GATE Chemistry Organic Chemistry Syllabus

What is gauche strain?

Gauche strain (a form of steric/van der Waals strain) is the repulsion between two bulky groups held at a $60^{\circ}$ dihedral angle in a gauche conformation, raising its energy relative to the anti conformer by about $3.8\ \text{kJ mol}^{-1}$ for butane.

Which conformation of cyclohexane is most stable and why?

The chair conformation is most stable because all bonds are perfectly staggered (no torsional strain) and bond angles are near the ideal tetrahedral $109.5^{\circ}$, minimizing angle strain. Twist-boat and boat are higher in energy.

Compare axial vs equatorial substituents on cyclohexane.

Equatorial substituents are sterically favored because axial substituents suffer 1,3-diaxial interactions. At equilibrium the conformer with the larger group equatorial predominates; e.g. methylcyclohexane is ~95% equatorial.

What is the A-value of a substituent?

The A-value is the conformational free-energy preference for a substituent to occupy the equatorial position on cyclohexane, defined as $A = -\Delta G^{\circ} = -RT\ln K$ for the axial$\rightleftharpoons$equatorial equilibrium. Larger A-value means stronger equatorial preference (e.g. $t$-Bu $\approx 4.9\ \text{kcal mol}^{-1}$).

What causes ring strain in cyclopropane?

Cyclopropane has severe angle strain (internal angle $60^{\circ}$ vs ideal $109.5^{\circ}$) plus torsional strain from fully eclipsed C–H bonds, giving bent ('banana') bonds and a total ring strain of about $115\ \text{kJ mol}^{-1}$.

Define constitutional (structural) isomers.

Constitutional isomers have the same molecular formula but differ in the connectivity (order in which atoms are bonded), e.g. n-butane vs isobutane, or ethanol vs dimethyl ether. They generally have different physical and chemical properties.

How do stereoisomers differ from constitutional isomers?

Stereoisomers have the same molecular formula AND the same connectivity but differ in the spatial arrangement of atoms. They subdivide into enantiomers and diastereomers (which includes geometric/cis-trans isomers).

What conditions are required for geometrical (cis–trans / E–Z) isomerism?

It requires restricted rotation (a C=C double bond or a ring) AND each of the two carbons of the rigid unit must bear two different groups. If either carbon has two identical groups, no geometric isomerism exists.

State the E/Z nomenclature rule.

Assign CIP priorities to the two groups on each doubly bonded carbon. If the two higher-priority groups are on the same side it is $Z$ (zusammen); if on opposite sides it is $E$ (entgegen). E/Z is unambiguous whereas cis/trans can be.

Define optical isomerism and optical activity.

Optical isomerism is stereoisomerism shown by chiral molecules existing as enantiomers. Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light; dextrorotatory ($+$) rotates clockwise, levorotatory ($-$) counterclockwise.

Give the formula for specific rotation.

$$[\alpha]_{\lambda}^{T} = \frac{\alpha}{l \cdot c}$$ where $\alpha$ is the observed rotation (degrees), $l$ is the path length in dm, and $c$ is the concentration in $\text{g mL}^{-1}$ (for solutions). $T$ is temperature and $\lambda$ the wavelength (often the Na D line).

What is a racemic mixture (racemate)?

A racemate is an equimolar (1:1) mixture of two enantiomers. It is optically inactive (rotations cancel), denoted $(\pm)$ or $dl$, and may differ in melting point/solubility from the pure enantiomers.

Define enantiomeric excess (ee) and give its formula.

Enantiomeric excess measures purity of one enantiomer over the other: $$\%\,ee = \frac{|[R] - [S]|}{[R] + [S]} \times 100 = \frac{[\alpha]_{obs}}{[\alpha]_{pure}} \times 100.$$ A 70:30 mixture has $40\%\ ee$.

What is atropisomerism?

Atropisomerism is a form of axial chirality arising from restricted rotation about a single bond (e.g. a biaryl C–C bond) where steric bulk creates a high enough rotational barrier that the rotamers are isolable as stable stereoisomers, e.g. substituted biphenyls and BINOL.

What rotational barrier is generally needed for atropisomers to be isolable at room temperature?

A barrier of roughly $\geq 93\ \text{kJ mol}^{-1}$ ($\approx 22\ \text{kcal mol}^{-1}$) gives a half-life of about $1000\ \text{s}$ at $300\ \text{K}$, the conventional threshold for atropisomers to be physically separable.

What structural feature gives rise to axial chirality in biphenyls?

Biphenyls are axially chiral when both ortho positions of each ring carry bulky groups that prevent coplanarity, and the substitution pattern lacks a symmetry plane. The chirality axis is the pivotal C–C bond; configuration is labeled $R_a$/$S_a$ (or $M$/$P$).

What is neighbouring group participation (NGP / anchimeric assistance)?

NGP is intramolecular assistance to a reaction by a nearby group with lone pairs or $\pi$ electrons, which attacks the reacting center to form a cyclic intermediate. It accelerates the reaction (anchimeric assistance) and controls stereochemistry.

How does neighbouring group participation affect stereochemistry of substitution?

NGP typically gives net retention of configuration via a double inversion: the neighbouring group displaces the leaving group with inversion forming a cyclic (e.g. bridged) intermediate, then the external nucleophile opens it with a second inversion.

How does NGP affect reaction rate and what evidence reveals it?

NGP produces rate enhancement (anchimeric assistance), sometimes by factors of $10^{3}$–$10^{6}$. Evidence includes abnormally fast rates, retention of configuration, rearranged products, and racemization through a symmetric bridged ion.

Give a classic example of a neighbouring group and its bridged intermediate.

A $\beta$-bromo or $\beta$-sulfide group forms a three-membered bromonium or episulfonium (thiiranium) ion; participation by an acetoxy group forms a cyclic acetoxonium ion; aryl participation gives a phenonium ion (norbornyl/2-arylethyl systems).

Distinguish kinetic control from thermodynamic control.

Under kinetic control the product distribution reflects relative rates of formation (lowest $\Delta G^{\ddagger}$ pathway dominates), favored at low temperature/short times/irreversible conditions. Under thermodynamic control the distribution reflects relative product stabilities ($\Delta G^{\circ}$), favored at high temperature/long times/reversible conditions.

In the relationship $\Delta G^{\circ} = -RT\ln K$, which type of control does $K$ describe?

$K$ (the equilibrium constant) describes thermodynamic control, since $\Delta G^{\circ} = -RT\ln K$ relates the equilibrium product ratio to the difference in product stabilities. Kinetic control instead depends on activation energies via $k = A e^{-E_a / RT}$.

Give the classic example illustrating kinetic vs thermodynamic control.

Electrophilic addition of HBr to 1,3-butadiene: the 1,2-adduct (3-bromo-1-butene) is the kinetic product (formed faster at low T, $-80^{\circ}\text{C}$), while the 1,4-adduct (1-bromo-2-butene) is the more stable thermodynamic product (dominant at high T, $40^{\circ}\text{C}$).

What is the Curtin–Hammett principle?

For two rapidly interconverting conformers/intermediates reacting to give different products, the product ratio is governed not by their populations but by the difference in free energies of the two transition states: $$\frac{[P_1]}{[P_2]} = e^{-(\Delta G^{\ddagger}_1 - \Delta G^{\ddagger}_2)/RT}.$$

What are the basic ways bonds break in mechanistic organic chemistry?

Homolysis cleaves a bond giving one electron to each fragment (forming radicals, shown with single-barbed arrows). Heterolysis cleaves a bond giving both electrons to one fragment (forming ions, shown with double-barbed curved arrows).

Define electrophile and nucleophile.

A nucleophile is an electron-rich species that donates an electron pair to form a bond (a Lewis base). An electrophile is an electron-poor species that accepts an electron pair (a Lewis acid). Curved arrows always point from nucleophile to electrophile.

Compare the stereochemistry of $S_N1$ and $S_N2$ reactions.

$S_N2$ is concerted, bimolecular, and proceeds with inversion of configuration (Walden inversion) through a backside attack. $S_N1$ is stepwise via a planar carbocation, giving racemization (often with slight excess inversion) due to attack on both faces.

What is the Hammond postulate?

The Hammond postulate states that the transition state resembles the species (reactant or product) nearest to it in energy. For an exothermic step the TS is reactant-like ('early'); for an endothermic step it is product-like ('late'), helping predict selectivity.

What is configurational vs conformational isomerism?

Configurational isomers differ in fixed spatial arrangement and interconvert only by breaking bonds (e.g. $R$/$S$, $E$/$Z$); they are isolable. Conformational isomers interconvert by rotation about single bonds without breaking bonds and are usually not isolable at room temperature.

What is a configurational effect versus a conformational effect on reactivity?

A configurational effect is reactivity governed by the fixed stereochemistry (e.g. trans-diaxial requirement in E2 of cyclohexanes). A conformational effect is reactivity governed by the populated conformer (e.g. axial vs equatorial orientation controlling whether elimination or substitution occurs).

Why does anti-periplanar geometry matter in E2 elimination of cyclohexanes?

E2 requires the H and leaving group to be anti-periplanar, which on a cyclohexane ring means both must be trans-diaxial. A substituent locked equatorial cannot undergo E2 in that direction; this conformational constraint dictates rate and regiochemistry (e.g. menthyl vs neomenthyl chloride).