Home / Courses / Organic Chemistry / Unit 2

Organic Chemistry Unit 2: Alkanes and stereochemistry

Alkanes adopt different conformations, and stereochemistry describes molecules that differ only in 3D arrangement.

Covers: Nomenclature, conformations of alkanes and cyclohexane, constitutional isomers and stereoisomers, chirality and R/S, enantiomers and diastereomers, optical activity, meso compounds

Study guide

Alkanes contain only C–C and C–H single bonds. IUPAC names start with the longest carbon chain (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-), number to give substituents the lowest numbers and list substituents alphabetically. Rotation around single bonds creates conformations. Newman projections show them: staggered conformations are more stable than eclipsed ones because of torsional strain, and in butane, the anti conformation is more stable than gauche because of steric strain. Cyclohexane's chair conformation has no angle strain; each carbon has an axial and an equatorial position. Large groups prefer equatorial positions to avoid 1,3-diaxial interactions, and ring flips swap axial and equatorial positions.

Isomers have the same molecular formula. Constitutional isomers differ in connectivity. Stereoisomers have the same connectivity but differ in 3D arrangement. A chiral molecule is not superimposable on its mirror image, usually because it has a chirality center (stereocenter), a carbon bonded to four different groups. Configuration is assigned R or S using Cahn–Ingold–Prelog priorities, based on atomic number: point the lowest priority away, then trace 1 → 2 → 3; clockwise is R and counterclockwise is S.

Enantiomers are non-superimposable mirror images with opposite configurations at every stereocenter. They have identical physical properties except that they rotate plane-polarized light in opposite directions (optical activity), and they can react differently with other chiral molecules, such as enzymes. A racemic mixture has equal amounts of both enantiomers and doesn't rotate light. Diastereomers are stereoisomers that aren't mirror images and have different physical properties; cis/trans (E/Z) isomers are diastereomers. A molecule with n stereocenters has at most 2ⁿ stereoisomers. A meso compound has stereocenters but is achiral because of an internal mirror plane.

When a molecule has several functional groups, the highest-priority group gets the suffix and the lowest possible number. Approximate priority: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine. Double and triple bonds are named in the parent chain with -en- and -yne, and stereodescriptors such as (2R,3E) go at the front of the name. Bicyclic alkanes are named bicyclo[x.y.z]alkane. The bracketed numbers give the count of carbons in each bridge between the two bridgehead carbons, largest first, and the alkane name counts all ring carbons. Numbering starts at a bridgehead and goes around the largest bridge first.

An A-value is the energy cost of putting a substituent axial instead of equatorial, and bulkier groups have larger A-values (tert-butyl is so large it essentially locks the ring). To compare two chairs, add up the A-values of the axial groups in each. The chair with the smaller total is more stable. Alkanes react with Cl₂ or Br₂ under light or heat by a radical chain mechanism. Initiation splits X₂ into radicals, two propagation steps make the product and regenerate a radical, and termination combines radicals. ΔH is roughly the energy of bonds broken minus bonds formed. Bromination is much more selective than chlorination for the most substituted (3° > 2° > 1°) hydrogen.

In a Fischer projection, horizontal lines point toward the viewer and vertical lines point away. Swapping any two groups inverts the configuration, and rotating the drawing 180° in the plane keeps it the same.

Key ideas

• Staggered conformations are more stable than eclipsed.
• Bulky groups prefer equatorial positions on a cyclohexane chair.
• A carbon with four different groups is a chirality center.
• R/S is assigned by CIP priorities.
• Enantiomers are mirror images; diastereomers are not.

Formulas and rules

Common mistakes

• Forgetting to reverse R/S when the lowest-priority group points toward you.
• Calling a meso compound chiral; its internal mirror plane makes it achiral.

Words to know

Alkane

A hydrocarbon with only single bonds.

IUPAC nomenclature

The systematic naming system for organic compounds.

Conformation

A shape produced by rotation around single bonds.

Newman projection

A drawing looking down a C–C bond.

Staggered conformation

Conformation with bonds offset by 60°.

Eclipsed conformation

Conformation with bonds aligned.

Torsional strain

Strain from eclipsed bonds.

Chair conformation

The most stable conformation of cyclohexane.

Axial position

A position parallel to the cyclohexane ring's axis.

Equatorial position

A position around the cyclohexane ring's equator.

1,3-diaxial interaction

Steric strain between axial groups.

Constitutional isomers

Same formula, different connectivity.

Stereoisomers

Same connectivity, different 3D arrangement.

Chiral

Not superimposable on its mirror image.

Chirality center

An atom, usually carbon, bonded to four different groups.

Enantiomers

Non-superimposable mirror-image stereoisomers.

Diastereomers

Stereoisomers that aren't mirror images.

Racemic mixture

An equal mix of two enantiomers.

Optical activity

Rotation of plane-polarized light.

Meso compound

An achiral compound with stereocenters.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.