Isomers Lesson PDF

ISOMERS Isomers are molecules with the same chemical formula but different arrangements of atoms. Isomers have different shapes and can have different physical and chemical properties. For example, each of the following all have the chemical formula C6H12O6 There are...

ISOMERS Isomers are molecules with the same chemical formula but different arrangements of atoms. Isomers have different shapes and can have different physical and chemical properties. For example, each of the following all have the chemical formula C6H12O6 There are two main types of isomers: 1. STRUCTURAL ISOMERS Two or more molecules that have the same atoms, but the atoms are bonded together differently. This results in the compound having different properties. Example: butane and isobutane Butane Isobutane 2. STEREOISOMERS Two or more molecules that have their atoms bonded in the same way, but the atoms are arranged differently in space. The location (carbon) where the isomer occurs is called the STEREOGENIC CENTRE or STEREOCENTRE. Example: 1,2-dichloroethane (C2H4Cl2) WHICH CARBON IS THE STEREOCENTRE?  Two types of stereoisomers are: A. Geometric Isomers These isomers can have very different physical properties, but tend to have the same chemical properties. WHICH CARBON IS THE STEREOCENTRE? A. Geometric Isomers (continued): ALKENE STEREOISOMERS: The positioning of the groups around the C=C double bond allows alkenes to form stereoisomers. If the isomer has similar groups on the same side of the double bond, it is called a cis isomer. If the isomer has similar groups on the opposite sides of the double bond, it is called a trans isomer. Example: butene These isomers have similar chemical properties, but different physical properties. A. Geometric Isomers (continued): CYCLIC STEREOISOMERS: Stereoisomerism can also occur in cyclic compounds. Example: 1,2-dimethylcyclohexane Notice that in the trans isomer, the methyl groups are across from each other. In the cis isomer, the methyl groups are on the same side. WHICH CARBON IS THE STEREOCENTRE? B. Optical Isomers CHIRALITY Molecules with NON- SUPERIMPOSABLE mirror images are chiral. Molecules with SUPERIMPOSABLE mirror images are achiral. Achiral Structures: B. Optical Isomers i) ENANTIOMERS These optical isomers are non-superimposable mirror images (like your right and left hands) that have similar physical properties. In an achiral environment, they also have similar chemical properties. However, in a chiral environment, they have different chemical properties, meaning that enzymes and other proteins can distinguish between them. Therefore, they can have different effects on the body. B. Optical Isomers (Enantiomers) continued: ENANTIOMERS: A molecule that has a NON-SUPERIMPOSABLE mirror image is said to be CHIRAL. The most common type of chirality is observed when a carbon atom has 4 different groups attached to it. This carbon atom is called the chiral carbon or stereocentre. B. Optical Isomers (Enantiomers) continued: Although enantiomers have similar chemical and physical properties in an achiral environment (such as in a laboratory), they interact differently with plane polarized light. This gives them optical activity, which can be measured with a polarimeter. Enantiomers cause the plane of polarized light to rotate in opposite directions. One enantiomer rotates the light clockwise (+) and the other rotates the light counterclockwise (-). Note: Achiral molecules DO NOT rotate the plane of polarized light, and are optically inactive. B. Optical Isomers (Enantiomers) continued: Racemization results in the formation of a racemic mixture of the enantiomers. This racemic mixture contains equal amounts of left- and right-handed enantiomers. If only one enantiomer enters the body, the pH of the blood can make that enantiomer undergo racemization, forming both enantiomers in equal amounts. In general, one form of the chiral molecule is biologically active and leads to the desired effect, while the other is biologically inactive and may cause side effects. Organisms are very sensitive to small variations in molecular geometry. For example, the drug thalidomide was used to treat morning sickness in pregnant women. In the body, it formed a racemic mixture. One isomer produced the desired effects, while the other caused major birth defects. the sedative drug teratogen  B. Optical Isomers (Enantiomers) continued: THREE WAYS TO NAME ENANTIOMERS: 1. (+) vs. (-) Each enantiomer is named based on its ability to rotate polarized light (+) represents rotation to the right (clockwise) This is called dextrorotatory (-) represents rotation to the left (counterclockwise) This is called levorotatory B. Optical Isomers (Enantiomers) continued: THREE WAYS TO NAME ENANTIOMERS: 2. D vs. L Each enantiomer is named based on it actual geometry (spatial configuration) in comparison to glyceraldehyde. D-glyceraldehyde L-glyceraldehyde  B. Optical Isomers (Enantiomers) continued: THREE WAYS TO NAME ENANTIOMERS: 3. (R) vs. (S) Each enantiomer is named based on actual geometry and priority rules (based on atomic number) If the priority of the substituent groups on the chiral carbon DECREASES in the CLOCKWISE direction, the enantiomer is labelled R. If the priority of the substituent groups on the chiral carbon DECREASES in the COUNTERCLOCKWISE direction, the enantiomer is labelled S. B. Optical Isomers (Enantiomers) continued: Name the following enantiomers using the (R) vs. (S) convention. 1 2 3 4 B. Optical Isomers (Enantiomers) continued: Name the following enantiomers using the (R) vs. (S) convention. 4 R S 4 1 1 2 2 3 3 1 2 3 4 B. Optical Isomers (Enantiomers) continued: Name the following enantiomers using the (R) vs. (S) convention. B. Optical Isomers (Enantiomers) continued: Name the following enantiomers using the (R) vs. (S) convention. 4 4 1 1 3 3 2 2 S R B. Optical Isomers (Enantiomers) continued: Name the following enantiomers using the (R) vs. (S) convention. R S 1 2 3 4 Name the following enantiomers using the (R) vs. (S) convention. S R B. Optical Isomers (Enantiomers) continued: THREE WAYS TO NAME ENANTIOMERS: B. Optical Isomers (Enantiomers) continued: EXAMPLES OF ENANTIOMERS: Limonene is a hydrocarbon that is a colourless liquid. It is found in orange and lemon peels. ORANGES LEMONS D-limonene L-limonene (+) limonene (-) limonene (R) limonene (S) limonene B. Optical Isomers (Enantiomers) continued: EXAMPLES OF ENANTIOMERS: Carvone is found naturally in many essential oils. It is abundant in oils from caraway seeds and dill. CARAWAY SEEDS DILL D-carvone L-carvone (+) carvone (-) carvone (R) carvone (S) carvone B. Optical Isomers (Enantiomers) continued: EXAMPLES OF ENANTIOMERS: Limonene is a hydrocarbon that is a colourless liquid. It is found in orange and lemon peels. Carvone is found naturally in many essential oils. It is abundant in oils from dill and caraway seeds. The fact that the human nose can smell the difference between enantiomers means that olfactory receptors must contain chiral groups. The different molecules fit into certain receptor cells in your nose, and therefore the molecules smell different to us. B. Optical Isomers ii) DIASTEREOMERS Diastereomerism occurs when two or more isomers of a molecule can have different configurations at one or more (but not all) of the stereocentres. Stereoisomers that are NOT mirror images are diastereomers. Example: Glucose and galactose WHICH CARBON IS THE STEREOCENTRE?  B. Optical Isomers (Diastereomers) continued: If a molecule has more than one stereocentre, some of the isomers will be enantiomers, and those that are not are diastereomers. Diastereomers ENANTIOMERS VS. DIASTEREOMERS ENANTIOMERS VS. DIASTEREOMERS