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Chem 345 Reaction Sheets: Acetal formation Mechanism, Lecture notes of Organic Chemistry

The mechanism consists of hemiacetal formation followed by an SN1 reaction. The carbonyl oxygen is lost as water. Acetals are useful to protect ketones and ...

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reaction.%%The%carbonyl%oxygen%is%lost%as%water.%%Acetals%are%useful%to%protect%ketones%
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Download Chem 345 Reaction Sheets: Acetal formation Mechanism and more Lecture notes Organic Chemistry in PDF only on Docsity!

Acetal formation Mechanism: Summary (Key words): Acetals are made under acidic conditions, so no basic compounds can be used in the mechanism. The mechanism consists of hemiacetal formation followed by an SN 1 reaction. The carbonyl oxygen is lost as water. Acetals are useful to protect ketones and aldehydes from nucleophiles such as Grignards. Acetal formation is reversible. O OH cat. H 2 SO 4 O O O OH cat. H 2 SO 4

Acetal Cleavage Mechanism: Summary (Key words): Acetals are cleaved by acid and water. The carbonyl oxygen comes from water. Acetals are useful to protect ketones and aldehydes from nucleophiles such as Grignards. All steps in the mechanism are reversible. O O cat. H 2 SO 4 H 2 O O HO OH O O cat. H 2 SO 4 H 2 O

Imine Hydrolysis Mechanism: Summary (Key words): Imines are slightly basic. It is not necessary for there to be an acid catalyst. Imines are water sensitive and are readily converted to carbonyls. All steps are reversible. NH 2 cat. H 2 SO 4 H 2 O N (^) O cat. H 2 SO 4 H 2 O N

Cyanohydrin Formation Mechanisms: Summary (Key words): Reaction is reversible. Cyanohydrins are base sensitive. Cyanide is a good nucleophile. It attacks the carbonyl directly. O AcOH, KCN or cat. NaOH, HCN CN OH racemic O AcOH,^ KCN O cat. NaOH, HCN

Reduction of a ketone with sodium borohydride Mechanism: Summary (Key words): Addition of the hydride to carbonyl is irreversible. The hydrogen that attacks the carbonyl carbon comes from sodium borohydride. The proton that adds to the oxygen comes from the alcohol solvent. Sodium borohydride is basic. It can also deprotonate OH’s (cyanohydrins, hemiacetals) NaBH 4 EtOH O OH NaBH 4 EtOH O

Reaction of a ketone with a Grignard Reagent Mechanism: Preparation of a Grignard Reagent: Summary (Key words): Addition of the carbon group to the carbonyl is irreversible. Always a two step procedure. Grignards are really strong bases. It will react with acidic hydrogens very quickly. This includes OH’s on cyanohydrins or hemiacetals. OH O 1.) MeMgBr, Et 2 O 2.) H 3 O+ O 1.) MeMgBr, Et 2 O 2.) H 3 O+ MeBr MeMgBr O Mg

Acid catalyzed hydrolysis of an ester Mechanism: Summary (Key words): All Steps are reversible. If the ester is a tertiary ester, a different mechanism will likely take place. This reaction is acid catalyzed only. Base will not work. OMe O cat. H 2 SO 4 H 2 O OH O MeOH OMe O cat. H 2 SO 4 H 2 O

Saponification Mechanism: Summary (Key words): This contains an irreversible step. Deprotonation of the carboxylic acid by the alkoxide that is generated. You need one equivalent of sodium hydroxide, ie: catalytic base will not work. OH O OMe MeOH O 1.) NaOH 2.) H 3 O+ OMe O 1.) NaOH 2.) H 3 O+

Reaction of a ketone with a Wittig Reagent Mechanism: Preparation of a Wittig Reagent: Summary (Key words): Formation of the four member ring may be one step or two steps. The resulting alkene can be mono, di, or tri substituted. Tetra substituted alkenes are typically too hindered to form via a Wittig reaction. If two diastereomers is possible, you can get either E or Z depending on the structure of the Wittig reagent and the base used to form it. (You get to choose which diastereomer you get as the major product in Chem 345). Real life on the other hand… O PPh 3 O PPh 3 PPh 3 Br 1.) PPh 3 2.) KOtBu

Acid chloride formation Mechanism: Summary (Key words): Reaction is an irreversible reaction. Acid Chlorides are very reactive compounds. In synthesis type problems, once you make it, use it in the next step. Thionyl chloride will also change normal alcohols in chlorides via an SN1 or SN2 mechanism depending on the alcohol. OH O SOCl 2 Cl O OH O SOCl 2

Cleavage of a tertiary ester Mechanism: Summary (Key words): This is just an SN1 reaction. All steps reversible. If sulfuric acid is used, the mechanism switches to an E1 reaction. OH O O O HCl Cl O O HCl

Reaction of an acid chloride with an alcohol Mechanism: Summary (Key words): Irreversible reaction. You need a tertiary amine or pyridine if the alcohol is tertiary. It does not hurt to include an tertiary amine or pyridine with every alcohol O O Cl O EtOH NEt 3 Cl O EtOH NEt 3

Reaction of an acid chloride with an amine Mechanism: Summary (Key words): Irreversible reaction. Either an excess of amine or a sacrificial amine such as triethyl amine must be present otherwise poor yields (less than 50%) occur. Cl O MeNH 2 NEt 3 N H O Cl O MeNH 2 NEt 3

Hydrolysis of an amide Mechanism: Summary (Key words): Very slow process. Often needs heat. Amides are not very reactive to nucleophilic attack. Under acidic conditions, so strong bases such as negative nitrogens and negative oxygens should be avoided. N O H 2 SO 4 H 2 O OH O N O H 2 SO 4 H 2 O