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CHEM2213 Topic 9.docx

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Department
Chemistry
Course Code
Chemistry 1027A/B
Professor
Mark Workentin

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Topic 9 Aldehydes & Ketones What Are Aldehydes & Ketones?  Aldehyde: compound containing a carbonyl group bonded to H (CHO group)  Ketone: compound containing a carbonyl group bonded to 2 Cs  C-O double bond consists of one sigma bond formed by overlap of sp 2 hybrid orbitals of C and O and one pi bond formed by the overlap of parallel 2p orbitals What Are The Physical Properties Of Aldehydes & Ketones?  O is more electronegative than C, C-O double bond is polar with O bearing a partial negative charge and C bearing a partial positive charge  In reactions of a carbonyl group, C acts as an electrophile and a Lewis acid and O acts a nucleophile and Lewis base  Aldehydes and ketones are polar compounds and have higher boiling points  Interact with water molecules by H bonding, soluble in water What Is The Most Common Reaction Theme Of Aldehydes & Ketones?  Addition of a nucleophile to form a tetrahedral carbonyl addition intermediate What Are Grignard Reagents, And How Do They React With Aldehydes & Ketones?  Organometallic Compound: compound containing a C-metal bond  Grignard Reagent: organomagnesium compound of the type RMgX or ArMgX o Polar covalent, with C bearing a partial negative charge and Mg bearing a partial positive o Carbanion: anion in which C has an unshared pair of electrons and bears a negative charge  Grignard reagents are very strong bases and react readily with a wide variety of acids to form alkanes  Any compound containing an O-H, N-H, and S-H group or a relatively acidic H will react with a Grignard reagent by proton transfer (preventing reaction with carbonyl from occurring)  Grignard reagents cannot be made from any halogen-containing compounds that also contain them because they react so rapidly with these protons acids  A carbanion is a good nucleophile and adds to the carbonyl group of an aldehyde / ketone to form a tetrahedral carbonyl addition intermediate o Driving force is attraction of partial – charge on the C of the organmetallic compound to partial + charge on the C of carbonyl o Alkoxide ions formed in Grignard reactions are strong bases and form alcohols when treated with aqueous acid  Addition to aldehyde gives a 2° alcohol (except formaldehyde which gives 1°) while addition to ketone gives a 3° alcohol What Are Hemiacetals & Acetals?  Hemiacetal: molecule containing an –OH and an –OR or –OAr group bonded to the same C  Addition of a molecule of alcohol to the carbonyl group of an aldehyde / ketone forms a hemiacetal (half-acetal)  Reaction is catalyzed by both acid and base: O added to carbonyl C and H adds to the carbonyl O  Base-catalyzed formation of a hemiacetal: o Proton transfer from alcohol to the base gives an alkoxide ion o Addition of the alkoxide ion to the carbonyl gives a tetrahedral carbonyl addition intermediate o Proton transfer from water to the tetrahedral carbonyl addition intermediate gives the hemiacetal and regenerates the hydroxide ion catalyst  Acid-catalyzed formation of a hemiacetal: o Proton transfer from H-A to the carbonyl gives a resonance stabilized cation, positive charge on the C o Addition of the alcohol to the resonance stabilized cation gives an oxonium ion o Proton transfer from the oxonium ion to A gives the hemiacetal and regenerates the acid catalyst  Hemiacetals are generally unstable and are only minor components of an equilibrium mixture  When a –OH is part of the same molecule that contains the carbonyl group, and a 5- or 6- membered ring can form, the compound exists almost entirely in a cyclic hemiacetal form  Acetal: molecule containg two –OR or –OAr groups bonded to the same C  Hemiacetals can react further with alcohols to form acetals plus a molecule of water  The reaction is acid catalyzed o Proton transfer from the acid, H-A to the hemiacetal OH group gives an oxonium ion o Loss of water from the oxonium ion gives a resonance stabilized cation o Reaction of the resonance stabilized cation (electrophile) with methanol (nucleophile) gives the conjugate acid of the acetal o Proton transfer from the protonated acetal to A gives the acetal and generates a new molecule of H-A  Excess of alcohol pushe
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