Benzene
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State the type of reaction in step 1. Identify the reagent(s) and conditions needed for step 1. Which is turning a benzene into a phenolketone
Acylation/ electrophylic substitution M2 CH3COCl OR Ethanoyl chloride M3 AlCl3 OR Aluminium chloride (mark could be awarded in space for M2)
Compare benzene and the theoretical molecule cyclohexa-1,3,5-triene in terms of: • stability • shape • carbon–carbon bond lengths. For each of these properties, suggest reasons for any differences.
Benzene is more stable than cyclohexatriene M2 The enthalpy of hydrogenation of benzene is (152 kJ mol–1) less / less exothermic M3 Due to the delocalisation of electrons in benzene M4 Both are planar / hexagonal M5 Benzene has equal C-C bond lengths or regular hexagon whereas Cyclohexa-1,3,5-triene has bonds of different/varied length or the hexagon is distorted/irregular
Mechnism and reagents for turning a phenylketone into alcohol
Nucleophilic addition M2 NaBH4 Allow LiAlH4 for M2 M3 1-phenyl ethan(-1-)ol
In practice, ethanoic anhydride is used in the industrial synthesis rather than ethanoyl chloride. Give one reason why ethanoyl chloride is not used in the industrial synthesis.
Corrosive OR forms strong acid/HCl (fumes) OR vulnerable to hydrolysis OR dangerous (to use)
ompound K (C6H5CH2NH2) is a structural isomer of J. Explain why J is a weaker base than K.
Lone or electron pair on N in J spread / delocalised into ring (or not delocalised in K) I Allow interacts with Π cloud for M2 M2 less available (for protonation or donation in J)
Suggest an advantage of surgical sutures made from PGA rather than from poly(propene). Explain your answer. Pga is a condensation polymer and polypropene is addition
PGA sutures react/dissolve/break down/are biodegradable/ are hydrolysed / attacked by water or nucleophiles /no need to remove OR Polypropene not biodegradeable/ not hydrolysed / not attacked by water/nucleophiles (Ester links have) polar bonds
Explain, in terms of structure and bonding, why benzene is more thermodynamically stable than cyclo1,3,5triene 1mark
π) electrons delocalised
plain the bonding in and the shape of a benzene molecule. Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule. 6marsk
Stage 1 Bonding 1a) Each C has three (covalent) bonds 1b) Spare electrons (in a p orbital) overlap (to form a π cloud) 1c) delocalisation Stage 2 Shape 2a) Planar 2b) Hexagon/6 carbon ring/120° bond angle 2c) C–C bonds equal in length / C–C bond lengths between single and double bond Stage 3 Stability 3a) Expected ∆Ho hydrogenation of cyclohexatriene = –360 kJ mol–1 3b) ∆Ho hydrogenation benzene (is less exothermic) by 152 kJ mol–1 3c) Benzene lower in energy than cyclohexatriene / Benzene is more stable
(b)The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene. Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.
Value within range –239 to –121 Double bonds separated by one single bond / alternating (or shown in structure) Allows some delocalisation/overlap of p orbitals
The hydrocarbons benzene and cyclohexene are both unsaturated compounds. Benzene normally undergoes substitution reactions, but cyclohexene normally undergoes addition reactions. (a) The molecule cyclohexatriene does not exist and is described as hypothetical. Use the following data to state and explain the stability of benzene compared with the hypothetical cyclohexatriene.
Benzene is more stable than cyclohexatriene Expected ΔHο hydrogenation of C6H6 is 3(–120) = –360 kJ mol-1 M3 Actual ΔHο hydrogenation of benzene is 152 kJ mol-1 Because of delocalisation of electrons
The the following data to show the stability of benzene relative to the hypothetical cyclohexa-1,3,5-triene. Give a reason for this difference in stabilit
Cyclohexane evolves 120 kJ mol–1 (expect triene to evole) 360 kJ mol –1 (1) or 3 × 120 360 – 208 = 152 kJ (1) NOT 150
ate and explain the difference in base strength between phenylamine and ammonia.
Phenylamine weaker (1) if wrong no marks lone pair on N (less available) (1) delocalised into ring (1) or “explained”
State the type of reaction in step 1. Identify the reagent(s) and conditions needed for step 1. Benzene to phenylketone
Elecrophylic substitution Ethanoyl chlordie reagent Conditions anhydrouis aluminium chloride
Phenyl ketone —> phenyl ehtanol
Nucleophilic addition Nabh4