Difficulty: Intermediate | Prerequisites: Brønsted acid-base concepts (Part 3 of these notes), orbital hybridisation, Newman projections intro
Lewis acid-base theory generalises what you learned with Brønsted chemistry: instead of tracking proton transfer, you track electron-pair donation. This framework explains reactions that have no proton transfer at all, like a nucleophile attacking an electrophile. The conformations section then shifts to three-dimensional thinking. You will learn to rotate around C–C bonds, draw Newman projections, and identify which conformations are lowest in energy. Both topics are building blocks: Lewis acid-base logic drives every nucleophilic substitution and addition mechanism, while conformational analysis becomes critical for understanding ring strain in cycloalkanes (Chapter 3+).
A Lewis acid accepts an electron pair (has an empty or partially empty orbital); a Lewis base donates one (has a lone pair or bonding electrons to share). Newman projections depict rotation around a C–C bond, and the most stable conformation is typically anti (largest groups 180° apart). Torsional strain comes from eclipsing interactions; steric strain comes from groups physically crowding each other.
Lewis acid
An electron-pair acceptor. It has an unfilled valence shell or a sufficiently electrophilic site to receive electrons. In simple terms, any species hungry for electrons.
Lewis base
An electron-pair donor. It has a lone pair or a bond with available electron density. In simple terms, any species with electrons to share.
Lewis complex
The product formed when a Lewis base donates its electron pair to a Lewis acid, creating a new covalent bond.
Electrophile
An electron-poor species that accepts electron density. Functionally the same as a Lewis acid, but the term is used more in the context of organic reaction mechanisms.
Nucleophile
An electron-rich species that donates electron density to an electrophile. Functionally the same as a Lewis base in a mechanistic context.
Leaving group
A group that departs with a pair of electrons during a reaction. Good leaving groups are stable once they leave (weak bases, often from strong conjugate acids). In simple terms, stability factors from Brønsted acid-base chemistry predict leaving-group ability: the more stable the departing anion, the better the leaving group.
Newman projection
A way of drawing a molecule by looking straight down a C–C bond. The front carbon is shown as a dot (or the centre of a Y), and the back carbon as a circle. Substituents radiate out from each.
Dihedral angle (torsional angle)
The angle between substituents on the front and back carbons in a Newman projection. Measured as the rotation needed to eclipse one group over another.
Staggered conformation
A Newman projection arrangement where substituents on the front and back carbons are offset by 60° from each other. This is lower in energy than eclipsed because there is no torsional strain.
Eclipsed conformation
A Newman projection arrangement where substituents on the front and back carbons are directly aligned (dihedral angle of 0°, 120°, 240°). This is higher in energy due to electron–electron repulsion between the eclipsing bonds.
Anti conformation
A specific staggered conformation where the two largest groups are 180° apart. This is typically the lowest-energy arrangement.
Gauche conformation
A specific staggered conformation where two large groups are 60° apart. Higher in energy than anti due to steric interaction, but still lower than any eclipsed form.
Torsional strain (Pitzer strain)
The energy cost of eclipsing interactions, arising from electron–electron repulsion between bonds that are directly aligned.
Steric strain (steric hindrance)
The energy cost when atoms or groups physically occupy the same space. It is not about bond alignment (that is torsional strain) but about bulky groups being too close.
Totally eclipsed conformation
The highest-energy eclipsed arrangement, where the two largest substituents are eclipsing each other (dihedral angle = 0°).
A Lewis base donates an electron pair to a Lewis acid, forming a Lewis complex (a new bond)
Net charge must be the same on both sides of the equation
Examples:
BH₃ (Lewis acid, empty p-orbital on B) + NH₃ (Lewis base, lone pair on N) → H₃B–NH₃ (Lewis complex)
TiCl₄ (Lewis acid) + Cl⁻ (Lewis base) → TiCl₅⁻ (Lewis complex)
CH₃Br (electrophilic carbon, Lewis acid) + OH⁻ (nucleophile, Lewis base) → CH₃OH + Br⁻
Every Brønsted acid-base reaction is also a Lewis acid-base reaction (the proton is the Lewis acid)
But Lewis theory is broader: it covers reactions where no proton is transferred
In a Brønsted reaction with CH₃Br + OH⁻, you can view it two ways:
Brønsted: not a proton transfer, so Brønsted theory does not directly apply
Lewis: OH⁻ donates electrons to the electrophilic carbon on CH₃Br; Br⁻ leaves as a leaving group
Leaving-group ability connects back to Brønsted stability factors: a good leaving group is a weak base (stable anion), which means it comes from a strong acid
Good leaving groups: stable once they depart (e.g., Br⁻, Cl⁻, I⁻, H₂O)
Weak bases make good leaving groups; strong bases make poor ones
The same factors that stabilise a conjugate base (electronegativity, size, resonance) stabilise a leaving group
Alkanes can rotate freely around C–C single bonds, producing different conformations
Newman projections depict a specific conformation by looking straight down a C–C bond
The front carbon is drawn as a dot (with three bonds radiating out); the back carbon is a circle (with three bonds radiating from behind)
Connectivity matters: make sure the correct groups are attached to the front and back carbons
Staggered: all H–H dihedral angles are 60°, E = 0 kcal/mol (the reference)
Eclipsed: all H–H dihedral angles are 0°, E = 3 kcal/mol above staggered
Rotating 60° converts staggered to eclipsed and back; the energy oscillates between 0 and 3 kcal/mol as a smooth curve
The 3 kcal/mol barrier is the torsional strain from three pairs of eclipsing H–H bonds (each pair contributes about 1 kcal/mol)
For butane (looking down the C2–C3 bond), the two methyl groups are the largest substituents
Anti conformation: the two methyls are 180° apart, E = 0 kcal/mol (lowest energy, reference)
Gauche conformation: the two methyls are 60° apart, E = 0.9 kcal/mol above anti (steric strain between the methyls)
Eclipsed: methyls eclipsing H, moderate energy
Totally eclipsed: the two methyls directly eclipse each other, highest energy point on the rotation plot
Eclipsing interaction | Eclipsing strain (kcal/mol) | Gauche strain (kcal/mol) |
|---|---|---|
H–H | 1.0 | 0 |
CH₃–H | 1.2 | 0 |
CH₃–CH₃ | 3.4 | 0.9 |
Identify which C–C bond you are looking down
Place the substituents of the front carbon on the dot; place the substituents of the back carbon on the circle
Check connectivity: make sure each group is on the correct carbon
Identify the dihedral angle between the two largest groups to name the conformation (anti, gauche, eclipsed, or totally eclipsed)
Barrier to rotation for ethane:
3.0 kcal/mol (three eclipsing H–H interactions, each ≈ 1.0 kcal/mol)
Gauche interaction for butane (CH₃–CH₃):
0.9 kcal/mol above the anti conformation
General trend:
Anti (lowest E) < gauche < eclipsed < totally eclipsed (highest E)
Conformational analysis is how biochemists understand protein folding: the rotation around bonds in the peptide backbone determines the three-dimensional shape that governs function. In drug design, knowing whether a molecule prefers an anti or gauche conformation helps predict how it fits into an enzyme's active site. Lewis acid catalysis (e.g., using AlCl₃ as a Lewis acid in Friedel-Crafts reactions) is a cornerstone of industrial organic synthesis.
Students often confuse Lewis acids with Brønsted acids. A Lewis acid does not need to donate a proton; it simply accepts electrons. BF₃ is a Lewis acid but has no proton to give.
Students sometimes think staggered and anti are the same thing. All anti conformations are staggered, but not all staggered conformations are anti. Gauche is also staggered.
Students forget that torsional strain and steric strain are different. Torsional strain is about eclipsing bonds (electron repulsion); steric strain is about bulky groups physically crowding each other. Both can be present simultaneously.
When drawing Newman projections, students often put groups on the wrong carbon. Always verify which carbon is in front and which is in back before placing substituents.
⚠️ You will be asked to convert between a dash-wedge structure and a Newman projection, and vice versa. Practise both directions.
⚠️ Be able to identify anti, gauche, eclipsed, and totally eclipsed conformations by their dihedral angles.
⚠️ Know the strain energy values (they are usually given, but you must know how to use them to calculate the relative energy of any conformation).
⚠️ Lewis acid-base reactions are the foundation of nucleophilic substitution (SN2) and addition reactions. Recognise when a reaction is Lewis rather than Brønsted.
⚠️ Leaving-group ability questions combine Lewis theory with the five Brønsted stability factors. A good leaving group is a stable anion (weak base).
True or false: BF₃ is a Lewis base.
Fill in the blank: In a Newman projection, the front carbon is drawn as a ________ and the back carbon as a ________.
True or false: The gauche conformation of butane is higher in energy than the anti conformation.
Fill in the blank: Torsional strain arises from ________ interactions between bonds.
True or false: Every Brønsted acid-base reaction is also a Lewis acid-base reaction.
Answers: 1. False (BF₃ is a Lewis acid; it accepts an electron pair). 2. Dot (or point); circle. 3. True (gauche is about 0.9 kcal/mol higher). 4. Eclipsing. 5. True (the proton acts as the Lewis acid, accepting the electron pair from the base).
Q: NH₃ reacts with BH₃ to form H₃N–BH₃. Identify the Lewis acid and the Lewis base.
A: BH₃ is the Lewis acid (it has an empty p-orbital and accepts the electron pair). NH₃ is the Lewis base (it donates its lone pair on nitrogen).
Q: Draw or describe the most stable Newman projection for butane when looking down the C2–C3 bond.
A: The anti conformation, where the two methyl groups are 180° apart. All remaining substituents (hydrogens) are staggered. This arrangement minimises both torsional and steric strain.
Q: Calculate the relative energy of the totally eclipsed conformation of butane (CH₃ eclipsing CH₃, plus two H–H eclipsing interactions).
A: CH₃–CH₃ eclipsing = 3.4 kcal/mol; two H–H eclipsing = 2 × 1.0 = 2.0 kcal/mol. Total = 5.4 kcal/mol above the anti conformation.
Q: Why is Br⁻ a good leaving group?
A: Br⁻ is a weak base (it is the conjugate base of HBr, a strong acid). Weak bases are stable on their own, which makes them willing to depart with the bonding electrons. Bromine is also a large atom, which helps stabilise the negative charge through polarisability.
Q: Explain why a Brønsted acid-base reaction between OH⁻ and CH₃Br does not fit the Brønsted model well, but fits the Lewis model.
A: Brønsted theory requires proton transfer, and in this reaction no proton is transferred; instead, OH⁻ donates its electron pair to the electrophilic carbon of CH₃Br, displacing Br⁻. Lewis theory covers any electron-pair donation, so it describes this reaction naturally: OH⁻ is the Lewis base, the carbon in CH₃Br is the Lewis acid, and Br⁻ is the leaving group.
Lewis acid-base reactions lead directly into nucleophilic substitution (SN2 and SN1) in later chapters, where the nucleophile is the Lewis base and the substrate's electrophilic carbon is the Lewis acid. Conformational analysis becomes essential for cycloalkanes (Chapter 3), especially cyclohexane chair conformations, axial/equatorial positioning, and 1,3-diaxial interactions (which are gauche interactions viewed on a ring). The strain energies learned here carry over directly.
Lewis acid, Lewis base, Lewis complex, electrophile, nucleophile, leaving group, electron pair donor, electron pair acceptor, Newman projection, conformation, staggered, eclipsed, anti, gauche, totally eclipsed, torsional strain, Pitzer strain, steric strain, steric hindrance, dihedral angle, barrier to rotation, butane conformations, ethane conformations, organic chemistry chapter 2