Flashcards · 9701 · A Level Topic 29

An introduction to A Level organic chemistry (A Level) flashcards

18 cards on A Level Topic 29, subtopics 29.1 to 29.4. Pick a subtopic or study them all.

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Functional groups new at A Level (with an example of each): Arene (benzene), halogenoarene (chlorobenzene), phenol, acyl chloride (propanoyl chloride, CH₃CH₂COCl), amide (propanamide, CH₃CH₂CONH₂), amino acid (2-aminoethanoic acid), secondary and tertiary amines (e.g. dimethylamine).
How are substituents on a benzene ring named?: Number round the ring to locate them, e.g. 3-nitrobenzoic acid (nitro group at position 3) or 2,4,6-tribromophenol (Br at 2, 4 and 6).
Draw/describe the functional groups of an acyl chloride and an amide: Acyl chloride: –COCl (C=O with Cl on the same carbon). Amide: –CONH₂ (or –CONHR for an N-substituted amide).
Secondary and tertiary amines: Secondary: two alkyl or aryl groups on the nitrogen (R₂NH, e.g. (CH₃)₂NH). Tertiary: three (R₃N). 9701 asks you to recognise and draw them, not name them systematically.

Electrophilic substitution (arenes): The reaction of arenes: an electrophile replaces an H atom on the ring, and the delocalised ring is restored.
Addition–elimination: A nucleophile adds to a carbonyl carbon, forming a tetrahedral intermediate, which then eliminates a leaving group (e.g. Cl⁻) to restore C=O. Overall it looks like substitution. Typical of acyl chlorides.
Why do alkenes undergo addition but arenes undergo substitution?: An alkene's localised π bond is used up in addition, gaining two σ bonds. Benzene's delocalised ring is especially stable; addition would destroy it, while substitution keeps it.
Three stages common to electrophilic substitution of benzene: Generate the electrophile; the delocalised ring attacks it, forming an unstable positive intermediate; the intermediate loses H⁺ to restore the delocalised ring.

Shape and bonding in benzene: Planar regular hexagon, bond angles 120°. Each carbon is sp² hybridised with one electron in a p orbital; the p orbitals overlap sideways to form a delocalised π system above and below the ring.
C–C bond length in benzene compared with single and double bonds: All six are 139 pm, between C–C (154 pm) and C=C (134 pm).
Three pieces of evidence for delocalisation in benzene: 1 All C–C bond lengths are equal. 2 Its enthalpy of hydrogenation is about 152 kJ mol⁻¹ less exothermic than 3 × cyclohexene predicts. 3 It resists addition and does not decolourise bromine water.
σ and π bonding in benzene: Each carbon forms three σ bonds (two C–C, one C–H) in the plane; the six p electrons form one delocalised π system rather than three separate π bonds.

Which of butan-2-ol, propan-2-ol, lactic acid and glycine have a chiral centre?: Butan-2-ol: yes (C2: H, OH, CH₃, C₂H₅). Propan-2-ol: no (two CH₃). Lactic acid, CH₃CH(OH)COOH: yes. Glycine: no (two H on the CH₂).
How do two enantiomers differ in properties?: Identical physical and chemical properties except: they rotate plane-polarised light in opposite directions by the same angle, and they may have different biological activity.
Why can enantiomers have different biological activity?: Receptor sites and enzymes are themselves chiral, so only one enantiomer may have the right 3D shape to fit.
Optically active, and racemic mixture: Optically active: rotates the plane of plane-polarised light (a single enantiomer). Racemic mixture: an equimolar mixture of two enantiomers; optically inactive because the rotations cancel.
Why do many drug syntheses give a racemic mixture?: When a chiral centre is created from non-chiral reactants (e.g. HCN adding to a planar C=O), attack from either side is equally likely, giving equal amounts of both enantiomers.
Why make a drug as a single enantiomer, and how?: Smaller dose, fewer side effects from the other enantiomer. Separating a racemate is hard and expensive (identical physical properties, up to half wasted), so chiral catalysts are used to make mainly one isomer.