Conformations of Alkanes & Newman Projection

Newman Projection

A Newman projection is just a way to look straight down a carbon–carbon single bond so you can clearly see how groups are arranged in 3D

(see image).

Conformations

Different 3D shapes of the same alkane molecule caused by rotation around a single (sigma) bond.

So:

• same molecule

• same connectivity

• just different rotations in space

Staggered Conformation

• Hydrogen atoms are as far apart as possible (you can see all Hydrogens)

• Less electron repulsion

Lowest energy → most stable (If something is not exactly eclipsed, it is staggered)

Eclipsed Conformation

• Hydrogens line up behind each other (think of an eclipse) (can’t see all Hydrogens like in staggered because they are directly behind each other)

• More electron repulsion (torsional strain)

Highest energy point → least stable

Gauche Conformation

when two bulky groups (usually CH₃ groups) are 60° apart around a single bond.

Anti Conformation

An anti conformation is when two bulky groups in a Newman projection are 180° apart from each other.

Torsional Strain

The repulsion that occurs when electron pairs in bonds on adjacent atoms line up too closely during rotation. ONLY happens in eclipsed confirmations

Torsional interactions

Any interaction between bonds on adjacent atoms as they rotate relative to each other.

Steric Strain

Repulsion when atoms or groups are physically too close together in space

In a Newman projection, kcal/mol

is a way to measure how unstable a conformation is. In a Newman projection, unfavorable interactions have an energy “cost” (kcal/mol), and you can add them up to estimate stability. Not every interaction has a cost — only the unfavourable ones.

Assigning Costs in Newman Projections

Step 1: Identify what’s interacting:

Look at the front carbon vs back carbon. You are checking:

Which groups are directly eclipsed (lined up)?

Which bulky groups are close (gauche, 60° apart)?

Step 2: Mark eclipsed interactions (biggest cost)

If bonds are directly on top of each other, that’s eclipsing.

You look for:

H behind H

H behind CH₃

CH₃ behind CH₃

Then assign costs:

H–H eclipsed ≈ 1 kcal/mol

CH₃–H eclipsed ≈ 1.5 kcal/mol

CH₃–CH₃ eclipsed ≈ 3–4 kcal/mol

• You “circle each overlap” and assign a number.

Step 3: Look for gauche interactions (60° apart)

Only matters if bulky groups (usually CH₃) are:

staggered BUT close together (60° apart)

Assign:

CH₃–CH₃ gauche ≈ 0.9 kcal/mol

Step 4: Anti = no cost

180° apart

no penalty, don’t count anything

Barrier Rotation

the energy difference (in kcal/mol) between the lowest and highest energy conformations during rotation. EX: barrier rotation = 3kcal/mol, which means, you need about 3 kcal/mol of energy for the molecule to rotate through the highest-energy eclipsed state. EX: Ethane. Lowest point, staggered, = 0 kcal/mol, highest point, eclipsed = 3 kcal/mol, 3-0 = 3kcal/mol