Organic Chemistry

Isomerism Isomers have identical composition but different structures Two forms of isomerism – –Constitutional (or structural) – –Stereoisomerism

Map of Organic Isomerization

compounds that have the same molecular formula but different structural formulas (connected differently) Constitutional isomers compounds that have the same molecular formula but different structural formulas (connected differently)

Constitutional Isomerism

Stereoisomerism Same atom-to-atom connections but different arrangement in space

Cis-2-butene Stereoisomers: Geometric Geometric isomers can occur when there is a C=C double bond. Trans-2-butene

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. Stereoisomers: Optical

Chiral Compounds—Polarized Light

Chirality generally occurs when a C atom has 4 different groups attached. These molecules are non- superimposable mirror images.

Lactic acid

Lactic acid isomers are non- superimposable

These amino acids are non- superimposable mirror images.

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

Hydrocarbons Alkanes and Cycloalkanes Alkanes and Cycloalkanes

Alkanes hydrocarbons containing only carbon-carbon single bonds

Hydrocarbon: a compound composed of only carbon and hydrogen

Alkanes –the first 10 alkanes with unbranched chains Generic Alkane Formula: C n H 2n+2

Alkanes are colorless gases, liquids, and solids Generally unreactive (but undergo combustion) Not polar (or low polarity) and so are not soluble in water. Alkanes

Nomenclature

IUPAC Names Alkyl group: a substituent group derived from an alkane by removal of a hydrogen atom –commonly represented by the symbol R- –named by dropping the -ane from the name of the parent alkane and adding the suffix -yl

IUPAC Names The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts: –(1) a prefix: the number of carbon atoms in the chain –(2) the suffix -ane: shows that the compound is a saturated hydrocarbon

IUPAC Names 1. The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms and the ending - ane

IUPAC Names 2. For branched-chain alkanes, longest chain of carbon atoms is the parent chain and its name is the root name

IUPAC Names 3. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name

Nomenclature Example

IUPAC Names 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number

IUPAC Names 5. If the same substituent occurs more than once, –number the parent chain from the end that gives the lower number to the substituent encountered first

IUPAC Names 5. If the same substituent occurs more than once, –indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on

IUPAC Names 5. If the same substituent occurs more than once, –use a comma to separate position numbers

IUPAC Names 6. If there are two or more different substituents –list them in alphabetical order

IUPAC Names 6. If there are two or more different substituents –number the chain from the end that gives the lower number to the substituent encountered first

IUPAC Names 6. If there are two or more different substituents –if there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number

IUPAC Names 7. Do not include the prefixes di-, tri-, tetra-, and so on, or the hyphenated prefixes sec- and tert- in alphabetizing; –alphabetize the names of substituents first, and then insert these prefixes

Nomenclature Example

Cycloalkanes a hydrocarbon that contains carbon atoms joined to form a ring Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring

Cycloalkanes

Cycloalkanes Nomenclature –to name a cycloalkane, prefix the name of the corresponding open- chain alkane with cyclo-, and name each substituent on the ring –if there is only one substituent on the ring, there is no need to give it a location number

All compounds are flexible. Cyclohexane, C 6 H 12, has interconverting “chair” and “boat” forms. Cycloalkanes

Alkenes Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds –ethylene is the simplest alkene

Alkenes - IUPAC Names To name an alkene –use the ending -ene to show the presence of the C=C –branched-chain alkenes are named in a manner similar to alkanes; substituted groups are located and named

Alkenes - IUPAC Names Examples

Alkenes Cis-trans isomerism because of restricted rotation about a carbon- carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism How many isomers are possible for a compound with the formula C 4 H 8 ?

Alkynes Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds –Acetylene (ethyne) is the simplest alkyne

Benzene and its Derivatives Benzene, C 6 H 6, in the top 25 chemicals produced in the U.S. Starting point for hundreds of other compounds.

Aromatic Compounds Aromatic compound: a hydrocarbon that contains one or more benzene-like rings –arene: a term used to describe aromatic compounds –Ar-: a symbol for an aromatic group derived by removing an -H from an arene

Kekulé structure for benzene (1872)

The electrons of Benzene are Delocalized sp 2 hybridized

Other Aromatic Hydrocarbons TolueneNaphthalene

Benzene Derivatives Aniline C 6 H 5 NH 2 Phenol C 6 H 5 OH TNT trinitrotoluene C 6 H 4 CH 3 (NO 2 ) 3

Naming Benzene Derivatives 1,4-dimethylbenzene Common name: p-xylene

Nomenclature Example