Examples of acyl in the following topics:
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- The overall transformation is defined by the following equation, and may be classified either as nucleophilic substitution at an acyl group or as acylation of a nucleophile.
- Reactivity: acyl halides > anhydrides >> esters ≈ acids >> amides
- Before proceeding further, it is important to review the general mechanism by means of which all these acyl transfer or acylation reactions take place.
- The first three examples concern reactions of acyl chlorides, the most reactive acylating reagents discussed here.
- No acylation reactions of amides were shown in these problems.
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- The Friedel-Crafts acylation reagent is normally composed of an acyl halide or anhydride mixed with a Lewis acid catalyst such as AlCl3.
- Some examples of Friedel-Crafts acylation reactions are shown in the following diagram.
- The first demonstrates that unusual acylating agents may be used as reactants.
- The second makes use of an anhydride acylating reagent, and the third illustrates the ease with which anisole reacts, as noted earlier.
- Since the nitro group is a powerful deactivating substituent, Friedel-Crafts acylation of nitrobenzene does not take place under any conditions.
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- Acyl halides and anhydrides are more easily halogenated than esters and nitriles, probably because of their higher enol concentration.
- Thus, conversion of the acid to its acyl chloride derivative is followed by alpha-bromination or chlorination, and the resulting halogenated acyl chloride is then hydrolyzed to the carboxylic acid product.
- This simple modification works well because carboxylic acids and acyl chlorides exchange functionality as the reaction progresses.
- The final product is the alpha-halogenated acid, accompanied by a trace of the acyl halide.
- To see a mechanism for the acyl halide-carboxylic acid exchange, view the second diagram below.
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- Most of the electrophiles are good acylating reagents, so it is reasonable to expect an initial acylation of the sulfoxide oxygen.
- (The use of DCC as an acylation reagent was described elsewhere. ) The electrophilic character of the sulfur atom is enhanced by acylation.
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- Triglycerides having three identical acyl chains, such as tristearin and triolein (above), are called "simple", while those composed of different acyl chains are called "mixed".
- If the acyl chains at the end hydroxyl groups (1 & 3) of glycerol are different, the center carbon becomes a chiral center and enantiomeric configurations must be recognized.
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- The following illustration shows the decomposition of a bicyclic bridgehead acyl peroxide.
- The concurrent formation of ester and dimeric cycloalkane products from acyl peroxides is common, and reflects a cage effect in homolysis reactions.
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- Cleavage reactions of β-lactones may take place either by acid-catalyzed acyl exchange, as in 4a, or by alkyl-O rupture by nucleophiles, as in 4b.
- Finally, the β-lactam cleavage of penicillin G (reaction 6) testifies to the enhanced acylating reactivity of this fused ring system.
- Most amides are extremely unreactive acylation reagents, thanks to stabilization by p-π resonance.
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- Three examples of acyl groups having specific names were noted earlier.
- Esters: The alkyl group is named first, followed by a derived name for the acyl group, the oic or ic suffix in the acid name is replaced by ate. e.g.
- Acid Halides: The acyl group is named first, followed by the halogen name as a separate word. e.g.
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- The rearrangement of acyl nitrenes to isocyanates that is the crux of the Hofmann, Curtius and Lossen rearrangements, is paralleled by the rearrangement of acyl carbenes to ketenes, a transformation called the Wolff rearrangement.
- The starting acid, written on the left, is converted first to an acyl chloride derivative, and then to a diazomethyl ketone.
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- In a similar sense, acyl chlorides are the most reactive substrate.