CHEM 2070 Alkanes, Cycloalkanes, IUPAC, Cyclohexanes

IUPAC naming

The standard for organic and inorganic compounds

International Union of Pure and Applied Chemistry

Parent chain

The longest continuous chain of carbons. The name is based off the number of carbons and suffix ‘-ane’

Substituent

Hydrogen atom or group coming off your parent chain

Methane

1 carbon

Ethane

2 carbons

Propane

3 carbons

Butane

4 carbons

Pentane

5 carbons

Hexane

6 carbons

Heptane

7 carbons

Octane

8 carbons

Nonane

9 carbons

Decane

10 carbons

How do you name rings?

Using prefix ‘cyclo-’ then the number of carbons

Steps of IUPAC naming

1. Identify the parent chain

2. Number the parent chain in a way to give substituents the lowest possible numbers

3. Name the substituents with the locator number

4. Assemble name: Locator numbers use a hyphen, no spaces between the substituents and parent chain

More than one substituent

Use the parent chain and choose the numbers that give the lowest locator numbers

If its a tie between two or more substituents?

Look at the second substituents

If the tie can’t be broken between two or more substituents?

Use alphabetical order

Naming Rule of more than one of the same substituent

1. Determine parent chain

2. Order substituents, order alphabetically (excluding the prefix)

3. Add multiplying prefix

4. Add locator number for each substituent, separated by a comma

Naming Cycloalkanes

1. Distinguish between ring carbons and chain carbons (parent chain)

   1. Ring C >= alkyl C, the ring is the parent

   2. Ring C < alkyl C, the ring is the substituent

      1. The carbon with the most substituents (if possible), then go in direction that gives the lowest locator numbers

Naming substituents

Alkane chains coming off a parent chain can also be substituents. Naming it with the prefix from the number of carbons plus ‘-yl'

1 carbon atom in substituent

Methyl

2 carbons atom in substituent

Ethyl

3 carbons atom in substituent

Propyl

4 carbons atom in substituent

Butyl

5 carbons atom in substituent

Pentyl

6 carbons atom in substituent

Hexyl

7 carbons atom in substituent

Heptyl

8 carbons atom in substituent

Octyl

What is this?

Butyl

What is this?

sec-butyl

What is this?

Isobutyl

What is this?

tert-butyl

Naming Complex Branching

1. Name separately first

2. Number the longest carbon chain within the substituent. Always start with C attached to the parent chain where the complex branch is attached to the parent

3. Name the substituent and add the side groups. () says it’s one unit

Isomers

Having the same formula, but different structure

Constitutional isomers

Having the same structure, but different connectivity

Conformers

Single bonds that can rotate, resulting in multiple 3D shapes

Newman Projections

Compare the relative stability of possible conformations (single-bond rotations)

Dihedral angle (torsional angle)

The angle between atoms on adjacent carbons

Steric strain

When groups come together, electrons are forced to occupy the same space (repulsion)

Ideal bond angles for sp³ hybridized carbon

109.5

Angle strain

The deviation from the ideal angle because of a ring

Torsional strain

Due to the restricted rotation

Cyclopropane characteristics

3 carbons

Significant strain, energy=9.2 kJ/mol

Planar ring

Bonds are eclipsed

Cyclobutane characteristics

4 carbons

Lots of strain, a little less torsional strain

Bonds mostly eclipsed

Cyclopentane characteristics

5 carbons

Little torsional strain

“Envelope conformation”

Partly eclipsed

Cyclohexane

6 carbons

Essentially no angle strain, still no torsional strain

Axial

Being parallel to an axis, like x or y

Equatorial

Being relatively to the “equator” of a cyclohexane

What four things NOT to do when drawing chair structures?

1. Up axial on a down carbon (mixing ups/downs)

2. Wedges/dashes

3. Equatorial in the wrong direction (must be parallel)

4. Something in between axial and equatorial

1,3-diaxial interactions

Steric strain from substituents in the axial position

Dashes indicate what when drawing chair conformations?

Down

Wedges indicate what when drawing chair conformations?

Up

Bicyclic

Compounds that contain two fused rings

Bridgehead carbons

Carbons where rings are fused together

Naming bicyclic compounds

Prefix ‘bicyclo-’ + # of carbons in each piece of the rings + parent chain

Declain

Two 6-member rings fused together; found in many naturally occurring compounds