Structures

• Electrophile attacks the π bond (weak), forming a carbocation on one carbon.

• Nucleophile then bonds to the carbocation, stabilizing the compound.

• Rate-determining step: breaking π bond (rate = k[alkene][H⁺]).

• More stable carbocations (e.g., tertiary) are favoured → Markovnikov's rule.

What happens in addition reactions to alkenes?

• Double bond breaks, forming free radicals which propagate a chain reaction.

• Initiation → Propagation → Termination (radicals meet and neutralise).

• More stable radicals form on carbons with more alkyl groups.

• Polymer stereoisomers: isotactic (same side), syndiotactic (alternating), atactic (random).

What is free radical polymerisation and how does it proceed?

• Carbon in C=O has δ⁺ charge; attacked by nucleophiles.

• Strong nucleophiles attack first; weak nucleophiles need protonation of oxygen.

• Ketones are less reactive than aldehydes due to steric hindrance and charge stabilisation.

• Alcohol + aldehyde → hemiacetal → (with more alcohol) acetal (via acidic conditions).

How do addition reactions to carbonyl compounds occur?

• NH₂ (a strong nucleophile) attacks δ⁺ carbon of aldehydes.

• Breaks C=O bond → negative oxygen picks up H⁺, NH₂ loses H⁺ → neutral intermediate.

• Further protonation leads to imine formation (C=N), with water as byproduct.

How do amino groups react with carbonyls and form imines?

• Order of reactivity: acid halide > anhydride > carboxylic acid > ester ≈ amide.

• Reactive derivatives: nucleophile attacks δ⁺ carbon → leaving group departs.

• Unreactive derivatives: need acidic protonation to activate → then nucleophile attack.

• Products: esters, amides, etc., depending on nucleophile.

What determines the reactivity of acid derivatives?

• Alkane, Alkene, Alkyne: single/double/triple C bonds.

• Carboxylic acid, aldehyde, ketone: carbonyl-containing groups.

• Esters, amides, acid halides, ethers: carbonyl derivatives with different substitutions.

• Alcohols, amines, thiols, phosphates, disulfide bridges: important polar/reactive sites.

What are the key functional groups in biological molecules?

• 1° alcohols → aldehyde → carboxylic acid; 2° alcohols → ketone; 3° alcohols = no oxidation.

• Oxidation reagent: Cr₂O₇²⁻; Reduction: LiAlH₄.

• Tollen’s test (Ag⁺): distinguishes aldehydes (silver mirror forms) from ketones.

How can alcohols be oxidised and distinguished?

• Polar regions attract water and ions; non-polar regions fold inward.

• Disulfide bonds (S–S) and hydrogen bonding help maintain 3D shape.

• Enzymes have active sites with specific steric, polar, and electrostatic features for selectivity.

How do shape and selectivity arise in large biological molecules?

• Monosaccharides = single sugar units; aldoses (aldehyde), ketoses (ketone), alditols (reduced).

• Fischer projections: D = OH on right of penultimate carbon; L = OH on left.

• Aldehyde/ketone top; CH₂OH at bottom.

What defines saccharide structure and Fischer projections?

• Aldehyde + alcohol → hemiacetal (ring); second alcohol + hemiacetal → acetal (glycoside).

• Glycosidic bonds form via loss of H₂O; reversible only under acidic conditions.

• Furanose = 5-membered ring; Pyranose = 6-membered ring (glucose is pyranose).

How do hemiacetals and acetals form in sugars?

• OH on anomeric carbon: same side as CH₂OH = β; opposite side = α.

• α = axial OH (less stable), β = equatorial OH (more stable).

• Glucose interconverts between α and β via ring opening/closing.

What’s the difference between α and β forms of sugars?

• Hemiacetals can open to aldehydes → reducing sugars.

• Acetals (glycosides) are non-reducing → no free aldehyde to react.

• Reducing ability is linked to the ability to revert to aldehyde form.

What are reducing and non-reducing sugars?

• Disaccharide = 2 sugars; trisaccharide = 3; polysaccharide = many.

• Glycosidic bond: alcohol of one sugar + hemiacetal of another → acetal + H₂O.

• Bonds can be hydrolysed with H⁺ or enzymes.

What are polysaccharides and how are glycosidic bonds formed?

• ⍺-Amylose: ⍺ 1–4 bonds; digestible; found in starch.

• Cellulose: β 1–4 bonds; not digestible; found in plant walls.

• Different glycosidic linkages → different biological roles and digestibility.

What are examples of polysaccharides and their properties?

• Aromatic rings: delocalised π electrons (6 or 10), high stability, low reactivity.

• Benzene = classic example; all bonds equal length; 6 delocalised e⁻.

• Reactivity depends on electron density and substituents.

What are aromatic compounds and their properties?

• Heterocyclic: contain non-carbon atom (e.g., N); affects electron count and reactivity.

• Fused rings: share carbons; 10 delocalised e⁻ (e.g., purines).

• Pyrimidines = heterocyclic; Purines = fused aromatic → components of DNA/RNA bases.

What are heterocyclic and fused aromatic compounds?

• Nucleoside = nucleobase + sugar; bond at anomeric carbon (β configuration).

• Nucleotide = nucleoside + phosphate on 5' carbon.

• ATP = nucleotide with 3 phosphates; releases energy by breaking P bond.

What are nucleosides and nucleotides?

• Nucleotides polymerise via phosphate ester bonds (5' to 3' direction).

• Phosphate of one nucleotide binds to 3' OH of another → sugar-phosphate backbone.

• Directionality: 5' → 3'; crucial for replication and transcription.

How do nucleotides form nucleic acids?

• RNA: OH on 2' carbon → can attack adjacent phosphate → cleavage.

• DNA: only H on 2' carbon → no internal attack → more stable.

Why is RNA less stable than DNA?