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Carbonyl chemistry
Uni of Notts, fundamentals of inorganic & organic chemistry, first year
| Term | Definition |
|---|---|
| How nucleophilic addition changes carbonyl orbitals | Forces double bond domain to break & redistribute pi electrons into the 3 existing sp2 orbitals to hybridise to 4 sp3 orbitals |
| Main factor affecting carbonyl reactivity | Groups adjacent to the carbonyl. This depends on electronegativity, size, attached groups, & lone pairs |
| Role of the oxygen in carbonyls | Very electronegative double-bonded species forming a permanent dipole allowing for nucleophilic attack. The double bond can also be broken to form a resonance hybrid |
| How nucleophiles add themselves to carbonyl groups (Burgi-Dunitz trajectory) | Nucleophiles attack the π* LUMO at a 107* angle to maximise attraction to the LUMO while minimising repulsion from adjacent groups. The nucleophile places its HOMO in the LUMO to form a covalent bond |
| Grignard reagents | Akyl groups covalently bound to MgBr, formed by heating alkyl halides in dry ether with Mg. They make powerful nucleophiles to form alcohols with carbonyls |
| How Grignard reagents work | C-Mg bond is so MgBr+ dissociates & the C- attacks the LUMO to form a tetrahedral alkoxide. The MgBr+ forms a salt with the negatively charged oxygen until it's washed with protons during work-up |
| How borohydrides work | Since hydrogen is more electronegative than boron, it pulls electron density to it making it δ- & allowing the bond to break, forming nucleophilic hydride ions |
| Cyanide as a nucleophile | Once it attacks the LUMO, it forms a reversible tetrahedral intermediate, this becomes irreversible once protonated. This can be reversed though by deprotonating the alcohol & heating causing it to collapse into a carbonyl & expel the CN- |
| Reactivity of carbonyls - electron explaination | The closer in energy the HOMO & LUMO are, there more reactive due to lack of steric hindrance & increased attraction. Negative inductive elements (Cl) lower HOMO energy while vice versa (N) raises the energy gap |
| Selective reactivity of carbonyls with hydrides | Weaker hydrides (Like NaBH4) can only reduce the most reactive carbonyls whereas stronger hydrides (Like LiAlH4) can reduce any carbonyl, even amides |
| How reduction of cyclic esters (lactones) differs from cyclic amides (lactams) | Lactones have each oxygen reduced to alcohols which cleaves the C-O in the ring & forms a linear diol. Lactams have their oxygen removed as AlHO making them cyclic amines due to nitrogen being a better electron donor than oxygen |
| Reduction of esters part 1 | H- adds to carbonyl making an sp3 tetrahedral structure. Lithium forms covalent character bond with O- & is then replaced by aluminium hydride to form a salt called an alumane |
| Reduction of esters part 2 | puts pressure on Al-H causing it to break into an H- which adds to the carbonyl to cleave the C-O bond making an alkoxide leaving group leaving behind an aldehyde |
| Reduction of esters part 3 | A 2nd LiAlBH4 binds to the O- in the same way as the first step to reduce the ketone to an alkoxide which is protonated in workup. This is more efficient since ketones are more reactive than esters |
| Reduction of amides part 1 | H- adds to carbonyl forming tetrahedral, Li binds to the the oxygen before being replace with AlH3 to form an alumane. Since Al oxide is a better leaving group than the amine (higher pKa), the AlHO is removed like an elimination |
| Reduction of amides part 2 | N forms a double bond with an adjacent C, making it a carbocation that it neutralises with its lone pair, to form an iminium intermediate which is highly electrophilic |
| Reduction of amides part 3 | The iminium can be reduced to an amine by the remaining hydride, this is why the C-N bond isn't cleaved in a lactam |
| How ethanol forms hemiacetal with acetone by nucleophilic substitution | With no acid catalyst it forms a tetrahedral intermediate with deprotonated O on the carbonyl & protonated O on the ethanol. The proton moves from oxonium to alkoxide by tautomerisation |
| Mutarotation | Changing the optical rotation in cyclical acetals (such a sugars) by opening the ring with tautomerisation (acid catalysed) to reform the carbonyl, rotating the groups, then recyclizing. This is why glucose forms a racemate |
| Hemiacetals & acetals | Hemiacetal: Central C is bound to -OH & -OR simultaneously Acetal = Central C is bound to 2-OR groups from protonation of the hemiacetal -OH to -OH2+ to be substituted for another alcohol |
| Why acetal formation always uses E1 & never E2 | The σ* LUMO has too many groups around it repelling the nucleophile sterically which is why oxonium is used to produce a water leaving group & an acid catalyst is required |
| Industrial acetal formation problems | The reaction is in equilibrium since it produces water & can be hydrolysed back to hemiacetal making it difficult to keep the process anhydrous |
| Azeotrope | A mixture of 2 liquids which have the same concentration in the gas phase as the liquid phase due to lowering each other's boiling point to the same level |
| Dean-Stark method of acetal production | Toluene azeotropes water & they both boil at 84*C to prevent reaction reversal. A clay molecular sieve &/or excess alcohol can be used on a smaller scale though. A ration of 2:1 alcohol:carbonyl is used with acid catalysis |
| Geminal carbon | A carbon bonded to 2 of the same functional groups e.g., acetal but not hemiacetal |
| Iminium synthesis part 1 | 2* amine nucleophilic lone pair attacks carbonyl on aldehyde which protonates the oxygen to alcohol & amine is protonated. Tautomerisation transfers proton from nitrogen to hydroxyl to form an R-OH2+ ion which leaves as water |
| Iminium synthesis part 2 | Lone pair on N donates to carbocation to form a double bond & positive charge, this is iminium & can resonate between the positive charge on the N & C |
| Enamine | Stabilised iminium where the pi orbitals are between 2Cs to form a very conjugated nucleophilic & can be reversed with water. Synthesised from 2* amine & ketone or E1 of iminium, can cyclise to bicyclic iminium |
Created by:
Beech47
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