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A-Level Terminology and mechanisms of organic reactions
What the A-Level syllabus expects for Terminology and mechanisms of organic reactions, and how to practise it.
What the syllabus expects
- Read and apply the vocabulary attached to organic reactions:
- functional group
- degree of substitution: primary, secondary, tertiary, quaternary
- homolytic and heterolytic fission
- carbocation
- free radical
- electrophile (Lewis acid) and nucleophile (Lewis base)
- addition, substitution, elimination, condensation, hydrolysis
- oxidation and reduction
Scope: for organic redox equations the symbols [O] and [H] are acceptable - Read and apply the vocabulary attached to organic reactivity:
- delocalisation
- electronic effect, whether electron-donating or electron-withdrawing
- steric effect, that is steric hindrance
- Explain why alkanes are largely unreactive, even towards polar reagents.
- Explain why alkenes react readily with electrophilic reagents.
- Using the delocalisation of pi electrons, explain how benzene and an alkene differ in:
- how each responds to electrophiles
- why benzene would rather substitute than add
- Account for the differing reactivities of halogenoalkanes, keeping hydrolysis and the strength of the carbon-halogen bond in view.
- Explain why chlorobenzene resists nucleophilic substitution far more than a halogenoalkane does, pointing to delocalisation of the halogen's lone pair and to steric hindrance.
- Explain why carbonyl compounds react with nucleophilic reagents such as hydrogen cyanide.
- Bring the terms of (a) and (b) to bear on mechanisms, framed by organic structure and bonding.
- Recognise that in a polar reaction the electrons travel from electron-rich sites toward electron-poor ones.
- Set out the free-radical substitution mechanism through its initiation, propagation and termination stages, using ethane reacting with chlorine.
- Set out the electrophilic addition mechanism in alkenes, taking bromine (Br2 in CCl4) adding to ethene as the example.
- Set out the electrophilic substitution mechanism in arenes, with the single bromination of benzene as the example.
- Describe how the delocalised electrons of an arene bear on such reactions.
- Describe and account for how nucleophilic substitution proceeds in halogenoalkanes:
- SN1, resting on how stable the carbocation intermediate is
- SN2, resting on steric hindrance within the halogenoalkane
- Set out the nucleophilic addition mechanism when hydrogen cyanide reacts with aldehydes and with ketones.
How it's examined
Questions on this topic most often ask you to show, suggest. About 7% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Set out the mechanism for the decarboxylation of trichloroethanoic acid, including all lone pairs, charges and curly arrows. Step 1: water acts as a Bronsted-Lowry base towards the acid. Step 2: the conjugate base from step 1 loses CO2. Step 3: the intermediates from steps 1 and 2 combine to give CHCl3.
Show the worked answer
Step 1: A lone pair on the O of H2O attacks/removes the acidic H of CCl3COOH. Curly arrow from an O lone pair of water to the acidic H; curly arrow from the O-H bond of the carboxylic acid to the O. Products: H3O+ (oxonium, positive charge on O) and the carboxylate CCl3COO- (negative charge delocalised on the two carboxylate O atoms, each carrying lone pairs). Step 2: The carboxylate CCl3COO- loses CO2. Curly arrow from the C-C bond into the CCl3 carbon (forming the carbanion), and the electrons of the C-O reorganise to give O=C=O. Products: CO2 and the carbanion :CCl3- (trichloromethyl anion, lone pair and negative charge on C). Step 3: The carbanion :CCl3- (from step 2) takes a proton from H3O+ (the oxonium from step 1). Curly arrow from the C lone pair of :CCl3- to the H of H3O+; curly arrow from the O-H bond back to O. Products: CHCl3 + H2O.
Example 2 (3 marks)
Tautomerisation is a form of isomerism in which two species sharing one molecular formula but differing in atomic connectivity (constitutional isomers) rapidly interconvert at equilibrium in solution. The classic case is the shift between the keto form (bearing a carbonyl) and the enol form (bearing an -OH group next to a C=C double bond). Draw the acid-catalysed mechanism by which propanone converts into its enol tautomer, including every curly arrow, all relevant lone pairs and any charges.
Show the worked answer
Acid-catalysed keto-to-enol tautomerisation of propanone, CH3COCH3. Step 1 - protonation of the carbonyl oxygen: a lone pair on the carbonyl O attacks H+ (from H3O+ / acid). Curly arrow from an O lone pair to the H; the O-H bond forms and O becomes positively charged. This gives the protonated ketone CH3-C(+)(OH)-CH3 (with the positive charge delocalised onto carbon), i.e. C=O(+)H. Step 2 - loss of an alpha proton: a base (e.g. H2O) removes a hydrogen from the alpha carbon (a CH3). Curly arrow from a lone pair on H2O to the alpha C-H hydrogen; a second curly arrow from the alpha C-H bond into the C-C bond to form the C=C double bond; a third curly arrow from the C=O(+) pi bond onto the oxygen, neutralising it and forming the O-H of the enol. Product: the enol, CH2=C(OH)-CH3, plus regenerated H3O+ (catalyst returned). Show: O lone pairs, the + charge on protonated intermediate, and all three curly arrows in the deprotonation step.
Example 3 (2 marks)
In step 3 the triiodoethanal reacts with the nucleophile OH- by substitution rather than the addition you might expect. Suggest a reason for this.
Show the worked answer
OH- adds to the carbonyl carbon, but the resulting tetrahedral intermediate expels the CI3- group instead of just giving an alcohol. The -CI3 group is an unusually good leaving group because the negative charge on the departing CI3- carbanion is stabilised by the three highly electronegative/electron-withdrawing iodine atoms. This makes C-C cleavage (substitution) favourable, unlike ordinary aldehydes where no such stable leaving group is available and simple nucleophilic addition occurs.
More worked questions on this topic
- Lay out the mechanism by which benzene is converted to acetophenone, drawing the electron-pair (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry