MCAT (Medical College Admission Test) Organic Chemistry for the Biological Sciences Syllabus

What is keto-enol tautomerism and which form usually predominates?

Keto-enol tautomerism is the equilibrium between a keto form (C=O with alpha C-H) and an enol form (C=C-OH) via proton transfer. The keto form usually predominates because the C=O bond is stronger/more stable.

What is the aldol condensation, and what product results?

In an aldol reaction, the enolate of one carbonyl attacks the carbonyl carbon of another, forming a beta-hydroxy aldehyde/ketone (aldol). Under heat/base, dehydration gives an alpha,beta-unsaturated carbonyl (the condensation product).

Rank the relative reactivity of carboxylic acid derivatives toward nucleophilic acyl substitution.

Acid chloride > acid anhydride > ester ~ carboxylic acid > amide. Reactivity decreases as the leaving group becomes a worse leaving group and as the heteroatom donates more electron density into the carbonyl.

How are alcohols classified, and how does class affect oxidation products?

Alcohols are primary, secondary, or tertiary based on the number of carbons attached to the carbinol carbon. Primary alcohols oxidize to aldehydes then carboxylic acids; secondary oxidize to ketones; tertiary alcohols generally do not oxidize (no alpha-H on the C-OH).

What reagent selectively oxidizes a primary alcohol to an aldehyde (stopping before the carboxylic acid), and what reagent oxidizes fully to the acid?

PCC (pyridinium chlorochromate) in anhydrous conditions stops at the aldehyde. Strong aqueous oxidants like Jones reagent (CrO3/H2SO4) or KMnO4 oxidize a primary alcohol all the way to the carboxylic acid.

How do you convert an alcohol into a good leaving group for substitution/elimination?

Protonate it under acidic conditions (to make water the leaving group) or convert -OH into a tosylate (TsCl) or a halide (e.g., SOCl2, PBr3), turning the poor -OH leaving group into a much better one.

Compare the basicity of aliphatic amines, ammonia, and amides.

Aliphatic amines are more basic than ammonia (alkyl groups donate electron density). Amides are very weakly basic because the nitrogen lone pair is delocalized into the carbonyl by resonance, reducing its availability.

What is the Williamson ether synthesis?

It is an SN2 reaction in which an alkoxide (RO-) attacks a primary alkyl halide to form an ether (R-O-R'). Best yields use a primary, unhindered alkyl halide to avoid competing elimination.

Why do alcohols have higher boiling points than ethers or amines of similar molecular weight?

Alcohols form strong intermolecular hydrogen bonds via O-H. Ethers cannot hydrogen-bond as donors (no O-H), and N-H hydrogen bonds in amines are weaker than O-H, so alcohols boil highest.

In carbohydrate chemistry, what is mutarotation?

Mutarotation is the change in optical rotation as alpha- and beta-anomers of a sugar interconvert through the open-chain form in solution until reaching an equilibrium mixture of anomers.

What type of bond links amino acids in proteins, and how is it formed?

A peptide (amide) bond links amino acids, formed by a condensation (dehydration) reaction between the carboxyl group of one amino acid and the amino group of the next, releasing water.

What is the difference between an alpha and beta glycosidic linkage in carbohydrates?

In an alpha linkage the glycosidic oxygen at the anomeric carbon points down (opposite the CH2OH reference, axial in glucose); in a beta linkage it points up (same side, equatorial). Example: starch is alpha-1,4; cellulose is beta-1,4.

What functional-group transformation occurs in the formation of a triglyceride from glycerol and fatty acids?

Three fatty acid carboxyl groups undergo esterification with the three hydroxyl groups of glycerol, forming three ester bonds and releasing three water molecules.

What is the role of a coenzyme like NAD+/NADH in biologically relevant redox reactions?

NAD+ acts as an oxidizing agent, accepting a hydride (2 electrons + 1 proton) to become NADH; NADH is the reducing agent that donates the hydride back. They shuttle reducing equivalents in metabolic redox reactions.

What principle underlies separation by extraction (liquid-liquid extraction)?

Extraction separates compounds based on differential solubility (partitioning) between two immiscible solvents (usually aqueous and organic). Adjusting pH can ionize acids or bases to pull them into the aqueous layer (acid-base extraction).

How does distillation separate a mixture, and when is fractional distillation needed?

Distillation separates liquids by differences in boiling point: the more volatile component vaporizes and is condensed first. Fractional distillation is needed when boiling points are close (typically within ~25 degrees C), using a column for repeated vaporization-condensation cycles.

What is the principle of thin-layer chromatography (TLC) and the Rf value?

TLC separates compounds by their relative affinity for a polar stationary phase (silica) versus a mobile solvent. Rf = (distance traveled by spot)/(distance traveled by solvent front); more polar compounds have lower Rf on silica.

How does column chromatography separate compounds and in what order do they elute on normal-phase silica?

Compounds separate by polarity differences in affinity for the stationary phase. On normal-phase (polar) silica, less polar compounds elute first and more polar compounds elute last (eluted by increasingly polar solvent).

How does recrystallization purify a solid, and what solvent property is required?

Recrystallization dissolves the impure solid in a minimum of hot solvent; on cooling, the pure compound crystallizes while impurities stay in solution. The ideal solvent dissolves the compound well when hot but poorly when cold.

What information does each major spectroscopic region/technique provide: IR, 1H NMR, and mass spectrometry?

IR identifies functional groups via bond vibrations. 1H NMR reveals the number, environment, and connectivity of hydrogens. Mass spectrometry gives molecular weight and fragmentation pattern (structural clues).

What key IR absorption bands identify O-H, C=O, and N-H groups?

O-H (alcohol): broad ~3200-3550 cm-1; O-H (carboxylic acid): very broad ~2500-3300 cm-1; N-H: ~3300-3500 cm-1 (one or two peaks); C=O: strong sharp ~1700-1750 cm-1.

In 1H NMR, what do chemical shift, integration, and spin-spin splitting each tell you?

Chemical shift (ppm) indicates the electronic environment (deshielding by electronegative/anisotropic groups). Integration gives the relative number of protons. Splitting (n+1 rule) indicates the number of neighboring nonequivalent protons.

State the n+1 rule for NMR splitting and give the multiplicity for a proton next to 2 equivalent neighbors.

A signal is split into (n+1) peaks, where n is the number of equivalent adjacent protons. Two equivalent neighbors give n+1 = 3, a triplet.

What is the approximate 1H NMR chemical shift range for aromatic, vinyl, aldehyde, and carboxylic acid protons?

Aromatic H: ~6.5-8.5 ppm; vinyl (C=C-H): ~4.5-6.5 ppm; aldehyde H (CHO): ~9-10 ppm; carboxylic acid H (COOH): ~10-12 ppm (broad).

In mass spectrometry, what are the molecular ion peak (M+) and the base peak?

The molecular ion peak (M+) corresponds to the intact molecule that has lost one electron; its m/z equals the molecular weight. The base peak is the most intense peak (set to 100% relative abundance), often a stable fragment.

What does the M+2 peak pattern reveal in mass spectrometry for chlorine and bromine?

An M+2 peak about one-third the height of M+ indicates chlorine (35Cl:37Cl ~ 3:1); an M+2 peak roughly equal to M+ indicates bromine (79Br:81Br ~ 1:1).

What does UV-Vis spectroscopy detect, and what structural feature increases absorption wavelength?

UV-Vis detects electronic transitions in molecules with conjugated pi systems (chromophores). Increasing conjugation lowers the HOMO-LUMO gap, shifting absorption to longer wavelengths (bathochromic/red shift).

How is the degree of unsaturation (index of hydrogen deficiency) calculated and interpreted?

For CcHhNnOoXx: DoU = (2c + 2 + n - h - x)/2. Each degree equals one ring or one pi bond; for example, a value of 4 commonly suggests a benzene ring (3 pi + 1 ring).

In structure determination, why is 13C NMR a useful complement to 1H NMR?

13C NMR shows the number of chemically distinct carbon environments and a wide chemical shift range (~0-220 ppm), helping identify carbon skeletons and carbonyl carbons (~160-220 ppm); it gives signals even for carbons bearing no hydrogens.

How can melting point be used as a measure of purity?

A pure compound has a sharp, narrow melting range close to its literature value. Impurities lower the melting point and broaden the range (melting-point depression), so a depressed/broad range signals impurity.