Organic Chemistry: Section 2 Isomers and Stereochemistry

Short cut to R and S

1. when the fourth substituent is in front of the molecule you assign the opposite of the arc you draw. 2. when the 4th substituent is in the plane, but drawn next to a group that is pointing back, then reverse the arc that you see. 3. when the 4th substituent is in the plane and the group next to it, is in the front then take the arc that you see.

Stereisomerism

= 2^n, where n equals the number of chiral carbons

Rotation about sigma bonds

Conformational isomers are in dynamic equilibrium in room temprature, because the molecules are constantly rotating, the structure dosent just remain just in one of the conformation. but rather assumes all confirmations. two overall extreme structures are known as Staggered and Eclipsed.

Configurational isomers

identical bonds but they have different arrangement of their atoms no matter how the structures are contorted

Meso structure

individual structures which contain a mirror plane slicing through the middle of the compound and an even number of chiral center symmetrical display, meso compounds are optically inactive. Rap key word me so inactive

cyclobutane

is more stable than the cyclopropane. It has a ring strain of 26.2 kcal/ mole. however, it is considered reactive in comparison to normal bonding

Nucelophile

loves positive charge, small neucleophiles are better neucleophile

Optical rotation

measurement of the rotation of plane polarized light by a solution with a chiral molecule. + describes the clock wise rotation of the light. - describes the anticlock wise rotation of the light.

Optical isomers

molecules of the same formula and the exact same bonds but have a different arrangement of the atoms due to asymmetry. one optical isomer cannot be rotated, the isomers rotate plane polarized light differently from one another.

conformational isomers

molecules with identical bond that are non superimposable because of the rotation about a bond " Ring flipping".

cyclohexane

most stable cycloalkane, establishes sp3 bond with an angle of 109.5 degrees. it has two conformation chair and a boat. they interconvert through a process referred to as ring flipping.

the finger method

put your thumb toward the fourth compound direction and see what matches it, if left handed match then it is an S center. however, if it is a right handed match then it is an R center

steriosomers

same bonds but have different arrangement of atoms, configurational isomers have different prefixes in their IUPAC names

Staggered structure

substituents on the first atom do not block the substituents on the back atom of the sigma bond. 180 degree difference. most stable Anti

the number of possible structural isomerism depends on

the molecular formula, linear alkanes are the simplest case. each extra carbon, the number of structural isomerism increases.

Optical rotation measured

using polarimeter, the plate are rotated in a manner to allow for the greatest amount of light to pass through. [a]D=(obsereved rotation in degrees)/ [(l of sample in dm)(conc grams/ml)]

R-center

when you look down from the chiral carbon to the 4th substituent, the remaining substituent form a clock wise and it is considered as a right handed.

eclipsed structure

where the substituents on the first atom block the substituents on the back atom of the sigma bond. zero degrees difference. it is the least stable

Z

zusammen, together, cis, keyword Zis

cyclopropane

Ring strain is 27.4 kcal/mol, the carbon-carbon are bent (not collinear), making them weaker than the standard carbon-carbon single bond. their sigma bonds in which density is not between the two nuclei, the bond is weaker and much easy to break.

all nucleophiles substitution reaction

SN1 & SN2

SN1 and SN2 destinguishment

SN1 rate= k[elecrophile] SN2 rate= K[elecrophile][Nucleophile] SN2 is a one hill reaction, however, SN1 is two hills with one intermediate

S-Center

a carbon center when you look down from the chiral carbon to the 4th substituent, the remaining substituent form a counterclock wise and it is considered as a left handed.

Asymmetry

a molecule that have a site which there is uneven distribution of the bonded atoms.

Electrophile

a species that is attracted to a region of high electron density. its strenght can be aproximate by the stability of the leaving group and electrophilic carbon

not all confgurational isomers rotate to plane paralized light

but optical isomers differs in magnitude by which the direction they rotate.

Chirality

chiral is the term assigned to molecules with no plane of symmetry, therefore a chiral molecule has an asymmetric structure. Chiral molecule has at least one stereogenic center present. Chiral carbons are sp3 hybridized, with four unique substituents attached. the two configurational isomers (enantiomers) the structure are mirror images of each others, the plane mirror reflects a configurational isomer as its image. two structure with identical bonds may exhibit different chemical properties despite identical physical as one another. for example humane digest D-sugars, because enzymes bind to D-sugar only.

Sparating stereoisomers

chirally selective seperation teq, involves enzymes and the other include seperation technique using chromatography. where the tube used have a specfic anti body

stereoisomers are divided into two

configurational isomers and conformational isomers,

SN1 reaction

*unimolecular nucleophilic substitution reactions: 2 steps 1. Leaving group leaves forming a positively charged carbocation (rate limiting step) *The rate of rxn depends only on the concentration of the substrate rate = k [R-L] R-L is the alkyl group containing the leaving group *Anything that accelerates the formation of the carbocation increase the rate of rxn 2. Nucleophile attacks the carbocation (unstable) *results in substitution product the product could be r/s depending on where the neucleophile is going to attack. sp3--->Sp2--->SP3 hybirdization pattern SN1 reaction is complicated by the arrangement of atoms. it causes a racemic mixture of product

SN2

-1 step (concerted) -Reaction rate is dependent on concentration of R-X and Nuc -Prefers methyl, primary, or secondary carbons -Strong nuc/strong base (OH-) or strong nuc/weak base (I-, CH3CO2-) required for methyl, primary, and secondary. Also SN2 switches the compound's steriochemistry, called inversion product

Determinig Absolute configuration using the Cahn-Ingold-Prelog rules

1) prioritize the substituents that are attached to the chiral carbon based on their atomic mass, from the haviest atom to the lightest atom. 2) orient the molecule in such a way that the substituents with lowest priority points behind the plane of the molecule. 3) draw a semi-circular arc from substituent 1 through 3. if the arc is clock wise then it is R, however counter clock wise referred to as S.

Newman projection

A method of visualizing a compound in which the line of sight is down a carbon-carbon bond axis.

Racemic Mixture

A mixture that contains equal amounts of the (+) and (-) enantiomers. Racemic mixtures are not optically active.

Cycloalkanes

CnH2n, they form an sp3 bond with 109.5 degree. three and four member ring are reactive, while five and six member rings are stable.

enantiomers and diastereomers

E (R,R)--> (S,S) D (R,R)-->(R,S)

Geometrical isomers (cis / trans)

Groups fixed in different space as a result of the restricted rotation of double bonds etc. identical bonds with different spatial arrangement found in double bonds and rings.

Sterioisomerism

Molecules with the same structural formula but a different arrangement of atoms in space

Isomers

divided into different kinds: 1. Constutional isomers, structural isomer, it differ in the bond connection. Stereoisomers, isomers that differs in the spatial arrangment of atoms. it includes conformers and configurational isomers. Conformers differ in space, identical after rotation aboub-bond. Configurational isomers, isomers that differ by orientation in space and can not rotate to become identical, it include, optical and geometrical isomers. Optical isomers: differ by orientation in space cant rotate to become identical due to asymmetry in the structure. Geometrical isomers: differ by orientation in space and it can not rotate to become identical due to the precence of a ring or Pi bond. Both optical and geomatrical have both Diastereomers and Enantiomers. Diastereomers: nonsuperimposable and not mirror image. Enantiomers: non super imposable mirror image

Cyclopentane

does not require distortion of its bond and shape to accomedate 109.5 angle for sp3 hybrid. perfect pentagon has an angle of 108, there is a small discrepancy from 109.5 and this difference dosent contribute the ring strain of 6.8 kcal per mole. to achieve the correct angle cyclopentan forms an envelope shape where one of the carbon is not coplaner with the other four.

E

entgegen across from, trans

two substitution

equatorial and axial. the structure trying to reduce the static hendrence produced when the the substituent are alligned around the cyclohexan

Structural isomers divided into

functional group isomers, positional isomers and skeletal isomers.

leaving group

functional group that dissociates from the electrophile in a nucleophilic substitution reaction. the more stable the electrophile the easier for the neucleophile to leave the molecoule.

gauche

have a dihedral angle of 60 degrees from the other susbtituint on the other side of the sigma bond

Structural isomers (constitutional)

have the same atoms, but the atoms have different bonding. Different arrangement of atoms.