Source: Exam 1 Key, Organic Chemistry I, University of Minnesota Twin Cities
Difficulty: Intermediate | Prerequisites: Lewis structures, formal charge, electronegativity trends, basic 3D molecular geometry
Acid-base chemistry is the single most important reaction framework in organic chemistry. Nearly every mechanism you encounter will involve a proton transfer, a nucleophilic attack, or both, and both Bronsted-Lowry and Lewis definitions are used constantly. Conformational analysis (Newman projections and chair conformations) is how organic chemists reason about 3D molecular shape and predict which arrangement of atoms is most stable. These topics connect directly to reaction selectivity and product distribution in later chapters.
Bronsted-Lowry acid-base reactions are proton transfers; the equilibrium favours the side with the weaker acid. Lewis acid-base reactions are electron-pair donations to electron-pair acceptors. Newman projections let you compare staggered and eclipsed conformations around a C-C bond, with anti being the most stable. Cyclohexane chair conformations place substituents axial or equatorial, and the most stable chair puts the largest groups equatorial.
Tags: Bronsted-Lowry acid, Bronsted-Lowry base, conjugate acid, conjugate base, Lewis acid, Lewis base, electron pair donor, electron pair acceptor, pKa, Newman projection, dihedral angle, staggered, eclipsed, gauche, anti, torsional strain, steric strain, chair conformation, axial, equatorial, 1,3-diaxial interaction, ring flip
Bronsted-Lowry acid
A proton (H⁺) donor.
In simple terms, this means: any species that gives away a hydrogen ion.
Bronsted-Lowry base
A proton (H⁺) acceptor.
Think of it as: any species with a lone pair or negative charge that can grab a proton.
Lewis acid
An electron-pair acceptor. It has an empty or electron-deficient orbital.
Think of it as: a molecule that wants electrons. Carbocations, BH₃, and H⁺ are classic examples.
Lewis base
An electron-pair donor. It has a lone pair or π electrons to share.
In simple terms, this means: a molecule that has electrons to give away. Amines, water, and hydroxide are typical Lewis bases.
Newman projection
A way of viewing a molecule along a C-C bond axis. The front carbon is drawn as a dot, the back carbon as a circle. Bonds on each carbon are shown as lines radiating from the dot or circle.
Think of it as: looking straight down the barrel of a bond, so you can see how the groups on the front and back carbons relate to each other.
Anti conformation
The staggered conformation in which the two largest substituents on adjacent carbons are 180° apart (dihedral angle = 180°). This is the lowest-energy staggered form.
Gauche conformation
A staggered conformation in which the two largest substituents are 60° apart. Higher in energy than anti due to steric strain, but lower than any eclipsed form.
Chair conformation
The most stable conformation of cyclohexane, in which all bond angles are approximately 109.5° and all adjacent C-H bonds are staggered.
Axial and equatorial positions
In a chair conformation, each carbon has one axial bond (pointing straight up or down) and one equatorial bond (pointing outward, roughly in the plane of the ring). Large substituents prefer equatorial positions to avoid 1,3-diaxial interactions.
1,3-diaxial interaction
The steric strain between an axial substituent and the axial hydrogens (or other groups) on carbons two positions away in the ring. This is the main reason large groups prefer equatorial placement.
The base donates a lone pair to the proton on the acid.
Curved arrows go from the base's lone pair to the H, and from the H-A bond to the conjugate base.
Products: a conjugate acid (the base + H) and a conjugate base (the acid minus H).
Equilibrium favours the side with the weaker acid (higher pKa).
To determine which of two acids is stronger:
Electronegativity effect: a more electronegative atom stabilises the conjugate base better. An O-H acid is stronger than an N-H acid of similar structure.
Inductive effects: electron-withdrawing groups (e.g. Cl, F, CF₃) near the acidic site stabilise the conjugate base and increase acidity. Electron-donating groups (e.g. alkyl groups) decrease acidity.
Charge effects: a positively charged acid (e.g. NH₄⁺) is stronger than its neutral analogue (e.g. NH₃) because the conjugate base is more stable when it is neutral rather than negatively charged.
Resonance stabilisation: if the conjugate base can delocalise its negative charge through resonance, the acid is stronger.
The stronger base has the weaker conjugate acid.
A negatively charged nitrogen (N⁻) is a stronger base than a neutral nitrogen, because its conjugate acid (neutral NH) is weaker than the conjugate acid of neutral N (which would be NH⁺).
A tertiary amine reacting with a carboxylic acid: the nitrogen's lone pair attacks H on the O-H, producing an ammonium ion and a carboxylate.
An anionic nitrogen (N⁻) reacting with an alcohol: the nitrogen grabs the proton, producing a neutral amine and an alkoxide.
Comparing acidity: chloro-substituted benzoic acid is more acidic than methyl-substituted benzoic acid because Cl is electron-withdrawing (stabilises the carboxylate conjugate base) while CH₃ is weakly electron-donating.
Lewis acid-base reactions do not necessarily involve a proton transfer.
The Lewis base donates an electron pair to the Lewis acid. The product is a Lewis acid-base adduct, a single new species with a new covalent bond.
Carbocation + alcohol: the oxygen lone pair on the alcohol donates into the empty p orbital of the carbocation. The product has a new C-O bond, with oxygen now carrying a positive formal charge.
BH₃ + hydroxide (OH⁻): the lone pair on oxygen donates into the empty p orbital on boron. The product has a new B-O bond, boron now has four bonds and a -1 formal charge.
H⁺ + ketone: the lone pair on the carbonyl oxygen donates to H⁺. The product has a protonated carbonyl (O-H with a + charge on oxygen).
Always show the curved arrow from the electron-rich species (Lewis base) to the electron-poor species (Lewis acid).
The arrow starts from a lone pair on the base and points to the atom on the acid that accepts the electrons.
Draw the adduct product showing the new bond and any formal charges that result.
The front carbon is the central dot; its three bonds radiate from it.
The back carbon is the large circle; its three bonds radiate from the circle's edge.
The dihedral angle is the angle between two specified substituents when viewed along the bond axis.
Staggered: dihedral angles of 60° between adjacent groups. Lower energy because bonds are as far apart as possible.
Eclipsed: dihedral angles of 0° between adjacent groups. Higher energy due to torsional strain.
Anti: a staggered conformation where the two largest groups are 180° apart. This is the global energy minimum for rotation around that bond.
Gauche: a staggered conformation where the two largest groups are 60° apart. A local energy minimum, higher than anti due to steric strain.
As you rotate around a C-C bond from 0° to 360°, the energy profile shows alternating maxima (eclipsed conformations) and minima (staggered conformations).
The lowest minimum is anti (largest groups 180° apart).
The next-lowest minima are gauche.
The highest maximum is the eclipsed form where the two largest groups are directly overlapping (0° dihedral).
The other eclipsed maxima (where a large group eclipses a smaller one) are lower than the fully eclipsed maximum but still higher than any staggered form.
For 1-chloro-1-methylethane (or similar), with CH₃ > Cl > H by size:
Conformation A (Cl and CH₃ at 180°) is anti.
The energy diagram maps: Point I = lowest-energy staggered (E), Point II = gauche staggered (F), Point III = highest eclipsed (B), Point IV = intermediate eclipsed (C).
Always confirm the size ordering given in the problem. The exam may specify that CH₃ > Cl > H, which determines which eclipsed interactions are most destabilising.
In a chair conformation, every carbon has one axial bond and one equatorial bond.
A ring flip converts all axial bonds to equatorial and all equatorial bonds to axial.
During a ring flip, substituents that were "up" stay up and those that were "down" stay down. The up/down relationship does not change; only axial/equatorial switches.
Larger substituents prefer the equatorial position because it avoids 1,3-diaxial interactions (steric clashes with axial groups two carbons away).
When a substituent moves from equatorial to axial in a ring flip, the new conformation is less stable.
If two substituents are on the same ring, the most stable chair puts the larger substituent equatorial. If both can be equatorial (as in trans-1,2-disubstituted cyclohexane with both equatorial, or certain cis isomers), that conformation wins.
Monosubstituted cyclohexane with Cl equatorial and CH₃ equatorial: ring flip puts both axial, which is less stable.
Disubstituted cyclohexane with Cl and two CH₃ groups: the ring flip that puts the larger groups axial is less stable.
trans-1,2-dimethylcyclohexane: the most stable conformation (C in the exam) has both methyl groups equatorial (one up, one down, both equatorial).
Each axial CH₃ group creates two 1,3-diaxial interactions with axial H atoms on the same face of the ring.
Conformation B in the exam has four di-axial H-CH₃ interactions (two per axial methyl, with two methyls axial).
Students often think a ring flip changes the cis/trans relationship of substituents. It does not. Cis (both up or both down) and trans (one up, one down) are fixed. The ring flip only swaps axial and equatorial.
Students often think anti and gauche are eclipsed conformations. Both are staggered. Anti has the largest groups at 180°; gauche at 60°. Eclipsed means groups are directly aligned (0° or 120° depending on which groups overlap).
Students often confuse Lewis acid-base reactions with Bronsted-Lowry ones. If no proton is transferred, it is not Bronsted-Lowry. Lewis acid-base reactions involve any electron-pair donation, proton-related or not.
Students often think the strongest acid is always the one with the most electronegative atom bearing the H. Inductive effects, resonance stabilisation of the conjugate base, and charge all play a role. Consider all factors.
⚠️ Acid-base reactions (both Bronsted-Lowry and Lewis) are the most heavily weighted section of this exam at 28 points combined. Be able to draw electron flow arrows, identify products, and explain acid/base strength comparisons.
⚠️ Newman projection questions will give you several conformations and ask you to match them to points on an energy diagram. Practise identifying anti, gauche, and eclipsed by inspection.
⚠️ Chair conformation questions require you to draw a ring flip and determine relative stability. Always place the largest group equatorial in the more stable chair.
⚠️ For acid strength comparisons, the exam expects a brief written explanation. Name the key structural difference (inductive effect, charge, resonance) and explain how it affects conjugate base stability.
True or false: In a Bronsted-Lowry reaction, the equilibrium favours the side with the stronger acid. (False. It favours the side with the weaker acid.)
Fill in the blank: A Lewis acid is an electron-pair ____. (acceptor)
True or false: The gauche conformation is lower in energy than the anti conformation. (False. Anti is the lowest-energy staggered conformation.)
Fill in the blank: In a cyclohexane ring flip, all axial bonds become ____ and vice versa. (equatorial)
True or false: An electron-withdrawing group near an O-H makes the molecule a weaker acid. (False. Electron-withdrawing groups increase acidity by stabilising the conjugate base.)
Q: A tertiary amine reacts with a carboxylic acid. Draw the products and show the electron flow.
A: The nitrogen lone pair attacks the acidic H on the O-H of the carboxylic acid. Products: an ammonium ion (N-H⁺) and a carboxylate anion (COO⁻).
Q: BH₃ reacts with OH⁻. What type of reaction is this, and what is the product?
A: Lewis acid-base reaction. OH⁻ (Lewis base) donates a lone pair to the empty p orbital on B (Lewis acid). Product: [BH₃OH]⁻, with boron carrying a -1 formal charge.
Q: Compound A has a Cl substituent adjacent to an O-H group. Compound B has a CH₃ group in the same position. Which is the stronger acid and why?
A: Compound A (with Cl). Chlorine is electron-withdrawing, which stabilises the conjugate base through inductive effects, increasing acidity. Methyl is weakly electron-donating and does not help.
Q: In a Newman projection of 2-chlorobutane looking down the C2-C3 bond, which conformation is the most stable?
A: The anti conformation, where the two largest groups (CH₃ and Cl, or CH₃ and CH₃ depending on which end you view from) are 180° apart.
Q: For trans-1,2-dimethylcyclohexane, which chair conformation is more stable?
A: The one with both methyl groups equatorial. In trans-1,2-disubstituted cyclohexane, one substituent is up and one is down; the diequatorial arrangement satisfies this and avoids 1,3-diaxial strain.
Bronsted-Lowry acid-base chemistry connects directly to reaction mechanisms: every protonation and deprotonation step in a mechanism is a Bronsted-Lowry reaction. Lewis acid-base theory underpins nucleophilic addition, electrophilic addition, and coordination chemistry. Conformational analysis matters most when predicting stereochemical outcomes of reactions (e.g. which face of a cyclohexane ring a reagent attacks, or whether an E2 elimination requires anti-periplanar geometry).
Chair conformations come back in sugar chemistry (carbohydrates), steroid structure, and any reaction involving substituted cyclohexanes.
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