ORGANIC CHEMISTRY HOMEWORK

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An Introduction to

Organic

Compounds

Nomenclature,

Physical Properties,

and Representation

of Structure

Chapter 3

Alkanes are Hydrocarbons that contain only

single Bonds general Molecular Formula: CnH2n+2

Nomenclature of Alkanes

C4H10 can be arranged in two ways:

C5H12 can be arranged in three ways:

These are constitutional isomers:

the same molecular formula but the atoms are linked differently.

Alkyl Substituents

Replace “ane” of alkane with “yl.”

Removing a hydrogen from an alkane results in

an alkyl substituent.

Alkyl nomenclature

Primary, Secondary, and Tertiary Carbons

A primary carbon is bonded to one carbon.

A secondary carbon is bonded to two carbons.

A tertiary carbon is bonded to three carbons.

Primary hydrogens are attached to primary carbons.

Secondary hydrogens are attached to secondary carbons.

Tertiary hydrogens are attached to tertiary carbons.

Alkyl Group Names

Alkanes Systematic Nomenclature

First identify the longest continuous chain

(this is called the parent hydrocarbon)

Add the Name of the Substituent

Number the chain in the direction that gives the substituent

as low a number as possible.

Common versus Systematic

Nomenclature

• Common names never have numbers.

• Only systematic names can have numbers.

List Substituents in Alphabetical Order

• The correct name is the one that contains the lowest of the possible numbers.

Multiple Substituents

• Chain is numbered in the direction that puts the lowest

number in the name.

• Substituents are listed in alphabetical order (di- and tri- are

not alphabetized).

When both have the

same Lowest Number

When both names have the same lowest number, go for the

next lowest number.

When both have the same Number

When the same numbers are obtained in both directions, the

first group listed gets the lower number.

Cycloalkanes

Skeletal structures do not show Cs and Hs bonded to Cs.

Skeletal Structures

Mono-Substituted Cycloalkanes

A number is not needed

Di-Substituted Cycloalkanes

• Substituents are stated in alphabetical order.

• #1 goes to first-listed substituent.

Nomenclature of Akyl Halides

• Numbers are used only for systematic names.

• Common names never have numbers.

Nomenclature of Alkyl Halides

Classification of Alkyl Halides

• Primary = Halogen is on a primary carbon.

• Secondary = Halogen is on a secondary carbon.

• Tertiary = Halogen is on a tertiary carbon.

Classification of Alcohols

• Primary alcohol = OH is on a primary carbon.

• Secondary alcohol = OH is on a secondary carbon.

• Tertiary alcohol = OH in on a tertiary carbon.

Classification of Amines

The classification depends on how many groups

are bonded to N.

• Primary amine = one group bonded to N

• Secondary amine = two groups bonded N

• Tertiary amine = three groups bonded N

The Structure of an Alkyl Halide

The C—X bond of an alkyl halide becomes longer and

weaker as the size of the halogen increases.

The Structure of an Alcohol

Resembles the Structure of Water

An alcohol is structurally like water with one H replaced by an R.

An ether is structurally like water with both Hs replaced by Rs.

The Structure of an Alcohol

Resembles the Structure of Water

Structures of Amines

The greater the attractive forces between molecules, the

higher the boiling point.

attractive forces

van der Waals forces

dipole–dipole interactions

hydrogen bonds

Boiling Points

• Methane –167.7

°C

• Ethane –88.6 °C

• Propane –42.1 °C

• Butane –0.5 °C

• Pentane 36.1 °C

• Hexane 68.7 °C

• Heptane 98.4 °C

• Octane 125.7 °C

induced dipole-induced dipole interactions

van der Waals forces

The greater the surface area of the molecule, the higher the boiling

point

Branching Lowers the Boiling Point

“cigar” “tennis ball”

Dipole–Dipole Interactions

Van der Waals Van der Waals

dipole–dipole

Dipole–dipole interactions are

stronger than van der Waals forces.

Comparative Boiling Points

The larger the halogen, the more polarizable it is.

Hydrogen Bonds in H2O and NH3

CH4 –167.7 °C

no hydrogen bonds

H2O 100 °C

hydrogen bonds

Hydrogen bonds are stronger than other dipole–dipole interactions.

Hydrogen Bonds in Water

Solubility

like dissolves like

Polar compounds dissolve in polar solvents (H2O).

Nonpolar compounds dissolve in nonpolar solvents (hexane).

Solvation

Solvation is the interaction between solute molecules and

solvent molecules.

Alcohols

An Oxygen can Drag

Three Carbons into Water

Solubilities of Alkyl Halides

Rotation Occurs About

Single Bonds

Staggered and Eclipsed Conformers

of Ethane

Rotation Can Occur About the

Three Carbon—Carbon Bonds in Butane

Rotation About C-2—C-3 in Butane

Steric strain is repulsion between the electron clouds of atoms or groups.

Bond Angles in Planar Cyclic Alkanes

Angle Strain

tetrahedral bond angle = 109.5° bond angle < 109.5°

Angle strain results from poor orbital–orbital overlap

because bonds have to deviate from the ideal (109.5°) bond angle.

Cyclopropane

Cyclobutane

Molecules twist out of a planar arrangement to minimize angle

strain and the number of eclipsed hydrogens.

Cyclopentane

Molecules twist out of a planar arrangement to minimize

angle strain and the number of eclipsed hydrogens.

Chair Conformer of Cyclohexane

The chair conformer of cyclohexane is completely free of strain.

All bond angles are 111° and all adjacent bonds are staggered.

Axial and Equatorial Bonds

Ring Flip

Cyclohexane interconverts between two stable chair

conformers.

Conformers of Cyclohexane

Conformers of Monosubstituted

Cyclohexanes

A Substituent is More Stable in an

Equatorial Position

1,3-Diaxial Interactions