Conformations of ethane, butane — Newman projections

WHY does energy change as we rotate? (first principles)

Two effects fight as we twist:

Torsional strain (eclipsing) — when front and back bonds line up ( $\phi = 0°$ ), the bonding electron pairs are forced close → electron–electron repulsion + loss of favourable σ→σ* hyperconjugation. This raises energy.
Steric strain — when bulky groups are forced close in space, their electron clouds repel (van der Waals repulsion). Important only for big groups (butane's methyls), negligible for ethane's H's.

Ethane: the simplest case

Front C has 3 H's, back C has 3 H's. As we rotate the back carbon, energy oscillates between two extremes:

The barrier is too small to stop rotation at room temperature ( $RT \approx 2.5$ kJ/mol of thermal nudge per collision), so ethane spins freely — but it spends more time staggered.

Figure — Conformations of ethane, butane — Newman projections

Butane: looking down the C2–C3 bond

Now front carbon carries a CH₃ (plus 2 H) and back carbon carries a CH₃ (plus 2 H). Bulky methyls make steric strain matter, giving four distinct conformers per turn:

$\phi$ (CH₃–CH₃)	Name	Strain present	Relative energy
$0°$	Fully eclipsed (syn)	torsional + CH₃/CH₃ steric	highest (~19–20 kJ/mol)
$60°$	Gauche (skew)	mild CH₃/CH₃ steric	~3.8 kJ/mol above anti
$120°$	Eclipsed	torsional + CH₃/H	intermediate (~16 kJ/mol)
$180°$	Anti (trans)	none significant	lowest (0)

Stability order (memorise the logic, not the list)

$\textbf{Anti} > \textbf{Gauche} > \textbf{Eclipsed} > \textbf{Fully eclipsed (syn)}$

WHY (HOW to reconstruct it): rank by total strain = torsional + steric. Anti has none, syn has both at maximum.

Common mistake Steel-manning the common errors

Wrong idea 1: "Conformers are different compounds / isomers you can bottle separately." Why it feels right: they look different on paper. Fix: they interconvert by mere rotation (barrier < ~20 kJ/mol), so at RT they are one rapidly-flipping substance — not isolable isomers.

Wrong idea 2: "Staggered always = lowest energy, so butane's gauche should be 0." Why it feels right: true for ethane. Fix: gauche IS staggered but the two methyls still clash sterically → it sits above anti. Staggered kills torsional strain, not steric.

Wrong idea 3: "Eclipsed has the highest barrier always." Fix: in butane the fully eclipsed (CH₃ over CH₃, $\phi=0$ ) is higher than the other eclipsed ( $\phi=120$ , CH₃ over H) because both bulky groups overlap.

Wrong idea 4: Drawing front and back bonds overlapping in the staggered Newman. Fix: staggered bonds must be drawn $60°$ apart (a clean 6-pointed star).

Recall Feynman: explain to a 12-year-old (click to open)

Imagine two steering wheels joined by a short rod, one in front of the other. You can spin the back wheel. When the spokes of the back wheel hide exactly behind the front spokes, they're "fighting" for the same space — that's eclipsed, uncomfortable. When the back spokes peek through the gaps, everyone's relaxed — that's staggered, comfy. Now glue a fat tennis ball on one spoke of each wheel (the CH₃ groups). The molecule is happiest when those two balls are on opposite sides (anti) so they don't bump. That's all conformation is: how a molecule twists to feel least crowded.

Flashcards

What is a conformation?

A spatial arrangement of atoms arising only from rotation about single bonds; interconvertible without bond breaking.

In a Newman projection, what represents the front and back carbons?

Front = central dot with 3 lines; back = circle with 3 lines from its edge.

What is the dihedral (torsion) angle?

The angle between a front-carbon bond and the nearest back-carbon bond viewed down the C–C axis.

Why is staggered ethane lower in energy than eclipsed?

Less torsional strain (bonds in gaps) → minimal electron–electron repulsion + best hyperconjugation.

Ethane rotational barrier value?

≈ 12.5 kJ/mol (about 4.2 kJ per eclipsing H–H pair).

Equation for ethane's torsional energy?

E(φ) = (E_barrier/2)(1 + cos 3φ).

Why does butane have steric strain but ethane essentially doesn't?

Butane has bulky CH₃ groups that clash; ethane only has small H atoms.

Most stable butane conformer and its dihedral?

Anti (trans), φ = 180° between the two methyls.

Why is gauche butane not the lowest energy even though it's staggered?

Staggering removes torsional strain, but the two CH₃ groups at 60° still repel sterically (~3.8 kJ/mol).

Stability order of butane conformers?

Anti > Gauche > Eclipsed > Fully-eclipsed (syn).

Which equation gives the conformer population ratio?

Boltzmann: N₂/N₁ = exp(−ΔE/RT).

Why can't conformers be separated at room temperature?

Rotation barriers (<~20 kJ/mol) are easily overcome by thermal energy, so they interconvert rapidly.

Connections

Hyperconjugation — explains why staggered is electronically favoured (σ→σ* overlap).
Steric strain and van der Waals repulsion — the cause of gauche/syn destabilisation.
Cyclohexane conformations chair and boat — same logic extended to rings (axial/equatorial).
Stereochemistry — isomerism overview — distinguishing conformers from configurational isomers.
Boltzmann distribution — quantifies how energy gaps set populations.
Alkanes — structure and sp3 hybridisation — origin of the rotatable C–C σ bond.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, ek single C–C bond ek ghoomne wale joint jaisa hai — dono carbon aapas mein rotate kar sakte hain bina bond toote. Har rotation ka snapshot ek conformation hai. Isko clearly dekhne ke liye hum Newman projection banate hain: bond ke seedha upar se neeche dekhte hain. Aage wala carbon ek dot, peeche wala ek circle. Jab aage aur peeche ke bonds bilkul ek line mein aa jaate hain, usse eclipsed kehte hain — yeh crowded aur high energy hota hai (torsional strain ke karan). Jab bonds gap mein settle ho jaate hain, woh staggered hota hai — relaxed, low energy.

Ethane ka case simple hai kyunki sirf chhote H atoms hain. Energy barrier sirf ~12.5 kJ/mol hai, jo room temperature pe rotation rok nahi paata — toh ethane freely ghoomta hai, bas staggered mein zyada time bitata hai. Formula $E=6.25(1+\cos3\phi)$ — yahan $3\phi$ isliye kyunki teen identical H hain (3-fold symmetry).

Butane mein twist aata hai: ab dono carbon pe ek-ek CH₃ (methyl) hai, jo bada aur bhaari hai. Ab sirf torsional nahi, steric strain bhi matter karta hai. Sabse stable anti (φ=180°) hai jisme dono methyl opposite side pe hote hain — door door, no clash. Gauche (60°) staggered toh hai par methyls paas hone se thoda strain (~3.8 kJ/mol). Sabse worst fully eclipsed/syn (0°) jisme dono methyl ek doosre ke upar — maximum crowding.

Yeh topic important kyun hai? Kyunki yahi soch aage cyclohexane chair–boat, reaction mechanisms, aur stability problems mein use hoti hai. Yaad rakho: staggered hamesha lowest nahi hota — bulky groups ho toh steric strain bhi dekhna padta hai. "AGES" mnemonic se order yaad karo: Anti > Gauche > Eclipsed > Syn.

Conformations of ethane, butane — Newman projections

WHAT is a conformation?

WHY does energy change as we rotate? (first principles)

Ethane: the simplest case

Butane: looking down the C2–C3 bond

Stability order (memorise the logic, not the list)

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Hydrocarbons

Connections