centers

- optically inactive

Draw all the stereoisomers and label which are meso compounds.

plus-minus is racemic which means theres equal amounts of S and R.