Revision Session

Revision Session Organic Chemistry

Stereochemistry and mechanism Starter Past Paper Questions and Answers

Aromaticity Starter Past Paper Questions and Answers

Question 1 (a) Huckel’s Rule Discuss [100%]

Aromatics

Revision and Past Paper Questions and Answers Carbonyl Group Chemistry

Basic Carbonyl Chemistry • Carbonyl acts as both an electrophileand anucleophile • Carbonyl has resonance structure

Basic carbonyl chemistry Electrophile:

Basic carbonyl chemistry Formation of Enolates and Enols Removal of alpha-proton (most acidic)

Basic carbonyl chemistry Enolate Alkylation Nucleophile

Basic carbonyl chemistry Acidities of alpha and beta carbonyl protons

Basic carbonyl chemistry 1,3 – Dicarbonyl Compounds Why is central alpha proton more acidic? Conjugation and intramolecular H – bonding

Basic carbonyl chemisty Alpha substitution reactions Reaction with an electrophile (acid catalysed)

Basic carbonyl chemistry Alpha substitution reactions Reaction with an electrophile (base catalysed)

Basic carbonyl chemistry Bases used for enolate formation Carbonyl compounds weakly acidic Strong baseneeded to ensure complete enolate formation LDA Lithium diisopropylamide

Basic carbonyl chemistry Deuterium Substitution of alpha protons When enriched with D2O (acid or base), all alpha protonsare in equilibria with deuterium Used as an isotopic label in molecules

Basic carbonyl chemistry Racemization Stereochemistry at alpha carbon is lost during enolate formation Protonated unequally at two faces

Basic Carbonyl CHemistry Halogenation of Carbonyls (acid catalysed)

Basic carbonyl chemistry Halogenation of Carbonyls (lewis acid)

Basic carbonyl chemistry Unsymmetrical Ketones Two different enolsare formed Occurs preferentially at the more substitutedposition as more stable

Basic carbonyl chemistry Enolates of Unsymetrical Ketones Kinetic favoured due to unhindered alpha protons

Basic carbonyl chemistry 1,3 – Dicarbonyl Compounds Alkylate at different sites in different conditions

Basic carbonyl chemistry Decarboxylation of beta-ketoesters

Basic carbonyl chemistry Add a -CH2CO2H to an alkyl halide

Basic carbonyl chemistry Add a -CH2COCH3 to an alkyl halide

Basic carbonyl chemistry Add a -CH2CO2H then –COCH3 to an acyl halide

Aldol reaction https://learn.uea.ac.uk/webapps/portal/frameset.jsp?tab_tab_group_id=_4_1&url=%2Fwebapps%2Fblackboard%2Fexecute%2Flauncher%3Ftype%3DCourse%26id%3D_85894_1%26url%3D Click Vignettes – Level 1 – Aldol Reaction

Basic carbonyl chemistry Aldol Reaction Mixed Aldol Reaction Use LDA (one equivalent) to convert all of aldehyde to enolate Same Ketone Aldol Reaction Use NaOEt/EtOHto convert a small proportion to enolate

Basic carbonyl chemistry Aldol Dehydration Base Catalysed Acid Catalysed (more common)

Basic carbonyl chemistry Intramolecular Aldol Reactions Dicarbonyl compounds treated with base Leads to cyclic product

Basic carbonyl chemistry Intramolecular Aldol Reactions For Unsymmetrical Dicarbonyl Species more than one product is possible

Basic carbonyl chemistry Claisen Condensation Esters react in same fashion as aldol reaction Intramolecular Claisen Condensations aswell

Basic carbonyl chemistry Knoevenagel Condensation

Basic carbonyl chemistry Michael Reaction Enolates react with conjugated carbonyls through terminus double bonds rather than the carbonyl itself!

Robinson Annelation Micheal reaction followed by intramolecular aldol Micheal Addition Intramolecular Aldol Aldol Condensation (Base catalysed)

Basic carbonyl chemistry Condensation with Amine Derivatives Ketone  Imine

Basic carbonyl chemistry Condensation with Amine Derivatives Imine  Enamine

Basic carbonyl chemistry Enamine Reactivity React simular to enolatesin neutral form. Forms iminium salt, hydrolysis leads to alkylated carbonyl

basic carbonyl chemistry Enamine Reactivity

Basic carbonyl chemistry Wittig Reaction Introduces alkenes from ketones using phosphorus ylids.

Wittig reaction Stage 1 Preparation of a phosphonium salt from an alkyl halide

Wittig reaction Stage 2 Deprotonation of the phosphonium salt to form a ylid

Wittig reaction Stage 3 Reaction of the ylid with a carbonyl group

Wittig reaction E or Z alkenes? Ylids that have a conjugatingor anion-stablilizing substituent (such as a carbonyl) adjacent to negative charge give E – Alkenes Ylids without such groups are unstabilizedylidsand tend to give Z – Alkenes

Wittig reaction Example of Z – Alkene formation Example of E – Alkene formation

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