1 Types of Organic Compounds organic compounds.Vast majority of over 20 million known compounds are based on C: organic compounds. Generally contain C.

11Types of Organic Types of Organic CompoundsCompounds

• Vast majority of over 20 million known compounds are based on C: organic organic compounds.compounds.

• Generally contain C and H + other elements

22

IsomerismIsomerism• Isomers Isomers have identical composition but different have identical composition but different

structuresstructures• Two forms of isomerismTwo forms of isomerism

– Constitutional (or structural)Constitutional (or structural)– StereoisomerismStereoisomerism

• ConstitutionalConstitutional– Same empirical formula but different atom-to-atom Same empirical formula but different atom-to-atom

connectionsconnections

• StereoisomerismStereoisomerism– Same atom-to-atom connections but different Same atom-to-atom connections but different

arrangement in space.arrangement in space.

22.3 Isomers >22.3 Isomers >

3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Look below at the structures of butane and 2-methylpropane.

Constitutional IsomersConstitutional Isomers

Butane 2-methylpropane



• Even though both compounds have the formula C4H10, their boiling points and other properties differ.

• Because their structuresstructures are different, they are different substances.

• isomers have the same molecular formula but differentdifferent molecular

structuresstructures.




• 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.




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.




Butane 2-methylpropane

Which of these molecules will have a lower boiling point and why?

2-methylpropane will have a lower boiling point because it has a more highly branched structure.

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Structural IsomersStructural Isomers

99

Pentane

2-Methylbutane

2,2-Dimethylpropane

CH3CH2CH2CH2CH3

CH3CHCH2CH3

CH3

H3CCCH3

CH3

CH3

Hydrocarbons Hydrocarbons & Structural & Structural IsomerismIsomerism

CC55HH1212 has 3 has 3

structural structural isomers.isomers.

CC55HH1212 has 3 has 3

structural structural isomers.isomers.

Isomers of C5H12?Isomers of C5H12?

Note names of isomersNote names of isomersNote names of isomersNote names of isomers



Stereoisomers

What are two types of stereoisomers?

StereoisomersStereoisomers



• 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.




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

Two types of stereoisomers are cis-trans isomers and enantiomers.



• 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.


Cis-Trans Isomers



In the cis configuration, similar groups are on the same side of the double bond.


Cis-Trans Isomers

Cis configuration



But, when similar groups extend from opposite sides of the double bond, the isomer is in the trans configuration.


Trans configuration



• 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.


Cis-Trans Isomers

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• Optical isomersOptical isomers are molecules with are molecules with non-superimposable mirror images.non-superimposable mirror images.

• Such molecules are called Such molecules are called CHIRALCHIRAL• Pairs of chiral molecules are Pairs of chiral molecules are

enantiomersenantiomers..

• Chiral molecules in solution can rotate Chiral molecules in solution can rotate the plane of plane polarized light.the plane of plane polarized light.

Stereoisomers: Stereoisomers: OpticalOptical

1818

Chiral Compounds and Chiral Compounds and Polarized LightPolarized Light



• 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.


Enantiomers

CHFClBr




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.



• 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.


Enantiomers



Pairs of molecules that are mirror images and not superimposable are called enantiomers, or optical isomers.

These molecules are examples of enantiomers.


Enantiomers

2323

Stereoisomers Stereoisomers IsomersIsomers

Chirality Chirality generally occurs generally occurs when a C atom when a C atom has 4 different has 4 different groups attached.groups attached.

Lactic acid

2424Stereoisomers Stereoisomers IsomersIsomers

Lactic acid Lactic acid isomers are isomers are non-super-non-super-imposableimposable

2525

Chirality: Handedness Chirality: Handedness in Naturein Nature

These molecules are non-superimposable These molecules are non-superimposable mirror images.mirror images.

2626

Chirality: Handedness Chirality: Handedness in Naturein Nature

These amino acids are non-superimposable mirror images.These amino acids are non-superimposable mirror images.

2727Stereoisomers in Stereoisomers in NatureNature

Right- and left-Right- and left-handed seashellshanded seashells

The DNA here is The DNA here is right-handedright-handed



• 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.


Enantiomers



Sample Problem 22.4Sample Problem 22.4

Identifying Asymmetric Carbon Atoms

Which compound has an asymmetric carbon?

a.

b.




Analyze Identify the relevant concepts.

An asymmetric carbon has four different substituents attached.

1




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



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



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.

Key ConceptsKey Concepts



• 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

Glossary TermsGlossary 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

Glossary TermsGlossary Terms

1 Types of Organic Compounds organic compounds.Vast majority of over 20 million known compounds are based on C: organic compounds. Generally contain C.

Documents

structural isomers

isomers of c

isomerism isomers

optical isomers

branched isomers

category of isomers

geometric isomers

names of isomers