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SN1 reaction: key features?
Answer: Two steps via a carbocation intermediate; rate depends only on substrate; racemization; favored by tertiary substrates, weak nucleophiles, and polar protic solvents.
Carbocation stability: tertiary > secondary > primary.
What is a Grignard reagent and what does it do?
Answer: An organomagnesium halide (RMgX) that acts as a strong carbon nucleophile, adding to carbonyls to form alcohols.
Destroyed by water or acidic protons; use dry ether.
How do substituents direct electrophilic aromatic substitution?
Answer: Electron-donating groups (–OH, –NH2, –OR, alkyl) activate and direct ortho/para; electron-withdrawing groups (–NO2, –COOH, –CN) deactivate and direct meta.
Halogens are deactivating but ortho/para directing.
What is electrophilic aromatic substitution?
Answer: An electrophile replaces a hydrogen on an aromatic ring via a resonance-stabilized arenium ion.
Examples: nitration, halogenation, Friedel-Crafts alkylation and acylation.
What is a nucleophile and an electrophile?
Answer: A nucleophile donates an electron pair (electron-rich); an electrophile accepts one (electron-poor).
Arrows in mechanisms go from nucleophile to electrophile.
What does hydroboration-oxidation of an alkene give?
Answer: An anti-Markovnikov alcohol with syn addition.
BH3·THF, then H2O2/NaOH.
What is Markovnikov's rule?
Answer: In the addition of HX to an alkene, the hydrogen adds to the carbon with more hydrogens and X to the more substituted carbon.
Because the more stable carbocation forms. Anti-Markovnikov with HBr and peroxides (radical mechanism).
E1 vs E2 elimination?
Answer: E2: one step, strong base, anti-periplanar geometry, rate depends on base and substrate. E1: carbocation intermediate, weak base, competes with SN1.
Zaitsev's rule: the more substituted alkene is the major product (unless a bulky base is used).
SN2 reaction: key features?
Answer: One-step backside attack by a nucleophile; rate depends on both substrate and nucleophile; inversion of configuration; favored by methyl and primary substrates and polar aprotic solvents.
Strong nucleophiles like I⁻, CN⁻, RS⁻.
What does the molecular ion peak in mass spectrometry tell you?
Answer: The molecular mass of the compound (M⁺).
An M+2 peak about one-third as tall suggests chlorine; equal M and M+2 suggests bromine.
In ¹H NMR, what does the n+1 rule describe?
Answer: A proton with n equivalent neighboring protons splits into n+1 peaks.
Integration gives the relative number of protons; chemical shift gives the environment.
What does a strong, broad IR absorption around 3300 cm⁻¹ indicate?
Answer: An O–H stretch (alcohol or carboxylic acid).
C=O appears as a sharp peak near 1700 cm⁻¹.
What is an aldol reaction?
Answer: An enolate adds to a carbonyl, forming a β-hydroxy carbonyl; heating dehydrates it to an α,β-unsaturated carbonyl.
Base-catalyzed; requires an α-hydrogen.
What is a Diels-Alder reaction?
Answer: A [4+2] cycloaddition between a conjugated diene and a dienophile forming a six-membered ring.
Concerted, stereospecific; electron-poor dienophiles react faster.
How is an ester formed from a carboxylic acid?
Answer: Fischer esterification: acid + alcohol with acid catalyst, removing water.
Equilibrium driven by excess alcohol or removal of water.
What does oxidation of a primary alcohol give with PCC vs with chromic acid?
Answer: PCC stops at the aldehyde; chromic acid (Jones) oxidizes to the carboxylic acid.
Secondary alcohols give ketones; tertiary alcohols don't oxidize.
What defines an alkane, alkene, and alkyne?
Answer: Alkane: only C–C single bonds (CnH2n+2). Alkene: at least one C=C double bond. Alkyne: at least one C≡C triple bond.
Degree of unsaturation increases from alkane to alkyne.
What is an aromatic compound?
Answer: A planar, cyclic, fully conjugated ring with 4n+2 π electrons (Hückel's rule).
Benzene has 6 π electrons (n = 1).
What are amines and how are they classified?
Answer: Compounds with nitrogen bonded to carbons; primary, secondary, or tertiary by the number of carbon substituents on N.
Amines are basic; the lone pair accepts protons.
What is an ether and an epoxide?
Answer: Ether: R–O–R'. Epoxide: a three-membered ring containing oxygen.
Epoxides are strained and open readily with nucleophiles.
What are esters and amides?
Answer: Ester: carbonyl bonded to –OR. Amide: carbonyl bonded to –NR2.
Both are carboxylic acid derivatives; amides are the least reactive.
What is the structure of a carboxylic acid and why is it acidic?
Answer: A carbonyl bonded to –OH (–COOH); the conjugate base (carboxylate) is resonance-stabilized.
pKa around 4–5, compared with ~16 for alcohols.
Aldehyde vs ketone?
Answer: Both have a carbonyl (C=O); an aldehyde has it at the end of a chain (bonded to at least one H), a ketone has it between two carbons.
Aldehydes are more easily oxidized (to carboxylic acids).
What is the functional group of an alcohol, and how are they classified?
Answer: A hydroxyl group (–OH) on an sp3 carbon; primary, secondary, or tertiary by the number of carbons attached to that carbon.
Phenols have –OH on an aromatic ring and are more acidic.
What is resonance?
Answer: Delocalization of electrons described by multiple Lewis structures; the real structure is a hybrid.
Resonance stabilizes carbocations, anions, and conjugated systems.
What makes a conformation more stable in cyclohexane?
Answer: The chair conformation with bulky groups equatorial avoids 1,3-diaxial strain.
Ring flipping interconverts axial and equatorial positions.
What is cis/trans (E/Z) isomerism?
Answer: Different arrangements of substituents around a double bond or ring, which cannot rotate.
E = higher-priority groups on opposite sides; Z = same side.
Enantiomers vs diastereomers?
Answer: Enantiomers are non-superimposable mirror images; diastereomers are stereoisomers that are not mirror images.
Diastereomers have different physical properties; enantiomers differ only in optical rotation and chiral interactions.
How do you assign R and S configuration?
Answer: Rank substituents by atomic number (CIP rules), point the lowest priority away, and read 1→2→3: clockwise is R, counterclockwise is S.
If the lowest priority points toward you, reverse the answer.
What is a chiral center?
Answer: A carbon bonded to four different groups, making the molecule non-superimposable on its mirror image.
Enantiomers are mirror-image stereoisomers.
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