Types of Organic Compounds Vast majority of over 20 million known compounds are based on C: organic compounds. Generally contain C and H + other elements

Isomerism Isomers have identical composition but different structures Two forms of isomerism Constitutional (or structural) Stereoisomerism Constitutional Same empirical formula but different atom-to-atom connections Same atom-to-atom connections but different arrangement in space.

Constitutional Isomers Look below at the structures of butane and 2-methylpropane. Butane 2-methylpropane Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Constitutional Isomers Even though both compounds have the formula C4H10, their boiling points and other properties differ. Because their structures are different, they are different substances. isomers have the same molecular formula but different molecular structures. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Constitutional Isomers Butane and 2-methylpropane represent a category of isomers called constitutional isomers, or structural isomers. Constitutional isomers are compounds that have the same molecular formula but the atoms are joined together differently. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Constitutional Isomers Constitutional isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities. In general, the more highly branched the hydrocarbon structure is, the lower the boiling point of the isomer will be compared with less branched isomers. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Which of these molecules will have a lower boiling point and why? Butane 2-methylpropane 2-methylpropane will have a lower boiling point because it has a more highly branched structure. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Structural Isomers

Hydrocarbons & Structural Isomerism Isomers of C5H12? C5H12 has 3 structural isomers. Note names of isomers

Stereoisomers What are two types of stereoisomers? Stereoisomers Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Because molecules are three-dimensional structures, molecules with the same molecular formula and with atoms joined in exactly the same order may still be isomers. Stereoisomers are molecules in which the atoms are joined in the same order but the positions of the atoms in space are different. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Two types of stereoisomers are cis-trans isomers and enantiomers. Two arrangements are possible for the methyl groups and hydrogen atoms with respect to the rigid double bond in 2-butene. Cis configuration Trans configuration Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Cis-Trans Isomers Cis-trans isomers, also known as geometric isomers, have atoms joined in the same order but the spatial orientation of the groups differs. The most common example of cis-trans isomerism occurs in molecules with double bonds. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Cis-Trans Isomers In the cis configuration, similar groups are on the same side of the double bond. Cis configuration Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers But, when similar groups extend from opposite sides of the double bond, the isomer is in the trans configuration. Trans configuration Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Cis-trans isomers have different physical and chemical properties. Stereoisomers Cis-Trans Isomers Cis-trans isomers have different physical and chemical properties. You should be able to identify cis-trans isomers of alkenes when each carbon of the double bond has one substituent and one hydrogen. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers: Optical Optical isomers are molecules with non-superimposable mirror images. Such molecules are called CHIRAL Pairs of chiral molecules are enantiomers. Chiral molecules in solution can rotate the plane of plane polarized light.

Chiral Compounds and Polarized Light

Stereoisomers Enantiomers A carbon with four different atoms or groups attached is an asymmetric carbon. In the compound shown here, H, F, Cl, and Br atoms are attached to a single carbon atom, so the carbon is an asymmetric carbon. CHFClBr Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Enantiomers The relationship between the two molecules is similar to the relationship between right and left hands. Sometimes the terms right-handed and left-handed are used to describe compounds with an asymmetric carbon. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Enantiomers By contrast, a pair of hands is distinguishable even though the hands have identical parts. When you try to stack your hands on top of one another, the thumb of one hand lines up with the little finger of the other hand. No matter how you turn your hands, you can’t get them to look exactly alike. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Enantiomers Pairs of molecules that are mirror images and not superimposable are called enantiomers, or optical isomers. These molecules are examples of enantiomers. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Stereoisomers Isomers Chirality generally occurs when a C atom has 4 different groups attached. Lactic acid

Stereoisomers Isomers Lactic acid isomers are non-super-imposable

Chirality: Handedness in Nature These molecules are non-superimposable mirror images.

Chirality: Handedness in Nature These amino acids are non-superimposable mirror images.

Stereoisomers in Nature Right- and left-handed seashells The DNA here is right-handed

Stereoisomers Enantiomers Unlike other isomers, enantiomers have identical physical properties such as boiling points and densities. Enantiomers do, however, behave differently when they interact with other molecules that have asymmetric carbons. Many molecules in your body have asymmetric carbons, so each enantiomer can have a different effect on the body. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Identifying Asymmetric Carbon Atoms Sample Problem 22.4 Identifying Asymmetric Carbon Atoms Which compound has an asymmetric carbon? b. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Analyze Identify the relevant concepts. Sample Problem 22.4 Analyze Identify the relevant concepts. An asymmetric carbon has four different substituents attached. 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Solve Apply the concepts to this problem. Sample Problem 22.4 Solve Apply the concepts to this problem. Draw the structure in a way that makes it easier to compare the four different groups attached to the central carbon. a. b. 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Solve Apply the concepts to this problem. Sample Problem 22.4 Solve Apply the concepts to this problem. Compare the groups. If all four groups are unique, the central carbon is asymmetric. If any two are the same, the central carbon is not asymmetric. a. b. Compound (b) is asymmetric. 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Key Concepts Constitutional isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities. Two types of stereoisomers are cis-trans isomers and enantiomers. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Glossary Terms isomers: compounds that have the same molecular formula but different molecular structures constitutional isomers: compounds that have the same molecular formula, but whose atoms are bonded in a different order stereoisomers: molecules that have atoms in the same order, but which differ in the arrangement of the atoms in space cis-trans isomers: compounds that have atoms in the same order, but differ in the orientation of groups around a double bond Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Glossary Terms cis configuration: the configuration in which substituent groups are on the same side of a double bond trans configuration: the configuration in which substituent groups are on the opposite sides of a double bond asymmetric carbon: a carbon atom that has four different atoms or groups attached enantiomers: molecules that differ from one another in the way that four different groups are arranged around a carbon atom Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.