Isomers Are different compounds with the same molecular formula Not mirror images Mirror images All stereocentres have opposite configuration Alkenes Cyclic? see  ≥ 2 stereocentres Only some have opposite configuration

Constitutional isomers Constitutional (STRUCTURAL ISOMER) isomers Are compounds have the same molecular formula, but their atoms are bonded in a different orders Thus they have the SAME MOLECULAR FORMULA but DIFFERENT STRUCTURAL FORMULA Character of structural isomer Same molecular formula Structural isomers have different physical properties Different arrangement of atoms Or different position of functional group Or containing different functional groups Constitutional isomers

Examples of Constitutional Isomers 1-Butene 2-Butene

pentane 2-methylbutane 2,2-dimethylpropane

The C6H14 Isomers hexane 2-Methylpentane 3-Methylpentane CH3CH2CH2CH2CH2CH3 hexane (CH3)2CHCH2CH2CH3 2-Methylpentane (CH3CH2)2CHCH3 3-Methylpentane (CH3)2CHCH(CH3)2 2,3-Dimethylbutane (CH3)3CCH2CH3 2,2-Dimethylbutane 9

II-Stereoisomers: Optical isomers (chiral molecules) A- Enantiomeres Stereoisomers are isomers that have the same molecular and structural formula but differ only in the arrangements of atoms in space Types of Stereoisomers are: Optical isomers (chiral molecules) A- Enantiomeres B- Diastereomeres 2. Cis/trans isomer ( achiral molecules) II-Stereoisomers: 6

Geometric Isomerism (cis/trans) If two compounds have the same molecular and structural formula, but differ in the spatial arrangement of different atoms or groups about a ring or about a double bond Example of Ring Cis-Trans Isomers Cis: groups on same side of ring Trans: groups on opposite side of ring trans-1,2-dibromocyclohexane cis-1,2-dibromocyclohexane If two identical groups occur on the same side of the double bond the compound is cis If they are on opposite sides the compound is trans Example of Alkene Cis-Trans Isomers cis-1,2-dibromoethene trans-1,2-dibromoethene

Chirality is a property present in an object that cannot be superimposed on its mirror image. Chiral objects or chiral molecules do not have a plane of symmetry, i.e. they are asymmetric. Chirality is typically seen in molecules that have a chiral or asymmetric carbon atom.

Optical isomers Enantiomers are optically active, non-superimposable mirror image molecules A molecule that has a non-superimposable mirror image is chiral. Objects (and molecules) that are superposable on their mirror images are achiral

Asymmetric Carbon Many chiral compounds contain an asymmetric carbon atom – Chiral or asymmetric carbons have four different atoms or four different group attached to it. It is a tetrahedral carbon atom

Optical activity Enantiomers are optically active; that is, they rotate plane-polarized light. One isomer of an enantiomeric pair rotates polarized light to the left (counterclockwise). The other isomer rotates polarized light to the right (clockwise). The degree of rotation is the same but in opposite directions. Rotation of polarized light to the right is indicated by a (+) dextrorotatory Rotation of polarized light to the left is indiated by a (-) levorotatory .

rotated polarized light Optical Isomers polarizing filter plane polarized light optically active sample in solution Polarimeter: a device for measuring the extent of rotation of plane-polarized light Dextrorotatory (d) = right rotation Levorotatory (l) = left rotation rotated polarized light

RACEMIC MIXTURE = equal amounts of two enantiomers; These mixtures are optically inactive. Because two enantiomers rotate plane-polarized light to an equal extent but in opposite directions, the rotations cancel, and no rotation is observed. RESOLUTION: Separation of a racemic mixture into its optically active forms (pair of enantiomers)

DIASTEREOMERS (non-enantiomeric stereoisomers ) 1. Molecules have more than one stereogenic (chiral) center: 2. are stereoisomers that are not mirror images of each other. 3. Diastereomers must have opposite configurations at some (one or more) stereogenic centers, but the same configurations at other stereogenic centers DIASTEREOMERS

Enantiomers & Diastereomers For a molecule with 1 chiral center, 21 = 2 stereoisomers are possible For a molecule with 2 chiral centers, a maximum of 22 = 4 stereoisomers are possible For a molecule with n chiral centers, a maximum of 2n stereoisomers are possible

Meso Compounds A meso compound is a stereoisomer that is superimposable on its mirror image due to an internal plane of symmetry (is an imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other). A meso compound is an optically inactive compound even through it possesses more than one chiral centre. (2R,3S)- (2R,3R)- (2S,3S)-

The biological importance of chirality 1) All but one of the 20 amino acids that make up proteins are chiral and left-handed . 2) The molecules of natural sugars are almost all classified as right-handed (R configuration). 3) the active site of enzyme is chiral, and only one enantiomer of a chiral reactant fits it Thus Enzymes are capable of distinguishing between stereoisomers The biological importance of chirality