Some examples from this set of practice questions
1.
What is the molecular formula of benzene
Answer: C6H6
2.
Describe the bonding in a molecule benzene
Answer: Each carbon has 3x sigma bonds (2x to carbon, 1x to hydrogen) Forming a planar hexagonal ring Sideways overlap of 6x adjacent p-orbitals forms a delocalised pi ring electron system above and below the plane of the carbon ring
3.
Evidence for delocalised model:
Answer: Lack of reactivity in comparison to alkene (eg benzene does not decolourise bromine water) Enthalpy change of hydrogenation less exothermic than expected (compared to expected 3x value for cyclohexene) C-C bond lengths are the same length instead of expected 3x single (long) and 3x double (short)
4.
Give the name and formulae of the oxidising agent used for oxidising alcohols and aldehydes
Answer: Acidified potassium (or sodium) dichromate H2SO4 / K2Cr2O7 Acid usually conc.
5.
Name a reducing agent for the reduction of an aldehyde or ketone
Answer: Sodium borohydride (NaBH4) and water NaBH4 is a source of hydride ions :H-
6.
Which reagent is used to test for both aldehydes and ketones
Answer: 2,4-DNPH (also called Brady’s reagent)
7.
What is the positive result for an aldehyde or ketone with 2,4-DNPH
Answer: Yellow/Orange Precipitate
Some examples from this set of practice questions
1.
What are the reagents and positive result for an alkene?
Answer: Bromine water Orange -> colourless solution
2.
What are the reagents and positive result for a phenol?
Answer: Bromine water Orange -> colourless solution AND white precipitate
3.
What are the reagents and positive result for a Ketone or Aldehyde?
Answer: Brady’s Reagent (2,4-DNPH) Yellow/orange precipitate produced
4.
How can you confirm the exact identity of the aldehyde/ketone?
Answer: Isolate the precipitate by filtration Purify by recrystallisation Measure melting point Compare to known values
5.
What are the reagents and positive result for a aldehyde?
Answer: Tollens reagent Silver mirror
6.
What are the reagents and positive result for a primary or secondary alcohol?
Answer: Sulfuric acid & Potassium dichromate H2SO4 & K2Cr2O7 Colour change: orange -> green
7.
What are the reagents and positive result for a haloalkane?
Answer: Aqueous silver nitrate and ethanol (solvent) 50 C Precipitate formed: White – chloro Cream – bromo Yellow - Iodo
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