Chem 123 - Unit 11 Relationships, Chiral Properties, Fischer Projections

What is a chiral molecule?

A molecule that is not superimposable on its mirror image.

Define achiral.

A molecule that is superimposable on its mirror image.

What are enantiomers?

Stereoisomers that are non-superimposable mirror images of each other.

Enantiomers will have ALL of their bonds changed conformations

What are diastereomers?

Stereoisomers that are not mirror images of each other.

Diastereomers will not have all of their bonds changed conformations

What is a meso compound?

A compound with multiple stereocenters that is superimposable on its mirror image.

an achiral moleucle with asymmetric centres

What is the relationship between constitutional isomers?

They have the same molecular formula but different connectivity of atoms.

What does the term 'stereoisomers' refer to?

Compounds with the same molecular formula and connectivity but different spatial arrangements.

What is a Fischer projection?

A 2D representation of a molecule that shows the configuration of its stereocenters.

How do you determine the configuration (R/S) of a stereocenter using a Fischer projection?

Assign CIP priorities, and if the lowest priority is vertical, assign normally; if horizontal, reverse the answer.

What is optical activity?

The ability of a chiral substance to rotate plane-polarized light.

What is a racemic mixture?

A 1:1 mixture of two enantiomers that is optically inactive.

What does dextrorotatory mean?

A substance that rotates plane-polarized light to the right (clockwise).

What does levorotatory mean?

A substance that rotates plane-polarized light to the left (counter-clockwise).

What is specific rotation?

The angle of rotation of plane-polarized light by an optically active compound at a specific concentration and path length.

What is the relationship between two molecules if they have the same IUPAC name?

They are identical molecules, possibly shown in different orientations.

mirror images of molecules will usually (depending on where the mirror is)

keep the wedges and dashes

will only switch if the mirror is in the plane of the page

What is the difference between optically active and optically inactive substances?

Optically active substances can rotate plane-polarized light (chiral), while optically inactive substances cannot (achiral or racemic).

What is the significance of asymmetric centers in chiral molecules?

They provide the basis for chirality and the existence of enantiomers.

What is the relationship between two compounds if they are identical?

They are the same molecule, possibly represented in different orientations.

What is the relationship between optical rotation and enantiomers?

Enantiomers rotate light in equal magnitudes but in opposite directions.

oprtical roation = (rotation to the +)(degree of rotation) + (rotation to the -)(degree of rotation)

What happens to the optical activity of a racemic mixture?

It is optically inactive due to equal contributions from both enantiomers.

cis configuration in alkenes

Z conficguration (highest priorities are both on the same side)

Trans configuration on alkenes

E configuration

highest priority groups are on opposite sides

what is a stereocenter

an atom where if you rotate its bonds, a new conformer is formed

what is an asymmetric center

a stereocentre that is an sp3 hybridized carbon atom wit h4 unique substituents bonded to it

superimposable

two molecules that can be placed direcly ontop of each other and overlap perfectly

non-superimposable

two object placed on top of eachother does not overlap perfectly

in fisher projection bonds that are on dashes

are verical

in fisher projections, bonds that are on wedges

are horizontal

reactive centre

sp3 hybridized carbon atom that is bonded to the leaving group

nucleophile

species with at least one lone pair that will form a bond with the electrophile

electrophile

species bonded to the leaving group

leaving group

species that is most polar on a molecule and will be a stable anion

kinetics of an Sn2 reaction

rate= k [electrophile][nucleophile]

what influences Sn2 reactions?

if the electrophile is larger with more groups it will be slower

good leaving groups

how many carbons are attached to the reactive centre (less = faster reaction, tertiary does not happen)

steps of an Sn1 reaction

1- leaving group leaving electrophile

2- electrophile bonding to nucleophile

3- acid- base clean up

kinetics of an Sn1 reaction

rate = k [electrophile]

properties of an Sn1 reaction

first order rate

will have heat added

happens in 3 steps

properties of an Sn2 reaction

second order reaction

happens in 1 step

nucleophile will attack 180 degrees around the leaving group

factors influencing and Sn1 reaction

stability of the electrophile (more degrees is better, will not happen with a methyl or primary)

good leaving group