Organic Compounds - Alkanes

Dr. Sean BonnessChemistry Department

El Camino College

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Organic compounds

• All contain carbon.• Organic chemistry is the study of

carbon containing compounds.• The essential needs of daily human

life are food, fuel, shelter, and clothing - all are organic in nature, except water (H2O).

Bonding characteristics and Isomerism

• What is the electronic configuration of Carbon

• Mixing of the 2s and 2p orbitals for carbon produce sp3 hybrid orbitals

• The hybrid orbitals point towards the corners of a regular tetrahedron. For example methane (CH4).

Bonding characteristics and Isomerism (Cont.)

• Carbon atoms bond covalently to other carbon atoms to form chains or networks.

• A third carbon can join the end of this chain.

• The process can continue to form chains of almost any length.

Bonding characteristics and Isomerism (Cont.)

• Carbon can also share more than on pair of electrons to form multiple bonds

• Structural Isomers - compounds that have the same molecular formula but in which the atoms bond in different patterns.

• For example, what are the isomers for the compound with molecular formula C4H10?

Bonding characteristics and Isomerism (Cont.)

• What are the structural isomers of C2H6O?

• Structural isomers exert a significant influence on properties. Ethyl alcohol is a liquid at room temp., whereas dimethyl ether is a gas.

• What is a structural isomer of C3H6O?

Functional groups

class Functionalgroup

Expanded

structure

Condensed

structure

Name

Alkane

Alkene

Alkyne

Functional groups (cont.)

class Functionalgroup

Expanded

structure

Condensed

structure

Name

Aromatic

Alcohol

Ether

Functional groups (cont.)

class Functionalgroup

Expanded

structure

Condensed

structure

Name

Amine

Aldehyde

Ketone

Functional groups (cont.)

class Functionalgroup

Expanded

structure

Condensed

structure

Name

Carboxylic Acid

Ester

Amide

Functional Groups (cont.)

• Write condensed structural formulas for the following compounds.

Alkane structures

• Hydrocarbons - simplest of all organic compounds. Only contain Carbon and Hydrogen.

• Are called Saturated Hydrocarbons.• Unsaturated hydrocarbons have double and triple

bonds. Which of the functional groups are unsaturated?

• We also have normal alkanes and branched alkanes, i.e. butane.

Conformations of alkanes

• Conformations are the different arrangements of atoms in space achieved by rotation about a single bond.

• Butane

• Ethane

• Are these pairs structural isomers?

Alkane Nomenclature

• Assigning IUPAC names.• Step 1 - name longest chain.

• Step 2 - number the longest chain

• Step 3 - locate and name attached alkyl groups.

• Step 4 - combine longest chain and alkyl groups into name

Alkane Nomenclature (cont.)

• Step 5 - indicate the number and position of attached alkyl groups. If two or more alkyl groups, then use prefixes di, tri, tetra, etc.

Cycloalkanes

• For alkanes the structural formula is CnH2n+2

• Is C3H6 an alkane?• If form a ring than

• And has the structural formula

Cycloalkanes

Name Structural formula

Condensed formula

cyclopropane

cyclobutane

cyclopentane

cyclohexane

Shapes of cycloalkanes

• Stereoisomers are compounds with the same structural formula but different spatial arrangements of atoms.

• For example 1,2-dimethylcyclopentane

• Geometric isomers are stereoisomers in which the geometry of their groups are maintained by rings and are called cis-trans isomers.

Shapes of cycloalkanes (cont.)

• Show geometric isomers for 1,2-dimethylcyclobutane

• What are the geometric isomers for 1,3-difluorocyclobutane?

Physical properties of alkanes

• Alkanes are composed of nonpolar carbon-carbon and carbon-hydrogen bonds.

• Have lower melting points and boiling points than other organic compounds due to weak attractions. However as chain length increases, the dispersion forces increases and which is why very long chain structures are solids (see figure 11.18 in S&S text)