Isomerism
Chemistry. the relation of two or more compounds, radicals, or ions that are composed of the same kinds and numbers of atoms but differ from each other in structural arrangement (structuralisomerism) as CH3OCH3and CH3CH2OH, or in the arrangement of their atoms in space and therefore in one or more properties.

Aplane of symmetryis an imaginaryplanethat bisects a molecule into halves that are mirror images of each other. eg. 1: In 1, the verticalplanethat bisects the methyl group, the carbon atom, and the hydrogen atom bisects the molecule into halves that are mirror images of each other.

centre of symmetry(plural centres ofsymmetry) (mathematics) A point, within an object or figure, through which any straight line also passes through two points on the edge of the figure at the same distance from thecentrebut on opposite sides.

Absolute Configuration: R-S Sequence Rules

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07:18, 7 Aug 2015

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ShareTable of contentsTo name the enantiomers of a compound unambiguously, their names must include the "handedness" of the molecule. The methodforthis is formally known as R/S nomenclature.

Introduction

The method of unambiguously assigning the handedness of molecules was originated by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and, as such, is also often called the Cahn-Ingold-Prelog rules. In addition to the Cahn-Ingold system, there are two ways of experimentally determining theabsolute configuration of an enantiomer:

X-ray diffractionanalysis. Note that there is no correlation between the sign of rotation and the structure of a particular enantiomer.
Chemical correlation with a molecule whose structure has already been determined via X-ray diffraction.

However, for non-laboratory purposes, it is beneficial to focus on the R/S system. The sign ofoptical rotation, although different for the two enantiomers of a chiral molecule,at the same temperature,cannotbe used to establish the absolute configuration of an enantiomer; this is because the sign of optical rotation for a particular enantiomer may change when the temperature changes.

Stereocenters are labeled R or S

The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral compound. The stereocenters are labeled as Ror S.



Consider the first picture: acurved arrow is drawn from the highest priority (1) substituent to the lowest priority(4) substituent. If the arrow points in a counterclockwise direction (leftwhen leaving the 12o' clock position), the configuration at stereocenter is consideredS("Sinister" → Latin= "left"). If, however, the arrow points clockwise,(Rightwhen leaving the 12o' clock position) then the stereocenter is labeledR("Rectus" → Latin= "right"). TheRorSis then added as a prefix,in parenthesis, to the name of the enantiomer of interest.

EXAMPLE 1

(R)-2-Bromobutane

(S)-2,3- Dihydroxypropanal

Sequence rules to assign priorities to substituents

Before applyingthe R and S nomenclature to a stereocenter, the substituents must be prioritized according to the following rules:

Rule 1

First, examineat the atomsdirectly attached to the stereocenter of the compound. A substituent with a higher atomic number takes precedence over a substituent with a lower atomic number. Hydrogen is the lowest possiblepriority substituent, because it has the lowest atomic number.

Whendealing with isotopes, the atom with the higher atomic mass receives higher priority.
When visualizing the molecule, the lowest priority substituentshould always point away from the viewer(a dashed line indicates this). To understand how this works or looks, imagine thata clock and a pole.Attach the pole to the back of the clock, so that when when looking at the face of the clock the pole points away from the viewer in the same waythe lowest priority substituent should point away.
Then, draw an arrow from the highest priority atom to the 2nd highest priority atom to the 3rd highest priority atom. Becausethe 4th highest priority atom is placed in the back, the arrow should appear like it is going across the face of a clock. If it is going clockwise, then it is an R-enantiomer; If it is going counterclockwise, it is an S-enantiomer.


When looking at a problem with wedges and dashes,if the lowest priority atom is not on the dashed line pointing away, the molecule must be rotated.
Remember that

Wedges indicate coming towards the viewer.
Dashes indicate pointing away from the viewer.

Rule 2

If there aretwo substituents with equal rank,proceed along the two substituent chains until there is a point of difference. First, determine which of the chains has the first connection to an atom with the highest priority (the highest atomic number). That chain has the higher priority.

If the chains are similar, proceeddown the chain, untila point of difference.

For example: an ethyl substituent takes priority over a methyl substituent. At the connectivity of the stereocenter, both have a carbon atom,which are equal in rank. Going down the chains, a methyl has only has hydrogen atoms attached to it,whereas the ethyl has another carbon atom.The carbon atom on the ethyl is the first point of difference and has a higher atomic number than hydrogen;therefore the ethyl takes priority over the methyl.

Rule 3

If a chain is connected to the same kind of atom twice or three times, check to see if the atom it is connected to has a greater atomic number than any of the atoms that the competing chain is connected to.

If none of the atoms connected to the competing chain(s)at the same pointhas a greater atomic number: the chain bonded to the same atom multiple times has the greater priority
If however, one of the atoms connected to the competing chain has a higheratomic number: that chain has the higher priority.

EXAMPLE 2

A 1-methylethyl substituenttakes precedence over an ethyl substituent.Connected to the first carbon atom, ethyl only has one other carbon, whereas the 1-methylethyl has two carbon atoms attached to the first; this is the first point of difference. Therefore, 1-methylethylranks higher in priority than ethyl, as shown below:



However:

Remember that being double or triple bonded to an atom means that the atom is connected to the same atom twice. In such a case,follow the same method as above.



Caution!!
Keep in mind that priority is determined by thefirstpoint of difference along the two similar substituent chains. After the first point of difference, the rest of the chain is irrelevant.


Whenlooking for the first point of difference on similar substituent chains, onemay encounter branching. If there is branching, choose the branch that is higher in priority. If the two substituents have similar branches, rank the elements within the branches untila point of difference.



After all your substituents have been prioritized in the correct manner, you can now name/label the moleculeRorS.

Put the lowest priority substituent in the back (dashed line).
Proceed from 1 to 2 to 3. (it is helpful to draw or imagine an arcing arrow that goes from 1--> 2-->3)
Determine if the direction from 1 to 2 to 3 clockwise or counterclockwise.

i) If it isclockwiseit isR.
ii) if it iscounterclockwiseit isS.

USEYOURMODELINGKIT:Models assist invisualizingthe structure. When usinga model, make sure the lowest priority is pointing away from you. Then determine the directionfrom the highest priority substituent to the lowest: clockwise (R) or counterclockwise (S).

IF YOU DO NOT HAVE A MODELING KIT: remember that the dashes mean the bond is going into the screen and the wedges means that bond is coming out of the screen. If the lowest priority bond is not pointing to the back, mentally rotate it so that it is. However, it is very usefulwhen learning organic chemistry to use models.

If you have a modeling kit use it to help you solve the following practice problems.

Problems

Are the following R or S?

Solutions

S:I > Br >F > H. The lowest priority substituent, H, is already going towards the back. It turns left going from I to Br to F, so it's a S.
R:Br > Cl > CH3> H. You have to switch the H and Br in order to place the H, the lowest priority, in the back. Then, going from Br to Cl, CH3is turning to the right, giving you a R.
Neither R or S: This molecule is achiral. Only chiral molecules can be named R or S.
R:OH > CN > CH2NH2> H. The H, the lowest priority, has to be switched to the back. Then, going from OH to CN to CH2NH2, you are turning right, giving you a R.
(5) S:−COOH−COOH>−CH2OH−CH2OH>C≡CHC≡CH>HH. Then, going from−COOH−COOHto−CH2OH−CH2OHto−C≡CH−C≡CHyou are turning left, giving you a S configuration.